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.
Boreosphenidans seem to have originated in
the northern hemisphere. They’re mammals equipped with highly derived and complex cheek
teeth; tribosphenic molars.
Tribosphenic comes from Greek refers to dental characteristics. 'Tribo' means grinding,
whilst 'sphen' refers to cutting, shearing and wedge. The teeth in question are
suitable for both functions, which is an elegant and useful combination, as you can
experience when eating or gnashing your teeth.
It was assumed that this sort of chopper could have evolved only once. New research
suggests that a similar condition may actually have developed twice; once in the north,
(thus 'boreo'), and once in the south, (
Australosphenida). Both routes led to similar, though differing gnashers. The
correctness of this view can only really be tested with the help of further finds,
especially from the southern hemisphere. This directory’s built upon the assumption that
this ‘dual evolution’ happened, though there are other views.
One of the attractions of the theory is that it interprets the available evidence very
neatly. Early australosphenidans led to monotremes,
those egg-laying mammals now restricted to Australasia. Early boreosphenidans gave rise to
therians, from which two main lines developed;
Eutheria -which means nearly the same as
placental mammals, and
Metatheria -nearly the same as marsupials.
Therians, placentals and marsupials are within
Boreosphenida, along with their more immediate predecessors.
With regards to the genus Tribactonodon, I'm not sure this is part of Boreosphenida.
Clarification would be welcome. |
A. Boreosphenidans B. Pappotheriidae
| Taxon: Within Boreosphenida? sensu Luo et al,
2001
Comparative anatomy is central to the interpretation of fossils, as well as living
organisms. Organisms have traits in common with each other which allow them to be logically
referred to species, genera, families, orders, classes and kingdoms, (clades if you want to
be more with it). There is a prerequisite, of course. You need something to compare them
with. The fossil record is frustratingly incomplete. The following are hard to place, which
is also what you’d logically expect with basal representatives.
Genera: Aegialodon,
Hypomylos, Kielantherium,
Mozomus, Tribactonodon,
other reports
Time-Line:
Lower Cretaceous: Aegialodon, Hypomylos, Kielantherium,
Mozomus, Tribactonodon |
| Genus: Aegialodon Kermack
KA, Lees PM & Mussett F, 1965
'sea shore tooth'
Family: Aegialodontidae Kermack, Kermack & Mussett, 1968 |
| Species: | Aegialodon dawsoni Kermack KA, Lees PM & Mussett F, 1965 |
| Place: | Wealden, Cliff End, Hastings |
| Country: | England |
| Age: | Wealden, Lower Cretaceous |
| Remarks: | The following is based upon my reading of Kermack
et al, 1965.
Considering its length is barely more than a millimetre, this single tooth was accorded an
unusually appreciative welcome. There were a couple of contributing factors. Firstly,
mammal fossils are rare. Secondly, Lower Cretaceous specimens were known only from two
places in the world at the time, (disregarding the Dorset Purbecks, then considered as
Upper Jurassic). Thirdly, this was precisely the type of tooth people had spent a lot of
effort looking for. It's a fully tribosphenic molar.
Tribo what?
It's a dual-purpose tooth capable of both cutting and refined grinding. All existing
mammals except for the egg-laying monotremes, are
descendants of a mammal with molars broadly like this one.
(Monotremes may also be descendants of an earlier episode of tribosphenic developments,
but that's a matter of dispute.) As the lines leading to both
placentals and
marsupials had already diverged before 125 million years ago, (as ably demonstrated by
Eomaia and
Sinodelphys), Aegialodon is too late to be a candidate for an ancestor.
Nevertheless, the morphology of the single molar is a plausible approximation.
A brief glimpse
The tooth has a collection of three main cusps known as a
trigonid, (p.535). Behind is located a fully basined heel termed a
talonid. This is positioned so as to enable a cusp on the
corresponding upper to use it as a grinding bowl, in order to enhance the exploitation of
food. The patterns of wear on this talonid are fully consistent, and only consistent, with
the attentions of such a cusp; the protocone. Since this description, Lower Cretaceous
mammal fossils have been found in a considerable number of further localities, and those
from Liaoning, China are of such exceptional quality and
completeness to qualify as gobsmacking.
