The geologically oldest Archaeopteryx (#12)

Not published yet in any academic journal,
but making the news in the popular press in Germany to promote a dinosaur museum (links below) is the geologically oldest Archaeopteryx specimen (no museum number, privately owned?). Found by a private collector in 2010, the specimen has been declared a Cultural Monument of National Significance. It is 153 million years old, several hundred thousand years older than the prior oldest Archaeopteryx. It is currently on  display at a new museum, Dinosaurier-Freiluftmuseum Altmühltal in Germany, about 10 kilometers from where the fossil was found.

Figure 1. The new oldest Archaeopteryx in situ with color tracings of bones.

Figure 1. The new oldest Archaeopteryx in situ with color tracings of bones. The ilium has been displaced to the posterior gastralia, or is absent. I cannot tell with this resolution.

So is it also the most primitive Archaeopteryx?
No. But it nests as the most primitive enantiorinithine bird. As we learned earlier, the Solnhofen birds formerly all considered members of the genus Archaeopteryx (some of been subsequently recognized by certain authors as distinct genera) include a variety of sizes, shapes and morphologies (Fig. 3) that lump and separate them on the large reptile tree. The present specimen has been tested, but will not be added to the LRT until it has a museum number or has been academically published (both seem unlikely given the private status).

The fossil is wonderfully complete and articulated
and brings the total number of Solnhofen birds to an even dozen.

Figure 2. Reconstruction of the geologically oldest Archaeopteryx, now nesting at the base of the Enantiornithes.

Figure 2. Reconstruction of the geologically oldest Archaeopteryx, now nesting at the base of the Enantiornithes. The cervicals may look odd and supernumerary. They likely are as most were overlain by the skull in situ. All bones are roughly traced and laid in. That’s a robust pelvis.

Compared to other Archaeopteryx specimens
you can see the new one is among the smallest (Fig. 3) and has a distinct anatomy.

Figure 3. The geologically oldest Archaeopteryx to scale alongside other Archaeopteryx specimens. The new one is among the smallest.

Figure 3. The geologically oldest Archaeopteryx to scale alongside other Archaeopteryx specimens. The new one is among the smallest.

 

References
Spektakulaerer-Fund-kommt-in-Ausstellung-article
originalskelett-eines-archaeopteryx-zu-sehen.html
auf-zum-archaeopteryx

Eomaia needs a makeover

With the recent nesting
of Eomaia (Ji et al. 2002) closer to Thylacinus (Fig. 1) than to eutherians (click here). Thylacinus is the longn-legged marsupial wolf AND it’s a basal marsupial with an unspecialized dentition. So maybe that slinky rat-like appearance originally given to Eomaia (Fig. 1) needs an update. I mean, look at those long legs! (Then again, the proximal sister, Didelphis, (Fig. 2) also has long legs, but you’d never know it the way it slinks around.

Maybe the carriage of Eomaia was a bit more upright,
like Thylacinus, despite the great size difference. The morphology was similar enough to nest the two together to the exclusion of all other 783 taxa. The metacarpals and metatarsals appear to trend toward digitigrade, as in so many marsupials, not flat-footed as originally reconstructed. The PILs align either way.

Let’s see what happens
when we let the bones and phylogenetic bracketing tell another tale.

Figure 1. Eomaia in situ, as originally reconstructed, as reconstructed here and compared to Thylacinus. This is what this blog and ReptileEvolution.com do best. Show sister taxa together to scale and not to scale to drive home their similarities with a strong visual impression. You can always ask for the data matrix later.

Figure 1. Eomaia in situ, as originally reconstructed, as reconstructed here and compared to Thylacinus. This is what this blog and ReptileEvolution.com really do best. Show sister taxa together to scale and not to scale to drive home their similarities with a strong visual impression. You can always ask for the data matrix later.

 

I note
long neural spines in Eomaia around the shoulders. The tibia and fibula appear to be able to be closely appressed, despite their disturbance post-mortem. The slender cervicals are unlike those of Didelphis (Fig.2). The lumbar region appears to be more supple, like Thylacinus, built for galloping.

Figure 2. Didelphis, the extant opossum, a slinky marsupial more primitive than Eomaia.

Figure 2. Didelphis, the extant opossum, a slinky marsupial more primitive than Eomaia.

