MESOZOIC MAMMALS; SHUOTHERIIDAE & AUSTRALOSPHENIDA, an internet
directory:
| MESOZOIC MAMMALS; SHUOTHERIIDAE &
AUSTRALOSPHENIDA, 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.
Looking for books?
You could visit the
Book Centre and look around.
| If you’re a wearer of dentures, please
put them in now…
Let’s start with some readily available evidence. Touch your lower, front teeth with the
tip of your tongue. There either are, or were four of them. These are your
incisors. The next tooth to either side is a sharp,
sticky-up one known as a canine. Next along are a couple of
exciting ones with a pointy bit at the front and a kind of basin behind; the
premolars. Finally come two large
molars, (or perhaps three including the wisdom tooth). Feel
around with your tongue and you’ll find the architecture’s rather involved. These molars
are one of the most dramatic evolutionary events in the history of mammals.
(If you don’t yet possess adult teeth, or can’t find your dentures, MI-Stupid.com
provide a helpful map: many thanks.)
These complex molars are tribosphenic teeth. This strange sounding word comes from the
Greek. 'Tribo' means grinding, whilst 'sphen' refers to cutting, shearing and wedge.
Unsurprisingly, the teeth in question perform both functions, which is an elegant and
useful combination, as you can experience when eating or gnashing your teeth. Only
mammals have truly dual-functional teeth like these.
It was assumed that this sort of chopper evolved only once. New research suggests that a
similar condition may actually have developed twice; once in the northern hemisphere and
once in the southern, (Luo Z-X, Cifelli RL & Kielan-Jaworowska Z, 2001: Dual origin of
tribosphenic mammals. Nature: Vol. 409, p. 53-57). This matter will probably only be
resolved with the help of further finds, especially from the south. More evidence is
emerging. This directory is built on the assumption that this dual evolution happened,
whilst acknowledging other points of view. (It certainly makes for tidier book-keeping.)
Nobody is arguing that exactly the same feature developed twice. Rather, that a similar
feature emerged in parallel, via two different routes; similar but not identical. This
discussion is very much on-going amongst mammal paleontologists. |
| Links:
T Mike Keesey, The Ages of the Mesozoic
http://dinosauricon.com/times/index.html
A map of the Mesozoic.
Mikko Haaramo's Australosphenida
Mikko Haaramo's Australosphenida
A modern day tribosphenidan seeks to instil order amongst his distant relatives. If the
Australosphenida theory is fully correct, than modern
monotremes are included in the taxon. However, should doubts concerning the affinities
of monotremes prevail, (eg. Woodburne, 2003 concludes they're pretribosphenic, rather than
tribosphenic mammals), then Australosphenida may be more limited in scope, but not
necessarily invalidated.
Carnegie Museum org
http://www.carnegiemuseums.org/cmnh/news/01-jan-mar/010301luo.html
The tooth, the whole tooth and nothing but the tooth, (a phrase I've disgracefully nicked
from David Marjanovic.) Mullings on tribosphenic molars.
Informative and reasonably readable.
(With thanks to Cathy Klingler, who kindly informed me of the new location of this
page.) |
A. Shuotheriidae B.
Australosphenida
| Genus: Pseudotribos
Luo Z-X, Ji Q, & Yuan C-X, 2007
'false grinding'
Family: Shuotheriidae Chow & Rich, 1982
Remarks: The generic name celebrates the pseudotriobosphenic qualities of the molar
teeth; dual-functioning cutters and grinders. The significance of that rather
ferocious looking word will be addressed below.
Thanks for that are due to Tim Williams for posting a notification of the
publication. |
| Species: | Pseudotribos robustus Luo Z-X, Ji Q, &
Yuan C-X, 2007 |
| Place: | Daohugou Locality, Jiulongshan Formation, Inner Mongolia |
| Country: | China |
| Age: | ?Middle Jurassic |
| Remarks: | The following is based upon my reading of
Luo et al, 2007. As I write this, a pdf version of the description is freely
accessible on-line and, as long as it hasn't since disappeared or I've been forgetful,
careless, or both, the link below should take you to it.
This genus, the third mammal to be salvaged from
the local fauna, doesn't match the outrageous preservation of the first two
weirdlings; semi-aquatic Castorocauda
and semi-airborne Volaticotherium.
Pseudotribos is thus potentially disappointing, and merits no more than
'exceptionally very good indeed'; marks no higher than an average ten out of ten.
It's a specialized digger, Mesozoic parallels of which have been done before, and
no more glorious than easily the most complete shuotheriid known. All that's been
discovered of it is about half a small corpse with only traces of occasional hairs
rather than more elegant ancient fur coats. In short, Pseudotribos excites
no more astonishment than would result from AFC Bournemouth winning the European
Cup Final 15-0 against the combined forces of AC Milan, Benfica, Bayern Munich,
Manchester United, Real Madrid and ten or twenty other teams. It's within the bounds
of possibility according to my imagination.
False cutter-grinders
One of the big stories that Pseudotribos relates is that of pseudotribosphenic
molars sported by shuotheriid mammals. These are
parallels to tribosphenic teeth but different.
Previously, the global supply of shuotheriid fossils consisted of a partial lower
jaw and eleven molars (or more assuredly molariforms
in the stricter terminology of the paper). Uppers and lowers had never been found
in direct association, so thoughts on how they may have interacted and, indeed, if
uppers and lowers had been correctly assigned to Shuotherium
were hypothetical musings awaiting robust testing. P. robustus gave the
structure of those ideas a strong kicking, and it remained standing. Not only was
it able to provide the teeth in association. It also managed to pass the time
through thousands of millennia by keeping several in occlusion.
As well as having an impressive adjective, those pseudotribosphenic things also
possess a suite of impressively derived characters
(p.93), and they appear somehow incongruous stuck, as they are, on essentially
basal mammalian jaws. Other skeletal features,
obviously not previously known due to the drastic effects of the fossil famine of
skeletons, are also primitive for mammals. An easy enough one to grasp is the
pronounced sprawling posture of the front legs. As is the case for other
therians, your front legs hang down relatively
straight from the body; a prediction you could test for yourself by reverting to
quad-pedal locomotion, aka crawling around on all fours. That's by no means the
limit of the basal-osity, but merely a reader-friendly example.
Age
This remains disputed. The local geology is complex as much folding of strata has
occurred. Evidence has been presented to support anytime between Middle Jurassic
to Lower Cretaceous. Presently, active mammal researchers all appear to favour a
date relatively early in that spectrum. Some other researchers clearly disagree.
If the later date were correct, then this would be a drastic chronological
extension for the otherwise poorly known shuos. All other localities, at least two
in China and one in England, are Jurassic. That isn't a significant part of the
evidence used for determining dates, and isn't a point mentioned by the authors.
The continuing paucity of shuo remains doesn't recommend the critters as reliable
indicators. However, as more mammals turn up at the Daohugou Locality, they could
become significant for faunal comparisons to help reading the time.
Teeth
As far as is known, the tooth formula per side is: (uppers): at least 2
incisors, 1 canine,
5 premolars and 3 molars;
(lowers): 4, 1, 5 and 3 respectively.
The upper molars have three main cusps. On the buccal
side are located a paracone to the front and a smaller metacone behind. That side
is the 'top' of a heart-shaped outline. Towards the 'base', actually the
lingual side, is a raised cusp termed a
pseudo-protocone. It functioned much as later
boreosphenidan protocones did but, in evolutionary terms, it's an entirely
independent development; an analogue and not a homologue. While a cusp in that
position may serve some function or other, there'd have been no point in developing
a large one merely for decoration. It was a functioning grinder and, for that
purpose, it required a corresponding area below to work against.
Thoughtfully, the lower molars took this into account by providing conveniently
located rimmed basins. If this had been at the rear, then it'd would've been
tempting to see these molars as transitionally on the way towards we therian mammals,
and some humans would've taken this more personally. However, this basin is
positioned in front of the trio of main cusps and, therefore, has no connection
with the development of the coincidental talonid of
boreosphenidans. As it's in entirely the 'wrong' place, it's a
pseudotalonid; a true pseudotalonid unlike the
broadly similar structure some docodont mammals
came supplied with. Part of its rim is provided by a buccal cusp termed the
pseudo-hypoconid but, in contrast to Shuotherium, there's no pseudo-entoconid
on the lingual side. The trigonid of cusps behind
this surprising state of affairs -a genuine trigonid you'll be relieved to hear- has
a buccally situated protoconid as its tallest
member, and it received lingual company from a
paraconid and a lower metaconid.
Affinities and size
Pseudotribos is the second shuotheriid genus to be established. As well as
lacking the pseudo-entoconid on lower molars, the teeth also happen to be differently
sized to all Shuotherium species, but it would fit into the full generic
range. It's around 30% smaller than S. shilongi and S. kermacki,
although it does get to beat S. dongi by 20%. The pseudo-protocone is
stronger than that of S. kermacki.
Lovers of the shoulder region might take joy from hearing that the: "interclavicle
(is) twice as long as the sternal manubrium, with two lateral processes and
an expanded posterior process." Many others will be left cold be this announcement.
It's a basal condition for mammals not found in
eutriconodonts, multituberculates, we
therians or most other extinct lineages. Other primitive traits include the presence
of lumbar ribs, and the lack of a clear neck on the femur
between the head and main shaft of the bone.
Molars
Isolated uppers had previously been assigned Shuotherium. They came from
the same localities as lowers, were the right sort of size and seemed complementary.
As Pseudotribos retains much of both dental teams, it's well qualified to say
those assignments were correct. It also confirms predictions as to how the teeth
went collectively to work. The pseudo-protocone of the left M1 is helpfully still
occluding with the pseudotalonid on the front of m2, and the same arrangement is
demonstrated by M2 and m3.
Further dental details include unimpressive canines described as incisiform, and
clear diastemata occurring along the series from
rear incisors to penultimate premolars. The teeth behind those become closer to
their neighbours.
Replacement
Replacement of teeth was occurring for a final lower incisor and the upper canine.
There's nothing to suggest any more than one episode for any position, so replacement
appears to have been diphyodont, as known only
for mammals.
Lower jaw
The dentary isn't well preserved, but it does provide
information that wasn't previously available for shuos. Its depth is comparatively
shallow, and becomes deeper beneath the molars. Its depth is 1.6mm below m2 compared
to 1.3 for a Shuotherium jaw (p.94, presumably the smaller S. dongi,
as the other species are only known from teeth). There's an angular process that's
more prominent than known from Shuo. A facet is present for the attachment of an
angular (ectotympanic)
bone, and a roughened area presumably housed a loosely connected coronoid. In contrast
to all living mammals, including monotremes such
as the platypus, these critters had multi-boned lower jaws. On the internal side,
beneath the coronoid process, there's a dentary trough for a rod of postdentary
bones and, from that, a Meckelian groove courses
forwards paralleling the lower jaw border until the front of the molar team.