The star of Wealden teeth
As the name implies, a trigonid has three main cusps. The
paraconid is on the lingual side at the front. About
halfway along the crown is the metaconid. Almost directly
buccal of that is found the largest cusp, the
protoconid. (It's slightly further forward and near to
the centre of the crown.) This arrangement is close to being a right-angled triangle. The
paraconid was probably a bit larger than the metaconid, which is worn, (p.540).
A small cusp is about halfway up the front (mesial) face of the paraconid. It's termed the
mesial cusp, (p.541). Buccal of that is a ridge extending down to gum level; the mesial
ridge. These two features are separated by a cleft. The cleft is part of an interlocking
mechanism with the tooth in front. It's positioned so as to fit together with a cusp on
the back of the next tooth called the hypoconulid. The point of this will be mentioned
shortly.
Talonid
The talonid heel is behind the trigonid. As it's basined, it provides a convenient grinding
bowl for the upper tooth. Well ground food can be exploited by the metabolism both more
quickly and thoroughly. It's biological high tech but isn't always the optimal approach in
every set of circumstances. Some descendants of critters with such sophisticated
dentitions have thrived by down-grading their teeth. A
few, eg. anteaters, have given up on the things entirely. The world is a complex place,
and no single strategy is the best in all situations.
This talonid is triangular from the occlusal perspective.
The rear edge is rimmed and carries three small cusps. The largest is the aforementioned
hypoconulid; the interlocking aid for the following tooth. Buccal
from that are two smaller and close cusps. These are part of the same structure, and it's
called the hypoconid ridge. In more derived forms, they
amalgamate into one cusp. Remains of a badly worn cusp are found on the lingual border.
Typical tribosphenic molars feature a cusp known as an
entoconid. However, the position of this mainly demolished structure in Aegialodon
would be strange, if that's what it is. Jumping briefly ahead to page 546, while shearing
was underway, this cusp would've been in front of the tip of the upper protocone. The
entoconid is characteristically behind. It could perhaps be a prototype but may be
something else.
Interlocking molars, (back to page 541)
The back of the talonid bulges outwards and, as judged from the front of the molar, that
bulge would have been accommodated by the hospitable cleft in the next tooth. Simply put,
the rear of the molar is overhung by the front of the following one. Interlocking methods
of one style or another were popular with Mesozoic mammals, (eg.
multituberculates and
triconodonts), and still are; for instance
Potomagale and Didelphis - the Virginian possum. The reason stated by the
authors is protection of the delicate peridontal membrane of the gum. If you were to ram
food particles down between your teeth, it'd bloody hurt. More significantly, it could
also cause much damage.
Certainly in triconodonts, the interlocking also enhanced stability of the tooth row. This
possibility isn't addressed for Aegialodon. Then again, a single molar is clearly a
very limited sample.
Wear facets
Patterns of wear are remarkably complex and clear facets are present all over the crown.
The authors go into more detail than I wish to digest, and notate various facets with
letters from a to g. Several are further subdivided. These all provide information on
the interaction of this tooth with the corresponding upper. I'm going to merely mention
facet e and two allies, all of which are situated on the talonid. The significance lies in
these being characteristic for the presence of a protocone on the upper molar, (p.544), and
corresponding facets aren't all found on less derived
molars. Facet e' shows that the tip of the upper cusp: "overlapped the lingual rim of
the talonid in the extreme buccal phase of occlusion", (p.546). This wouldn't be
consistent with a straightforward enlargement on the upper lingual
cingulum. There was a properly developed protocone. Also
identified is facet e'', and it matches wear found on some extant mammals, (eg.
Didelphis).
"To summarize: the wear facets on the lower molar of Aegialodon could only have
been produced by occlusion with a typically trituberculate upper molar in which a
definitive protocone had been evolved."
Peramus
The lower molar has most similarity with Peramus among Mesozoic mammals know at the
time of publication, but there's no sign of facet e' in that genus. The lingual cingulum
on the somewhat earlier peramurid was developing in the direction of becoming a protocone,
but it hadn't reached the stage of being a true cusp. That's the most significant contrast
between the equivalent teeth. The northern tribosphenic mammals want on to flourish. The
closely related peramurids went extinct.
