MESOZOIC MAMMALS; Tinodontidae and Spalacotheriidae,
an internet directory: |
PLEASE NOTE: THIS PROJECT IS NOT SCIENTIFIC. IT IS A HOBBY.
"I was looking for information on an old mammal and found this lot. What is this
project?"
It's got lots of information on old mammals. For a short bit of background information, see
here.
The following owes much to Kemp, 2005 (p.162-166).
Symmetrodonta as a distinct order of mammals isn't a concept which stood up to scrutiny,
but a vaguer version can still be useful. At one time the category contained the families
of Kuehneotheriidae,
Tinodontidae and Spalacotheriidae, with the
kuehneotheriids as the most basal members. A key feature was
held to be a triangulated arrangement of main molar cusps.
However, that's not in the least unusual for mammals. As
it's unclear whether the first family had an intimate relationship with the later two, it
was ejected. As it's also unclear whether the remaining two families are in any way
closely related, it might require a bit of effort to appreciate why the term is of any use
at all. Still, it does convey a rough idea of the sort of grade of molars meant, and it
has a long history. Consequently, 'symmetrodonts' is comprehensible (to those who are
familiar with it).
The molars
The molars of 'symmetrodonts' are sort of similar. They're tall, pointy and relatively
simple. Those of the later 'symmetrodonts' are usually more simpler than the
kuehneotheriid version, in that the cingulum is weakly
developed.
The triangulated arrangement can be roughly appreciated as follows. The most basal
mammalian molars had a straight line of three main cusps: xXx. Shifting the middle one
either up or down produces a triangle. In 'symmetrodonts' the central cusp shifted; it
moved to the labial in lowers and
lingually in uppers. In tindontids and kuehneotheriids, the angle at the apex is over
90°, which is obtuse. It's acute in spalacotheriids. However, as the obtuse condition is
primitive, it's not of itself particularly informative about relationships.
The cuter acutees
At least the acute angled 'symmetrodonts' are related with each other, as shown by that very
condition and further specialisations. A second family was established in 2003,
Zhangheotheriidae, but Kemp retained Zhangheotherium
within Spalacotheriidae in his outline, (p.163): "This group shares certain
features with the remainder of Holotheria, indicating a
relationship. The most important is the exaggeration of the postvallum-prevallid shearing
action, between the back of the upper molar and the front of the lower, which is the
functional reason for the development of the acute angulation of the cusps."
Additionally
The findings of Luo et al 2002, (see Bibliography for details of the paper), place both the
tinodontids and spalacotheriids within crown-group
Mammalia, as shown by the cladogrammes on pages 14 and 15.
Reference for Symmetrodonta Simpson, 1925:
Simpson GG (1925), Mesozoic Mammalia II: Tinodon and its allies. American Journal
of Science, Series 5 10, p.451-470. |
A. Tinodontidae B.
Spalacotheriidae & Zhangheotheriidae
| Taxon: Tinodontidae Marsh, 1887
Small mammals, which haven’t been found, with tiny teeth, which have. I was very sceptical
as to the integrity of this group. However, a further genus has recently been referred to
it. As presented here, the family does apparently serve some purpose.
In terms of mammalian evolution, the tinodontids seem to be less
derived than the spalacotheriids and Co. However, quite where tinodontids fit within
Mammaldom is unclear.
Tinodon is described as "a representative of "obtuse-angle"
symmetrodonts", as opposed to the spalacotheriids, which are "acute-angle
symmetrodonts", (Luo et al 2002, p.10-11). They go on to say, (p.16):
"Kuheneotheriidae and Tinodontidae include
generally similar taxa, with principal molar cusps arranged
in an obtuse triangle (in occlusal view), and upper
triangles reversed with respect to lowers."
This was previously held to be probably a one-off evolutionary trick, linking these critters
to 'descendants' such as Dryolestes, (a "eupantothere"), and the
therians, such as myself. However: "The degree of cusp
triangulation is quite variable among "obtuse-angle symmetrodonts". The degree
of cusp triangulation varies through the molar series in Kuehneotherium (see
Parrington 1978). In Tinodon, it is clear that the posterior lower molars show more
prominent triangulation than the first lower molar, which is hardly triangulated at all
(Crompton and Jenkins 1967). A similar variability in triangulation of cusps in now also
documented in Gobiotheriodon (Averianov in press)", (p.16-17).
Similar variability is also known from non-"obtuse-angle symmetrodonts" such as
Gobiconodon and to a lesser extent Megazostrodon. All this suggests this
feature is not the result of a unique evolutionary event, and doesn't reveal much about the
systematics, (ie. relationships), among mammalian groups. And that this "obtuse"
and "acute" stuff is old hat. Luo et al 2002, (p.17), reach this conclusion:
"In summary, we consider that these archaic, "obtuse-angle symmetrodonts"
represent a heterogeneous evolutionary grade, and that they lack reliable diagnostic
features. All that can be said is that they are probably unrelated to each other or to
"acute-angle symmetrodonts". They occupy some basal
positions..." (note the use of the plural) "... in mammalian
phylogeny that are not very well resolved by the limited anatomical data known from their
currently incomplete fossils."
Averianov's article on Gobiotheriodon has since been published. Some details are
contained in the relevant entry below.
Reference: Marsh (1887), American Jurassic Mammals. American Journal of Science 33,
p.326-348.
Genera: Amphidon (partly =Eurylambda =Tinodon?),
Eurylambda (=Tinodon?), Gobiodon (=Gobiotheriodon),
Gobion (=Gobiotheriodon), Gobiondon (=Gobiotheriodon),
Gobiotheriodon, Menacodon (=Tinodon),
Tinodon, Trishulotherium,
Yermakia, other reports
Time-Line:
Lower Cretaceous: Gobiotheriodon, Tinodon (England), Yermakia,
Morocco
Upper Jurassic: Tinodon (Wyoming)
Middle Jurassic: ?England
Lower Jurassic: Trishulotherium |
| Genus: Gobiotheriodon
(Trofimov BA, 1980) Trofimov, 1997
Aka: Gobiodon Trofimov, 1974 [nomen nudum] and Cassiliano ML & Clemens WA,
1979, [nomen nudum] and Trofimov, 1980 (preoccupied); Gobion (sic!) Trofimov,
1974 [nomen nudum]; Gobiondon (sic!) Yadagiri P, 1985
Remarks: What with one thing and another, this genus has suffered from some
nomenclatural discomfort! Gobiodon refers to a fish, a form of life which I’ve
never previously seen associated with the Gobi Desert. |
| Species: | Gobiotheriodon infinitus (Trofimov BA, 1980)
Trofimov, 1997 |
| Aka: | Gobiodon infinitus Trofimov, 1974 [nomen nudum] and
Cassiliano & Clemens, 1979, [nomen nudum] and Trofimov, 1980 (preoccupied);
Gobion (sic!) infinitus Trofimov, 1974 [nomen nudum]; Gobiondon
(sic!) infinitus Yadagiri, 1985 |
| Place: | Höövör (formally known as Khoboor) |
| Country: | Mongolia |
| Age: | Aptian-Albian, Lower Cretaceous |
| Remarks: | Much of what follows is based on Averianov
AO 2002, (and thanks are due to the generous donor).
This genus seems to be a fairly basal sort of mammal, lacking the
further derived dental specializations seen in
cladotherians, (eg.
dryolestids and myself). Gobiotheriodon retained the 'reptilian'
feature of a Meckelian groove on the inside of its
jaw, (for some possibly incompetent considerations upon the significance of that,
click here and scroll down a bit to the comments
following Dryolestes leiriensis). However: "The postdentary trough is missing
and there is no clear evidence for attachment of postdentary bones", (p.706). In the
broadest sense of the term, it's a "symmetrodont" of some kind or other.
Affinities
The lack of this postdentary trough thing is more than enough to rule out affinities with
the mega old-fashioned kuehneotheriids. Its
molars are too complex for its originally proposed membership
of the decidedly iffy Amphidontidae. Averianov rejects it as a member of Spalacotheriidae,
as it lacks that group's tooth specializations. Gobiotheriodon has too few molars
to pair up with Zhangheotherium, (four in comparison to at least six), and various
other possibilities are ruled out on the grounds of paucity of evidence.
The poor creature needs some company, surely? Happily, despite some dental differences,
(which may well have much to do with the more basal nature of Tinodon), Averianov
awards Gobiotheriodon a provisional placement within Tinodontidae. "All of
these characters distinguishing Tinodon from Gobiotheriodon may be considered
as plesiomorphic for the former; loss or modification
in Gobiotheriodon is unsurprising, considering its geologically younger age",
(p.710).
Holotype
The holotype, PIN 3101/80, is a 2,6cm long, near-complete
mandible containing three molars. This resides in the
collection of the Paleontological Institute of the Russian Academy of Sciences, Moscow,
although the original description also identified it as PIN 3101/50 in the text. Other
referred material includes part of an upper jaw, (which, in the opinion of Averianov, is an
unlikely possibility. He states it: "could be currently attributed to as cf.
Gobiconodon sp.", (p.707)); and two isolated
petrosals which "may belong to Gobiotheriodon", (p.706). A petrosal
is the bone case which houses the mammalian internal ear.
Catalogue number
For some reason, Kielan-Jaworowska et al, 2000 refer to the holotype as being
PIN 3101/50 on page 596. This is presumably a typo as it changes back to /80 for
the figure on page 597. I've rechecked the Averianov 2002 paper (namely in April
2007), and I didn't make a tieping eror with it. |
| References: | Trofimov (1974), in Belyaeva EI, Trofimov BA &
Reshetov VJ General stages in evolution of late Mesozoic and early Tertiary
mammalian fauna in Central Asia, (in Russian). Trudy Sovmestnoj
Sovetsko-Mongol'skoj Paleontologiceskoj Ekspedicii 1. |
| Cassiliano & Clemens (1979), Symmetrodonta. In Lillegraven JA, Kielan-
Jaworowska Z & Clemens WA (eds.), Mesozoic Mammals: the First Two-thirds of Mammalian
History, University of California Press, p.151-161. |
| Trofimov (1980), Multituberculata and Symmetrodonta from the Lower Cretaceous of
Mongolia, (in Russian). Doklady Akademii Nauk SSSR 251, p.209-212. |
| Yadagiri P (1985), An amphiodontid symmetrodont from the Early Jurassic Kota Formation,
India. Zoological Journal of the Linnean Society 85, p.411-417. |
| Trofimov (1997), A new generic name Gobiotheriodon for a symmetrodont
mammal Gobiodon Trofimov, 1980. Acta Palaeontologica Polonica 42, p.496.
|
| Genus: Tinodon Marsh, 1879
Aka: Amphidon (partly), Eurylambda Simpson, 1929; Menacodon
Marsh OC, 1887
Remarks: McKenna & Bell give eury as a junior synonym of tino. Another
of life's mysteries is resolved.
The Oklahoma Museum of Nat His houses several T.? specimens from the Upper Cretaceous,
(Albian), of Utah. I’ve also seen a reference to the Upper Jurassic of Portugal, (Krusat,
1989)
The systematic placement of this genus within the mammal scheme of things is unclear.
"Data are limited to partial dentition and
mandible", (Luo et al 2002, p.10).
References: Marsh (1879), Additional remains of Jurassic mammals. American Journal of
Science, 18, p.215-216, with 1 fig. in.
Marsh (1887), American Jurassic mammals. American Journal of Science, 33, p.326-348.
Simpson (1929), American Mesozoic Mammalia. Mem. Peabody Museum 3pt. 1pp I-xv + 171, 62
figs, 32p.
| Reassigned species: T. ferox Marsh, 1880 see
Priacodon ferox; T. grandaevus see
Priacodon grandaevus; T. lulli see
Priacodon lulli; T. robustus Marsh, 1879 see
Priacodon robustus; | |
| Species: | Eurylambda aequicrurius (Simpson, 1925) Simpson GG, 1929 |
| Aka: | Amphidon aequicrurius Simpson GG, 1925 |
| Place: | Como Bluff, Morrison Formation, Wyoming |
| Country: | USA |
| Age: | Upper Jurassic |
| Remarks: |
Part of an upper jaw with an incomplete tooth. This is very possibly the same genus as
Tinodon, (which is based on lower jaw material), though this is not yet proven. The
holotype, YPM 13639, is at Yale.
Rougier et al 2003 report a new find from the same location. This is an upper left,
molariform tooth. "The new specimen was collected
at Como Bluff Quarry in 1883...," (p.2). Subsequently, it made its way to the
collection of the Smithsonian Institute, where it was catalogued as an undetermined mammal
tooth, (USNM 2846). Despite tentatively following the interpretation of this genus as
being a junior synonym of Tinodon bellus, these authors found it convenient to
continue using the 1929 name. Coincidentally, so do I.
It's perhaps worth mentioning that upper jaw and teeth remains are far rarer than their
lower counterparts. The bone of the lower jaw is significantly stronger, and thus more
likely to fossilize. |
| References: | Simpson GG (1925), Mesozoic Mammalia. II. Tinodon and
its allies. Amer. Journ. Sci. 10, p.451-470.
|
| Simpson (1929), American Mesozoic Mammalia. Mem. Peabody Mus.
Nat. Hist. iii (i), p.1-235. |
| Species: | Tinodon bellus Marsh OC, 1879 |
| Place: | Morrison Formation, Wyoming |
| Country: | USA |
| Age: | Upper Jurassic |
| Remarks: |
"Tinodon Marsh 1879, from the Morrison Formation of Wyoming
(Jurassic-Cretaceous boundary), stands out among the symmetrodonts as being known by
several lower jaws; the type jaw of T. bellus Marsh 1879 has M/1-M/4 in place,
while the teeth of a referred specimen are preserved from P/1 to M/4...",
(Sigogneau-Russell & Ensom 1998, p.458).