Put some back into it
The former resident of the body may have stuck its neck out further than is healthy,
as it appears to have been decapitated. Other than for the jaws, the skull and
some neck bones are either missing or obscured by the broken shoulders. It was
more careful with the rest of its spine until the base of the tail which, as befits
specialized diggers, was presumably fairly short. Extravagant tails could be a
nuisance for serious tunnel builders. 22 spine bones are available from the breast
and lumbar regions, and at least thirteen of these come from the first mentioned
region. As is still the case for we mammals, those
thoracic vertebrae are associated with ribs. Undress a good friend and have a
look and a feel. Those ribs are firmly joined. The next five spine bones also
have ribs, but these are termed floating as they're not attached. Such floaters
occur with some living mammals. In this case, however, the morphological differences
between vertebrae suggest only the first trio are thoracic. The last pair seem to
belong to the lumbar vertebrae. Lumbar ribs, whether
floating or attached, have long gone out of fashion for mammals. Also present are
the first three tail vertebrae (p.95).
A call to arms (and legs)
The remains of the hips and rear legs are robustly built. Limb bones are extremely
wide and thick. Various characters are plesiomorphies
shared with basal mammals and their near relatives, the
tritylodontids. Included among them is the presence of so called marsupial bones;
the epipubes. Far from being restricted to marsups,
epipubic bones are typically mammalian, and their loss in
eutherians (around 94% of extant mammals) is exceptional in the context of
furry bodied history. Basal eutherians had them too,
but no living placental does.
Out with a quiet whimper
Should there be any surviving shuos then I'll be forced to retract the following
libel.
Despite their rarity, shuotheriids provide a fine demonstration of dental diversity
among basal mammals. Entirely independently of
boreosphenidans, they convergently developed dual-functional molars. That trick
was doable for different lineages of mammalian magicians. However, as suggested by
their very rarity, it doesn't seem to have been a roaring success for the shuos.
Rather than enabling them to overrun other mammals across Laurasia, they appear to
have rapidly died out; pushed aside by rivals with less sophisticated teeth.
It tempts me to think of an ingenious medieval munitions manufacturer coming up with
a rapid-fire, light-weight machine gun; a devilish device capable of spitting out a
hundred potential deaths a minute. The effects on Middle Age warfare could have
been devastating. Except, they never got around to bullets and it turned out to be
bloody useless against iron swords. The army they equipped was quickly slaughtered
by the less inventive.
Holotype
CAGS040811 A and B are the positive and negative plates of the same individual.
Luckily, the Chinese Academy of Geological Sciences, Beijing has provided a new
kennel for this critter. The specific name alludes to the tough guy qualities of
the animal's strongly built limbs. |
| Reference: | Luo Z-X, Ji Q, & Yuan C-X (2007), Convergent dental
adaptations in pseudo-tribosphenic and tribosphenic mammals, Nature, 450,
p.93-97. |
| Genus: Shuotherium Chow MC & Rich THV, 1982
Family: Shuotheriidae Chow & Rich, 1982
Aka: Shunotherium
Remarks: "Like Kuehneotherium, it retains a primitive jaw structure in
combination with a triangular arrangement of principal molar cusps," (Cifelli, 2001
with ref. to Chow & Rich, 1982).
The authors of Australosphenida see this genus as representing a closely allied line with
common ancestry. "We hypothesize that the mesial cingulid on molars of
Australosphenida is a highly plausible structural antecedent to the
pseudo-talonid of Shuotherium. This and other
shared, derived features support a relationship of Shuotherium and Australosphenida
as sister-taxa," (Kielan-Jaworowska Z, Cifelli RL &
Luo Z-X, 2002, see Bibliography).
Be that as it may, the following dental detail is decidedly eccentric: "The Middle
Jurassic mammal Shuotherium has lower molars that possess a
trigonid and talonid,
but are unique in having the talonid situated in front of the trigonid, rather than behind
it, as in molars of usual tribosphenic pattern", (Kilean-Jaworowska et al, 2002). This
odd feature seems to be unique to shuotheriids. As for what the funny words actually mean,
ask a dental expert such as:
The basic structure of cheek teeth, Phil Myers, University of Michigan
http://animaldiversity.ummz.umich.edu/anat/cheek_teeth_structure.html
The following quotes also come from Kielan-Jaworowska et al (2002)
"In our view, the most compelling evidence as to the affinities of Shuotherium
lies in the structure of the last premolar, which shares
striking similarities to that of Australosphenida."
"Lower molar structure of Shuotherium and Australosphenida is obviously quite
different, and for this reason we do not place Shuotherium within this Gondwanan
clade. Based on the limited evidence available, however, we suggest that Shuotherium
is a viable sister-taxon to Australosphenida." |
| Species: | Shuotherium dongi Chow MC & Rich THV, 1982 |
| Place: | Upper Shaximiao Formation, Sichuan |
| Country: | China |
| Age: | Middle Jurassic (or possibly early Upper) |
| Remarks: | Zhao 1984 provides a brief introduction. Remains
consist of a partial left mandible. It lacks an articular
process, a coronoid process and a reflective angle. Three
premolars and four molars were reportedly present, (but
see below). The trigonid and
'talonid' are well developed, but the latter is utterly weird. Nature demands, in
absolutely all cases, that a talonid must be at the back of a molar. Shuotherium
was obviously not paying attention and developed one at the front instead. This is termed
a pseudotalonid. This aberration is paralleled in
some docodonts, but that seems to have occurred
independently. When present, the docodont version is termed a "pseudotalonid".
The specimen was found by residents of Shilong Village in 1978, who donated it to the
Chongqing (aka Chungking and Zhongqing) Natural History Museum.
According to the original interpretation, the teeth were held to be three premolars and
four molars. However, Kielan-Jaworowska, Cifelli & Luo (2002) found: "we
reinterpret the premolar-molar boundary in the holotype of S. dongi, and propose a
dental formula of four (or more) premolars and three molars." They also state that
Sigogneau-Russell (1998) assigned two lower molars from the Middle Jurassic of Oxfordshire
to this species.
Holotype
The holotype is affectionately known as IVPP V6448. It hangs out in the collection of the
Institute of Vertebrate Paleontology and Paleoanthropology, Beijing. |
| Reference: | Chow & Rich (1982), Shuotherium dongi n. gen. and
sp., A therian with pseudo-tribosphenic molar from the Jurassic of Sichuan, China.
Australian Mammalogy 5, p.127-142. |
| Species: | Shuotherium kermacki Sigogneau-Russell D,
1998 |
| Aka: | S. kermackii |
| Place: | Forest Marble Formation, Oxfordshire |
| Country: | England |
| Age: | Bathonian, Middle Jurassic |
| Remarks: | Lower teeth. |
| Reference: | Sigogneau-Russell (1998), Discovery of a Late Jurassic Chinese
mammal in the Upper Bathonian of England. C R Acad Sci Paris, earth & Planetary Sci,
327, p.571-576. |
| Species: | Shuotherium shilongi Wang YQ, Clemens W,
Hu YM & Li CK, 1998 |
| Place: | Upper Shaximiao Formation, Sichuan |
| Country: | China |
| Age: | Middle Jurassic (or possibly early Upper) |
| Remarks: |
One upper molar, which is too large to be referable to S. dongi. |
| Reference: | Wang et al (1998), A probable pseudo-tribosphenic upper molar
from the late Jurassic of China and the early radiation of the
Holotheria. J of Vert Paleo, 18(4), p.777-787. |
| Species: | Shuotherium sp. Sigogneau-Russell D, 1998 |
| Place: | Forest Marble Formation, Oxfordshire |
| Country: | England |
| Age: | Bathonian, Middle Jurassic |
| Remarks: |
Several isolated teeth were assigned to an unnamed species. A couple of upper teeth may
also belong to this genus. |
| Reference: | Sigogneau-Russell (1998), Discovery of a Late Jurassic Chinese
mammal in the Upper Bathonian of England. C R Acad Sci Paris, earth & Planetary Sci,
327, p.571-576. |
The following is based upon my reading of Kielan-Jaworowska, Cifelli
& Luo, 2002.
Possible affinities of Shuotherium
Although remains are limited to a fragment of jaw and some isolated teeth, these fossils
are enough to qualify as weird. "The Middle Jurassic mammal Shuotherium has
lower molars that possess a
trigonid and talonid, but are unique in having the
talonid situated in front of the trigonid, rather than behind it, as in molars of usual
tribosphenic pattern", (p.479). This perhaps
paltry sounding detail hints at a whole evolutionary lineage of animals with eccentric
teeth. Shuotherium had ancestors and relatives regardless of their elusiveness.
Kielan-Jaworowska and colleagues think they may have identified some of the relations.
More funds will test their proposal.
The molars are such aberrations, that the original authors felt compelled to establish a
new family called Shuotheriidae; the order of Shuotheriidia; and an entire legion termed
Yinotheria. This is an unusual reaction to a small scrap of bone, but it doesn't appear to
have been anything but fully justified.
The first discovered shuotheriid, S. dongi
The mandible contains seven
postcanines and an alveolus. Although decidedly odd,
the molars are impressively sophisticated. In contrast, the
jaw is basal. A scar, tough and a ridge suggest the
postdentary bones were attached. Consequently, the malleus
and incus won't be found in the middle ear, (should one ever
become available).
The postcanines were interpreted as three
premolars and four molars, but the 'm1' was noted as
being significantly different and possibly a molar-like premolar. For one thing, it lacks
a pseudotalonid, (p.480). That was in 1982. At
the time, there was nothing else with anything even vaguely like these strange molars.
Of shuotheriids and docodonts
In 1987, a docodont called Simpsonodon was
described. There is a basin at the front of the molars, and this is at least a similar
kind of structure. However, it appears to have been produced by an entirely different
evolutionary route. As it's only known from some derived
docodonts, (especially from Asia), it seems to be a specialisation restricted to Docodonta,
and uninformative about their wider affinities.
Simpsonodon was found in the Forest Marble of Oxfordshire, where Middle Jurassic
mammal remains were first encountered in the eighteenth century. In 1998, it also yielded
two lower molars of Shuotherium dongi, and molars of a further, unnamed species.
Three upper teeth were tentatively assigned to the genus. In the same year, the original
Chinese locality was the supplier of S. shilongi; and upper molar too large for the
first named species. Back in 1982, Chow and Rich had attempted a hypothetical reconstruction
of the then unknown upper molars, and this turned out to be satisfyingly close. Yinotheria
was suggested to be the sister taxon of
Cladotheria. An alternative hypothesis supported
derivation from early 'symmetrodonts'.
Of shuotheriids and australosphenids
More recent fossils from the southern hemisphere have formed the basis of Australosphenida,
and provided a new possibility for comparison. To complicate matters, quite what (if
anything) qualifies as an australosphenid, is a matter of little consensus. Some of the
discussion is addressed in the appropriate section below.
Kielan-Jaworowska and colleagues include Ausktribosphenos
in this group. The last four lower postcanines include three molars and a molariform
premolar. If the 'm1' of Shuo is actually a p4, then that's a similarity, (as far as it
goes). Then Ambondro came along, and the final lower
premolar also has a molar-like
trigonid. That genus demonstrates fully
tribosphenic molars were around in the Middle Jurassic, 25 million years earlier than
expected. Of course, Shuotherium has not talonids
on any teeth. It has pseudotalonids.
Interlude: What's the difference between a premolar and a molar?