Holotype
The holotype resides at the Natural History Museum in London, and is affectionately known
as BMNH M23345. The specific name honours Charles Dawson: "who was the first man to
search deliberately for mammalian teeth in the Wealden", a hobby he began pursuing in
the latter years of the nineteenth century, (p.539). |
| Reference: | Kermack et al (1965), Aegialodon dawsoni, a new trituberculosectorial
tooth from the lower Wealden. Proceedings of the Roy. Soc., London, B, 162, p.535-554. |
| Genus:
Hypomylos Sigogneau-Russell D, 1992
Aka: Hypomylus
Remarks: The genus is represented by lower molars, (Canudo &
Cuenca-Bescós 1996, p.223). |
| Species: | Hypomylos micros Sigogneau-Russell D, 1994 |
| Place: | Anoual |
| Country: | Morocco |
| Age: | Berriasin?, Lower Cretaceous |
| Remarks: | Information welcome. |
| Reference: |
|
| Species: | Hypomylos phelizoni Sigogneau-Russell D, 1992 |
| Place: | Anoual |
| Country: | Morocco |
| Age: | Berriasin?, Lower Cretaceous |
| Remarks: | |
| Reference: | Sigogneau-Russell (1992), Hypomylos phelizoni nov.
gen. nov. sp. une étape précoce de l’évolution de molaire tribosphénique (Crétace basal
du Maroc). Geobios 25, p.389-393. |
| Genus:
Kielantherium Dashzeveg D, 1975
'Kielan’s beast'
Aka: Kielanotherium
Family: Aegialodontidae Kermack, Kermack & Mussett, 1968
Remarks: Although the derivation of the name isn't stated, it'd be surprising if it
referred to anybody other than the Polish paleontologist, Zofia Kielan-Jaworowska.
John H Burkitt’s presentation also includes the genus Kermackia within
Aegialodontidae, though I don’t know what the current view is.
"... its molar structure is widely regarded as
plesiomorphic among mammals with the tribosphenic
pattern ...", (Luo et al 2002, p.11). In other words, its cheek teeth are seen as
being basal for us derived
types. |
| Species: | Kielantherium gobiensis Dashzeveg D, 1975 |
| Place: | Höövör, near Guchin Us, Arvaykher |
| Country: | Mongolia |
| Age: | Aptian-Albian, Lower Cretaceous |
| Remarks: | The first block of the following is based upon
my reading of Dashzeveg, 1975, and thanks are due to the supplier. For unclear reasons, I
now know I was propagating a garbled version of the original citation and holotype number,
and I've no idea how that happened. It only took me five years to notice. Both are now
correct.
A short comment on terminology
Dashzeveg used Theria in a wider sense than is now
generally followed. His usage seems to approximate the subsequently coined
Boreosphenida presently prevailing in my directories.
I'm not all that good at fortune telling, but I suspect Tribosphenida could one day over
come that, especially as it happens to be an older
word. Unless Tribosphenida were to be massively expanded so as to include
australosphenidas and, by implication, large
numbers of non-tribosphenic mammals, then I'm now
struggling to recognize an actual difference between Boreosphenida and Tribosphenida.
Be that as it may, Dashzeveg concluded that mammals with tribosphenic
molars, presently boreosphenidans in this project, probably
already existed in time for the dawn of the Cretaceous (p.402). As
Tribactonodon eventually scurried along from the fossil fields of
Purbeckia, he's been
vindicated. His judgement was based upon a single tooth from the Wealden of Sussex,
Aegialodon. Although that genus is still represented by
just one tooth, it's a pleasingly tribosphenic lower molar with a basined
talonid at the rear.
Dashzeveg had been studying fossils from the Höövör locality of Mongolia, a place that's
been known by a number of variations of that name. While some of that material wandered
off to distant Moscow, other fossils roamed east to Ulan Baatar. Among those was a well
preserved lower molar. Although from deposits perhaps somewhat more recent than the
Wealden of Sussex: "It is almost identical to that of Aegialodon." For reasons not
actually discussed, Dashzeveg opted to establish a separate genus for it despite that very
close resemblance. The geographical distance and differing biotopes of the two faunas
would appear, to me, to make a genuinely common genus rather unlikely, as it'd suggest a
critter at home in both swampy and approaching desert conditions. Consequently, his
decision may well be in line with reality. That said, the Wessex Formation of the Isle of
Wight is at least closely contemporary with Aegialodon, and that place now does
have a genus in common with Höövör;
Gobiconodon. However, that genus does presently display a rather extreme
amount of variability.