T. bellus and T. lepidus material is housed at the Peabody Museum, Yale.
There is also a specimen notated as dryolestid indet., which once had the 'MS' name,
Tinodon nanus.
There are various tinodons and pseudo-tinodons in the
collection. |
| Reference: | Marsh (1879), Notice of new Jurassic mammals. American
Journal of Science 20, p.396-398. |
| Species: | Tinodon lepidus Marsh OC, 1879 |
| Aka: | Menacodon rarus Marsh, 1887 |
| Place: | Morrison Formation, Wyoming |
| Country: | USA |
| Age: | Upper Jurassic |
| Remarks: |
This species, should it be a distinct species, is also known
from lower jaw remains; "... the type of T. lepidus Marsh 1879 keeps only two
broken molars (M/2-3); but Menacodon rarus Marsh 1887,
which was synonymized with the latter taxa by Simpson (1929)
has C to M/3", (Sigogneau-Russell & Ensom 1998, p.458). Reportedly, Simpson also
doubted that this species differs specifically from T. bellus.
"... synonym of T. bellus according to Cifelli, pers. comm. 2002",
(Averianov 2002, p.710). |
| References: | Marsh (1879), Notice of new Jurassic mammals. American
Journal of Science 20, p.396-398. |
| Marsh (1887), American Jurassic Mammals. Amer. J. of Sci. 3 ,
p.326-348. |
| Species: | Tinodon micron Ensom P & Sigogneau-Russell D,
2000 |
| Place: | Durlston Bay, Swanage, Dorset |
| Country: | England |
| Age: | Lower Cretaceous |
| Remarks: |
The following is based on my interpretation of Ensom &
Sigogneau-Russell, 2000, (with more thanks to the kindly supplier).
The holotype, DORCM GS 1110, was described as a somewhat broken lower left
molar. It's missing the tips of two of its cusps. Update:
it's probably a right molar, (Sigogneau-Russell 2003b, p.524).
Five other teeth are also referred to this taxon, one of
which is an upper molar. Although not the holotype, "we probably would not have
created a new species were it not for the upper molar", (p.774). All known fossils
are residents of the Dorset County Museum, Dorchester and originally hale from the upper
horizon of the Cherty Freshwater Member at Sunnydown Farm.
These lower molars are smaller than those of T. bellus and T. lepidus. They
also feature small, lumpy bumpy bits, (cingula), and the
cusps are differently angled.
And now things get murkier. When it comes to preserved jaw material with teeth, the
distinctions between the lower molars of Tinodon and Spalacotherium are clear
enough. However, the main characteristics that distinguish Tinodon are also present
with a third genus, Spalacolestes, thus: "so that the teeth identified here as
belonging to Tinodon could as well be the anterior molars of Spalacotherium
evansae; all the more so that on these teeth, and contrary to what is seen in the two
other species of Tinodon, the cristids are unequal as they are in Spalacotherium
", (p.775). Similarly, five of the teeth referred to S. evansae
"could be posterior molars of this small species of Tinodon..." The
decisive element in this puzzle "was the presence or absence of a clear wear facet A...
" A strong influence on wear facets is the mode and degree of contact between the
upper and lower teeth. If this particular feature is present, Tinodon. If not,
Spalacotherium.
"Attributed upper molar: In fact, the justification for, and the main interest of,
this new Tinodon species resides in the attributed upper molar, DORCM GS 694. We
could have designated this tooth as the type, and considered the incomplete lower molars as
attributed, but of what genus? The synonymy Tinodon-'Eurylambda' is not proved,
but on the other hand it seems ill-advised to create a new 'Eurylambda' species."
The problems of paleo-book-keeping.
'Eurylambda' is known from one specimen; a piece of upper jaw with one, incomplete
tooth. The anterior labial corner's missing. This is
preserved in the Dorset specimen, (which is approximately a third smaller, length 1,2mm as
opposed to 1,9; width 0,6 : 0,9), and that's why it's seen as of special interest.
I like the conclusion. Whilst noting the discovery of more complete material from North
America and China: "Our specimens nevertheless contribute in substantiating the variety
of mammals during the middle Mesozoic and in throwing more light on relationships between the
faunas of the Morrison Formation of North America and the Purbeck Limestone Group of England,
while perhaps obscuring the concept of spalacotheriids!" (p.778).
'Micron' is Greek and means very small. This entry is one of the rare occasions upon which
I have spelt Dr Sigogneau-Russell's name correctly, at the first attempt. |
| Reference: | Ensom & Sigogneau-Russell (2000), New symmetrodonts
(Mammalia, Theria) from the Purbeck Limestone Group, Lower Cretaceous, southern England.
Cretaceous Research, vol 21, no 6, p.767-779. |
| Species: | Tinodon sp. |
| Place: | |
| Country: | Portugal |
| Age: | Tithonian-Berriasian, Upper Jurassic - Lower Cretaceous |
| Remarks: |
One isolated molar was reported, (Averianov 2002, p.710). |
| Reference: | Krusat G (1989), Isolated molars of a triconodont and a
symmetrodont (Mammalia), from the uppermost Jurassic of Portugal. Berliner
geowissenschaftliche Abhandlungen A 106, p.277-289. |
| Genus: Trishulotherium
Yadagiri P, 1984
'Trishul beast'
Remarks: A trishul is a three-pronged weapon favoured by Lord Sankara in Indian
mythology. The relevant teeth was an earlier parallel. |
| Species: | Trishulotherium kotaensis Yadagiri P, 1984 |
| Place: | Kota Formation, Andrah Pradesh |
| Country: | India |
| Age: | ?Middle Jurassic |
| Remarks: | The following is largely based upon my reading
of Yadagiri, 1984, and thanks go to the supplier. Unfortunately, the published article
contained significant flaws. As well as establishing this genus, it also included
Indotherium, now seen as a possible
morganucodontid.
The first Mesozoic mammal fossil to be described from India hit the news kiosks of the
world in 1981. You may be able to recall the huge queues of wannabe readers, all anxious
to get their hands on the reports. It was a small tooth from the Kota Formation of
Central India and, surprisingly enough, some editors caused outrage by neglecting to
include the story on either the front or back page or, indeed, on any of the
pages between them. Angered mobs took the law into their own hands, and burned down the
headquarters of some such miscreants. While not condoning such behaviour, I can
certainly emphasize with it. While it's true that many other things of some level of
significance were going on in the world that year, eg that party in Charminster when she
insisted upon showing me her etchings, you would've thought such negligence required
some heads to roll. My eyes were left rolling, but that was for a different reason. How
different when this description appeared three years later! Very different. She'd
moved to Swindon. However, the press reaction was broadly comparable.
The new specimens came from a second fossil locality about ten kilometres north of the
first one (p.514). Plenty of matrix was collected, soaked and then sorted through
sieves of varying mesh sizes. As the desired fossils fit within particular size
categories, that sort of treatment gets rid of a lot of the wrong-sized stuff, and that
can drastically reduce the amount that needs to be carefully examined. Isolated mammalian
molars were found among the remnants along with scraps of
jaw and shards of skull from other critters. Two particularly well preserved teeth
agreed to cooperate with detailed enquiries but, sadly, they couldn't prevent their
answers being somewhat misunderstood.
In the paper, the main central cusp is termed the protoconid, the front one the
paraconid, and the rear one was accused of being the metaconid. That reflected oft
used practice for lower molars at the time but, as these aren't
tribosphenic teeth, it would now cause eyebrows to
rise on my sides. (Thanks for that expression are due to Mr Usman Bello of Lagos, a
bank manager with a wealthy corpse. And Mr Bello Usman. And Dr Usman Williams. And
etc.) However, there are several problems. Firstly, the specimens happen to be upper
teeth rather than lowers. Secondly, Indotherium was subsequently convicted of
being closer to morganucodontid affinities than "
'symmetrodont' ones. I don't like tinkering with published terminology if I feel
it can be avoided but, in this instance, I think it's the most conservative course to
take. I'm going to use A, B and C respectively for the three relevant cusps. (I've
just checked Prasad et al, 2006 for current views on these fossils, which coincidently
arrived yesterday. Many thanks.)
'Trishul beast'
The original author described the molar as having a well developed
lingual and a weakly developed buccal
cingulum, and nobody disagrees with that (p.515).
However, as the possession of two cingula like these is the sort of behaviour associated
with upper molars of this kind of age, everybody seems to disagree about it being a lower
left tooth. It's an upper right one. Given the mirror-like qualities involved, that
fortunately makes no difference with identifications of front, rear, buccal or lingual
orientations. For example, if you'd now like to stare into the mouth of a willing
mammalian accomplice, the buccal side of a lower left molar, the external one, is to
your right. Now get a pair of pliers and place the tooth into the upper right row. (You
might have to make some space.) Buccal remains buccal, blood is spluttering from the mouth,
and the eyes of your beloved are now filled with pain, shock and shear horror.
Cusps B and C are of similar sizes while A is dominant (p.515). A crest connects A
with C, and A is positioned somewhat lingually. As mentioned, the lingual cingulum is
more strongly developed than the buccal one. The tooth can be fairly described as tiny:
length 0.5mm, width 0.2. It's double rooted.
Unfortunately, as the fossil isn't a lower molar, none of the comparisons discussed on
page 517 are particularly useful.
Holotype
GSI SR/PAL/10 is a right upper (not a left lower) molar housed in the paleontological
collection of the Geological Survey of India, Southern Region, Hyderabad. The specific
name pertains to the Kota Formation.
Additional notes
This genus seems to be somewhere within
'Symmetrodonta' Simpson, 1925, at least according to
McKenna & Bell, 1997. However, the fact that it's not mentioned in the review by
Averianov, 2002, leaves me somewhat puzzled. Other sources place the animal within
Kuehneotheriidae. My most recent source, (Prasad
& Manhas 2001, p.1235), plumps for Tinodontidae. I'm presently following this
interpretation, but I don't know how they might define the family.
This is based on what was interpreted to be a lower molar.
Sigogneau-Russell & Ensom, 1998, (p.458), concluded it was an upper. |
| Reference: | Yadagiri (1984), New symmetrodonts from Kota formation (Early
Jurassic), India. Journal of the Geological Society of India, 25 (8), p.514-621. |
| Genus: Yermakia
Lopatin AV, Maschenko EN, Averianov AO, Rezvyi AS, Skutschas PP & Leshchinkiy SV, 2005
Remarks: I found mention of this genus by chance on the webpage of Dino Hunter. Many
thanks for posting it. Further thanks are due to Jelle Zijlstra for finding the abstract. |
| Species: | Yermakia domitor Lopatin et al, 2005 |
| Place: | Shestakovo locality, Kemerovo Region, Western Siberia |
| Country: | Russia |
| Age: | Lower Cretaceous |
| Remarks: | The following is based purely on the abstract of
the paper, a full copy of which would be most useful.
This genus is known from at least one dentary and this seems
to be a relatively informative lower jaw. The bone turns dorsally in the area of the
incisors and the mandibular symphysis is described as short. An ancient feature is on the
inner face of the jaw. This is a Meckelian groove,
which was a handy place for storing a bit of cartilage we mammals no longer retain as we get
older. Yermakia and various primitive beasties had different views on
that. The groove in this case widens like an estuary at the back, grooves along the jaw
for a while and then thins progressively. A feature known to its friends as the pterygoid
crest is well developed, the pterygoid fossa small, and the coronoid process rises gently.
Teeth
Preserved are four small incisors and a not very impressive,
double rooted canine. A diastema
separates that tooth from the postcanine brigade. Its members consist of three
premolars and four
molars. The premolars are all double rooted and lack a cusp termed b. The angle
formed by the trigonid cusps of the molars is obtuse. It
ranges from 102° (m1) to 139° (m4). The increase is progressive from front to rear.
Affinities
The authors refer Yermakia specifically to the family of Tinodontidae. They find it
to be more derived than the earlier
Tinodon but somewhat primitive in comparison to its nearer
contemporary, Gobiotheriodon from Mongolia. This is
the first 'symmetrodont' to have been recovered from
Russia, and the eucynodont fauna from the locality is
presently enjoying a boom in diversity. |
| Reference: | Lopatin et al (2005), Early Cretaceous mammals from Western
Siberia: 1. Tinodontidae, Paleontological Journal, 35(5), p.523-534. |
| Other reports:
Anoual, Morocco
With regards to a dentary fragment with two
molars referred to Donodon, Averianov, 2002, includes:
"I think that the dentary specimen referred to D. perscriptoris should be
attributed to "Symmetrodonta". The reduction of the
lingual cingulid and mesial cingulid cuspule "f" are similarities shared with
Gobitheriodon", (p.713).
Oxfordshire, England
Sigogneau-Russell, 2003b refers two lower molariform
teeth from the Bathonian Forest Marble Formation to ?Symmetrodonta indeterminate, (p.524).
Of one specimen: "This tooth evokes the anterior molars or [sic] Tinodon
Marsh, 1879, but differs by a lower crown and a cingulum
not being angled in the middle." The specimens are poorly preserved and may be milk
teeth. |
A. Tinodontidae B.
Spalacotheriidae & Zhangheotheriidae
| B. SPALACOTHERIIDAE & ZHANGHEOTHERIIDAE |
| Taxa: Spalacotheriidae Marsh, 1887
Zhangheotheriidae Rougier GW, Ji Q & Novacek MJ, 2003
These small insectivores / carnivores are most commonly found in the Cretaceous rocks of
the northern hemisphere. In virtually all cases, remains consist of teeth, with occasional
fragments of jaw. The exceptions are the zhangheotheriids, which are known from near
complete specimens from China.