The answer depends upon context. In your mouth, the
premolars are the replacement cheek teeth (or those which you haven't yet got around to
replacing), while the molars are permanent teeth, (the big
ones at the end of the row). That's what placental
mammals do, (if they have teeth). However, that definition wouldn't work for
marsupials, as they're lazier and only replace the third
premolar.
In all toothed living mammals: "premolars developed from a secondary dental lamina and
replace deciduous precursors (which may or may not erupt),
whereas molars originate from the primary dental lamina, and by definition, neither replace
precursors nor are replaced by successor teeth", (p.481). However, except in some
very rare cases, Mesozoic mammals don't provide this sort of information. In some instances,
their teeth didn't necessarily conform. 'Molar' replacement has been reported in
Morganucodon, and that's one reason why the
word 'molariform' is often used instead.
Generally, the only criteria which can be used for such old remains are morphological and,
in some instances, the dividing line is less than obvious. Shuotherium is an
example.
Premolars usually have simpler crowns and the main cusp, (A or a depending upon uppers or
lowers respectively), dominates. Other cusps on molars are often enlarged and the crown
is more complex. There's also some pattern for interlocking evident between uppers and
lowers. With exceptions for particular specialised dentitions,
sequential variations can be seen along the row. Also helpful is that the main premolar
cusp is higher than the molars, but there are exceptions for this as well, (p.482). When
looked at as a series, the most abrupt morphological change on the jaw of Shuotherium
was between the third and fourth tooth, which is why the division was tentatively drawn
between them.
However, the general rules of thumb aren't always applicable. Sometimes there's no abrupt
change in morphology. For example, early eutherians have
molarized premolars.
In the holotype, the fourth tooth is damaged and incomplete, (p.483). Nevertheless, there
are visible differences from the last three postcanines,
which are molars. There's no pseudotalonid; the
angle of the trigonid is greater; and the
talonid (at the back) is more developed. The first of these
is held to be the most notable. Wear facets indicate those teeth had a crushing function,
which the debatable tooth didn't share. This supports the view it's a premolar. If so,
then the dental count is four or more premolars and three molars.
So what?
According to the view of these authors, most australosphenid lower molars have: "a
shelf-like mesial cingulid that wraps around the anterolingual corner of the teeth",
(secondarily reduced in ornithorhynchids).
If that feature were expanded and basined, then it could be seen as an antecedent to the
pseudotalonid.
Furthermore, (but less compellingly), australosphenidans have a molar count of three or
less, (p.484). The structure of the last premolar shares characteristics with the debatable
tooth in Shuotherium. In both instances, there's a fully triangulated
trigonid, which isn't known from other Mesozoic mammals.
It's wide, semi-molarized and the conids are well developed. These common traits could be
accounted for by common descent. Although clearly not an australosphenidan,
Shuotherium might be part of a sister-lineage.
On page 485, this argumentation is explicitly termed a hypothesis. There's presently
insufficient data to test its validity. The affinities of the mammal with the weird
molars are thus unresolved pending further evidence. |
| Other reports:
Junggar Basin, Xinjiang, China
This is a breaking news item, but sometimes news breaks very slowly. Paleontologists from
Tübingen, Germany have been fossiling in the desert of Inner Mongolia. Finds include five
mammal teeth, which are similar to Shuotherium, but seem to represent a new genus,
(pers. comm. Michael Maisch). Prof H-U Pfreitzschner is reported as saying, (my
translation), "A fossil site is considered 'rich' when one tooth is found amongst a
ton of stone. Here in the Junggar Basin we found five teeth in two bucketfuls of
stone." The site is most likely Callovian, Middle Jurassic. |
B. Shuotheriidae B.
Australosphenida
|
Taxon: Australosphenida Luo Z-X, Cifelli RL & Kielan-Jaworowska Z, 2001
According to the researches of Luo, Cifelli and Kielan-Jaworowska, (the results of which
can be reviewed in at least three papers from 2001-2002), a common ancestor gave rise to
both Shuotheriidae and Australosphenida, some time before the Bathonian of the Middle
Jurassic. The australosphenidans then spent their time populating the southern continent
of Gondwana with full-blooded, tribosphenically toothed, egg laying mammals. If correct,
they’re still at it. The monotremes of Australasia are
their descendants; the duck-billed platypus and the echidnas. Later, boreosphenidans
radiated in the north, where they eventually gave rise to ourselves,
(placentals), and
marsupials.
This theory is very attractive and based on a thorough examination of the available
evidence. It elegantly explains the origins of
monotremes. However, it hasn’t met with universal agreement and further evidence is
awaited eagerly by all concerned.
Australosphenid characteristics
With reference to Australosphenida, (and apologies for some possibly frightening language):
"This group is characterized by a well-developed, continuous mesial cingulid..."
(that's a kind of shelf running around the base of a lower tooth) "...that
wraps around and extends to the lingual side of the
trigonid;..." (a collection of cusps at the front
of the tooth,) "...a mesiodistally short, buccolingually broad
talonid;..." (a basin-like heel which allows
the cusps of the upper teeth to act as grinders) "... and reduced height of the
trigonid -at least when compared with other Jurassic/Early Cretaceous
holotherians. Notably, Ausktribosphenos retains
plesiomorphies..." (basal characteristics)
"... of the jaw as seen in stem mammals, and we provisionally recognize some of these
primitive characters to be preserved also in Steropodon." (A Lower
Cretaceous relative of the platypus.) "By contrast, the Boreosphenida, a clade of
northern mammals with tribosphenic molars, have distinctive cingulid cuspules (cuspule e,
capsule f or both) but lack a continuous mesial cingulid." (That's the previously
mentioned shelf. We don't have it.) "The mesial cingulid does not wrap around or
extend to the lingual side of the
molars. Furthermore, boreosphenidans and proximal relatives, such as
Henkelotherium, are more derived in that they have a mandibular angle far more
posteriorly positioned than australosphenidans, Kuehneotherium, morganucodontids and
Sinoconodon," (Luo, Cifelli & Kielan-Jaworowska 2001, p.53, see
Bibliography).
Those are some of the features which led these authors to conclude, (p.54): "These
australosphenidans occupy their own morphospace and are distinct from the stem taxa of
boreosphenidans, metatherians and
eutherians. Both parsimony analysis of discrete
characters and shape analysis suggest that Ausktribosphenos does not belong to the
crown group of therians, and that it may have close
affinities to the monotreme Steropodon."
Crown-group therians is a narrower concept than
crown-group mammalians.
Alternatively
In contrast, a few comments from Rich et al, 2002 might be of interest. This was a reply to
the paper quoted above, though the present form of this directory has been more influenced
by subsequent studies. Rich and Co still favour a closer relationship for ausktribosphenids
with placentals. They also point out some apparent
inaccuracies in the Luo et al 2001 study; eg. Steropodon (
Monotremata) does not have an internal groove on the
mandible. Rich et al found that almost all the characteristics cited in support of
Australosphenida required reconsideration, (p.467).
They didn't rule out the possibility of the dual evolution of tribosphenic teeth.
"However, the coincidence that three apparently independent major features of the
ausktribosphenids, namely the dental formula, submolariform nature of the most posterior
premolar, and nature of the mandibular angle, are typical
of early placental mammals, formed the basis for the allocation of the family to the
Placentalia..." (Regarding the dental formula of Bishops, I've never heard of
any placentals or eutherians with six premolars, as seems to be indicated in Fig.4C, p.468,
though the first has been termed as possibly "a premolarform canine" by Rich et
al, 2001.)
Rich & Co also highlight the problem posed by the incompleteness of the available
fossils. In order to resolve the matter to everyone's satisfaction: "That will probably
required the discovery of other parts of the anatomy of a member of this family."
They also briefly address a couple of points in Luo et al, 2002, (which had just been
published, and was more influential to the layout of this directory). On page 469 they
argue: 1. The lower molar of Teinolophos
(Monotremata) is illustrated in lingual view, rather
than labial. Seen from the perspective of the latter, no
"shelf-like cingulid" is evident. (However, this doesn't say anything about the
other taxa figured); 2. a number of hedgehog species show a
condition of the jaw cited as typical for australosphenidans, even if the European hedgehog
(Erinanceus europaeus) doesn't; 3. Steropodon still doesn't have an internal
groove on the mandible.
Also alternatively (but then again)
Woodburne, 2003 also casts doubt on close links between australosphenidans and monotremes.
He points out that the latter don't have, and appear never to have had tribosphenic
molars. As the paper directly concerns toothed monotremes, further details are included in
that directory.
Martin & Rauhut, 2005 also mention the lack of protocones on monotreme upper molars.
However, they widen the scope by citing doubts about the presence of protocones on any
australosphenidan. Dual-functional tribosphenic molars don't of necessity require such
structures. This is discussed in more detail in the entry below for Asfaltomylos.
Briefly put, the implication is that monotremes could indeed be descendants of a
distinctive form of tribosphenic mammals; namely australosphenidans.
Genera: Ambondro,
Asfaltomylos, Ausktribosphenos,
Bishops, Henosferus,
other reports
Time-Line:
Lower Cretaceous: Ausktribosphenos, Bishops
Upper Jurassic:
Middle Jurassic: Ambondro, Asfaltomylos, Henosferus |
| Genus:
Ambondro Flynn JJ, Parrish JM, Rakotosamimanana B, Simpson WF & Wyss AR,
1999
'Ambondro' (geographical)
Remarks: The generic and species names refer to the village of Ambondromahabo in northwest
Madagascar. |
| Species: | Ambondro mahabo Flynn et al, 1999 |
| Place: | Isalo 'Group', Mahajanga Basin |
| Country: | Madagascar |
| Age: | Bathonian, Middle Jurassic |
| Remarks: | You could gather from some literal creationist
pages that this is the sort of thing paleontologists live in dread of discovering. A
basal tribosphenic mammal from the Middle Jurassic of the
southern hemisphere was completely unexpected. This is actually the kind of find most
paleontologists would love to make.
"Ambondro and Ausktribosphenos share an unusual feature in which the
premolars seem to be molariform, with a distinctive
triangulation of the three main cusps and a prominent lingual
cingulid. The posterior part of the premolar is buccolingually wide. This derived
feature is shared by Obdurodon dicksoni (Australia, Miocene), an ornithorhynchid
monotreme widely accepted to be a relative of
Steropodon (in which the premolar is not preserved)", (Luo et al, 2001).
However, a relationship with Ausktribosphenos is not in line with the views expressed
in the original description. Flynn & Co observed: "its lack of numerous
autapomorphies rules out affinities to
Ausktribosphenos, an enigmatic (contested
placental) holothere from the Early Cretaceous of
Australia.
Holotype
The holotype is UA-10602. It resides in the collection of the University of Antananarivo
and consists of a "a dentary fragment with three
teeth: the ultimate premolar and the first two molars,"
(Kielan-Jaworowska et al, 2002). The molars are fully tribosphenic. |
| Reference: | Flynn et al (1999), A Middle Jurassic mammal from Madagascar.
Nature 401, p.57-60. |
| Genus: Asfaltomylos
Rauhut OWM, Martin T, Ortiz-Jaureguizar E & Puerta P, 2002
'Asfalto mill'
Family: Henosferidae Rougier GW, Martinelli AG, Forasiepi AM & Novacek MJ, 2007
Remarks: The name reflects the Formation, and also the fine grinding abilities of
the molars. |
| Species: | Asfaltomylos patagonicus Rauhut et al, 2002 |
| Place: | Canadon Asfalto Formation, Chubut, Patagonia |
| Country: | Argentina |
| Age: | Callovian, Middle Jurassic |
| Remarks: | The following is based upon my reading of
Rauhut et al, 2002.