To tell the tooth
The lower molar has a narrow, two cusped talonid. These were identified as being a small
hypoconid and the hypoconulid. The protoconid is
something like twice the height of its two colleagues found on the
lingual side of the crown and, of that pair, the
paraconid is a wee bit taller than the
metaconid (p.403). Their bases are "clearly separated".
The development of the cusps, the configuration of crests and comparison with a Moscow
specimen all indicate this tooth is a molar, and not a
premolar.
Holotype
The type fossil, GI PST 10-14, is a right lower molar imprisoned in the Institute of
Geology, Ulan Baatar. Although originally living alone, it has since received companionship
from further specimens. Dashzeveg (1975) mentions a lower jaw with two molars in Moscow,
and thought it probably belonged to the same genus. However, there was no description.
The next block is based upon my reading of Dashzeveg & Kielan-Jaworowska, 1984, and more
thanks are due to the same supplier. The authors consider a partial lower jaw known as
GI PST 10-14, another immigrant to Ulan Baatar.
Six years after delivering the brief description of the type fossil, Dashzeveg found part
of a lower jaw from the same locality. This provided a partial dental formula of four
molars and at least four (perhaps five) premolars (p.217). The description of that also
includes a review of relevant background, and that makes mention of Fox synonymizing
Kielantherium and Aegialodon in 1976, although he did accept the plausibility
of separate species (p.218). However, that was challenged by others as Höövör and the
Wealden were chronologically and geographically isolated from one another, and the strong
similarities could perhaps be inheritances from a common ancestor. A clear resolution of
this not overly significant matter could perhaps be provided by more informative specimens
of the English critter. Unfortunately, none have been forthcoming. It may be that
on-going research on the Isle of Wight will uncover something of relevance.
The jaw
The fragment of right mandible preserves the lower area of the coronoid process, four
molars in reasonable condition, eight alveoloi for the
same number of premolars, and a further alveolus for either another premolar or the canine.
It's a slender jaw with a depth fairly similar to the heights of the molar crowns, judging
from the accompanying photos; 1.7mm below the rear root of m1, 1.9mm in the case of m4
(p.219). The lingual face of the bone displays a distinct
Meckelian groove low down, and this runs along much
of the posterior half (p.221). Also on visible is a facet towards the base of the coronoid
process, an attachment area for a small coronoid, an 'extra' jaw bones we moderns don't
have.
Molars
The first pair in the team were similar in size and form, although number one has been more
worn down on its main cusp. As with the type fossil, protoconids are the taller cusps,
paraconids are higher than the metaconids (p.233), and their bases are well separated. At
the front of these teeth are small cuspules; one on either side. Talonids are low, small
but basined.
The third molar is smaller and little worn, and the cuspules on its front are more strongly
built. Two cusps can be discerned on the talonid; a heavily damaged hypoconulid and a much
healthier looking hypoconid. The metaconid is proportionately smaller on this tooth, and
its base is less well separated from that of the paraconid.
Molar number four is the smallest and least worn of the quartet. Its front cuspules are
less well developed, and the metaconid's relatively lower.
All four molars are double-rooted teeth, with the rear roots being the larger.
Premolars
No crowns are present, but there are roots still in the alveoli. Tentatively, the positions
were identified as being for p2-p5 rather then 1-4. This is based upon expectations
resulting from comparisons with other fossils. A further ancient resident of Höövör was a
eutherian since named
Prokennalestes. In common with a number of other
basal eutherians, eg. Eomaia (not known at the
time of publication), Kennalests (juveniles)
and Gypsonictops, the
postcanine formula is five premolars and four molars per side. Eocene sirenians are
also mentioned. While interesting in itself, I can't imagine that being due to anything
other than convergence. Anyway, five and four appear to have been the original numbers for
eutherians and, presumably, this was inherited from non-eutherian ancestors. As
Kielantherium was phylogenetically fairly close to that line, a neat way of saying
a none too distant relative that could leave your biology teacher temporarily speechless,
then this count is at least a contender for worthy consideration. Obviously, should more
revealing specimens turn up, the conclusion could be shown to be wrong.