Luo et al 2002, (p.26), place Spalacotheriidae, as represented by
Zhangheotherium), within a modified Trechnotheria McKenna, 1975, which they define
as: "the common ancestor of Zhangheotherium (and by extrapolation the
monophyletic group of Spalacotheriidae, see Cifelli
and Madsen 1999) and crown Theria, plus all of its
descendants." In other words, this lot and us share common ancestry.
"Spalacotheriids included forms with as few as two
premolariforms but as many as seven molariform teeth",
(Butler & Clements 2001, p.13. They're sometimes referred to as "acute-angle"
symmetrodonts.
Reference: Marsh OC (1887), Additional remains of Jurassic mammals. American Journal of
Science 18, p.215-216.
Shock, horror, non-molar molars
Rougier, Isah & Manabe, 2007 includes some disturbing news on page 75. As is widely
known, should your circle of acquaintances be wide enough, mammals aren't allowed
to replace their molars. Those, should you be wondering, are the large teeth to the
rear of your jaws. Indeed, that's reflected in present definitions of a molar; a
first generation tooth begind the permolars that experiences no replacement.
Mounting evidence indicates that zhangheotheriids (and likely lots of other ancient
mammals) hadn't been informed of this. The first 'molar' may look like and have
worked like a molar, but it was actually a replacement for an earlier tooth at that
position. This primitive form of dental delinquency would appall any right-minded
dentists, if that's not a contradiction in terms. This perverse trait is unlikely
to have occurred in this particuclar family. The habit could've been widespread
among "symmetrodonts" and other groups. For example, it's also now known from
gobiconodontid tricondonts. |
| Links:
Mikko Haaramo’s Spalacotherioidea
Mikko Haaramo's Spalacotherioidea
The cladogrammer’s cladogrammer.
Geodiversitas 1999, 21(2), p.167-214
http://www.mnhn.fr/publication/geodiv/g99n2a4.html
Spalacotheriid symmetrodonts (Mammalia) from the
medial Cretaceous (upper Albian or lower Cenomanian) Mussentuchit local fauna, Cedar
Mountain Formation, Utah, USA.
The abstract of an article featuring a number of genera by Cifelli RL & Madsen SK.
This proposes that Spalacotheriidae is a monophyletic group, (ie: all members are descended
from a common ancestor). Whether all the names below are valid is a different matter.
Genera: Akidolestes,
Heishanlestes, Kiyatherium,
Maotherium,
Mictodon, Peralestes (=Spalacotherium),
Shalbaatar, Spalacolestes,
Spalacotheridium,
Spalacotherium, Spalacotheroides,
Symmetrodontoides, Symmetrolestes,
Yaverlestes, Zhangheotherium,
other reports
Time-Line:
Upper Cretaceous: Mictodon, Shalbaatar, Spalacolestes,
Spalacotheridium, Spalacotheroides, Symmetrodontoides
Lower Cretaceous: Akidolestes, Heishanlestes, Kiyatherium,
Maotherium,
Spalocotherium, Spalacotheroides (Texas), Symmetrolestes,
Yaverlestes, Zhangheotherium, East Asia |
| Genus: Akidolestes Li G
& Luo Z-X, 2006
'Point thief'
Remarks: The generic name alludes to the pointed rostrum of the jaw. 'Lestes' (thief) has
been frequently used for mammals. If they could talk, then insects and worms might
explain about some of the problems they encounter. |
| Species: | Akidolestes cifellii Li G & Luo Z-X, 2006 |
| Place: | Yixian Formation, Liaoning |
| Country: | China |
| Age: | Barremian, Lower Cretaceous |
| Remarks: | The following is based upon my reading of Li &
Luo, 2006. I've added certain elements the authors sensibly neglected.
Yixian strikes again
The Yixian Formation is providing a barrage of fossils which defy superlatives, and it
shows no signs of breaking this extravagant habit. There are two mammal-bearing strata,
and Akidolestes is the latest addition from the upper fauna (until the next find
makes nonsense of this sentence). By the sensational local standards, I suppose this one
could be termed unspectacular. It's a small critter a bit over ten centimetres long,
with around a third of the length provided by a relatively short tail. It's the third
spalacotheroid, so that's no new claim to fame. Akidolestes is the fourth to
outrageously flaunt remains of its fur coat. Superficially, there's nothing that's
particularly original with yet another furry, terrestrial, insect-annihilating mammal
delivered, after a journey of 125 million years, in staggering completeness other than for
much of the left skull. Even staggering completeness is no longer surprising for the
Yixian.
This mammal does have lumbar ribs, but they've been previously sported by
Repenomamus from the lower fauna. Now there
was a mammal worth interviewing. That genus grew up to lengths of a metre, and ate baby
dinos anytime it damned well pleased. Or at least, when it could. That's the road to
megastardom. Akidolestes is more like a small face in the crowd, albeit with half
its head missing.
Lumbar ribs! What they?
There's no particular reason why most people would've heard of lumbar ribs. It sounds like
a supper for tree-fellers. I'm now going to write in line with my own biases, so a naked
woman is obviously essential at this juncture. Should you feel like leaning towards a naked
man instead, then feel free to grab one (but mind out where).
Time for nudity
My wife's not in the room at the moment, so some imagination will be required. As you can
see, if you look in through the monitor, I'm sat on the chair somewhat to the side of my
usual position. I've cleared a bit of space on the floor, and am now psychically
projecting the image of a naked woman standing there. The skilled anatomists among you
might notice they can see her face, so they'll be sure they're looking at the front. Most
readers will be concentrating on different parts of her body, and have doubtlessly reached
a similar conclusion.
I'm now letting my hand glide firmly, yet gently, down her neck. Definitely without
hurrying, my fingers are pleasingly admiring the warm flesh of her fine mammary glands.
This area of the torso is known as the breast region, and it's well worth spending time on
the subject. Allowing my hands to slide slowly down and outwards enables me to feel the
rib cage. These structures originate from the vertebrae
of the upper spine. They're thoracic ribs; the only kind this naked woman has.
I've spread my hand flat and am now caressing the skin of the stomach. This is the lumbar
region. The bones of this lower part of the spine have no ribs, and this is typical for
placental and
marsupial mammals. However, that wasn't the case for many non-mammalian
cynodonts, various basal
mammals or indeed Akidolestes. It also doesn't apply for the egg-laying
monotremes such as the
platypus. All those critters have or had ribs
down there too, although they're much shorter than the
thoracic ones, and don't form parts of a protective cage. (They did in earlier
synapsids.)
All this thinking about a naked woman has somehow distracted me from the paper. There will
now be a short interlude. I've got some pressing matters I urgently want to see my wife
about.
A while later
Good. I'm now de-invigorated and everybody knows where our lumbar ribs aren't. The
thing about this aspect of Akidolestes, is that their presence was unexpected.
Based on what was known from other mammals, nobody would've predicted a spalacotheriid
would have had them. The trait is too old fashioned. It's generally somewhat more
derived than its zhangheotheriid neighbours such as
Zhangheotherium, and they don't have lumbar ribs.
Several other features are also surprisingly familiar to fans of monotremes (p.195), whose
ancestors diverged from a 'lower' branch of the mammalian bush.
I enjoy the word counterintuitive, and such characteristics in this inoffensive, small
mammal certainly qualify for it. So unremarkable as this staggeringly complete fossil
may appear, he was clearly a wonder. The spur of bone on the back of the ankle means
this individual probably was male.
Teeth and identity
The dental formula per side is: (uppers): 4 incisors,
1 canine, ?5 premolars
and ?5 molars; (lowers): 4, 1, 5 and 6
respectively (p.196). The cusps of the last premolar and all the molars are acutely
triangulated, with the angle sharpening from front to back along the row. (This means the
three main cusps are arranged in the form of a triangle.) The angle's below 50° for the
final tooth. This and other features are typical of spalacotheroid
dentitions. A crest on the molars termed the
protocristid is higher than found in zhangheotheriids. A further distinction lies in the
relative sizes of the final premolar and first molar, with the former being the larger of
the pair in Akidolestes. This is already enough to debar the animal from admission
into the family of Zhangheotheriidae.
It has more premolars than Symmetrolestes and outdoes
the molar count of both Spalacotherium and
Spalacolestes. The coronoid process of the
dentary is a gracile construction. This contrasts with the
somewhat more recent Heishanlestes, but isn't dissimilar
to the zhangheotheriids.
In short, this mammal is confidently assigned to Spalacotheriidae.
Legs
The front legs and shoulders are much like those known from the zhangheotheriids. The back
legs are more surprising, as these display similarities with monotremes; presumably
convergences. Before reaching that area, I'm allowing my hand to linger low down on the
stomach of the naked woman. It's now moving to the side and around her body just above the
hip. Feeling the base of her spine is heavenly. I'm not sure how many lumbar vertebrae
she has, and I presently couldn't care less. My mind's on other matters. Involuntarily,
my fingers are probing the soft...
Excuse me. There will now be another interlude.
A further while later
I can confirm my wife definitely has no lumbar ribs. As established earlier, this contrasts
with Akidolestes. That mammal has six vertebrae in the lumbar region, and five are
associated with short ribs. These aren't attached to the spine. The bones are free
slithers slightly to the side of the vertebrae. A similar condition is found in monotremes,
Repenomamus, Fruitafossor and a
herd of non-mammalian cynodonts. This trait enjoys a long tradition which we latecomers
neglect to follow.
We also fail to develop small bones at the front of the hips called
epipubes, but all other known mammals other than
eutherians have or had them. (Epipubic bones are present
in basal eutherians and non-mammalian tritylodontids.)
Akidolestes unsurprisingly had these, and they're unusually wide (p.198). This
condition is reminiscent of the platypus, but the equivalent bones in other monotremes and
Mesozoic mammals tend to be narrower. A number of features are monotreme-like: "The
pubis has a prominent tubercule for the psoas minor
muscle..."; "In the head, neck and trochanteric area of the
femur, Akidolestes is most similar to
morganucodontids, eutriconodontans and
monotremes (although to a smaller extent for the latter)...". The
fibula also has a hypertrophied parafibular process. (That
particular bit of bone is much enlarged.) Again, this is also the case for monotremes.
Walking the walk
Some of these details have significance for how monotremes move. They ensure that the rear
legs can't be fully straightened. The limb sticks out somewhat sidewards from the body.
The similar construction suggests Akidolestes was a sprawler as well, and this is
starkly different to the proud, erect, Didelphis-like posture of
Zhangheotherium (p.199).
Pausing for paws
The relative proportions of bones in fingers and toes can provide revealing clues about
lifestyle. Fanatical diggers, swimmers or tree acrobats generally develop telltale
specialisations. For example, the latter often have elongated digits for grip. No
particular specialisations are evident for Akidolestes. This appears to have been a
generalised ground-dweller as with Zhangheotherium and morganucodontans. The
relatively short tail seems to me to strengthen this conclusion. It appears complete with
sixteen vertebrae. While that's more than I possess, it's not all that many for a mammal.
Lumbar ribs in a wider context
Lumbar ribs have an ancient pedigree. Sometimes, as in this case, they're found in mammals
with close relatives, which are abstainers. Another example is provided by
gobiconodontids. Their
fellow eutriconodont, Jeholodens, doesn't
share this trait. This seems to be an ancient characteristic which has been reactivated on
a number of occasions. As a consequence, it doesn't appear overly helpful when trying to
establish relationships between groups of mammals.
What are they for?
Having ribs free of the vertebrae may sound odd but, in monotremes, they come in very
useful. By providing anchorage for muscles and the diaphragm they help with movement and
breathing. Presumably, especially in the light of further postcranial convergences,
Akidolestes benefited in broadly similar ways.
The hips of my naked woman are built differently. Her gorgeous skin... Excuse me
again.
Holotype
The type fossil, NIGPAS 139381A and B, is an employee of the Nanjing Institute of Geology
and Paleontology. There are two slabs of stone involved, and these are positive and
negative impressions of the same animal. The specific name honours the pioneering Utah
symmetrodontologist, Rich Cifelli. |
| Reference: | Li G & Luo Z-X (2006), A Cretaceous symmetrodont therian
with some monoteme-like postcranial features, Nature, 439, p.195-200. |
| Genus: Heishanlestes Hu YM,
Fox RC, Wang YQ & Li CK, 2005
'Heishan thief'
Remarks: The generic name refers to the county from which the fossils came. |
| Species: | Heishanlestes changi Hu YM, Fox RC, Wang YQ &
Li CK, 2005 |
| Place: | Badaohao, Heishan County, Liaoning |
| Country: | China |
| Age: | Aptian, Lower Cretaceous |
| Remarks: | The following is based upon my reading of Hu et al,
2005.
Heishanlestes is the third new 'symmetrodont'
genus to appear from the strata of Liaoning Province in the last eight years, but this is
both the most recently alive and poorest known of them. Rather than being from the
fabulous treasury of the Yixian Formation, Heishanlestes
hails from a less salubrious coal mine. As well as being geologically younger, the
taxon is more closely related with the spalacotheriids of
North America. Remains are restricted to lower jaws with teeth, but at least they're
relatively well preserved, (p.1).
The animal possessed four premolars which were closely
packed together, and probably acted to crush foodstuff. This was then passed back to the
six molars for cutting. An interesting feature is that the
molars are differentiated into three distinct subgroups. The first displays a
trigonid with obtuse-angled triangulation, while the other
five are acute-angled. The last pair have a large cusp at the centre of the trigonid, and
this isn't homologous with the anatomy of any other known mammal.
A further point of significance is that all postcanines
are in situ (p.2). That presently qualifies as extremely unusual.