When published, this fossil constituted the entire South American mammal fauna for the
Jurassic, which is roughly 65 million years of time. There were some paw prints from
Argentina, but no body fossils whatsoever, (p.165). This paucity is a stark contrast to
the fabulous contribution the continent has provided for Triassic
eucynodonts. Although one-and-a-half centimetres of
lower jaw might not sound like much of an improvement, it was a fairly spectacular opening
effort. The entire southern hemisphere has a remarkably poor fossil record for Mesozoic
mammals, especially more recent than the Lower Jurassic. Absolutely anything has a good
chance of revealing novelties.
The authors had no reservations about referring this genus to Australosphenida, a somewhat
controversial taxon established only in the previous year.
Provenance
The fossil was obtained from mudstone a few kilometres away from the village of Cerro
Condor in Chubut Province. As these strata are lacustine in origin, its owner presumably
ended up in a lake. The age now appears to be Callovian. Previously, the strata were
thought to possibly be a bit later (Oxfordian).
The critter
A scrap of small jaw can provide a surprising amount of details about its former possessor.
For one thing, as the dentary was tiny, so was the rest of
the animal. In some regards it was a rather basal mammal in
comparison to many of its northern contemporaries but, in at least one respect,
Asfaltomylos was a sophisticate. Its molars were
like a glimpse into the distant future.
Molars are very useful for endothermic animals, as they can give food a thorough processing
before consumption. This allows it to be turned into usable energy quickly and
efficiently. They're not the only solution for high-octane animals, (as 9,000 species of
birds and a few modern mammals would confirm), but they're an effective solution in many
circumstances. The earliest mammals wore molars of a sort, but the ones in this mouth were
considerably more efficient than anything around in the northern hemisphere at the time. In
a word, they were tribosphenic.
The talonids are fully basined, and feature a hypoconid and
hypoconulid. For those who may be concerned with such matters, the
trigonid of the m1 is obtuse-angled, while those on its
colleagues are acute-angled. All molars have a cingulid on the internal side at the base
of the paraconid. As far as it's known, the dental
formula is: (lowers): at least three
premolars and only three molars; (uppers): no idea. The
animal had a slender dentary and the coronoid process ascends gently.
Advanced teeth, but what's old fashioned?
The answer, my friend, is on the back of the jaw. There's a shallow trough on the
lingual side, (p.166), and it's clearly subdivided. This
was functional. It housed postdentary bones. These would have included a small
articular. I keep that in my ear and call it a
malleus. The subdivision suggests this trough must have
contained further accessory lower jaw bones as well. No living mammal would be seen dead
running around with such a feature, and Middle Jurassic northerners were generally too
advanced for it, (with Shuotherium being an exception).
One primitive characteristic that isn't visible is a
Meckelian groove. However, this absence may simply be due to less than ideal
preservation. (Update: Three years after this publication, the aforementioned shallow
trough unexpectedly turned up on a monotreme, as can be
found in the Teinolophos hutch.)
Wider significance
As well as being the first to fly the mammalian flag in the Jurassic of South America,
Asfaltomylos strengthens the case for an endemic radiation of tribosphenic critters
in Gondwana, (p.167). This occurred at least 25 million years before a similar
characteristic emerged in the northern lands of Laurasia. The authors added this genus to
the data set published by Luo et al, 2001, in order to analyse possible relationships. The
results suggested it's the most basal australosphenidan,
although Ambondro is somewhat earlier. Despite the
continuing shortage of fossils, (and regardless of the affinities of Ausktribosphenidae),
the presence of this group in Argentina and Madagascar indicates a wide geographical range.
A funny thing happened on the way to the Cretaceous
Although these 'southern grinder-cutters' had a head start in tooth technology,
South American australosphenidans appear to have been phased out in favour of northern,
non-tribosphenic invaders. This would suggest that high-tech choppers aren't enough to
ensure a long-term future. The Upper Cretaceous Mammaldom in Patagonia was dominated by
eccentric dryolestoideans. Also present were
mysterious gondwanatherians and
'triconodonts' (in a descriptive sense). The
diversification of placentals and
marsupials appears to have waited until the South
American Paleocene.
World affairs
Although very limited, southern hemisphere fossils indicated a strongly contrasting
mammalian fauna when compared to the north, but some interchange was possible.
Limitations of high-tech teeth
Tribosphenic molars permit a strong increase in
the efficient exploitation of food, and they're frequently complimented for playing a
major role in the ultimate success of therian mammals.
Therians now account for over 99.9% of all species. Assuming monotremes are
australosphenidans, (which I don't think is necessarily correct), the pioneers of this
tooth style have been reduced to a mere three species. Furthermore, they've all found
better means of dealing with diet. No monotremes have
been found with tribosphenic teeth. Efficient chewing isn't necessarily advantageous in
all circumstances, (eg. when dealing with lots of ants or soup). Nor can tribosphenic
teeth alone account for therian domination. If that were the sole factor, then the world
would now be awash with 'southern grinder-cutters'.
Further developments (and more detail)
The following is based upon my reading of Martin & Rauhut, 2005 (and my thanks go to the
supplier).
This study again concerns the holotype, (p.414). Originally, only the
lingual side of the fossil was visible for study, as the
rest was buried in matrix. Extraneous rock has since been fully cleared and the remains
chemically strengthened. This proved to be extremely demanding work, and the authors pay
due credit to the expert efforts of the preparator Ellen Eggert, (formerly Drescher), of the
Freie Universität, Berlin. A wealth of further details became available.
Key points
The dental formula now appears to be: (lowers): ?five
premolars and three molars with no information for the
incisors or possible canine.
On the molars (m2 and m3), the talonids are wider than long
as in other australosphenidans. Wear is limited towards the top with no facets present in
the basins. Apical wear also typifies other 'southern cutters' including toothed
monotremes. In the m3, the classic boreosphenidan
wear facets 5 and 6 aren't present. This lack suggests a functional protocone may not have
existed on the corresponding upper molar. While being dual-functional, the morphology of
these teeth isn't necessarily homologous with that of the boreosphenidans. (It should be
borne in mind that upper molars aren't yet known for any australosphenidans other than
monotremes.)
Finally, the last premolar of Asfaltomylos doesn't have a triangulated arrangement of
cusps. This seems to be a primitive characteristic in comparison to other 'southern
cutters'.
Terminology
The authors adopt the standard terms for tribosphenic molars suggested by Crompton in 1971.
However, as strong doubts exist regarding whether the structures are homologous, new
terminology may be required when upper teeth are found. These standard terms are used
descriptively, (p.416), and they refer to cusps, wear facets and so on.
Preservation
The labial side of the jaw was embedded in rock, and this
left it better preserved than the more weathered lingual face. Nevertheless, some damage
and distortion has occurred. The whole fossil has been somewhat compressed, the coronoid
process is incomplete and the angular process has been displaced into an unlife-like
alignment.
Teeth - premolars
Originally, the three remaining premolars were interpreted
as p?4-6. Comparison with a close relative from the same locality suggests p?3-5 is more
likely. (The question mark shouldn't be overlooked.) The premolars are double-rooted, with
the rear root being a bit larger than the front one. The middle tooth, (p?4), is somewhat
more complete than the others and slightly larger than p?3. p?5 has been turned sideways by
a breakage in the jaw and the central cusp (a) is the largest. The arrangement of what's
left shows the three principle cusps weren't triangulated, and this seems to be a
basal characteristic for australosphenidans. Whether further
cuspules were present is unclear.
Molars
There's a large, natural gap between the third molar and the
coronoid process of the jaw, and x-rays revealed no trace whatsoever of a further tooth.
There were only three molars in life and all were a bit less than a millimetre in length.
Each was double-rooted.
In contrast to the proceeding pair, the trigonid cusps of
m1 form an obtuse angled triangle from the occlusal
perspective. The crown is roughly rectangular in outline. The
talonid isn't preserved but it was wide; probably more so than the trigonid, (p.419).
As with the premolars, the rearmost root is slightly larger than the anterior one.
The trigonids of the other two molars are both triangulated at 80° and thus acutely angled.
On the m2, the main cusps (protoconid) is complete
whereas the paraconid (front) and
metaconid are broken at the bases. The mesial side of
the paraconid does preserve a faint, partly interrupted cingulid. In contrast to the
condition known from other australosphenidans, (Ambondro,
Ausktribosphenos,
Steropodon), this feature is weakly developed and doesn't wrap around the
mesial side. The crown also has a lingual cingulid which terminates at a gap between the
paraconid and metaconid. This narrow gap means the trigonid basin is open on the lingual
side.
The talonid is completely basined and wider than long. There's a large hypoconid with a
pointed tip, and the peak of this has an ovoid wear facet which doesn't match any typical
facet found on boreosphenidan molars. There's no sign of facets termed 5 or 6 in the basin.
The m3 is the smallest of the molars. The trigonid has acute triangulation of the cusps,
all of which are damaged. Again, there's a lingual cingulid at the base of the paraconid
which doesn't wrap around the mesial side, and the trigonid basin is open lingually.
Jaw
The dentary is slender but snapped off in front of the
three premolars. It's crushed and various parts are missing. About halfway down on the
external side of the mandible is a mental foramen, which is in front of the p?3, (p.421).
At the back of the jaw, the angular process has been pushed into a distorted position; it
points more downwards instead of backwards. Rather than tapering towards the end, this
process widens (flares). That isn't caused by damage and is in contrast to the condition
in many basal mammals.
The articular process is the part of the bone which includes the lower jaw joint. This is
positioned relatively far up, rises steeply and reaches a height fairly similar to the top
of the coronoid process. For the latter, the presence of only three molars allows the
ascent to begin gently before the bone rises steeply at around 120°. A large posterior
chunk of the coronoid process is missing, but its approximate height and dimensions are
obvious enough. Indentations and scars indicate a tendon was positioned at the top.
'Extra' jaw bones
The interior side (lingual) of the jaw features a postdentary
trough which is broad and shallow, and has a mandibular foramen at its front end. Some
details are obscured by damage. One ridge could be an attachment point for a bone called
the prearticular. A less likely possibility is this is the border of the trough. Faint
scars show where a surangular bone was inserted. The postdentary trough narrows and deepens
towards the back before becoming shallower and ascending the articular process. A triangular
area could represent a connection for part of an angular bone, but this would require
confirmation from better preserved specimens. A hole occurs where a coronoid bone attachment
might be expected, but this seems to be due to breakage. Further damage means the presence
or otherwise of a Meckelian groove or splenial bone
is unclear.
A feature lacking on the lingual face of the coronoid process is a distinct pterygoid fossa.
This contrasts with eutriconodonts, multituberculates,
spalacotheriids,
dryolestoideans and more derived northerners.