Distinctions
In as far as the known specimens tell us, Kielantherium is so similar to
Aegialodon, it's been accused of being a synonym. The molars are much the same size
(p.224). Those of the Mongolian are higher, but this isn't necessarily significant given
the greater degree of wear the Sussex molar experienced. It may be due to that.
Kielantherium has a better degree of basal separation for the protoconid and
metaconid, the talonid's set lower and its shape differs. It also has less cusps.
A handy summary is provided for the talonid distinctions viewed by Dashzeveg and
Kielan-Jaworowska: Aegialodon -roughly triangular, wider and with three or four
cusps; Kielantherium -roughly rectangular, much narrower and with two cusps close
together at the back.
And there's more
Another look at this critter resulted from further specimens, and the account was published
in 2006. It's somewhere in my files and I've not yet read it. If I remember rightly, it
goes into how the chewing processes would've worked. Doubtlessly, I'll get round to it
some time or other. Presumably.
Additional notes
Known from lower molars and an incomplete
dentary. This is perhaps the same as, or
closely related to, Aegialodon. A skull length of a couple of cm.
Kielan-Jaworowska et al, 2000 contains a brief summary on pages 599-600.
The trigonid of the three main cusps takes up more
of the molar crown than the talonid is permitted.
Its protoconid is the tallest cusp and the
metaconid the lowest. The basin of the talonid
is narrow, and its rim houses only two cusps rather than three. It appears the
entoconid failed to show up. |
| Reference: | Dashzeveg D (1975), New primitive therian from the Early
Cretaceous of Mongolia, Nature 256, p.402-403. |
| Genus: Mozomus Li C-K, Setoguchi T,
Wang Y-Q, Hu Y-M & Chang Z-L, 2005
Family: Mozomuridae Li et al, 2005
Remarks: Thanks for the tip are due to George G (who was right as usual), and Dino Hunter
for posting the citation. |
| Species: | Mozomus shikamai Li, Setoguchi, Wang, Hu &
Chang, 2005 |
| Place: | Badaohao, Heishan County, Liaoning |
| Country: | China |
| Age: | Aptian, Lower Cretaceous |
| Remarks: | Wang et al, 2006 provides some information
on page 198. This genus is from the same locality as the
'symmetrodont' named
Heishanlestes. In that paper, the
authors stated that the fauna included a relative of
Aegialodon. Further examination, however, showed that wasn't quite correct
and a new family was required.
Not Tribosphenida
The genus is based on a lower jaw preserving two
premolars and four molars, and it has most
similarity with Kielantherium of Mongolia. The
final premolar is submolarized. The molars are close to being tribosphenic, but
they don't manage to meet the full requirements. Talonids
are large but unbasined, and a wear facet known as 5 is lacking. Assuming that
corresponds to facet e of Kermack et al, 1965 (mention is made in the entry for
Aegialodon), then that facet -lacking here- requires the assistance of a
corresponding proper protocone. This absence suggests Mozomus is somewhat
less derived than originally thought.
If the paper were to arrive, that would be most welcome. |
| Reference: | Li et al (2005), The first record of "Eupantotherian;"
(Theria, Mammalia) from the Late Early Cretaceous of western Liaoning, China, Vertebrata
PalAsiatica, 43(4), p.245-255. |
|
| Species: | Tribactonodon bonfieldi Sigogneau-Russell D,
Hooker JJ & Ensom PC, 2001 |
| Place: | Durlston Bay, Dorset |
| Country: | England |
| Age: | Berriasian, Lower Cretaceous |
| Remarks: |
Based on one molar. To quote the abstract, "this tooth
shows characters which have an important bearing on the distinction between the two
infraclasses of Holotheria
(Australosphenida and
Boreosphenida) as defined by Luo et al. and which lead us to question the timing and
place of their emergence."