Collection and conditions
Three specimens were obtained from small coal mines in the field seasons of 1994 and '95.
These are in Badaohao, Heishan County, and correlate with the Shahai Formation. Fossils
recovered include further mammals (triconodonts,
multituberculates, a relative of
Aegialodon and
eutherians), various molluscs, fish, lizards and teeth and egg shell from dinosaurs.
Below is found the Jiufotang Formation which shares elements with the famous Yixian Jehol
fauna. The remains from Badaohao must be younger than that, and an estimated age for the
Shahai is Aptian, (p.3). That's about 121 - 112 million years ago.
Size
The 'Heishan thief' was typically sized for a Mesozoic mammal; small. The holotype is a jaw
a touch longer than 1.7cm, although the front of this
dentary, (where the incisors and canine were housed), is missing. A
canine is included with another specimen.
Dental formula
The known numbers of teeth per side are: (lowers): ? incisors,
1 canine, 4 premolars and 6 molars, (p.4). Anybody anxious for information on the
upper teeth should make their way to the coal mine in question and find some.
Incisors
As not much of the jaw is missing, the front teeth were presumably relatively small. This
would be in line with Spalacotherium. In contrast,
Zhangheotherium boasted of an enlarged first incisor.
There probably wasn't room for such an implement in this mouth.
Canine
The tooth is preserved on one jaw fragment, but it was in the early stages of eruption.
The alveolus is larger than the unicusped canine within it,
and that suggests this is a replacement for a deciduous
predecessor. The number of roots is unknown. Its equivalents are double-rooted in
Spalacotherium tricuspidens and single-rooted in zhangheotheriids.
Premolars
The four premolars are unusually crowded together. A projection at the front of each
overlaps with the rear of the tooth in front, (p.7), forming a tighter unit a bit like a
line of interlocking shields. The first molar also overlaps with the p4, but whether the
situation is similar for the p1 and canine is unclear. Each tooth has an ovoid area of wear
behind and to the lingual side of the main cusp. The whole
unit seems to have provided a crushing process for breaking down foodstuff.
The largest premolar is p4. The p1 was probably a bit
bigger than ps 2 and 3. Each is oval in outline from the
occlusal perspective. With the exception of p1, they are certainly double-rooted. Only
one root is visible for the first tooth in the series, but that could conceivably divide
inside the bone. Wear on p4 suggests this tooth may also have participated in some cutting
of food in concert with its upper partners, (p.8).
Molars
Each of the six molars is double-rooted. A surprising feature
is that they're divided into three distinct models. Had they not been found on the jaw the
authors state these would probably have been assigned to at least different genera. The m1
has an obtuse-angled arrangement of trigonid cusps (140°),
with the paraconid and
metaconid marginally more lingually situated than the
protoconid. All other molars have acute triangulation, with it being most extreme in
the second type (55°, 48° and 46° for ms 2, 3 and 4 respectively). These teeth are
considerably taller on the labial side than they are on the
lingual, as is known from other spalacotheriids.
The angles in m 5 and 6 are 50° and 60°. This pair are adorned with an extra and strong
cusp on the lingual aspect of the trigonid. A parallel with other Mesozoic mammals is
unknown. Wear on m1 suggests some crushing activity; evidence of the functional transition
along the postcanine row as a whole, (p.10). As with the premolars, projections at the
front of the crown interlock with the proceeding tooth.
The two rearmost molars are the smallest and lowest in the row, (p.12), with the m6 being
much smaller than the m5. The tallest feature of these crowns is that presently unique cusp
between the paraconid and metaconid. There's no hint of anything similar on the teeth in
front, let alone in other mammals.
Jaw
The dentary is relatively slender, (p.13), with the
maximum depth of the horizontal ramus below the m6. Small mental foramina are present on
the external side of the bone beneath the canine and the second and third premolars.
There's no sign of a Meckelian groove
Behind the tooth row is a nearly complete coronoid process. This has suffered some
crushing and part of the rear border is missing. The process ascends more or less at 90°
behind the molars, and there's little rearwards curvature until about two-thirds of the
way up. A small, rough depression is on the medial surface, (p.14). This is perhaps an
attachment point for an 'extra', small jaw bone; a coronoid. Other possibilities include it
being an artefact of distortion. More specimens could provided clarity.
The absence of an angular process below the coronoid process is usual for 'symmetrodonts'.
A spalaco molar stock take
Six lower molars is the number also reported from zhangheotheriids, (although one
interpretation placed this as five for Zhangheotherium).
Symmetrodontoides canadensis manages six or seven. The latter figure is
favoured by Spalacotherium triscuspidens and
Spalacolestes cretulablatta. However, in all those
animals, size and shape of molars alter gradually along the row. None have three distinct
types as in Heishanlestes.
The unusual m1 is similar to a tooth assigned to Symmetrodontoides candensis, although
this identification may not be correct. It could be from a closer relative of this 'Heishan
thief', and the same possibility exists for some further specimens from Utah, (p. 16). The
m2-m4 are more typical of spalacotheriids. The crowns are short and wide with a protoconid
towards the rear and no talonid. Other features, such as the acute triangulation of cusps
and lack of a cusp f, (in contrast to mammals such as Tinodon),
occur in all spalacos as well, (including zhangheotheriids).
Affinities and geography
As Heishanlestes is more similar to Spalacolestes and Symmetrodontoides
than Spalacotheroides is, the authors place the genus
within the otherwise North American subfamily of Spalacolestinae, although this produces
doubts concerning the monophyletic status, (p.18).
Be that as it may, the relationship is clear enough. Closely related mammals were scampering
around in both North America and China. This is far from being a unique case.
It's been speculated that spalacolestines were able to survive in North America because
eutherians were rare. However, Heishanlestes
lived in an area alongside of eutherians. Nevertheless, spalacotheriids became rarities
beyond the middle of the Cretaceous, and their disappearance coincides with the radiation
of we therians.
Holotype
The type fossil, IVPP V 7480, is an employee of the Institute for Vertebrate Paleontology
and Paleoanthropology, Beijing. The specific name honours Professor Zhenglu Chang, who
has done much over the past five decades for the studies of stratigraphy and paleontology
in the region. |
| Reference: | Hu et al (2005), A new spalacotheriid symmetrodont from the
Early Cretaceous of northeastern China, American Museum Novitates, 3475, pp.1-20. |
| Genus: Kiyatherium
Maschenko EN, Lopatin AV & Voronkevich AV, 2002
Remarks: Originally referred to the family of Peramuridae, additional speciemens reuslted
in this genus as being identified as zhangheoterheriid (Lopatin, Averianov, Maschenko &
Leschinksky, 2010). |
| Species: | Kiyatherium cardiodens Maschenko EN, Lopatin AV
& Voronkevich AV, 2002 |
| Place: | Shestakovo 1, Kemerovo Region |
| Country: | Russia |
| Age: | Lower Cretaceous |
| Remarks: | This entry, short as it is, requires a rewrite.
Further specimens and research have out the beast as a zhangneotheriid.
This genus is also mentioned in a further paper by the same
authors from 2002, (and they're listed in the same order). They state it's a peramurid,
(p.76). Details are in the bibliography at the end of this directory. As the description
of this genus is referenced in that, the Kiyatherium paper must have been published
first, so I'll refer to it here as 2002a.
Maschenko et al, 2002a was on-line (though the page numbering differs to
the reference in Maschenko et al, 2002b). As it's in Russian, my understanding is even
more limited than usual. However, the authors clearly place this critter within
Peramuridae.
Holotype and teeth
The holotype is part of an upper jaw and has the number 16/2-50. I can't decipher which
collection it's within for sure, but I think it's the Paleontological Museum of the Tomsk
State Museum. As upper jaw fragments go, it appears to preserve a good array of teeth.
The description seems to mention the canine,
premolars 1-5 and molars
1-3. Hatched lines on the drawing might well indicate that the second premolar is absent.
Premolar 5 is clearly double rooted, and the roots are broadly similar in the length. The
distance from the canine to the final molar is 14mm. |
| Reference: | Maschenko, Lopatin & Voronkevich (2002a), [New Early Cretaceous
mammal from West Siberia.] Doklady Akademii Nauk. T.386 No.5, p.715-716, [in Russian]. |
| Genus: Maotherium Rougier GW,
Ji Q & Novacek MJ, 2003
'Mao's beast'
Family: Zhangheotheriidae Rougier et al, 2003
Remarks: I haven't seen the paper, but this is reportedly named in honour of the late
Charirman Mao Tse-Deng. Unfortunately or otherwise, I'm informed that may be incorrect.
(With thanks to Mark Isaak's
Curiosities of Biological Nomenclature ('Named after People: Political and Military
Figures).
Brief information is provided by Wang et al, 2006 (p.197-198). This sounds like a
hum-dinger of a fossil. It's complete (and even furry). In essence, it's a somewhat
inflated version of Zhangheoterhium with a
few differing details. Lower jaw halves are supplied with one more molar. Skeletal
differences include the proportions of limb bones, the number of
vertebrae and, in this critter, an unfused
sternum. |
| Species: | Maotherium sinensis Rougier GW, Ji Q & Novacek
MJ, 2003 |
| Place: | Yixian Formation, Liaoning |
| Country: | China |
| Age: | Barremian, Lower Cretaceous |
| Remarks: |
This genus is mentioned in the recent Carnegie Museum Press Release, which accompanied the
publication of Sinodelphys szalayi. It's referred to as a
symmetrodont. It's in the family of
Zhangheotheriidae, (Tsubamoto et al 2004, p.337). Thanks are due to Dinohunter for posting
notice of the description, which will hopefully turn up somehow or other.)
From the abstract: "This paper describes a complete specimen of a symmetrodont mammal
with well-preserved hairs and soft tissue from the basal part of the Yixian Formation in
the Sihetun area, Beipiao, western Liaoning." |
| Reference: | Rougier, Ji & Novacek (2003), A new symmetrodont mammal
with fur impressions from the Mesozoic of China, Acta Geologica Sinica, 77 (1), p7-14. |
| The Yixian and Jiufotang Formations, China,
Lower Cretaceous
The following is loosely based upon my reading of Luo, 1999. However, various bits and
pieces of information aren't mentioned in that paper; for example...
I live about forty miles from
Solnhofen, which is one of the most incredible places in the world for stunningly
well-preserved, Mesozoic fossils. Its most famous Upper Jurassic resident was
Archaeopteryx. Seven specimens have turned up over the last 150 years, though one is
presently regarded as belonging to Wellnhoferia, (a close relative). That makes
Archie a well-represented dino-bird. Seeing the quality of the fossils in the
Bürgermeister-Müller-Museum and other collections provides a staggering experience, and
mind-blowing's an appropriate term. However, the fossils from the Yixian Formation of
Liaoning, northwest China, have conspired to make Solnhofen seem relatively mundane, which
it certainly isn't. They make all superlatives woefully inadequate and keep on coming.
The Times they are A-changing
The Upper Jurassic / Lower Cretaceous transition produced profound changes within both the
terrestrial flora and fauna of the world. The flowering plants (angiosperms) budded,
blossomed and then went on to gloriously seed. This seems to have powered the emergence of
nectar-feeding flies and subsequent adaptation among many other insects; eg. bees and
butterflies. The pterosaurs were joined on the wing and challenged by strange dinosaurs
known as birds. New groups of outlandish mammals took to
their paws; metatherians and
eutherians, lineages that later gave rise to the
marsupials and
placentals respectively.
And this locality, slap bang in the chronological middle (ca. 124 million years BP), provides
exquisite evidence of these episodes. When Dr Luo describes the Yixian Formation as:
"one of the richest and most important sources of fossils from the Mesozoic era...",
I feel compelled to object on the grounds of false modesty. These fabulous finds are
powerful enough to condemn even the hard-hearted of paleontologists to tears of joy. I
can hear Thomas Huxley
sobbing with pleasure in his grave.
Shooting Star
Fossil fish, arthropods and plants have been coming from these beds since the 1920s, though
these were mostly forms which were purely Asiatic during the Lower Cretaceous. This
endemicism made biostratigraphic comparisons with specimens from further afield difficult,
and didn't shed much light on wider evolutionary matters. But, after relatively modest
beginnings, Liaoning began firing on the most astounding cylinders and more than made up
for lost time.
The aforementioned Huxley had linked dinos with birds in the 1860s but, perhaps as the
evidence available consisted of similarities between a couple of specimens of
Archaeopteryx and a small, non-birdy compatriot called Compsognathus, this
reasonable interpretation fell somewhat out of favour for nigh-on a century. Further work
and more fossils helped revive its fortunes from the 1960s. Then Liaoning began batting
in earnest and it was soon game over (though there are still some dissenters).
One of the most basal coelurosaurs, Sinosauropteryx,
seemed to be equipped with fluffy feathers. The same was the case for the primitive
maniraptors Protarchaeopteryx and Caudipteryx. And, since Luo's paper was
written, we've even been blessed with the appearance of Microraptor gui from the
overlying Jiufotang Formation; a non-birdy dromaeosaur with aerodynamically adapted
feathers and leg wings. A four-winged dinosaur may sound bizarre, but it's appropriate
enough for Liaoning. Also present in the Yixian faunas are Confusciusornis and
Changchengornis, both of which are more derived
birds than my friend Archaeopteryx von Solnhofen.
Desolation Row
The increasing mammalian element (two more species added in the first nine months of 2005)
is an intriguing cocktail of 'old' and 'new'. Sinobaatar is a relatively
old-fashioned 'plagiaulacidan'
multituberculate. Fairly late occurring
'symmetrodonts' (this directory) and triconodonts are
also known. More derived are Sinodelphys (
Metatheria) and Eomaia (Eutheria). Both animals
are the earliest and most basal representatives of their respective lineages. As was
Maotherium ('Symmetrodonta'), these fossils
came supplied with what were then oldest directly evidenced fur coats in the world.