A non-northern jaw
At least three features distinguish the lingual face of the lower jaw of Asfaltomylos
from boreosphenidans and even more basal northern
trechnotherians: the mandibular foramen is set further
forward; there's no pterygoid fossa; a postdentary trough is present. A foramen within the
postdentary trough is found in various non-mammalian
eucynodonts (eg. Probainognathus),
basal mammals including Morganucodon,
docodonts (Haldanodon),
Kuehneotherium,
Shuotherium, Ausktribosphenos,
Bishops and (according to these authors at least)
Steropodon. This is
plesiomorphic in comparison with stem-boreosphenidans
such as Peramus and
Nanolestes, (p.422).
The steeply rising articular process is a further characteristic in common with
Ausktribosphenos, Bishops and
Teinolophos. Turning to the angular process, this tapers in stem-boreosphenidans
and boreosphenidans. A posteriorly directed angular process is an australosphenidan trait
and not a boreosphenidan one.
Southern postcanines
Asfaltomylos has tribosphenic molars which could
grind as well as shear foodstuff. The width of the talonids
exceeds the length as in australosphenidans. Further
details are also not known from boreosphenidans until
after the Cretaceous, (eg. a slightly procumbent hypoconulid).
Several features suggest Asfaltomylos is more basal than other australosphenidans.
The lingual cingulid at the base of the paraconid fails
to wrap around the mesial side. For the premolars, the cusps on the ultimate tooth aren't
triangulated regardless of whether it's a p5 or otherwise.
Size ain't everything
These molars are a bit less than a millimetre in length, and that's about one fifth the size
of a corn of black pepper. Human senses aren't made to take much notice of stuff this small,
so it's just as well people developed microscopes. Size isn't necessarily a good reflection
of significance. The different wear patterns on tiny talonids, (the heel at the back of
these molars), may be microscopic, but they provide important information on how these
lower teeth worked in conjunction with the corresponding uppers, even though upper molars
are presently unknown. The differences in the traces of wear are fundamental. They
appear to represent a stark, evolutionary dichotomy deep within Mammaldom; more evidence
for the dual evolution of tribosphenic molars.
Differences in talonid wear
A striking contrast between northern and southern mammalian methodologies is apparent in the
depths of the talonid basin. No evidence exists for wear
within that basin for any australosphenidan
(including toothed monotremes). All observed wear is
apical; towards the top. In the case of Asfaltomylos, it's towards the top of
features descriptively termed the hypoconid, hypoconulid, hypocristid and the oblique
cristid. Don't worry if those terms sound offensive. It's easy enough to remember the
wear is restricted to the upper rim of the basin. Several standard facets on
boreosphenidan lower molars occur down in the basin; facets 5 and 6. As stated, that's not
the case for 'southern cutters'.
Tribosphenic protocones are not de rigueur
With regards to 'northern cutters', a protocone is a cusp on an upper molar which is
correspondingly positioned with the talonid of a lower. (The condition has been secondarily
lost in some cases. A few boreosphenidans have even dispensed with teeth.) In 2003,
Woodburne pointed out that no evidence indicates the presence of such a feature in any
monotreme. Martin & Rauhut observe that the talonid wear found in non-monotreme
australosphenidans indicates that a protocone wasn't necessarily part of their armoury
either, as none of the wear is found within the basin, (p.423). They suggest four
hypothetical models which could account for the pattern, none of which would require an
upper protocone. I'll content myself with a list of these suggestions based upon:
# Shuotherium,
Laolestes (Dryolestida), Simpsonodon
(Docodonta) and Monotrematum (Monotremata).
Note: all are explicitly termed 'hypothetical'. When upper molars come to light, they
could be singing in a different melody.
Holotype and acknowledgements
The holotype, MPEF-PV 1671, is an employee of the El Museo Paleontológico Egidio Feruglio,
Trelew. The specific name honours Patagonia.
(Thanks for notification are due to Marcel Opitz and Simon Clabby. The formation details
were originally gleaned from Tim Williams via the DML. Further assistance has been
generously provided by David Marjanovic.) |
| Reference: | Rauhut et al (2002), A Jurassic mammal from South America.
Nature, 416, p. 165-168. |
| Links:
Jurassic mammal unearthed in Patagonia, Cosmiverse
http://www.cosmiverse.com/science03140206.html
Based on the Associated Press report dated 13.3.2002.
Nature 416, 2002
http://www.nature.com/nlink/v416/n6877/abs/416165a_fs.html
This abstract contains a reference to mammalian "trackway evidence from Argentina," which sounds interesting. There's no indication of any association with this particular fossil or location, though.
You'll have to register to view this link, but registration is free.
The first Jurassic mammal from South America, Nature, (Supplementary Information)
Some of the data behind the publication.
For those with a thirst for detail.
"To test the phylogenetic position of Asfaltomylos patagonicus, we inserted
this taxon into the data matrices made available by Luo et
al. (2001) in the supplementary data of that paper." (That paper being Dual origin of
tribosphenic mammals. Nature 409, p.53-57, Luo et al, 2001).
"The inclusion of Asfaltomylos drastically reduced the number of equally
parsimonious trees, helped to resolve the polytomy within Australosphenida, and placed
Asfaltomylos as the most basal member of this
group." Rauhut et al, (2002).
Science Now, Mining Mammalian Evolution
http://www.calacademy.org/science_now/archive/headline_science/fossil_tooth.html
This article includes some good photos.
Trelew, History and Legends
http://www.welcomeargentina.com/trelew/history.html
Curiosity Corner: My first name, Trevor, is Welsh. So is Trelew. Welsh is still
spoken by some people in the area, and there's an annual Eisteddfod. This area was partly
settled by immigrants from Cymru. Perhaps it's just as well that nobody established a
Patagonian Llanfairpwllgwyngyllgogerychwyrndrobwyllllantysiliogogogoch. |
| Genus:
Ausktribosphenos Rich TH, Vickers-Rich P, Constantine A, Flannery TF, Kool L
& van Klaveren N, 1997
'Australian Cretaceous tribosphenic' (Aus = Australia, K = Cretaceous)
Aka: Ausktriboshenosi, Ausktribosphenus
Family: Ausktribosphenidae Rich et al, 1997 |
| Species: | Ausktribosphenos nyktos Rich TH et al, 1997 |
| Place: | Flat Rocks, Victoria |
| Country: | Australia |
| Age: | lower Aptian, Lower Cretaceous |
| Remarks: |
Much of the following is based upon my understanding of Rich et al, 1997.
The authors point to differences in dental details to
monotremes, as well as to the fact that the dentary
lacks an internal coronoid process, (p.1439). Dental charecteristics also differentiate
this critter from marsupials and deltatheroids,
(metatherians). The most reader-friendly of these is
the tooth count. Ausktribosphenos has three, and not four, lower
molars; and at least four, not three,
premolars. For lovers of obscure words, there are various
contrasts observable in the trigonid, "the absence of
a posterolabial cingulum on m1-3..." and so on and so forth.
The authors also report differences to all other
placentals, (a formulation of words in line with their diagnosis of this animal as a
possible placental mammal): " Ausktribosphenos nyktos is unlike all other
Placentalia, including Prokennalestes, in having (i) a remnant of the surangular
facet, (ii) a hypoconulid located close to entoconid on the lower molars, and (iii) a crest
on m1-2 linking the hypoconulid and metaconid buccal to the entoconid, which is separately
linked to the metaconid."
Ancient touches
It has several archaic features in common with basal
eutherians, which their descendants lack. The inside of the
dentary has a thing known as a
Meckelian groove and a coronid bone. However, there
are also features which differentiate it from undisputed basal eutherians, such as
Prokennalestes and the subsequently described Eomaia; eg. "the presence
of four, double-rooted lower premolars instead of five". As the authors make clear
later on the same page, there may have been a fifth premolar present in Ausktribo. A tooth
hole, (an alveolus), might have housed either a
single-rooted premolar or a canine. (The thickness of the bone present in front of the
alveolus seems to preclude the possibility of a further a double-rooted tooth.) Various
details, including its small size, were found to favour the premolar option.
In line with this interpretation, the authors label the rearmost premolar p5. "There
is no hint of a talonid-like structure on this tooth."
In general, final placental lower premolars have a heel-like talonid of a sort. However,
there are exceptions. Some erinaceids (hedgehogs) don't. Thus: "The form of the p5
of A. nyktos is unusual, but not unknown in placentals." (According to McKenna
& Bell 1997, p.276, Erinaceidae is unknown from the fossil record until the Paleocene,
which began about fifty million years later. However, Rich et al were pointing out a
similarity of structure, and not proposing a steady relationship.)
Page 1441: "Among tribosphenids, as indicated in the diagnoses, A. nyktos is not a
deltatheroid, a marsupial, or a tribothere. Those diagnostic features, coupled with a
postcanine dental formula unknown, other than in placentals and highly characteristic of
them, P1-4 or P1-5 and M1-3 are the bases for regarding A. nyktos as a placental."
(A typographical error seems to have temporarily transformed these lower teeth into upper
ones!)
Monotremes?
Ruling out a relationship with monotremes, however,
strikes me as less emphatic. "The lack of an angular process on the
dentary is not a feature that unites A. nyktos with
the monotremes." Whilst duckbilled platties (Ornithorhynchus tend to also lack
this characteristic, it's known from some specimens. It's also present in other fossil
and living monotremes, which seems to render the point inconclusive. They authors cite
differences in the premolar count and the number of roots for the molars, which might
distinguish Ausktribosphenos from monotremes. But they immediately seem to neutralize
this argument. They point out that Steropodon galmani, a Lower Cretaceous monotreme,
also shows more than two premolars and molars with only two roots; numeric similarities.
Placentals?
It might be a placental, or proto-placental mammal, which would be very interesting. With
the exception of bats and marine mammals, placentals
were not thought to have reached Australia until about five million years ago- by me at
least! (Tingamarra
porterorum was present in the Lower Eocene of Queensland). However, at present
the evidence concerning the true nature of A. is not conclusive.
Holotype
The genus and species was based upon a well-preserved, 16mm fragment of lower jaw, found by
the volunteer worker Nicola Barton. It resides with some friends in the Museum Victoria,
Melbourne, and is affectionately known as NMV P208231. The specific name, nyktos,
means 'night'. This intriguing genus features on over a hundred websites. |
| Reference: | Rich et al (1997), A Tribosphenic Mammal from the Mesozoic of
Australia. Science vol 278, 21.11.97, p.1438-1442. |
| Links:
Dr Tom Rich, Cretaceous Mammal Discovered
http://www.earth.monash.edu.au/dinodream/inverloc/jaw/press001.htm
An interesting report on the find and location. There are a fair number of similar pages
around.
Dann’s Dinosaurs
http://www.alphalink.com.au/~dannj/seozenv.htm
A listing of Lower Cretaceous non-dinosaur fossils from Victoria. This also cites a second
genus, Bishops.
Do-While Jones, Science Against Evolution
http://www.ridgenet.net/~do_while/sage/v2i3n.htm
A fine example of misrepresentation and muddle, posing as science. This article manages to
expand the size of the fossil to five inches, (Try imaging a shrew-like critter with a 13cm
jawbone. A fox would be shredded.), apparently resolve all questions regarding
classification without any worthwhile consideration and suggest that "evolutionists"
might interpret the Cretaceous rock as being only five million years old. None have. It
goes on to maintain that if placentals were present in Australia 115 million years ago,
only to have gone extinct, then this would mean that they must have evolved twice, in order
to repopulate the landmass. This is tripe. For example, marsupials seem to have previously
died out in North America, although opossums live there now. This didn’t necessitate
anything evolving twice. Later forms walked, or clambered in via Panama.