The placement of this genus within the scheme of things is unclear to me. It’s a
member of Crown-group Mammalia, and it
has to go somewhere. Common sense suggests it probably belongs near to or within
Boreosphenida. That's in line with a paper I've read but not yet added information
from. |
| Reference: | Sigogneau-Russell et al (2001), The oldest tribosphenic
mammal from Laurasia (Purbeck Limestone Group, Berriasian, Cretaceous, UK) and its bearing
on the 'dual origin' of Tribosphenida. Comptes rendus de l'Académie des sci, IIa, Sci de
la Terre et des planètes 333 (2), p.141-147. |
Boreosphenids B. Pappotheriidae
| Genus:
Pappotherium Slaughter, 1965 |
| Species: | Pappotherium pattersoni Slaughter, 1965 |
| Place: | Paluxy Formation, Texas |
| Country: | USA |
| Age: | Albian, Lower Cretaceous |
| Remarks: | There's also a possible
sister species. 99% sure = Slaughter BH. This taxon is
sometimes regarded as a eutherian. It’s based on
"upper molars, referred lower molars, and (possibly) a
referred distal lower premolar", (Luo et al 2002,
p.11).
Kemp, 2005 (p.173) mentions partial jaws as well. |
| Reference?: | Slaughter BH (1965), Postilla 93 |
Seven Phases of Teeth
(Postcanines)
VII Tribosphenic dentition
The following is derived from and inspired by my reading of Butler & Clements, 2001,
(p.11).
Tribosphenic teeth are rather like high tech; employed
appropriately and the results can be impressive, but they're not necessarily the best
available equipment. Some descendants of mammals with such
teeth, eg. anteaters, have both wisely dispensed with them. Nevertheless, if it makes sense
to talk in terms of evolutionary trends, occlusion, infrequent replacement and refinement
of teeth would all be of significance when considering mammalian and pre-mammalian
history.
With tribosphenic teeth, the protocone of the upper molar
occludes with a basined talonid on the lower. This
theoretically simple sounding concept greatly enhances their effectiveness. If you feel
like crushing grapes quickly and without wasting any more of the fruit or juice than can be
helped, this is best done in a basin, rather than on a flat cutting board. It also helps if
your crusher is positioned directly above the basin. This applies to crushing or grinding
all kinds stuff. Given the shortage of grapes during the Mesozoic, this is just as well.
It may account for the success of tribosphenic
boreosphenidans; over 99.9% of existing mammals. The other three genera seem to be
specialized australosphenidans
Tribosphenic postcanines are divided into two basic sorts of tooth. There are
premolars and molars "In
all extant mammalian clades (monotremes,
marsupials and eutherians)
tooth buds of the primary generation appear at about the same time as ossification centres
in the jaws," (p.11). Molars grow directly from the dental lamina, and get added on at
the back as the jaw length increases.
Young mammals have shorter jaws than older ones. Before the molars fully assume their
duties, chewing is the responsibility of the deciduous
teeth, which have more complex crowns than their replacements, the secondary premolars. For
example, the last deciduous tooth is usually shaped like a molar. The molars emerge at the
back of the line as the jaw grows. This has the effect of maintaining the most effective
grinders in the region where the maximum force is available; nearest the muscles.
In adults, the premolars are generally secondary teeth, whilst the molars are primary ones.
They're usually distinctive in structure too, but that's not always so; ungulates are not
alone in having molarized premolars. Compared to existing reptiles, mammal teeth develop
late. It follows hatching for platypussies, and starts in the suckling stage for marsupials. For
placentals, the deciduous teeth mainly develop before
birth, but we have extended gestation.
Go to Phase: I Carnivorous non-mammalian cynodonts,
II Basal mammals,
III Kuehneotheriids (basal Holotheria),
IV Cladotheria,
V Dryolestidae,
VI Amphitheriida and Zatheria,
VII Tribosphenic dentition. |
| Genus:
Slaughteria Butler PM, 1978
'for Slaughter' |
| Species: | Slaughteria eruptens Butler PM, 1978 |
| Place: | Paluxy Formation, Texas |
| Country: | USA |
| Age: | Albian, Lower Cretaceous |
| Remarks: | Much of the following is
based upon my reading of Kobayashi et al, 2002.
This is mainly a story of archaic, boreosphenidan
tooth replacement. That placental mammals naturally
replace their teeth only once during a lifetime is something people learn from personal
experience. However, it's easily forgotten that we don't replace them all. The action's
concentrated at the front, seeing as our molars are
permanent. This is in contrast to out non-mammalian ancestors, who weren't of
diphyodont habits. Our
marsupial cousins have gone even further with the retention of primary gnashers, in
that they only replace the third premolar, (p.369). In
this respect they could fairly claim to be more derived.