(Still older preserved fur has since turned up on fossils from Inner Mongolia.)
An interesting flowering plant (angiosperm) is Archaefructus. The female reproductive
structure, the carpel, is typical enough. However, this plant lacked petals, and it seems
to be a transitional proto-flower.
Watching the River Flow
The basal nature of some of these mammals and dinosaurs shows affinities with groups known
from earlier times in North America and Europe, and this is also reflected by other
vertebrates. The pterosaurs resemble those from Solnhofen. Such patterns might suggest an
Upper Jurassic age for Yixian too, but that's not the case. Volcanic material has supplied
a reliable dating, (Argon 40 / Argon 39 basis). This formation is Lower Cretaceous.
Consequently, some of these lineages were effectively 'living fossils' of their time, which
supports the view that Central and Eastern Asia were isolated during the Upper Jurassic and
lowest Cretaceous. (Further evidence for this has been provided by the persisting presence
of non-mammalian tritylodontids from Lower Cretaceous
locations in Siberia and Japan. As yet, there's no indication of any such fossils in
Liaoning, but I live in admittedly diminishing hope.)
I'll be your Baby Tonight
"Primitive fossil taxa may not always be older than
their more derived relatives." According to my understanding, populations aren't
reliable analogues of individuals. They don't come with built-in lifespans. A lineage can
continue for as long as it has individuals willing and able to produce viable off-spring.
A duck-billed platypus is far more
basal than a woolly mammoth, but I know which species today
derives the most satisfaction from sex.
Tangled Up in Blue
A note for fossil collectors. Occasionally, Yixian 'mammals' are offered for sale on
the open market. Up until now (July 2005), they've all had one thing in common. They're
fakes. Some are complete forgeries, while the better efforts are composites of perhaps
a few mammalian remains, enhanced with much other microvertebrate material. Just because an
offered specimen happens to be composed of fossil bone, it would be hasty to assume it's
genuine. Probability suggests otherwise.
I'm not going to mention a list of faults with the photographed 'Zhangheotherium' I
saw this month, as that could be a useful resource for producing better quality forgeries.
However, as a clue, it'd be worth thinking about whether the 'animal' depicted could have
ever managed to walk.
My understanding of the legal situation isn't extensive. Nevertheless, as things stand,
should a genuine specimen somehow make it onto the market (and that is highly unlikely),
it will have been obtained by theft somewhere along the line. No Yixian mammals have ever
been granted an export licence.
Should you be informed that an appropritate authority, such as the Institute of Vertebrate
Paleontology and Paleoanthropology, Beijing, has verified the fossil, it could well be a
good idea to check. This would almost certainly be news to them.
Ideally, there should be no market for such fossils.
Jehol mammals are far too important for any one person to claim ownership.
Further Mesozoic site summaries can be found at Localities.
Meet the mammals of the Yixian and Jiufotang Formation (14 genera and 15 species)
To be accurate, the Yixian Formation has a number of members. The Repenomamus
sisters, Gobiconodon and Meemannodon are from the lower fauna, and
this dates to between 128 and 139 million years ago. Most the other mammal fossils listed
here are younger; around 125 million years.
To complicate the picture still further, the Yixian Formation is immediately below the
somewhat more recent Jiufotang Formation and, as of 2011, that has also become a source of
mammal remains with the eutriconodont genus of Liaoconodon. Its age is about
120 million years.
Collectively, both those formations are usually now referred to as comprising the Jehol
Group (Wang & Zhou p.22-24 in Chang et al, 2008). In earlier texts, the Jehol Group was
often used to incorporate two further, still more recent formations, the Shahai and Fuxin,
both of which also happen to yield mammal fossils from the Lower Cretaceous. Those don't
feature in this list.
Multituberculata (1 genus)
Sinobaatar lingyuanensis
Triconodonta (7 genera)
Gobiconodon zofiae;
Jeholodens jenkinsi;
Juchilestes liaoningensis;
Liaoconodon hui (Jiufotang Formation);
Meemannodon lujiatunensis;
Repenomamus robustus; R. giganticus;
Yanoconodon allini
'Symmetrodonta' (3 genera)
Akidolestes cifellii; Maotherium
sinensis; Zhangheotherium quinquecuspidens
Metatheria (1 genus)
Sinodelphys szalayi
Eutheria (2 genera)
Acristatherium yanensis;
Eomaia scansoria |
| Genus: Mictodon Fox, 1984 |
| Species: | Mictodon simpsoni Fox, 1984 |
| Place: | Upper Milk River Formation, Alberta |
| Country: | Canada |
| Age: | Campanian, Upper Cretaceous |
| Remarks: |
Known from a single tooth. The cusp arrangement and slender
roots suggest a milk tooth of a spalacolestine symmetrodont. Symmetrodontoides
canadensis would be a possibility, seeing as it's known from the same formation,
(Averianov 2002, p.712).
The holotype, UALVP 16273, is in the collection of the University of Alberta.
"Assignment to Tinodontidae is uncertain," (McKenna & Bell, 1997). Quite.
That's why I've relocated this entry to Spalacotheriidae. |
| Reference: | Fox (1984), A primitive, "obtuse-angled" symmetrodont
(Mammalia) from the Upper Cretaceous of Alberta, Canada. Canadian Journal of Earth Sciences
21, p.1204-1207. |
| Genus: Shalbaatar Nessov LA,
1997 |
| Species: | Shalbaatar bakht Nessov LA, 1997 |
| Place: | lower Bissekty Formation,
Dzharakuduk |
| Country: | Uzbekistan |
| Age: | middle-upper Turonian, Upper Cretaceous |
| Remarks: | Originally, the remains were held to be a
multituberculate, which are commonly called
Something-or-other-baatar. However: "According to Z. Kielan-Jaworowska (personal
communication 2000), the posteriorly placed masseteric fossa excludes Shalbaatar
from Multituberculata, because this indicates absence of the "backward masticatory
power stroke" that is characteristic of the group", (Averianov 2002, p.713).
On re-examination, Averianov identified several features known from spalacotheriids. He
places it unreservedly within Spalacotheriidae, and possibly even within the otherwise
American subfamily, Spalacotheriinae. He also points to similarities between the Bissekty
fossils and those known from the Upper Cretaceous of North America. Several vertebrate
taxa are common to both places, including the
eutherian mammal Paranyctoides, (p.713-714).
A further fragment of toothless dentary has been recovered
from the Aitym Formation, (possibly upper Turonian-Coniacian). This formation is about 30
metres above the Bissekty. The second specimen is thus somewhat later than S. bakht,
and does display some contrasts. It may represent a different species, but this will only
become clear with further and better specimens, (Averianov & Archibald 2003, p.5).
Holotype
The genus is based on an edentulous fragment of lower
jaw, which means it ain't got any teeth left. This is ZIN 82622 and lives at the
Zoological Institute of the Russian Academy of Sciences, St. Petersburg. |
| Reference: | Nessov (1997), Cretaceous Non-marine Vertebrates of Northern
Eurasia, (in Russian). University of Saint Petersburg, Institute of the Earth Crust, Saint
Petersburg. 218pp. (Posthumous edition by Golovneva LB & Averianov AO.) |
| Genus: Spalacolestes
Cifelli RL & Madsen SK, 1999
'mole-like thief' |
| Species: | Spalacolestes cretulablatta Cifelli RL & Madsen SK,
1999 |
| Place: | Cedar Mountain Formation, Utah |
| Country: | USA |
| Age: | Albian (late) - Cenomanian (early), Upper Cretaceous |
| Remarks: |
This mighty beast probably weighed about four grammes, (two paperclips). A number of
mini-teeth are in the collection of the Oklahoma Museum of Natural History, some of which
were previously assigned to Spalacotheroides sp. Kirkland et al, 1997.
About fifty enigmatic, smaller premolars (and two
associated jaw fragments) have also been recovered from this location. They're unusual for
Mesozoic mammals, in that they're low crowned, and the probably posterior specimens show a
highly molarized form, (Cifelli, 1999 -see Bibliography for details). These are probably
milk teeth. They can be confidently grouped into three types, (Taxa 1-3), and probably
represent the juveniles of the three spalacotheriids from the local fauna. "Most
groups of mammals known from the Cedar Mountain Formation, (
Multituberculate, Triconodonta,
Tribosphenida) can be immediately dismissed from
consideration, because their dentitions contain no teeth
that are remotely similar to those described herein. The only mammalian group known from
the Cedar Mountain Formation by taxa of appropriate size, morphology, and abundance is the
Symmetrodonta", (p.257-258).
The sizes and relative quantities of these teeth correspond closely to those of the three
spalacotheriid species, and the fossils come from precisely the same sites. Therefore,
the most abundant and middling-sized Taxon 1 probably represents young Spalacolestes
cretulablatta; the smaller specimens of Taxon 2, Spalacotheridium noblei; the two
larger specimens of Taxon 3, Spalacolestes inconcinnus, (p.260). |
| Reference: | Cifelli & Madsen (1999), Spalacotheriid symmetrodonts
(Mammalia) from the medial Cretaceous (upper Albian or lower Cenomanian) Mussentuchit local
fauna, Cedar Mountain Formation, Utah. Geodiversitas 21 (2), p.167-214. |
| Species: | Spalacolestes inconcinnus Cifelli RL & Madsen SK,
1999 |
| Place: | Mussentuchit, Cedar Mountain Formation, Utah |
| Country: | USA |
| Age: | Albian (late) - Cenomanian (early), Upper Cretaceous |
| Remarks: |
This holotype also lives at the Oklahoma Museum of Nat Hist. |
| Reference: | Cifelli & Madsen (1999), Spalacotheriid symmetrodonts
(Mammalia) from the medial Cretaceous (upper Albian or lower Cenomanian) Mussentuchit local
fauna, Cedar Mountain Formation, Utah. Geodiversitas 21 (2), p.167-214. |
| Genus: Spalacotheridium
Cifelli RL, 1990
'small Spalacotherium'
A species is also present in the Santonian Straight Cliffs Formation of Utah, (Eaton,
2005 see link). |
| Species: | Spalacotheridium mckennai Cifelli RL, 1990 |
| Place: | Cedar Mountain Formation, Utah |
| Country: | USA |
| Age: | Albian (late) - Cenomanian (early), Upper Cretaceous |
| Remarks: | Known from at least two locations. |
| Reference: | Cifelli (1990), Cretaceous mammals of southern Utah. III.
Therian mammals from the Turonian (early Late Cretaceous). J. Vert. Paleontology. 10,
p.346-360. |
| Link:
Vertebrate Paleontology in Utah, Medial Cretaceous vertebrates from the Cedar Mountain
Formation, Emery County, Utah: the Mussentuchit local fauna
Vertebrates from the Cedar Mountain Formation
An extensive report on fossil vertebrates from Cifelli
& Co. This is the text version of a pdf file, from the Utah Geological Survey. |
| Species: | Spalacotheridium noblei Cifelli RL & Madsen SK,
1999? |
| Place: | Cedar Mountain Formation, Utah |
| Country: | USA |
| Age: | Albian (late) - Cenomanian (early), Upper Cretaceous |
| Remarks: |
A seriously small creature, with an estimated bodyweight of about
2,5 grammes. This species was named in honour of the Noble Foundation, in recognition of
financial support. Remains can be admired at the Oklahoma Museum of Nat Hist. A
microscope would be useful.
The fauna from this locality is known as the Mussentuchit local fauna, (Cifelli 1999,
p.247-248). About 80 vertebrate
taxa have been identified on hand of over 7000 specimens, over a thousand of which are
mammalian, (p.257). These are still subject to studies, but there are more than 20 species.
These include multituberculates,
triconodontids, spalacotheriids and various
tribosphenic critters. Amongst these are Kokopellia, which may be an early
metatherian. A radiometric dating suggests the remains
are about 98 million years old. |
| Reference: | Geodiversitas |
| Link:
The Daily Ardmoreite
http://www.ardmoreite.com/stories/092999/new_dino.shtml
'Dinosaur now a 'Noble' creature'. This is a good, readable report. Mussentuchit, I’m
delighted to discover, was named with reference to the local water, which is rich in
alkaline and undrinkable, (Musn’t-touch-it). If only the inflictor of the atrocious
headline had been a twentieth so inspired. |
| Genus: Spalacotherium
Owen R, 1854
'mole-like beast'
Aka: Peralestes Owen R, 1871
Remarks: Based on the evidence provided by Zhangheotherium, Peralestes
appears to represent the upper teeth of Spalacotherium, which enjoys seniority.
(This is included in the link to Z. by Hu et al, 1997). S. & P.
are known from the same Dorset site.
Reference: Owen (1854), On some fossil reptilian and mammalian remains from the Purbecks.
Quarterly Journal of the Geological Society of London 10, p.420-433.
Apparently, a further publication from the same year is Owen (1854), Discovery of
Spalacotherium in the Purbeck beds. Bull. Soc. géol. France XI (2), 482.
| Reassigned species: S. minus Owen, 1854 see
Peramus tenuirostris | |
| Species: | Spalacotherium tricuspidens Owen, 1854 |
| Place: | Durlston Bay, Dorset, and Wealden,
Hastings Beds |
| Country: | England |
| Age: | Lower Cretaceous |
| Remarks: | This creature was something like 15cm long and is
known from several lower jaws. Collectively, these preserve the teeth from the
canine to the m6, (Sigogneau-Russell & Ensom, p.459).