Link:
Opossum Recipes
http://www.justgamerecipes.com/opposum/index.shtml
Things you can do with North American marsupials, though I’d suggest garlic rather than
onion. |
| Genus:
Bishops Rich TH, Flannery TF, Trusler P, Kool L, van Klaveren NA &
Vickers-Rich P, 2001
'Bishop’s'
Remarks: The genus is named in honour of the late Dr Barry Bishop, the former Chairman of the Committee for Research and Exploration, National Geographic Society.
Family: Ausktribosphenidae Rich et al, 1997 |
| Species: | Bishops whitmorei Rich et al, 2001 |
| Place: | Flat Rocks, Victoria |
| Country: | Australia |
| Age: | lower Aptian, Lower Cretaceous |
| Remarks: | monotremes or
placental mammals. The authors take the latter position. "B. whitmorei is
more like the vast majority of placental mammals than is Ausktribosphenos nyktos,"
(Rich et al, 2001). This is a reference to grooves on the jaw and dental characteristics.
Additionally, and from the same paper, "if these six premolariform teeth are actually
a premolarform canine followed by five premolars, then
B. whitmorei possesses a known primitive character state for Placentalia". Of
course, if is an important word.
"A second ausktribosphenid, Bishops whitmorei, from the same locality as
Ausktribosphenos (Rich et al. 2001), is represented by a nearly complete
dentary with six premolars and three
molars, together with three other dentary fragments."
This quotation is drawn from Kielan-Jaworowska et al (2002). It suggests there's broad
agreement on the number of premolars, if not on the implications of them.
Thanks are due to: Dann Pigdon, for information on the holotype, Dr Tim Flannery of the South Australia Museum, who supplied the citation and Dr Tom Rich, for forwarding the paper. |
| Reference: | Rich et al (2001), A second tribosphenic mammal from the
Mesozoic of Australia. Records of the Queen Victoria Museum 110, p.1-9. |
| Links:
Dr TH Rich, Monash University, Fossil Mammal Lectures
http://www.earth.monash.edu.au/ESC2032/LECTURES/Lec21/L21p2.htm
According to these lecture notes, this second, (since named) genus of ausktribosphenid
shows clearer signs of being more like a typical placental. I’m going to have to look more
closely at this homepage. A quick perusal suggests it’s of great interest.
Scientific American, Mammal Melee
New fossils impugn leading model of early mammal origins
This is a reasonable summary of the state of play in 2000. It’ll be interesting to see how
things develop, in the light of the publication of Bishops. |
Ausktribosphenida or Australosphenida?, State of play, October 2002:
Puzzling small beasties from down under.
"Sigogneau-Russell et al. (2001) supported the sister-taxon relationship of
Ausktribosphenos and toothed monotremes, as
implied in Luo et al. (2001) and explicitly stated in our present paper, but they presented
a variant version of the "dual origin hypothesis" by excluding Ambondro
from Australosphenida. These authors considered only two characters diagnosing
Australosphenida (lingual molar cingulid and angulation of premolar cusps). We note that
Boreosphenida and Australosphenida differ in many more ways, including apomorphic
characters at 4-6 nodes of our earlier work (Luo et al. 2001: fig. 1) and 6-10 nodes herein
(Figs. 1, 2). Their suggestion that a lingual cingulid is plesiomorphic was based on an
eclectic and incomplete sampling of taxa and variants (e.g., polymorphisms in certain
specimens of Palaeoxonodon). As for the other character, Sigogneau-Russell et al.
(2001) failed to make the important distinction between molariform appearance and cusp
angulation: the main cusps a, b, c are not triangulated on the mediolaterally compressed
p5 in Peramus, differing fundamentally from the ultimate lower premolar in
Australosphenida," (Luo et al, 2002, page 47).
(Luo et al, 2001 and 2002 are in the Bibliography. Sigogneau-Russell et al (2001) is the
description of Tribactonodon.)
The Rich et al (2001) paper on Bishops argues for Ausktribosphenida within
Placentalia Owen, 1837. Another possibility is Australosphenida Luo, Cifelli &
Kielan-Jaworowska, 2001, should the monotreme affinities prevail.
To make things more fun, Pascual et al (2002), came to the following conclusion: "In
our opinion, dental evidence contradicts, rather than supports, a close relationship of
monotremes and therians, as apparently suggested by
basicranial evidence (Wible and Hopson 1993). Consequently, Steropodon galmani,
Teinolophos trusleri (a species recently described by Rich et al. 2001a) and, by
extension, the more advanced monotremes, cannot be grouped with Gondwanan tribosphenic
mammals, the Ausktribosphenidae (Ausktribosphenos and Bishops; see Rich et
al. 1999, 2001b) and Ambondro (Flynn et al. 1999)."
The Pascual et al study was of Palaeocene monotreme teeth, (Monotrematum), which
aren't generally associated with the relevant location, which is in Patagonia. The use of
the word, Ausktribosphenidae, would seem consistent with either Rich et al or Luo et al.
This is very possibly because Pascual & Co weren't examining ausktribosphenids.
However, if correct, their findings would leave monotremes outside of Australosphenida,
which would be a pity. Still, that's life.
On page 490, they do state: "For the reasons given above, we suggest that monotremes
should be excluded from the Australsphenida," which seems to imply an acceptance of
that taxon, (to me at least).
Reference for Ausktribosphenida: Rich et al (1997), as for Ausktribosphenos.
Reference for Australosphenida: Luo, Cifelli & Kielan-Jaworowska (2001), Dual origin
of tribosphenic mammals. Nature 409, p.53-57. |
| Update, December 2002:
A paper has just been published by Averianov, which contains some interesting and differing
ideas. He doesn't accept a connection between Shuotherium and Australosphenida.
Rather, he draws attention to the following:
"Pascual et al (2000) and Pascual and Goin (2001) argued for the close affinities of
Docodonta and Australosphenida..." I must read that
first paper more thoroughly. This is something I didn't notice! I'm not acquainted with
the second paper.
"Docodonts retain a primitive, Morganucodon-like mandible with a trough for
postdentary bones and anteriorly placed angular processes... These mandibular features
were retained in primitive australosphenidans, such as Asfaltomylos,
Ausktribosphenos, and Bishops Rich et al., 2001, but reduced in living
monotremes (an angle is still present in the fossil monotremes Teinolophos Rich et
al., 1999 and Obdurodon Woodburne and Tedford, 1975 see Musser and Archer 1998 and a
review by Luo et al. 2002). In "symmetrodonts" the angular process, present in
"eupanthotheres" and more advanced therians,
therefore appears to be secondarily derived. Summing up, among Mesozoic mammals the
combination of an angular process with the trough for the postdentary bones is found only
in Morganucodontidae, Docodonta, and Australosphenida and close relationship(s)
between these group(s) should be seriously considered. Shuotherium lacks an
angular process and in molar appearance (is) more reminiscent that of primitive
holotherian ("symmetrodont") mammals and
should be included to that group", (Averianov 2002, p.710).
This argumentation strikes me as being very interesting. Averianov also points out that
the Luo et al 2002 analysis is based on limited data, (which is possibly something of a
tautology, seeing as data's always limited. Such is life.) Nevertheless, more data could
clearly have profound effects upon results.
Satisfyingly, this discussion's becoming increasingly complex. |
| Genus: Henosferus
Rougier GW, Marinelli AG, Forasiepi AM & Novacek MJ, 2007
'Old animal'
Family: Henosferidae Rougier et al, 2007
Remarks: The generic name comes from Gratin, a language spoke by dead people on
islands exactly halfway between Italy and Greece. Henos is Greek for 'old'
and Latin contributes ferus for 'animal'. At least, that's the translation
given by the authors. A Gratin speaker informs me that ferus actually means
'wild'. I suspect the intended name had some connection with the specimen's oldness
and animalosity. |
| Species: | Henosferus molus Rougier et al, 2007 |
| Place: | Canadon Asfalto Formation, Chubut, Patagonia |
| Country: | Argentina |
| Age: | Callovian, Middle Jurassic |
| Remarks: | The following is based upon my reading of
Rougier et al, 2007 and thanks go to Mikko for sending a copy.
The paper by Rougier and Co is directly based upon three small fragments of jaw from
an ancient pond in Patagonia. Collectively, they present a reasonably full photo-fit
of the lower jaw, but the authors' concerns extend rather more widely. They take
the occasion to contribute to the on-going controversies concerning the possible
origins of descriptively tribosphenic molars
(note: 'origins' is a plural), and the implications that they may (or may not) have
for the family tree of the platypus, the
wonderful egg-layer from down under. Nevertheless, the descriptive part of the
study is restricted to three scraps of dead
dentary.
Henosferus was the latest addition to the published mammal fauna of the
Canadon Asfalto Formation, which actually translates as being only the second Jurassic
mammal from the entire continent of South America (p.1). Shortly afterwards came
the expected birth of a third, a triconodont.
While not exactly providing enough for teaming crowd scenes, the stadium was entirely
empty a few years ago excepting for possibly mammalian paw prints. This 'old beast'
is closely related to the first named, Asfaltomylos,
but it's distinct enough to form the basis of a separate genus within the newly
erected family of Henosferidae. Personally, I'd have thought the family name
would've been based upon the earlier discovery. However, there's no rule stipulating
anything of that kind, lots of precedents and, taking factors other than mere
seniority into account, the new genus could win the vote on the grounds of more
specimens. It's also the star of the relevant study. Besides, I'm far more
handsome than any of my older brothers, so there! In short, the family's
Henosferidae, damit basta!
Ancient and modern
In various ways the jaw's archaic. Uncertainly still obscures the actual geological
age of this site which, rather than dating from late during the Middle Jurassic,
could actually hail from the Oxfordian; early days of the Upper. The inside of the
bone carries a 'reptilian' Meckelian groove and
a deep trough towards the rear, and that latter feature held a set of postdentary
bones. I keep some of the ones involved in my middle ears, but Henosferus
still wore them on the jaw. At least, probability suggests so, and the preservation
of such bones in situ on a docodont,
the exquisite Castorocauda, indicates
probability knew what it was talking about.
Despite that sort of old hat, the molar teeth were
about the most complex the world had ever known featuring, at the rear of lowers, a
three-cusped, basined talonid. They perform most
excellent impersonations of tribosphenic molars which, in their true form, are
apparently first encountered during the Lower Cretaceous of the northern hemisphere,
several tens of millions of years later.
The authors provide a summary of Mesozoic mammals from all over the southern landmass(es)
of Gondwana and, should anybody be requiring such a thing, it's a useful and concise
one (p.2). Included are two possibly mammalian ichnotaxa from South America
(animals known only from preserved paw prints). Argentina's Middle Jurassic La
Matilde Formation waltzed in with Ameghinichnus pataganicus. Outraged by
this unwelcome one-upmanship by their ne'er-do-well neighbours, Brazil's Lower Jurassic
Botucatu Formation equalized with Brasilichnium elusivum.