Still, we can both crow about old-timers like the Montana
Gobiconodon, which did have replacements
for molars.
Slaughteria is a more basal boreosphenidan than either
myself or a koala, and presumably outside of Theria. The
specimen discussed here is a piece of lower jaw with four teeth in place, (which are termed
t1-t4). A replacement is preparing for lift-off beneath the third in the series.
The High-Resolution x-ray gismo at the University of Texas allowed the discovery of the
unerupted premolar beneath the first molar-like tooth. It also showed: "a mental
foramen is present on the lateral surface of the ramus between t3 and t4", (p.370),
which wasn't previously apparent. This characteristic isn't found in more basal mammals.
It's worth mentioning that all the erupted teeth are double-rooted.
The t1 (not fully erupted) and t2 are premolariform, with the latter having both the larger
crown and roots. t3, (the one due for replacement), has a molariform crown. This is common
enough for deciduous premolars, which are more complex
than their replacements. The front root is clearly reduced, and seems to have been caught
in the process of resorption. t4 is the largest tooth present.
The first two teeth show no or little wear, while t3 is more worn than t4. In combination
with the pattern of the crowns, this suggests the first pair and the unerrupted tooth are
secondary premolars, the number of which is unknown for this genus. No therians from the
Lower Cretaceous of Texas with less than four are known. Furthermore, t1 is fairly large
and complex when compared to the p1 of metatherians,
(from which none are known which have more than three lower premolars). Therefore,
Slaughteria probably didn't possess a metatherian dental formula. There's no sign
of a replacement tooth for the t4, and that is likely the first permanent molar. The
position of the aforementioned mental foramen varies in therians, but it's found below the
m1 in two near-contemporary North American taxa,
(Montanalestes and
Kokopellia. Assuming four premolars to be
appropriate, the order of tooth eruption seems to have been: (dp3?) dp4-m1 / p3-p2-p4,
(p.371). That premolar sequence is known from Jurassic
dryolestids.
The size of the t4 (?=m1) fits within the range known for lower molars ascribed to
Pappotherium, (the entry above), and other distinguishments between these genera:
"are, at best, open to interpretation." This raises the possibility that S.
could be the juvenile form of P.
"Slaughteria is the closest sister taxon to the evolutionary divergence of modern
mammal clades for which the tooth-replacement pattern is known. It demonstrates that a
sequence of simple posterior premolars replacing molariform antecedents was present 110 Myr
ago... and provides a plausible model of tooth replacement from which both clades of
modern therian mammals could be derived", (p.372, references omitted). Should anyone
think that means Slaughteria is proposed as my ancestor, then please read more
carefully.
One of the more violent sounding zoological names. There's a second possible species. The
holotype is a partial jaw which was found by Bob Slaughter in 1967. It's known to its fans
as SMU 61992, and resides in the collection of the Southern Methodist University, Dallas. |
| Reference: | Butler (1978), A new interpretation of the mammalian teeth
of tribosphenic pattern from the Albian of Texas. Breviora 446, p.1-27. |
| Links:
SMU Researchers discover tooth replacement pattern in primitive mammal
http://www2.smu.edu/newsinfo/releases/01223.html
A Southern Methodist University press release announcing the research results, which were
published in early 2002. This includes a number of photos.
Proceedings of the Royal Society B, 269, p.369-373
Kobayashi, Winkler & Jacobs, 2002
The paper is presently on-line and freely accessible.
Shuler Museum of Paleontology
http://www.smu.edu/geology/shulerlab.htm
A short item on the work of the museum, part of the Southern Methodist University, Dallas.
List of Airline Nicknames
http://www.cuug.ab.ca:8001/~busew/airlines.html
Those of a nervous disposition might like to ignore this, especially if they’re about to
travel by plane. For some reason, Slaughteria is also a junior synonym of the airline
Saudia, though I don’t think it appears in their advertising. |
| 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, November 2002. Last updated: 27.12.2007
Ktdykes@arcor.de |
| With further thanks due to:
Mr Simon Clabby, for generously supplying a paper.
Dr Zhexi Luo, for the kind words, encouragement and papers.
The animations are courtesy of HitBox Central,
Animation Library and best animations.com.