The holotype works at The Natural History Museum in London. It's known to its friends as
BMNH 46019, (Gill, 2004) and is part of a dentary. |
| Reference: | Owen (1854), On some fossil reptilian and mammalian remains
from the Purbecks. Quarterly Journal of the Geological Society of London 10, p.420-433.
|
| Species: | Spalacotherium longirostris (Owen, 1871) |
| Aka: | Peralestes longirostris Owen, 1871 |
| Place: | Durlston Bay, Dorset |
| Country: | England |
| Age: | Lower Cretaceous |
| Remarks: |
Peralestes appears to represent the upper dentition
of Spalocotherium. The type specimen seems to reside in the Natural History Museum,
London. It's an upper jaw and shows the teeth P3-M6, (three
premolars and six molars), though they have been slightly
damaged, (Sigogneau-Russell & Ensom 1998, p.459). |
| Reference: | Owen (1871), Monograph of the fossil Mammalia of the Mesozoic
formations. Palaeontographical Society 24, i-vi +, p.1-115. |
| Species: | Spalacotherium taylori Clemens WA & Lees PM,
1971 |
| Place: | Wealden, East Sussex |
| Country: | England |
| Age: | Lower Cretaceous |
| Remarks: |
Known from a single, and very small tooth. It's a lower molar
with a complete labial
cingulum, which is a shelf-like bump on the side, (Ensom & Sigogneau-Russell 2000,
p. 774). |
| Reference: | Clemens & Lees (1971), A review of English Early
Cretaceous mammals. In Early mammals (eds Kermack DM & Kermack KA). Linnean Society
of London 50, (Supplement I to the Zoological Journal), p.117-130. |
| Wealden
mammals, Southeast England - Lower Cretaceous
The following is very loosely based upon part of Clemens WA, 1963.
The Weald is an area in southeast England. In the nineteenth century, geologists found a
sequence of rock layers were particularly well exposed there, and took to calling them
Wealden. These rocks, (which are in places generous suppliers of fossils), were
deposited during the Lower Cretaceous. A more precise age was difficult to pin-point. In
the paper by Clemens they're referred to as pre-Aptian. More recent research indicates
they're Valanginian - Berremian (132 - 121 million years ago).
There can be little room for dispute about the most widely known Wealden genus.
Iguanodon was first named in 1825 on the basis of sparse fossils from Sussex. Better
material turned up in Belgium in 1878. For some reason, the discovery of forty of the
things in a coalmine was deemed somehow newsworthy. This was clearly blown out of all
proportion, as not all of them were complete. Iguanodon became a global celebrity,
and that just goes to show the power of advertising. I could write more about this animal
but who wants to be bored by ten metre dinosaurs?
Animals with far more personality were living in the ancient Weald, and you don't need
telling they were mammals. Remains are ever so slightly less than complete, but Clemens
reported on ten undoubtedly Mesozoic mammalian specimens and a few of less certain pedigree;
all of them teeth. In keeping with this tasteful minimalist approach, I should mention
that I haven't got a complete copy of his paper. I'm restricted to a few pages. Still,
some is better than none. I also have the benefit of some subsequent studies by other
authors.
Bone Beds
As well as being very rare, mammalian fossils from the Wealden are difficult to find due to
their tiny size. Further complications are provided by the elusiveness of the source rock,
and the temperamental nature of the eroding cliffs. Promising spots can disappear due to
collapses. With perseverance and luck, the Cliff End Bone Bed can be found near Hastings
in Kent, which is immediately east of Sussex. This mighty layer of hard and coarse-grained
sandstone can attain thickness of over ten centimetres, but not much over. (Clemens
states: "never more than 4 or 5 inches thick".) Despite that apparent paucity,
researchers still managed to collect up an impressive quantity of rock to process; several
hundredweight. (Considerably more was subsequently worked through.)
Processing the rock involved the use of diluted formic acid for dissolving extraneous
material. The remainder was then washed, dried and sieved through differently sized
meshes. Smaller particles then required further chemical treatment prior to examination.
This yielded five mammalian teeth. A second location, the Paddockhurst Bone Bed, donated
three further specimens in 1960. (More detail on the processing is given below.)
Clearly, a great deal of muscle work must have been required. Furthermore, as these
fossils are a millimetre or so in length and had to be picked from among loads of other
tiny things, it's obvious that an enormous amount of intense concentration was also called
for.
It sounds like a lot of trouble for a few teeth
It does, but the research didn't only turn up mammal fossils. At the time, this was the
only known source for mammals of this age in the world. Upper Jurassic specimens were known
from the Morrison Formation of North America and, (as
interpreted at the time), Dorset in England. (The Purbeckian of Dorset is now thought to
be lowermost Cretaceous.) Apart from the two teeth found previously in Kent, the next
oldest fossils were from the Trinity Sand of Texas, (Albian). This left a gap in the
record of around 30 - 35 million years, and the Wealden provided the chance of partly
bridging it.
A brief history: some right, some less so
The first announcement of a Wealden mammal was made in 1891 by A Smith-Woodward. I've
heard he was good when it came to fossil fish, but not so sure with us hairy animals. One
of Smith-Woodward's later publications was Eoanthropus, the notorious 'Piltdown Man'
forgery.
Smith-Woodward wrote about a specimen from the collection of the Natural History Museum,
London (BMNH M13134). He interpreted it as a molar from
Plagiaulax, a
multituberculate. Subsequently, the tooth
suffered damage, and all that's left is a bit of root. The identification was less than
convincing and, as far as can be told, it's probably not mammalian.
Lydekker published further news in 1893. He assigned an incisor, (BMNH M5691), to
Multituberculata. This was done with the observation that it was indistinguishable from
the corresponding tooth of a rodent. As rodents had never been found in the Lower
Cretaceous, he concluded it was most likely from a multi.
More specifically, the incisor resembles those known from
Theriodmys so closely, that's probably what it is. The problem here is provenance.
The fossil was found in a block among a heap of stone on the shore, and then spent three
decades neglected in a private collection. Doubts were expressed as to whether the
remaining matrix was from the locality. Although it was said to have been encased in
Tilgate Grit, this was incorrect. That would be sandstone. As GG Simpson pointed out in
1928, the matrix was crumbly clay. The block of stone was presumably transported. This
could have occurred due to either tidal action or as a consequence of road building.
Smith-Woodward described three further specimens in 1911. These were collected by Teilhard
de Chardin and Peiletier at Cliff End, and two are from Wealden mammals. The bone bed of
origin couldn't be found at the precise location in 1960. Cliff erosion had very possibly
either destroyed or buried it. BMNH M10480 is the holotype of
Loxaulax valdensis. BMNH M10481 is part of a
lower molar, (m2), and probably from the same species. The third tooth wasn't catalogued,
but the short description matches what's now BMNH M20241, which was collected in 1911 near
Hastings. This is regarded as indeterminate and not clearly mammalian.
Until collecting resumed in 1960, remains of undoubted mammals from the Wealden (and much
of the Lower Cretaceous) were limited to two molars of Loxaulax valdensis. In the
terminology of Clemens (1936), the new finds provided further information on that species,
and attested to the presence of a symmetrodont and a
eupantothere. Unfortunately, my partial copy preserves no further details.
Kermack et al, 1965 provides further information on the locality, and goes into events
Clemens had departed.
A water bed?
The Cliff End bonebed may well be in the sea, which would account for difficulties with
precisely locating it, (p.536). Bits of it are most commonly found on the beach following
spring and autumn storms. These can be enthusiastic. In one instance, a block weighing a
couple of tons was left balancing on a ledge about three metres up the cliff face.
The location was rediscovered by Professor P Allen and collecting recommenced in 1960.
That's when Dr Clemens arrived on a post-doctoral placement at the University College of
London. Of the five mammalian specimens recovered between then and the autumn of the
following year, four teeth were from multis and the other was an upper molar at least
allied with Laolestes (Melanodon).
Fieldwork continued, (p.537).
The Paddockhurst Park locality proved to be very limited in scope and was probably
exhausted. All attentions turned to Cliff End and nine more teeth were recovered. One of
them was immediately recognised as something special. This was
Aegialodon, a close relatives of animals
such as myself and a wombat.
The rock
In line with the information above, the fossils are reported as coming from a layer of
rough sandstone with a thickness of between 5 and 12.5 centimetres. Ripple marks show
which side was up. The grains are of variable sizes; 0.24 to 5 millimetres in diameter,
and the black fossils are of similar dimensions.
Ancient conditions
This material was probably deposited in a brackish area, where fresh and salt waters chose
to network with each other. A river estuary is a good candidate. The London Platform was
being drained by rivers flowing southwards, and that land may have been home sweet home for
the terrestrial vertebrates. They surely didn't live in
the sea.
Various fish account for 90% of the recognisable vertebrate fossils, and most come from
Lepidotes mantelli. Pycodont teeth are well represented, and 15 to 20% of fossils
have been contributed by sharks; at least four species of hybondont. In percentage terms
the mammalian contingent doesn't merit a quiet squeak. Bits of reptile are more common.
However, quality is of great importance.
How to find Wealden mammals
First catch your rock. That's the relatively straightforward part. The grains and fossils
are held together by a calcareous cement, and it needs dissolving, (p.538). A solution
involving 10% formic acid is a good option. It's: relative speedy, easier to clean off,
and not as poisonous as other possibilities. Speedy in this context means a block of about
75cm² with a thickness of 10cm will require a mere two or three months of continual
submergence in order to disintegrate. (The figures in the paper are in inches; 12 square
inches and 4 inches thick.)
Continual submergence isn't quite meant literally, as the acid periodically gets tired and
needs replenishing. Blocks were removed and washed when bubbling stopped, and any loose
material was collected. This spent a day in running water to cleanse off any calcium salts
and silt. Such impurities can cause damage and clogging when sieving. The rock was then
placed in fresh solution and so on and so forth. Eventually, the block was fully
dissolved and sorting could begin.
Sieves
After cleaning the loose material required light baking to dry it (60°F). It was then sent
through a set of three sieves with differing meshes, (approximately between 2.4mm down to
0.42mm). Anything passing through the whole series was discarded. Elements as substantial
as about 1.4 millimetres and larger were sorted purely by eye and hand. However, most
material wasn't as large. It was also mixed in with a lot of quartz.
Sink or swim
The fossils are denser than most the other material, and that quality caused them to sink
more eagerly. Fossils and much unwanted debris can thus be sorted by floatation. In this
case, the floatation liquid was a mixture of tetrabromoethane and dibromoethane, and this
was a relatively cheaper option. The drawback is the poisonous fumes. A fume cupboard,
a convenient draft and as little exposure to the toxins as possible helped minimalize
danger to researchers. (They can be difficult to replace.) It's now time to add your
debris and stir, (preferably without breathing in).
In the unlikely event that the liquids are mixed to the correct proportions, the fossil
fragments and some extraneous material will obligingly sink. The mixture will more probably
require refining adjustments. This procedure left 95% of the loose material on the surface
to be sieved off. All of that is rubbish. Now add more material.
Drying
Eventually, there'll be a satisfying quantity of heavier material in your toxic soup, (p.539).
This can be retrieved and bathed in pure alcohol, which both limits exposure to the fumes
and allows as much as possible of the floatation liquid to be recycled. ('Relatively
cheaper' still meant expensive.) The lighter material was also treated for similar reasons.
As a further detox stage, both accumulations were left to dry at their leisure. The
uninteresting rubbish, (95% of the total), could then be disposed of. The potentially
interesting 5% was left for to air for two more days.
Efficiency
As is clear, this entire process is intensely time consuming and sounds amazingly dull in
comparison to prospecting á la Jurassic Park. It was the most efficient means available.
Finding tiny teeth can require sorting through extraordinary quantities of rubbish.
And now for the laborious bit...
Having isolated and detoxified the heavier material, each seriously small particle requires
looking at individually. This is best done with the help of a numbered grid of squares as
a background for orientation and a good microscope. Afterwards, the researchers may be
left wanting to bathe in alcohol.
The following informal list of Wealden mammals has come from subsequent sources.
Further Mesozoic site summaries can be found at Localities.
Meet the Mammals of the Wealden (Lower Cretaceous) (4 genera, 5 species)
Multituberculata (1 genus, 1 species)
Loxaulax valdensis
Dryolestoidea (1 genus, 1 species)
Laolestes (Melanodon) hodsoni
'Symmetrodonta' (1 genus, 2 species)
Spalacotherium taylori; S. tricuspidens
Boreosphenida (1 genus, 1 species)
Aegialodon dawsoni
|
| Species: | Spalacotherium henkeli Krebs B, 1985 |
| Place: | Castellar Formation, Galve |
| Country: | Spain |
| Age: | lower Barremian, Lower Cretaceous |
| Remarks: |
This species is based on a few cheek teeth. These somewhat
larger than those of the other species, (Sigogneau-Russell & Ensom 1998, p.459). |
| Reference: | Krebs (1985), Theria (Mammalia) aus der Unterkreide von Galve
(Province Teruel, Spanien). Berliner Geowissenschaften Abh., Reihe A 60, p.29-48.
|
| Species: | Spalacotherium evansae Ensom P & Sigogneau-
Russell D, 2000 |
| Place: | Durlston Bay, Swanage, Dorset |
| Country: | England |
| Age: | Berriasian, Lower Cretaceous |
| Remarks: |
The following is based upon my reading of Ensom & Sigogneau-Russell, 2000, (with thanks
to the kindly supplier).
The Purbeck Limestone Group of southeast Dorset was the second source of Mesozoic mammals
discovered in the world. Only the
Forest Marble Formation in Oxfordshire began work earlier, (and that also occurs in
west Dorset). Descriptions of Purbeckian mammals from Durlston Bay near Swanage date back
to 1854. A new inland site was discovered roughly five kilometres west at Sunny down Farm
in 1986, (p.767). Among the finds are fossils assigned to both Spalacotherium and
Tinodon. Most came from two strata of the Cherty Freshwater
Member at Sunnydown. Four additional specimens had been recovered at Durlston Bay.