I’d like to see original data
for the reflected process of the dentary on Eomaia. Sister taxa don’t have a ventral protrusion, but they do have a sharp little ascending curl of bone, and I don’t see it in the fossil.

References
Ji et al 2002. The earliest known eutherian mammal, Nature 416:816-822.m online here.

wiki/Eomaia

Eomaia and Monodelphis sort of switch places and wombats are added to the LRT

Today
I added Vombatus (Geoffroy 1803; 98 cm long; extant) and Thylacoleo (Owen 1859; 114 cm long; <2mya;  ) to the large reptile tree (now 783 taxa). Here (subset Fig. 1) they nested together as they do traditionally and share with Macropus protruding dentary incisors that are only incipient in Anebodon.

Revision to the earlier posted basal mammal cladogram. Here two wombats are added. Eomaia and Monodelphis switch places.

Here two wombats are added. Eomaia and Monodelphis switch places.

While rummaging around the marsupials…
I checked data for Monodelphis (Burnett 1830) based on newly found Digimorph.org photos. Previously I had relied on drawings. The differences in data got rid of several autapomorphies and shifted Monodelphis closer to traditional placentals and Eomaia (Ji et al. 2002) closer to Thylacinus, with which it apparently shares a digitigrade manus and pes. You might remember Monodelphis has no pouch. Eomaia was hailed as the basalmost eutherian. That I’ll have to look into further and will report when the dust has settled. I have not updated the online pages yet for these taxa.

Figure 2. Monodelphis skull in three views. Note the supra occipital is narrower than the exoccipitals, like other mammals, not like the data from the figure previously used.

Figure 2. Monodelphis skull in three views. Note the supra occipital is narrower than the exoccipitals, like other mammals, not like the data from the figure previously used.

References
Burnett GT 1830. Illustrations of the Quadrupeda, or Quadrupeds, being the arrangement of the true four-footed Beasts indicated in outline. Quarterly Journal of Science, Literature and Art, July to December, 1829, 336–353.
Ji et al 2002. The earliest known eutherian mammal, Nature 416:816-822.
Pine RH, Flores DA and Bauer K 2013. The second known specimen of Monodelphs unistriata (Wagner) (Mammalia: Didelphimorphia), with redescription of the species and phylogenetic analysis. Zootaxa3640 (3):425-441.

wiki/Monodelphis
wiki/Eomaia

Microsaurs in the Viséan and Middle Devonian footprints

Figure 1. Which came first? The tracks or the trackmakers? In this case the tracks came first, strong indications that the variety of Devonian trackmakers we have found were all commonplace in the Late Devonian. The variety of basal reptiles and microsaurs found in the Visean must also reflect a wide radiation of derived taxa, pointing to an earlier origin.

Figure 1. Which came first? The tracks or the trackmakers? In this case the tracks came first, strong indications that the variety of Devonian trackmakers we have found were all commonplace in the Late Devonian. The variety of basal reptiles and microsaurs found in the Visean must also reflect a wide radiation of derived taxa, pointing to an earlier origin.

The earliest known microsaur,
Kirktonecta milnerae (Clack 2011, UMZC 2002, Viséan, 330 mya), is not the basalmost microsaur, nor is it a basalmost lepospondyl, the parent clade. In the large reptile tree, Kirktonecta nests with Tuditanus, phylogenetically nesting much more recently than the Utegenia(Lepospondyl) /Silvanerpeton (stem-reptile) split.  That means what we have as taxa in the Visréan represents these taxa when they were commonplace, long after their origination and radiation.

On a related note,
the earliest known tetrapod trackways, the early Middle Devonian Zachelmie trackways, precede all known Devonian trackmakers in the Late Devonian. That means we no longer have to wait for the Late Devonian taxa to begin to evolve the earliest reptiles, but we can still use their morphologies. Now we can begin to evolve reptiles earlier, likely during the Tournasian, the first part of Romer’s Gap, a time for which there are (strangely) few to no fossils during the first 15 million years of the Carboniferous. This time succeeded a major extinction event, the Hangenberg event, in which most marine and freshwater groups became extinct or reduced, including the Ichthyostegalia. Evidently the places where these rare survivors were radiating are currently unknown in the fossil record. These survivors include basal temnospondyls and lepospondyls that also include basal microsaurs.