???????????, Japan
http://epp.eps.nagoya-u.ac.jp/~seicoro/bio/mammalia.html
The organization of this page has been extensively assisted by this HUGE and elegant file.
John H Burkitt, Mammals, A World Listing of Living and Extinct Species
http://cougarhillweb.org/mammals.pdf
The Prehistoric Data Files
http://www.angellis.net/Web/PDfiles/marsups.pdf
Dr John Alroy, North American Fossil Mammal Systematics Database
http://www.nceas.ucsb.edu/~alroy/nafmsd.html
A source of citations, possible second species, etc.
BIOSIS, The Index to Organism Names
http://www.biosis.org.uk/triton/indexfm.htm
The Society of Vertebrate Paleontology BFV Online, (John Damuth)
http://www.bfvol.org/ |
Bibliography:
Averianov AO & Archibald JD (2003), Mammals from the Upper Cretaceous Aitym
Formation, Kyzylkum Desert, Uzbekistan. Cretaceous Research 00 (2003), p.1-21.
Butler & Clemens (2001), Dental morphology of the Jurassic holotherian mammal
Amphitherium, with a discussion of the evolution of mammalian post-canine dental
formulae. Paleontology, 44 (1), p.1-20.
Canudo JI & Cuenca-Bescós G (1996), Two new mammalian teeth (Multituberculata
and Peramura) from the Lower Cretaceous (Barremian) of Spain. Cretaceous Research 17,
p.215-228.
Cifelli RL (2001), Early Mammal Radiations, Journal of Paleontology, vol 75 (6),
p.1214-1226
Cox B, Dixon D, Gardiner B & Savage RJG (1989): Dinosaurier und andere Tiere der
Vorzeit, Mosaik Verlag (Sonderausgabe für Gondrom Verlag, 1994), ISBN 3 8112 1138 2.
Dashzeveg D (1975), New primitive therian from the Early Cretaceous of Mongolia,
Nature 256, p.402-403.
Dashzeveg D & Kielan-Jaworowska Z (1984), The lower jaw of an aegialodontid mammal
from the Early Cretaceous of Mongolia, Zoological Journal of the Linnean Society, 82,
p.217-227.
Hu YM, Fox RC, Wang YQ & Li CK (2005), A new spalacotheriid symmetrodont from the
Early Cretaceous of northeastern China, American Museum Novitates, 3475, p.1-20.
Kemp TS (2005), The Origin and Evolution of Mammals, Oxford University Press,
pp.331.
Kermack KA, Lees PM & Mussett F (1965), Aegialodon dawsoni, a new trituberculosectorial
tooth from the lower Wealden. Proceedings of the Roy. Soc., London, B, 162, p.535-554.
Kielan-Jaworowska Z, Hurum JH, & Badamgarav D (2003), An extended range of the
multituberculate Kryptobaatar and distribution of mammals in the Upper Cretaceous
of the Gobi Desert. Acta Palaeontologica Polonica 48(2), p.273-278.
Kielan-Jaworowska Z, Novacek MJ, Trofimov, BA & Dashzeveg D (2000), Mammals
from the Meozoic of Mongolia, p.573-626 in Benton MJ, Shishkin MA, Unwin AM
& Kurochkin EN (Eds.), The age of dinosaurs in Russian and Mongolia, Cambridge
University Press.
Kobayashi Y, Winkler DA & Jacobs LL (2002), Origin of tooth-replacement pattern
in therian mammals: evidence from a 110 Myr old fossil, Proceedings of the Royal Society B,
269, p.369-373.
Luo Z-X, Kielan-Jaworowska Z & Cifelli RL (2002), In quest for a phylogeny of
Mesozoic mammals. APP 47 (1), p.1-78.
McKenna MC & Bell SK (1997), Classification of Mammals Above the Species Level.
Columbia University Press.
Wang Y-Q, Hu Y-M & Li C-K (2006), Review of recent advances on study of
Mesozoic mammals in China, PalAsiatica, 44(2), p.193-204.
Wible JR, Rougier GW, Novacek MJ & McKenna MC (2001), Earliest Eutherian Ear
Region: A Petrosal Referred to Prokennalestes from the Early Cretaceous of Mongolia.
American Museum Novitates 3322, p.1-44.
|