S. evansae
The new species is represented by a fragmentary partial lower right jaw and remains of a
further thirty or so isolated teeth, (p.768). As well as being smaller than their equivalents
in S. tricuspidens and S. henkeli, they differ in the incompleteness of the
labial cingulum, and the
even greater equality of root size (as far as can be told). The incomplete cingulum is also
a distinction from the only known specimen of S. taylori, which falls within the
size range of this species. Some of the molars have a
tinodontid-like cuspule f.
A pipsqueak
The jaw fragment is 3.7 millimetres long, and the molar lengths run from 0.37 to 1.1mm,
averaging out at around 0.73, (Table 1, p.769). This was a seriously small mammal.
Jaw
The front of the fragment contains two alveoli for roots,
with the second being a bit longer than the foremost, (p.770). Next in line is a complete
molar. The protoconid is tall and thin. The
paraconid was probably not as high as the
metaconid, but the latter cusp is broken. The tooth has
a lingual cingulum which features a cusp e and cusple f at the front. The rear of the crown
possesses a low, sharp hypoconulid cusp. The remnants of the roots suggest similar sizes
and the division is halfway along the paraconid.
The next molar has lost most of its crown, but it was considerably smaller. The third
preserved tooth is even tinier. It shares a suite of characteristics with the final molar
of Spalacolestes.
Referred teeth
Most specimens are fragments. Among them are lower and upper molars. Four were found to
probably represent juvenile gnashers: "They are very thin transversely, with
particularly long and thin roots", (p.773). But: "Whether these milk teeth
belong to Tinodon or Spalacotherium is uncertain; however, the similarity
with the tooth from Galve, where S. henkeli was identified (Krebs, 1985), has led us
to prefer the latter attribution."
Lower molars of 'symmetrodonts' typically feature two similarly sized roots, a lingual
cingulum and a small, sharp hypoconulid. The
labial cingulum of these teeth, (albeit partial), the
compressed trigonid and the sharp high protoconid are
signatures of Spalacotherium. Generally, spalacotheriids favoured a complete labial
cingulum, (p.774). However, that also doesn't apply for
Spalacotheroides or Zhangheotherium (if so
classified).
The abstract is linked to Tinodon micron, (above).
Holotype
The holotype, DORCM GS 355, is in the collection of the Dorset County Museum, Dorchester.
The specific name honours Dr Susan Evans, "in recognition of her major contribution to
the discovery and study of Mesozoic microvertebrate remains in Great Britain." |
| Reference: | Ensom & Sigogneau-Russell (2000), New symmetrodonts
(Mammalia, Theria) from the Purbeck Limestone Group, Lower Cretaceous, southern England.
Cretaceous Research, vol 21, no 6, p.767-779. |
| Link:
The Dorset County Museum Geology Department
http://home.clara.net/dorset.museum/page9.html
Nostalgia break: I wasn't very old when I first went to this museum. It's full of
stuff on the archaeology, history, culture, geology, paleontology, natural history and etc
of Dorset. Dino footprints; Thomas Hardy memorabilia; ichthyosaurs; an old font from the
church of Melbury Bub, which is where I'm reliably informed my father was baptized; dead
people from the old days and stuffed animals from the surrounding countryside.
We took our daughter along when she was about three. A couple of German tourists were
struggling somewhat with reading the stuffed animal labels. Our daughter, who's Anglo-
Deutsch, childed around the room with great enthusiasm, gleefully announcing
"Fledermaus - bat. Fuchs - fox. Dachs - badger..." The German tourists were
much astounded and stopped struggling. On leaving the museum, turn right and walk a couple
of hundred yards further up the road. The 17th century Old Tea House does a great cream
tea. |
| Species: | Spalacotherium hookeri Gill, P, 2004 |
| Place: | Durlston Bay, Dorset |
| Country: | England |
| Age: | Berriasian, Lower Cretaceous |
| Remarks: | The following is based upon my reading of Gill,
2004,
Most Purbeckian Mesozoic mammals were either collected in the 1850s or since the late
1980s, (p.748). This one was found by the author in 1969, but further searching of the
precise location hasn't turned up more material. It's probably marginally older than
S. evansae, as it came from a slightly lower layer of rock. The main difference
from the type species, S. tricuspidens, is size. The new species is 40% or so
smaller, although both individuals were adult. Distinctions to the other species are
stronger, and include details of the molars.
The only specimen is part of a lower left jaw with the rear five molars in place. The last
in the series is the smallest tooth. Along with its distinctive shape, this allows a
confident identification of it as being the m7. (This genus has seven lower molars.)
Consequently, the foremost preserved tooth must be the m3. An
alvelous attests to the former presence of the m2, and that pair were of similar size.
Also included in the paper are descriptions of four isolated teeth from the Wealden strata
of southeastern England. These fossils were found some decades ago, and also belong to the
genus, (p.749-750). One may be a deciduous, lower
premolar. The trigonid
has an obtuse angle and the enamel is relatively thin.
Although the teeth of this species are similar in form to S. tricuspidens, there
are a couple of differences on the jaw, (p.751). In the new species,
Meckel's groove extends further forward until at least
below the m2. Behind the final molar, a feature termed the coronoid process seems to
ascend more gradually. However, crushing could have provided some distortion, and the
features may represent variation in individuals. Further specimens would be required to
show whether these characteristics are of diagnostic value.
Considering they've been dead for a long time, spalacotheriids and friends seem to be
breeding profusely at present. (Thanks are due to the supplier of the paper.)
The holotype, BMNH 44970, resides at The Natural History Museum, London, where Dr Jerry
Hooker can also be found. The specific name is in recognition of his contributions to the
study of fossil mammals. Judging by the photo, the holotype's a bit over half a centimetre
long. |
| Reference: | Gill P (2004), A new symmetrodont from the Early Cretaceous of
England, Journal of Vertebrate Paleontology, 24(3), p.748-752. |
| Genus: Spalacotheroides
Patterson, 1955
'Spalaco form' |
| Species: | Spalacotheroides birdwelli Patterson, 1955 |
| Place: | Paluxy Formation, Texas & Massentuchit, Utah and Wyoming |
| Country: | USA |
| Age: | Lower Cretaceous - Upper Cretaceous (early) |
| Remarks: |
According to Cifelli et al, 2000, this is a nomen dubum.
Several specimens, including the holotype, live in the collection of the Peabody Museum,
Yale. |
| Reference: | Patterson (1955), A symmetrodont mammal from the Early
Cretaceous of northern Texas. Fieldiana. Zool. 37, p.689-693, fig. 145. |
| Genus: Symmetrodontoides
Fox RC, 1976
'symmetrodont form'
Remarks: The latest known and most specialized North American spalacotheriid genus, if
that's where this genus belongs. JF Bonaparte has apparently expressed the opinion that
this taxon belongs within
Dryolestida, (with thanks to Mikko Haaramo for the notification.)
McKenna & Bell give Fox, 1976 as the year of description. I'm sure they have their
reasons and they're probably correct. Averianov confirms this. I have also seen 1979
cited.
Reference: Bonaparte (1999), New Dryolestida (Theria) from the Late Cretaceous Los
Alamitos Formation(Argentina)and paleogeographical comments. in Leanza HA, (ed.) 1999:
Abstracts of VII Int. Symposium on Mesozoic Terrestrial Ecosystems. Buenos Aires, A1-A65
|
| Species: | Symmetrodontoides canadensis Fox, 1976 |
| Place: | Upper Milk River Formation, Alberta |
| Country: | Canada |
| Age: | early Campanian, Upper Cretaceous |
| Remarks: | This holotype, UALVP 8588, is a partial right
dentary in the collection of the University of Alberta.
There are a couple of lower jaw fragments with teeth in situ, (Sigogneau-Russell
& Ensom 1998, p.461). |
| Reference: | Fox (1976), Additions to the mammalian local fauna from
Upper Milk River Formation (Upper Cretaceous), Alberta. Canadian Journal of Earth
Sciences 13, p.1105-1118. |
| Species: | Symmetrodontoides foxi Cifelli RL & Madsen,
1986 |
| Place: | Wahweap Formation & Paunsaugunt Plateau,
Utah |
| Country: | USA |
| Age: | Santonian (late) or Campanian (early), Upper Cretaceous |
| Remarks: |
Weighed about four paperclips, (8g). The type fossil, a lower
molar, is housed at the Oklahoma Museum of Nat Hist. |
| Reference: | Cifelli & Madsen (1986), An Upper Cretaceous symmetrodont
(Mammalia) from southern Utah. J. Vert. Paleont. 6, p.258-263. |
| Link:
A New Vertebrate Fossil Locality Within the Wahweap Formation (Upper Cretaceous) of Bryce
Canyon National Park and Its Bearing on the Presence of the Kaiparowits Formation on the
Paunsaugunt Plateau
http://www2.nature.nps.gov/geology/paleontology/pub/grd3_3/brca1.htm
An interesting article by Eaton JG, Munk H, & Hardman MA. This relates to finds made in
1997. |
| Species: | Symmetrodontoides oligodontos Cifelli RL, 1990 |
| Place: | Straight Cliffs Formation, Utah |
| Country: | USA |
| Age: | Turonian, Upper Cretaceous |
| Remarks: |
The holotype, also in Oklahoma, is a corroded right lower molar.
According to Sigogneau-Russell & Ensom, 1998, this is an even smaller species, (p.461).
|
| Reference: | Cifelli (1990c), Cretaceous mammals of southern Utah. III.
Therian mammals from the Turonian (early Late Cretaceous). J. Vert. Paleontology. 10,
p.332-345. |
| Genus: Symmetrolestes
Tsubamoto T, Rougier GM, Isaji S, Manabe M & Forasiepi AM, 2004
'symmetric hunter' |
| Species: | Symmetrolestes parvus Tsubamoto T, Rougier GM,
Isaji S, Manabe M & Forasiepi AM, 2004 |
| Place: | Kitadani Formation, Fukui Prefecture |
| Country: | Japan |
| Age: | Barremian?, Lower Cretaceous |
| Remarks: | The following is based upon my reading of Tsubamoto
et al, 2004.
Following the establishment of Zhangheotheriidae for the
'symmetrodonts' of Liaoning late in 2003,
Shalbaatar was left on its own as the only Asiatic member
of the sister family, Spalacotheriidae. To avoid feelings of isolation Japan agreed to
yield this new genus as a companion. The complete shearing surfaces, higher crowns and
fuller cingulids of the lower
molars mean it's more advanced than the Chinese zhangheotheriids, (p.329). However, it
also differs from its closer relatives in the presence of more lower
premolars (five) and less molars (four). In addition, the transition between the two
types of tooth is more gradual.
The presence of both this relatively basal spalacotheriid and
zhangheotheriids in East Asia prompts the authors to raise the possibility of Asiatic
origins for Spalacotheriidae. If correct, then this would imply a presently unevidenced
history stretching back to the Jurassic.
Remains presently consist of a lower right jaw, which was found in a block of stone in
central Japan, (p.330). This also contained a fragment of bone, bits of egg shell and the
tooth of an iguanodont dinosaur. The age is not absolutely clear, but Barremian is at
least likely, (p.331). The specimen is fragmentary and preserved in two pieces. As they
were prepared from opposing sides, re-assembly is made difficult. Five teeth are present.
These are probably the final premolar (p5) and four molars (m1-m4). However, this diagnosis
is based upon the shape of the teeth as evidence regarding replacement patters is lacking.
The premolar has a length of just over a millimetre. The molars range from about 0.6 (m4)
to 0.7mm.
Family concerns
Page 332 offers a revised diagnosis for the family. Differences to other 'symmetrodonts'
include: the strong pterygoid crest on the dentary and a wide
and upright coronoid process. The postcanine teeth have
a continuous shearing crest and high crowns. The lower molars contain reduced
talonids and near continuous cingulids on the
buccal and lingual sides, with
that latter characteristic being lost in
Spalacotheroides.
Dentition
As well as the teeth, the jaw also contains alveoli for
further gnashers. Nine holes are complete with three other probables, (p.333). The likely
lower dental formula was four incisors, one
canine, five premolars and four molars, (although i3+ and
p4+? is safer -p.334). The first incisor is pretty much horizontal. As it doesn't protrude
beyond the front of the jaw and lacks wear, this was probably unerupted and is visible due
to a convenient break in the bone, (p.335).
The best preserved tooth is the final premolar, (p5), but it's damaged. As premolars go, it's
relatively complex. Of the molars, m4 is the smallest and simplest. It was probably
single-rooted. Although upper teeth aren't known, its structure suggests if there was an M4
counterpart, then it was most likely to have been much reduced, (p.336).
Postcanine peculiarity
With spalacotheriids in general, molars tend to be more generously supplied than premolars,
(p.337). However, although the p5 in this case can be reasonably termed molar-like, it seems
to be a premolar and resembles what was originally termed the m1 in
Zhangheotherium. That interpretation was doubtful for a couple of reasons.
Firstly, the tooth was much less worn than its colleagues which suggests it was more
recently erupted. Secondly, an additional specimen has been found which shows replacement
occurring at that position. That's very un-molar-like behaviour.
Among Spalacotheriidae, a more usual number of premolars is three, (p.339), while molars go
up to seven. If Symmetrolestes does possess five and four respectively, this may be
a reflection of its relatively primitive position within the family.