Fortunately,
the Ichthostegalia had already given rise to a wide range of stem-amphibians and stem-reptiles that ultimately produced all the post-Devonian tetrapods. Those Zachelmie trackways dated 10-18 million years earlier, give more time for reptilomorphs and reptiles to have their genesis and radiation. Post-extinction events traditionally produce new clades. So it appears to be with the genesis of the Reptilia (= Amniota).

The Early Devonian
is where we find Meemannia eos, an early ray-finned fish that was originally classified an early lobe-finned fish. So it didn’t take long after the origin of such fish to develop fingers and toes and move onto land.

This just in:
Recent work by Sallan and Galimberti 2015 showed that only small fish survived the Devonian / Carboniferous extinction event. Read more here. And a paper on Late Devonian catastrophes, impacts and glaciation here.

References
Clack JA 2011. A new microsaur from the early Carboniferous (Viséan) of East Kirkton, Scotland, showing soft tissue evidence. Special Papers in Palaeontology. 86:1–11.

Sallan L and Galimberti AK 2015. Body-size reduction in vertebrates following the end-Devonian mass extinction. Science, 2015; 350 (6262): 812 DOI: 10.1126/science.aac7373

Anebodon: another symmetrodont or kangaroo ancestor?

Updated Sept 23, 2016 with a new information on the septomaxilla in marsupials and a new basal mammal cladogram (Fig. 2).

Bi et al. 2016
reported on “Anebodon luoi (STM 38-4, Tianyu Museum of Nature, Shandong Province, China, Fig. 1) a new genus and species of zhangheotheriid symmetrodont mammal from the Lujiatun site of the Lower Cretaceous Yixian Formation, China. The fossil is represented by an associated partial skull and dentaries with a nearly complete dentition.”

Figure 1. Anebodon luoi subjected to DGS tracing and phylogenetic analysis nests with Macropus, the extant kangaroo, not with Zhangheotherium.

Figure 1. Anebodon luoi subjected to DGS tracing and phylogenetic analysis nests with Macropus, the extant kangaroo, not with Zhangheotherium. The kangaroo kink is just starting here with a concave/convex maxilla. The canines are present, but tiny. The anterior dentary teeth extend anteriorly, the first step toward the kangaroo’s ‘tusks’.

Bi et al. noted
Anebodon lacked the high molar count typical of derived symmetrodonts. Their diagnosis focused on dental differences compared to Maotherium and Kiyatherium.

By contrast
the large reptile tree (LRT) nested Early Cretaceous Anebodon with the extant kangaroo, Macropus (Fig. 1). A septomaxilla was identified in Anebodon and this cause me to reevaluate the septomaxilla situation. Apparently in continues in basal marsupials, but is lost several times in derived taxa. A septomaxilla has been figured for Vincelestes, but I cannot confirm a septomaxilla due to damage in the rostrum. Phylogenetic bracketing argues against it.

Revision to the earlier posted basal mammal cladogram. Here two wombats are added. Eomaia and Monodelphis switch places.

Figure 2. Revision to the earlier posted basal mammal cladogram. Here two wombats are added. Eomaia and Monodelphis switch places.

With this in mind, 
it’s worthwhile to look at intervening basal kangaroos (Fig. 3).

Figure 3. Anebodon, Nambaroo, Dendrolagus, Macropus, kangaroo ancestors. The purported 'wolf-like' fangs are not apparent here. Were they referring to the long dentary incisors? After phylogenetic analysis Nambaroo nested basal to Plesiadapis and rabbits.

Figure 3. Anebodon, Nambaroo, Dendrolagus, Macropus, kangaroo ancestors. The purported ‘wolf-like’ fangs are not apparent here. Were they referring to the long dentary incisors? After phylogenetic analysis Nambaroo nested basal to Plesiadapis and rabbits.

Nambaroo gillespieae (N. tarrinyeri Flannery & Rich, 1986; Kear et al. 2007; Late Oligocene, 25 mya) was reported to be the “granddaddy of kangaroos” and to have had fangs, ‘probably for display,’ and mostly ate soft food such as fruit and fungi (Kear 2007). It was found in Australia.

One wonders
if the fangs they refer to are indeed the lower incisors (Fig. 4), because I see no canine fangs here. And with that short rostrum, one wonders if a soft tissue tapir-like trunk extended its length.