Home sweet home
All three jaw fragments and an isolated upper premolar, which may belong to the same
genus, came from a single locality near the village of Cerro Condor. The fauna of
the whole Formation is becoming pleasingly varied (p.5). Obviously, the present
count of three mammals provides easily the most interesting attractions, but a
number of minor background performances are spiritedly provided by
sauropods, theropods,
pterosaurs, crocs, fish, frogs, turtles and
others. That they merit little more than polite applause isn't due to any fault of
the performers concerned. It simply results from their natural inadequacies and
general inferiority. We eucynodonts shouldn't place too much blame upon other
animals for being boring, as they can't help it. As stated, that's the cast drawn
from the whole Formation, not necessarily from this single locality.
Jaws
The description uses information from three jaws rather than a single specimen and,
for lovers of symmetry, both left and right mandibles
have been recovered. A table on page 40 contains various dimensions for two of
them. The type specimen has a length of 22.24mm, and depths of 1.67 below the first
premolar, 2.14 below the m1 molar. A second individual was a bit larger; bone length
about 22.5mm. For people none too keen on numbers, that helps add up to a shallow,
gracile jaw. The length tempts me to speak of
mouse-like
dimensions, and that means fairly large should you happen to be a shrew of some of
Henosferus' relatives. For example, it's double the size of
Asfaltomylos.
The state of preservation is rather good and, odd as it may sound to some, features
towards the rear of the jaw provide fuel for investigations on evolutionary developments
concerning the middle ear. The external side is convex to some degree whereas the
lingual surface is generally flat. The area of
least depth is found to the rear of the canine. Four mental foramina (small holes)
punctuate the buccal surface in reasonable consistent
positions among the various specimens (p.10); between the third and fourth incisors,
beneath the front of the canine, between that tooth and the following premolar, and
below the p2. The last mentioned is the largest. Behind the molars, the bone rises
to form a broad coronoid process for the attachment of muscles. Its presence
doubles the depth of the bone, and its fairly straight roof is well above the level
attained by any of the teeth.
Mammalian 'extras'
Had he ever had the opportunity to see such a thing, then the lingual surface of the
dentary could well have incited Charles Darwin into performing a jig of joy or, during
his worst health periods, a painful cough of pleasure accompanied by a grimacing
grin. This is the sort of fossil his theory suggested must've existed.
Near the base at the front of the coronoid process is found a small concavity, and
the like of this graces the jaw of no known mammal born during the last hundred
million years or so. It's the attachment area for an 'extra' jaw bone called the
coronoid (not to be confused with the coronoid process, a branch of the
dentary. A bit below and behind that is found a
mandibular foramin, and there's a projecting ridge running diagonally upwards from
that towards the condyle (the lower part of the jaw-skull joint), which is the
rearmost feature of the mandible. Along part of its course, this flange juts out
above a trough, and below that a groove runs forward at a level somewhat above the
lowest part of the dentary. Should anybody feel a bit lost by that concise list,
then you're welcome to fetch a cup of tea, relax, and re-read it more slowly.
That groove thing is the critter's Meckelian groove.
Having allowed Darwin a jig for the just mentioned home of the coronoid, those
further details should leave him leaping his way through a full-blown Highland fling.
No jaw from any living mammals has a postdentary trough or a Meckelian groove.
The dentary trough is deep and subdivided into two areas, albeit rather indistinctly.
If this trough weren't for housing a rod of postdentary bones, then its purpose
would be bafflingly obscure. To the fore, it connects with the Meckelian groove
which proceeds (p.13), with a minor loss of width, along until level with the rear
premolars. At that point, it curves up towards the front of the dentary. This
groove also grows increasingly shallow along its course forwards. Generally, terrestrial
vertebrates are proud bearers of Meckelian grooves. We mammals, however, opted to
do away with them.
Teeth
The full formula is available for each side of the lower jaw: 4
incisors, 1 canine, 5
premolars and 3 molar. The division between the
two types of postcanine teeth was drawn on the grounds of differing morphology.
Premolars and molars don't look the same. No evidence on replacement patters
happens to be available.
There are four incisors. Although close to one another, some space occurs between
them, and more so for the rear ones. Roots are all that remains of the first two,
and they serve to show these teeth were round at their bases, and directed more-or-less
forwards rather than vertically. The final pair show more upright yet still diagonal
inclinations. None happen to be particularly well preserved. The
alveoli, however, are all close to one another in
terms of size (p.14), and that presumably indicates much the same could've been said
of the crowns. The first may have been modestly larger.
The anterior brigade is completed by a canine, and two obliging specimens remain
intact. This tooth was single-rooted and taller than all other lower teeth (p.15).
It tilts slightly towards the rear, perhaps in order to offer arriving food morsels
clues as to the desired direction of travel. It's a robust, not overly frightening
sort of canine, at least not to me, and diastemata
separate it from both the incisors and premolars.
Premolars
There's perhaps an element of uncertainty about premolar numbers, but five is the
probable number for the adult set. The slight difficulty is provided by an extra
alveolus in front of the first on the type specimen. Nothing of the sort occurs on
the other two, and levels of wear suggest the holotype was probably the oldest
individual regardless of this apparently rather childish behaviour. Certainly, I'm
old than either of our children despite any retained juvenility. This odd
alveolus is probably a relic retained from a former
deciduous premolar as it's partly plugged with bone. You always get some
individuals with dental peculiarities among a population. Apparently, there's a
specimen of Peramus in the Natural History
Museum, London, and the former owner grew one of its premolars the wrong way round.
The intended front's facing the rear.
The sensibly behaving premolars are regularly spaced along 45% of the tooth-bearing
dentary, and isolated from one another by large diastemata. As lengths beat widths
by nearly a factor of four, they certainly qualify as being narrow. Each premolar
is hosted by two fairly similarly sized, cylindrical roots. For the first trio, the
main cusp is positioned above the front to centre of the first root. However, it's
closer to between both roots on the final pair. The p1 and 2 have no accessory
cuspules or cingula as far as can be seen (just
buccally), and the former's a bit more strongly
built. Both are shorter than the p3, with a further height increase for the p4.
The cusp of p3 is less symmetrical than's the case for the preceding premolars.
Its rear slope is somewhat more concave than the straighter front slope. An
accessory cuspule occurs. The main cusp of the p4 returns closer to symmetry, and
the worn accessory cusp is both larger and more lingually
positioned. The crown also experiments with a small cingulum at its front
lingual base. The fifth in the series is close in height with its predecessor, but
of a greater length. Similar sets of cuspules inhabit the front and rear regions of
the buccal side. There's a minute accessory cuspule to the fore of the main cusp,
nearly centrally positioned, and a larger rear cusp set both aft and more
lingually (p.16). Sadly for any fans, the tooth has no basined
talonid.
Molars
All three molars in the series are double-rooted and, in contrast to other teeth,
they're positioned closely to one another. The first is the largest and the third
much the smallest. The middle molar's nearer in size to the m1. There's also little
space between this trio and the start of the coronoid process behind. The best
preserved specimen is the m1 of the type fossil.
Its crown has a trigonid and basined talonid heel
which can sensibly be described with the specialized terminology applicable for
tribosphenic teeth. However, whether that's
strictly appropriate is a different matter. Ideally, for example, 'talonid' should
refer only to structures resulting from the same evolutionary development; results
of shared ancestry. When used in this sense, words have their meanings blurred by
applying them to features which happen to be similar for other reasons. Still, the
terminology is descriptively available.
The talonid of m1 is a touch wider than its trigonid.
Some wear occurs on its buccal side, but not enough to have removed cuspule f at the
front. That cuspule appears to be associated with a not very impressive cingulum,
which is strongest by the root. The protoconid
is larger than the paraconid, and the latter can
laugh at the puny physique of the metaconid. There's
a bit of a crenulated cingulum on the lingual side, but it gives up its travels
prior to the metaconid. Some damage obscures the entertainment provided by the front
of the cingulum. Nevertheless, it projected forward to some degree, and got involved
in producing an interlocking system with part of p5. Friends of a cusp termed e
may be saddened to hear none could be made out. The trigonid is wide, unbasined and
mostly taken up by the base of the protoconid.
The talonid is a wider than long oblong (p.17). It's completely basined with all
fixtures and fittings a tribosphenic heart could desire; the highest of high-tech
models. Facilities include a hypoconid at the rear of the buccal side and a friend,
the hypoconulid, from which its base isn't completely separated. Part of the latter
juts back to perform interlocking duties with the following tooth; finding place
snugly between the cusp f and a projection from the paraconid. An entoconid was
presumably available although heavily worn down. It's probably represented by a
rounded prominence on the lingual side. In any case, these cusps and associated
crests conspire to completely enclose the basin, and an entoconid is reasonably
distinct in the corresponding tooth of another Henosferus individual.
However, there is a measure of variation in position and relative proportions of m1
talonid cusps.
Detailed descriptions of further molar positions are provided in the paper (p.17-18)
and, unless I've forgotten to include one, a link to the study is available below.
It's presently freely available in pdf format. Even should I indeed have forgotten,
copy the title into a search ending and hope I haven't mistryped that. (Note: that
typo was deliberate.)
Wear facets
As some people might not be sure about what wear facets actually are, we may as well
go for a brief walkies with that theme in the company of Patch, the dead fox
terrier. Sometimes, back in the 1970s, he used to take me for walks on part of St
Catherine's Hill which, as all will doubtlessly know, is wooded and close to Hurn
Airport on the eastern edge of Bournemouth, England. The top of that hill is
something like a plateau. As Patch attended to important stuff, such as rabbit
reports, he left me to explore the remains of bits of the airbase that'd been
temporarily located up there during the Second World War; several ack-ack gun
emplacements, some barrack huts and so on. By 1970, the only buildings left intact
on that part of the hill were air raid shelters, and they subsequently had their
doorways bricked up. There wasn't much to see in them, but it was fun looking
regardless.
For the seeker of wear facets, however, the left over floors of long demolished huts
in broad daylight are more instructed. Only the concrete surfaces remained, but
working out where the door had been was dead easy. They'd been erected in something
of a hurry, and making the ground beneath them as smooth as a billiard table hadn't
been a priority. You can search all you like, but the doors obviously never quite
fitted properly with the floor, they always closed along precisely the same route
(unless the hinges fell apart), and the noise that resulted, again and again,
must've been bloody annoying. There was regular occlusion between the bottom
corner of the door and the floor, and this resulted in a characteristic groove.
Decades later, that told anybody who wanted to listen where the door had been. It
was an unmistakable wear facet.
Unlike for other toothed vertebrates, typical
mammals have teeth that occlude regularly during chewing, again and again and again
along the same routes. That activity also produces tell-tale wear facets. The
principle involved is easy enough to grasp, but the resultant patterns can be
complex due to regular occlusion along more than one plane of movement, and from
more than a single feature.
Tribosphenic molars produce an array of wear facets in different places, and they've
received identifying numbers. However, the facets can get blurred by too much
wear, and that's happened on the m1 of the holotype. Despite having analogues of
all major boreosphenidan cusps (tribosphenids
is the word used by the authors, and I think it probably would be the more
appropriate choice), there are differences among the wear facets (p.19). Clarity
is aided by teeth from other jaws.