The holotype, NSM PV 20562, is a resident of the National Science Museum, Tokyo. The
specific name is Latin for 'small'. As the fossil has a length of roughly 1.8cm,
mouse-sized
shouldn't be too misleading. |
| Reference: | Tsubamoto et al (2004), New Early Cretaceous spalacotheriid
"symmetrodont" mammal from Japan. Acta Palaeontologica Polonica, 49 (3),
p.329-346. |
| Genus: Yaverlestes Sweetman
SC, 2008 |
| Species: | Yaverlestes gassoni Sweetman SC, 2008 |
| Place: | Wessex Formation, Isle of Wight |
| Country: | England |
| Age: | Barremian, Lower Cretaceous |
| Remarks: |
The paper has arrived and is being read. Thanks are due to David N. Remains include a partial
dentary and some isolated teeth. In contrast to other
euro-spalacos, this one fits within the presently largely North American subfamily of
Spalacolestinae. |
| Reference: | Sweetman SC (2008), A spalacolestine spalacotheriid
(Mammalia, Trechnotheria) from the Early Cretaceous (Barremian) of southern England and
its bearing on spalacotheriid evolution, Palaeontology, 51(6), p.1367-1385. |
| Genus: Zhangheotherium Hu Y,
Wang Y, Luo Z & Li C, 1997
'Zhang He’s beast'
Family: Zhangheotheriidae Rougier GW, Ji Q & Novacek MJ, 2003
Remarks: The generic name honours Mr Zhang, who both harvested and generously donated the
holotype from his personal collection of fossils. He has reportedly donated all his
extensive collection to one institution or another.
The genus briefly summarised: "Its hands, however, have relatively short, straight
fingers, with gently curved, broad claws.", (Weil, 2002), the inference being that it
was a moderate ground dweller-clamberer by habit, rather than any kind of arboreal acrobat. |
| Species: | Zhangheotherium quinquecuspidens Hu et al, 1997 |
| Place: | Jiansahngou Beds, Yixian Formation, Liaoning |
| Country: | China |
| Age: | Barremian, Lower Cretaceous |
| Remarks: | The following is based upon my reading of Hu et al,
1997.
Until this publication, 'symmetrodonts' had been
subjects of plenty of studies, but the available evidence was severely restricted to
occasional fragments of jaw and isolated teeth. Naturally enough, this left much room for
speculation. Teeth are generally a good indication of favoured foodstuff, and so a diet
based largely upon creepy-crawlies was a safe enough conclusion. The small size of the
teeth, (molars with lengths of about a millimetre or so), and
the jaw fragments suggested animals of roughly shrew- or mouse dimensions, rather than
anything likely to rival dinosaurs. However, the rest of the anatomy was hypothetical;
presumably more derived than basal
mammals, but less so than ones with tribosphenic
or near-tribosphenic teeth. (This is ignoring
australosphenidans, as that concept postdates this description.) Mr Zhang, an
experienced and self-taught amateur, stepped in where professionals hadn't managed to tread,
and changed the state of knowledge. He found a specimen more complete than anybody had
probably ever dared to dream of, and donated it for study, (p.137). Aptly, there's now a
family named in his honour.
The first specimen consisted of a near complete skeleton, a partial skull and teeth. This
allowed the hypothetical expectations to be tested, and they were pretty accurate. In the
terminology as used in the paper, the animal was an 'archaic therian', although this
project uses Theria in a more restricted sense. (The nearest
equivalent term I presently employ is Holotheria.)
Getting the measure of things
As a table of lengths is provided, it would be churlish not to mention some. The
dentary is a bit over three centimetres. This is longer
than the corresponding element in the Liaoning eutherian,
Eomaia. Taking a shot in the arm, the
humerus measures 2.2cm, the ulna
half a millimetre less, and the radius 1.7cm. The legs are
a bit longer: femur 2.2cm, tibia
and fibula 2.35cm. For the teeth, the lower molar lengths
range from 1.1 to 1.8 millimetres (1.5 is the mean), and the uppers (M3-M5 only) 1.6 to
1.8mm. There are six lowers.
Ignoring the less well represented tail, the complete animal had a length of about fifteen
centimetres, (nose to other end). This is actually quite large compared to many 'small'
insectivores. (The bodylength is based on the sketch and photos on page 138.)
Family concerns
Although originally assigned to Spalacotheriidae, a new family was established in 2003;
Zhangheotheriidae. Its members are more basal than spalacos,
even the earlier ones. ["The Chinese Zhangheotherium, having rounded, conical
molar cusps that lack connecting crests (Hu et al. 1997), apparently did not achieve the
dental specialization characteristic for Spalacotheriidae... and should be excluded from
this taxon, although it may be a sister taxon to
Spalacotheriidae", (Averianov 2002, p.713).]
Teeth
The dental formula per side is given as: (uppers): 3
incisors, 1 canine, 2
premolars and 5 molars; (lowers): 3, 1, 2 and
6 (5 is correct) respectively.
Dental formula updates
However, Tsubamoto et al, 2004 (p.339) suggests the holotype has five
lower molars rather than the six originally described. The front-most is likely to be a
premolar. In a subsequently identified specimen, it was in the process of being replaced.
Another possibility is that the holotype could be a juvenile and not in possession of a
complete set of adult teeth. The closely related
Maotherium has six molars, the final one of which is extremely reduced in
size.
Wang et al, 2006 (p.197) has the lower formula revised to 3, 1, 3, 5 -not 6, but the
revison isn't their doing. They do see a further complicating factor. In terms of
its construction, the 'premolar' presently designated as p3 is similar to the
molars, and there's a possibility it's actually the first of those teeth. This
uncertainty reflects the fact that, in contrast to extant mammals, some more basal
versions replaced at least some 'molars'.
Returning to the original description, (p.138), the lower molars have relatively robust and
rounded main cusps. The uppers also differ in that a cusp termed B' is hypertrophied; ie
particularly big. That doesn't mean it's anything like the largest cusp on the tooth. The
central cusp, (A or a respectively), dominates the crown.
Referring to the lower molars, (p.139): "Zhangheotherium resembles other known
spalacotheriid symmetrodonts... in that the central cusp (a) and the two accessory
cuspules (b,d) of the lower molar form an acute triangle. [I suspect the accessory
cuspules are actually b and c, rather than d.] The associated upper and lower teeth of
Zhangheotherium could occlude into the embrasures of the opposing tooth row, a
pattern unique to spalacotheriids among archaic therians... This suggests that the teeth
were used more for crushing and puncturing than shearing..." (The omissions are
reference number, 'spalacotheriids' includes what are now zhangheotheriids, and 'archaic
therians' equates to holotherians in this project.)
Judging by the sketch, the other lower teeth are fairly unremarkable. The first incisor is
a couple of millimetres in height, and slants forwards and upwards. The other two are
about half the size. The canine's fairly tame. Seen from the side, the profile of the
premolars is roughly triangular. They're about the same height as the canine and molars.
They're nothing like the morphology of the tooth designated m1. However, should it be a
premolar, it must be a deciduous one. They typically
have more complex crowns.
Implications for Dorset
The cusp pattern of the upper molars is reminiscent of Peralestes. As mentioned in
the text to Figure 2, (p.138), the lowers are more similar with
Spalacotherium. These Purbeckian genera are known from the same Dorset location,
and only from upper and lower material respectively. By inference, there's a strong
probability that the fossils represent only one genus. However, probability and certainty
aren't the same.
Lower jaw development
"The dentary has a dorsally curved condylar process
and a posteriorly tilted coronoid process", (p.139). There is no angular process, and
that lack is old fashioned.
I appreciate many people will have little idea what these 'processes' might be. In this
instance, they're particular branches of the lower jaw, and they serve some function or
other. Anchoring points for muscles is a common use. An angular process can often be found
low down on and towards the rear of mammalian dentaries. However, it emerged in animals
more derived than this genus. Characteristics typical of
existing mammals didn't arrive as a single package. They evolved at various times.
There are other features of this lower jaw which won't be found in modern mammals. There's
a channel on the inside called the Meckelian groove.
In this case, it's narrow and becomes shallower and fainter towards the front. A rugose
area along the anterior of the groove could indicate the presence of an 'extra' bone known
as a splenial, and a rough area at the base of the coronoid process is consistent with the
retention of a poorly developed coronoid bone.
It's often stated that mammalian lower jaws are composed of a single bone called the dentary,
and this is now the case. Relatively early non-mammalian synapsids had lower jaws made from
up to eight separate bones per side. (Between four and six are still popular with reptiles,
including birds who generally favour five, Sidor 2003, p.605.) During synapsid history,
most these bones gradually faded and then vanished, or were relocated, but this took time.
Multi-boned lower jaws were still in use by some mammals of the Cretaceous.
Dryolestidans may be anatomically more derived than
spalacos and zhangheoteriids, (although there's no consensus on this as yet). At least in
the case of Henkelotherium, a much reduced
coronoid bone has been inferred as still present.
Ears
In mammals, the inner ear is encased in one bone called a
petrosal, and this seems to be universal across
Mammalia. The cochlea is housed in a feature termed the
promontorium, and the shape provides an indication
as to the form of its contents. In more derived mammals, the promontorium is oval and
bulbous. This allows room for a cochlear canal which
is coiled by at least 270°.
In Zhangheotherium, Sinoconodon,
morganucodontids,
triconodonts and multituberculates, this
feature is cylindrical and finger-like in shape. The cochlea is straight or slightly
curved, but not properly coiled.
Vertebrae
Like virtually all living mammals, the neck consists of seven
cervical vertebrae. As this was also the case in the nearly-
eucynodont Thrinaxodon, (pers comm Dr Ferdinand Abdala, 2004), this basic
number was presumably already established at least 250 million years ago. Strictly speaking,
it's no longer universal. Both sloths and manatees are non-conformists in this regard, and
there may be others. (Mit bestem Dank an Frank, who raised the subject at some stage in
2003.)
Going further down the body, there are thirteen
throacic
vertebrae, on which can be found the ribs. Six lumbar and
three or four caudal vertebrae are represented by
impressions.
Arms
The upper arm bone is robust, (p.140). "Distally, the
humerus has an incipient trochlea for the ulna, a
therian apomorphy... that is absent in
multituberculates..." (reference numbers
omitted). A prominent, spherical unlnar condyle on the humerus resembles the condition in
'non-therian' mammals, (non-holotherian in this
project.)
Hips and feet
The bones of the pelvis are slender, and the rod-like
ilium is about three times the length of the
ischium. On the back of the foot is a feature known from
Gobiconodon, existing
monotremes and (to my knowledge) at least some multis,
(eg Kryptobaatar). This is a spur of bone on
the heel.
In living monotremes, this feature is generally restricted to adult males. With reference
to only the platypus, it's associated with
venom. The presence of this spur suggests the holotype is also a male, (although that
point isn't directly expressed in the description).
Zhang-she-otherium
For those of a romantic disposition:
There's some good news for Zhangheotherium from Luo et al 2002, p.68. He, (and it
very probably was a he), can look forward to female companionship. That spur of bone is
missing in the case of a second specimen. Assuming this is s a sexual dimorphic
characteristic, then we might be able to look forward to the patter of tiny paws, as long
as they fancy each other. While no breeding programme is planned, experience suggests that
such formalities are sometimes superfluous. Where there's a will there's a way.
Systematics
The description reports the following, (p.140): "The sister-taxon relationship between
therians and multituberculates... is strongly
supported by the evidence from Zhangheotherium, such as the
clavicle-interclavicle joint and some features of the
femur, corroborate the therian affinities of
multituberculates..." (reference numbers omitted). However, it's now 2004 rather
than 1997 and there's little consensus. The available evidence is improving, but it's
still thin.
Acute-angled 'symmetrodonts' are generally regarded as holotherians, and are probably
within Crown-group Mammalia; the most recent
common ancestor of myself and a platypus, and all of its descendants. In some respects,
(eg the retention of an interclavicle in the pectoral girdle), the anatomy is more
basal than known from
Henkelotherium, a dryolestidan. However, that's insufficient to indicate a more
basal position within Mammalia. The interclavicle is
present in existing, adult monotremes and, embryonically, in
marsupials.
Holotype
The holotype, IVPP V7466, works at the Institute of Vertebrate Paleontology and
Paleoanthropology, Beijing. The specific name refers to the three main and two accessory
cusps on the lower molars. |
| Reference: | Hu et al (1997), A new symmetrodont mammal from China and its
implications for mammalian evolution. Nature 390, p.137-142. |
| Other reports:
East Asia
Abstracts submitted to The Society of Vertebrate Paleontology Annual Meeting 2004 include:
Early Cretaceous spalacotheriid "symmetrodont" (Mammalia) from East Asia, lead
author Takehisa Tsubamoto. This may have something to do with
Symmetrolestes, but I'm not certain. |
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| Help:
Should anybody have any further information, I'd be pleased to hear of it.
Regarding references and Bibliography:
I haven't and can't verify all the references, so beware. Traditional papers used in
constructing this page are in the bibliography. If you feel these are too few, then send
some more.
With thanks to all the featured sources.
Trevor Dykes, October 2001 Last update: 23.9.2010
Ktdykes@arcor.de |
Bibliography:
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Averianov AO & Archibald JD (2003), Mammals from the Upper Cretaceous Aitym
Formation, Kyzylkum Desert, Uzbekistan. Cretaceous Research 00 (2003), p.1-21.
Butler & Clemens (2001), Dental morphology of the Jurassic holotherian mammal
Amphitherium, with a discussion of the evolution of mammalian post-canine dental
formulae. Paleontology, 44 (1), p.1-20.
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and Peramura) from the Lower Cretaceous (Barremian) of Spain. Cretaceous Research 17,
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p.1214-1226.
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http://www.uhmc.sunysb.edu/anatomicalsci/paleo/terms.html |