A contrast concerns facet 5. This should adorn the rear face of the
metaconid and then proceeds down to the front of
the lingual area of the talonid. Its probable origin
is a cutting stroke performed by the upper protocone along a crest. It couldn't be
clearly found on any Henosferus lower molar. Facet 6 should form on the front
buccal part of the entoconid. Even when plenty of
wear has occurred in that neighbourhood, that facet also doesn't result for this
critter.
Affinities
Along with 47 other genera, Henosferus was questioned concerning 271 anatomical
characters (p.20), although it was unable to provide all the answers due to missing
body parts. Still, it did its best to oblige. The results of these enquiries
placed it as closest to Asfaltomylos within a
monophyletic group called Australosphenida
which, apparently, includes monotremes such as
the still living platypus.
While the resemblance with Asfaltomylos is close, it's not close enough for
them both to be allocated to the same genus. Henosferus differs in a number of
features including: the large spaces between the premolars,
details of jaw such as a lower coronoid process with a less vertical front edge, and
less room behind the molars. There's also the matter
of being double the size. That seems too extreme to be due to different sexes of the
same species.
Tribosphenic tribulations
Used in a loose sense, tribosphenic molars have a basined
talonid heel at the back of the lowers, and uppers equipped with a suitably
positioned protocone pounder. That bashes foodstuff up in the basin. Australosphenidan
lower molars, including those of Henosferus, fit in with that general idea
(excepting for monotremes). However, whether the uppers conformed to type is a
different matter. The shortage of specimens is less than helpful (p.24). If
monotremes are considered as included, then upper molars are known for just three
genera; Monotrematum,
Obdurodon and the
platypus (juveniles). The first mentioned,
dating from the Paleocene, has a small cuspule in about a protocone sort of position,
and it could perhaps be a persisting remnant. That conclusion, however, is disputed.
The other two genera mentioned have no sign of a protocone.
Rudely leaving the monotremes out of the picture, then (other) australosphenidans
all share a characteristic of their upper molars. These teeth happen to be unknown.
Henosferus, Asfaltomylos,
Ambondro,
Ausktribosphenos, Bishops... Knowledge
about them all is presently restricted only to lower jaws and teeth. And at least
four of those five show no wear in the talonid caused by a possible protocone.
Ambondro may be an exception, but most of its talonid wear occurs
buccally of a crest termed the crista obliqua. While
that doesn't mean there can't have been a protocone-like cusp, it does suggest a
lack of "mortal-to-pestle grinding". Traditionally, that factor's been an essential
component of this tribosphenic business.
Of jaws and middle ears
The dentary of Henosferus contains a postdentary
trough towards the rear, as previously mentioned. This is subdivided into two surfaces
by a ridge, although not as markedly so as known for the more ancient
Morganucodon (p.28), a very
basal mammal. This trough is also proportionately for
that comparative old timer (p.29). In general terms, however, the structures on
both jaws are similar. Presumably, both retained postdentary bones in these troughs,
and attachment surfaces are also identifiable for so called paradentary bones as
well; the coronoid and splenial. At least, in the case of Henosferus, the
point of attachment for a coronoid is clear. The shorter trough of this Patagonian
suggests the postdentary bones were still further reduced in size and, as it's the
more derived of the pair, this is in line with
expectation.
Modern mammals have but a single lower jaw bone, and many of these 'extra' small
components have been incorporated into the middle ear. For example, the
malleus is the recycled articular and the
incus developed from the quadrate, the previous upper
element of the non-mammalian jaw joint. (As it's not actually either a para- or
postdentary bone, my writing's fallen a bit off-target, but never mind.) As these
various bones appear to have seen been on the jaw of Henosferus, then they
couldn't simultaneously have been housed in the ear. Therefore, the hearing
hardware didn't include three small ossicles in the ear; the incus, malleus and
stapes. Presumably, although already serving such
functions, the first two of that trio weren't within the
petrosal
And that leads us to Teinolophos
These next few comments should perhaps be built into the appropriate entry for Teino,
a Lower Cretaceous monotreme. However, I think I'll leave them here as they are
intimately connected with information listed above, and I don't like splitting up
articles drawn from single studies if conveniently avoidable. It plays havoc with
my bookkeeping methods. It concerns hearing.
All living mammals, whether placentals,
marsupials or monotremes, have three sound-processing
ossicles in the middle ear, and it was long assumed that this arrangement resulted
form common descent. Doubt was cast upon that by an interpretation of a monotreme
named Teinolophos in 2005. As can be seen in the directory entry for
Teinolophos, doubt was then cast upon that doubt, and from several quarters
(p.36). To be brief here, one objection questions the identity of the specimen
involved and suggests that individual could actually be a non-monotreme australosphenidan.
Another challenges the alleged retention of a postdentary trough (it could be the
"floor of the mandibular canal"), and finds no compelling evidence for the presence
of a facet for the angular bone. As the second objection involved a study of all
known Teinolophos specimens, both could be valid.
On the face of it, if there's no reason to suppose any monotreme had 'extra' para-
or postdentary bones on the lower jaws, then placentals, marsupials and monotremes
could've all inherited the basic arrangement of incus, malleus and stapes from their
most recent common ancestor. That would certainly be in line with my present
expectations.
However, my expectations play little significant role (beyond my wife sometimes
ending up naked), and they've frequently been wrong. The picture changes should
monotremes actually be derived australosphenidans as, presently, lots of studies
are indicating. The evidence for the presence of para- and postdentary bones on the
jaws of more basal australosphenidans is presently indirect, but nevertheless seems
overwhelming. I've no option but to except it. With those circumstances, logic
demands either monotremes aren't australosphenidans or there was a remarkable bout
of convergence in the evolution of mammalian inner ears (p.31). At least, it would
seem remarkable to me.
Holotype
The type fossil, MPEF 2353, is a piece of lower jaw preserving parts of the first
two premolars and a near complete
molar (m1). It's been granted a furnished apartment
in the Museo Paleontologico Egidio Feruglio, Trelew, and I've heard it's attempting
to learn both Spanish and Welsh. Welsh happens to be a minority language in that
particular part of Argentina. The specific name comes from mola, a Latin
word for 'millstone', and this refers to the well developed
talonid on the molar. |
| Reference: | Rougier et al (2007), New Jurassic mammals from Patagonia,
Argentina: A reappraisal of australosphenidan morphology and interrelationships,
American Museum Novitates, 3566, p.1-54. |
| Other reports:
Canadon Asfalto Formation, Argentina
Abstracts submitted to The Society of Vertebrate Paleontology Annual Meeting 2004 include:
A new mammal from the Canadon Asfalto Formation, Chubut Province (Argentina), lead author
Analia M Forasiepi. Martin & Rauhut, 2005 refers to this fossil as being similar to
Asfaltomylos, (p.415).
Update: The animal has been descriptively born, and the name chosen by its
parents is Henosferus. |
| Bonus link:
Mammalian Phylogeny, The University of Louisville
http://www.louisville.edu/medschool/anatomy/1-faculty/mammal_phylogeny.html
This is a bonus link for any specialists or masochists. Collaborative Research: A
Morphological Database for the Higher-Level Relationships of Fossil and Recent Mammals.
Here you can check up on the orientation of narial apertures, should you feel inclined.
|
| 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.
back to top
Trevor Dykes, October 2002. Last update: 10.11.2007
Ktdykes@arcor.de |
Bibliography:
Averianov AO, (2002), Early Cretaceous "symmetrodont" mammal
Gobitheriodon from Mongolia and the classification of "Symmetrodonta".
Acta Palaeontologica Polonica 47 (4), p.705-716.
Cifelli RL (2001), Early Mammal Radiations, J. of Paleontology, vol 75 (6), p.1214-
1226
Flynn JJ, Parrish JM, Rakotosamimanana B, Simpson WF & Wyss AR (1999), A Middle
Jurassic mammal from Madagascar. Nature 401, p.57-60.
Kielan-Jaworowska Z, Cifelli RL, & Luo Z-X (2002), Dentition and relationships of
the Jurassic mammal Shuotherium. Acta Palaeontologica Polonica 47(3), p.479-486.
Luo Z-X, Cifelli RL & Kielan-Jaworowska (2001), Dual origin of tribosphenic
mammals. Nature, vol.409, p.53-57.
Luo Z-X, Ji Q & Yuan C-X (2007), Convergent dental adaptations in
pseudo-tribosphenic and tribosphenic mammals, Nature, 450, 93-97.
Luo Z-X, Kielan-Jaworowska Z & Cifelli RL (2002), In quest for a phylogeny of
Mesozoic mammals. Acta Palaeontologica Polonica 47 (1), p.1-78.
Martin T & Rauhut OWM (2005), Mandible and dentition of Asfaltomylos
patagonicus (Australosphenida, Mammalia) and the evolution of tribosphenic teeth,
Journal of Vertebrate Paleontology, 25(2), p.414-425.
McKenna MC & Bell SK (1997), Classification of Mammals Above the Species Level.
Columbia University Press.
Pascual R, Goin FJ, Balarino L & Udrizar Sauthier DE (2002), New data on the
Paleocene monotreme Monotrematum sudamericanum, and the convergent evolution of
triangulate molars. Acta Palaeontologica Polonica 47(3), p.487-492.
Rauhut OWM, Martin T, Ortiz-Jaureguizar E & Puerta P (2002), A Jurassic mammal
from South America, Nature, 416, p. 165-168.
Rauhut OWM, Martin T, Ortiz-Jaureguizar E & Puerta P (2002), The first Jurassic
mammal from South America, (Supplementary Information). See Asfaltomylos for the
link.
Rich TH, Flannery TF, Trusler P, Kool L, Klaveren van NA & Vickers-Rich P (2001),
A second tribosphenic mammal from the Mesozoic of Australia. Records of the Queen Victoria
Museum 110, p.1-9.
Rich TH, Flannery TF, Trusler P, Kool L, Klaveren van NA & Vickers-Rich P (2002),
Evidence that monotremes and ausktribosphenids are not sistergroups, Journal of Vertebrate
Paleontology, 22(2), p.466-469.
Rich TH & Vickers-Rich P (2004), Diversity of Early Cretaceous Mammals from
Victoria, Australia, Chapter 3, Bulletin American Museum of Natural History, 285,
(p.35-53).
Rich TH, Vickers-Rich P, Constantine A, Flannery TF, Kool L & Klaveren van NA
(1997), A Tribosphenic Mammal from the Mesozoic of Australia. Science vol 278, 21.11.97,
p.1438-1442.
Rougier GW, Martinelli AG, Forasiepi AM & Novacek MJ (2007), New Jurassic
mammals from Patagonia, Argentina: a reappraisal of australosphenidan morphology and
interrelationships, American Museum Novitates, 3566, p.1-54.
Woodburne MO (2003), Monotremes as Pretribosphenic Mammals, Journal of Mammalian
Evolution, 10 (3), p.195-248.
Zhang Fakui (1984), The Fossil Record of Mesozoic Mammals in China. Vertebrata
PalAsiatica, Vol XXII (1), p. 29-38, (as translated by Will Downs, May 1986 with
minor revisions in 1999). This paper was obtained courtesy of the Polyglot Paleontologist
.
http://www.uhmc.sunysb.edu/anatomicalsci/paleo/terms.html |