One also wonders
whether Nambaroo is indeed a kangaroo at all given the short rostrum and comb-like surface of premolar 3. The jugal does not reach the jaw joint, as in other marsupials including kangaroos. Not sure why, but in their diagram of the skull and mandible with measurements Kear et al. reduced the scale of the mandible so the rostrum did not look so short relative to the dentary. Here (Fig. 4) that scaling problem is eliminated.

Figure 3. Nambaroo skull in several views. Colors added using DGS methods. Dentary scaled to skull.

Figure 4. Nambaroo skull in several views. Colors added using DGS methods. Dentary scaled to skull and the molars occlude. Using this data, Nambaroo nested with rabbits and Plesiadapis.

During the course of this writing
I added Nambaroo to the large reptile tree and it nested far from kangaroos, at the base of Plesiadapis + rabbits, close to the base of rodents + multituberculates. It’s not the first time paleontologists, including yours truly, have gotten things wrong, That’s why we need independent testing with larger taxon lists.

Figure 6. Nambaroo mandible. Note premolar 3 and its striking resemblance to the same tooth in multituberculates.

Figure 5. Nambaroo mandible. Note premolar 3 and its striking resemblance to the same tooth in multituberculates. Kangaroos don’t have such a tooth.

Figure 8. Nambaroo nests at the base of Plesiadapis + rabbits in the LRT.

Figure 8. Nambaroo nests at the base of Plesiadapis + rabbits in the LRT. See figure 2 for updated cladogram of basal mammals. 

Dendrolagus ursinus (Müller 1840, extant; 50 cm long + 60 cm tail) is the tree-kangaroo. More at home in trees than on the ground, Note the skull is very similar to that of the larger gray kangaroo and makes a good transitional taxon from Anebodon to Macropus.

Macropus giganteus (Shaw 1790, extant) is the eastern gray kangaroo. The forelimbs were elongated. The hind limbs even more so. The pedal digits were reduced, all but the second digit, which was robust. Kangaroos hop bipedally and rest tripodally with the tail. The canines were absent and a diastema separated the incisors from the molars, convergent with rodents and multituberculates. The lower incisors were elongated and procumbent.

The clade
Symmetrodonta is considered paraphyletic at Wikipedia. The clade is based on teeth characterized by the triangular aspect of the molars when viewed from above and the absence of a well-developed talonid. Perhaps such teeth were common to basal placentals.

When I first started
ReptileEvolution.com, about five years ago, I never thought I’d find so many new nestings for published taxa. It’s been an interesting run, but I really think I’m about out of taxa to examine (up to 780 at present for the LRT, plus 59 for therapsids and 229 for pterosaurs). Let me know of any enigmas that need testing.

References
Bi S-D, heng X-T, Meng J, Wang X-L, Robinson N and Davis B 2016. A new symmetrodont mammal (Trechnotheria: Zhangheotheriidae) from the Early Cretaceous of China and trechnotherian character evolution. Nature Scientific Reports 6:26668 DOI: 10.1038/srep26668
Kear BP, Cooke BN, Archer M and Flannery TF 2007. Implications of a new species of the Oligo-Miocene kangaroo (Marsupialia: Macropodoidea) Nambaroo, from the Riversleigh World Heritage Area, Queensland, Australia, in Journal of Paleontology 81:1147-1167.
Shaw G 1790. Macropus giganteus. Nat. Miscell. plate 33 and text. kangaroo online pdf

wiki/Nambaroo
wiki/Tree-kangaroo
wiki/Kangaroo

Zhangheotherium: a pangolin ancestor

Today we’ll look at
Zhangheotherium quinquecuspidens (Hu et al. 2009; Late Jurassic/Early Creteacous; dentary length 3 cm; IVPP V7466; Fig. 1). It was originally described as a symmetrodont mammal, an ‘archaic’ taxon typically represented by only tooth and dentary scraps. Here (Fig. 1) a complete skeleton provided new insight to the original authors. They reported Zhangheotherium did not travel in a parasagittal posture and the cochlea (an organ of the inner ear) was not fully coiled.

Figure 1. Zhangheotherium reconstructed. The tail is unknown. The high scapulae indicate great strength in the pectoral region, likely for arboreal locomotion in a taxon of this size. Zhangheotherium nests as a basal pangolin. It was preserved in ventral view. Here the epipubes are identified as pubes, which is otherwise not shown.

Figure 1. Zhangheotherium reconstructed. The tail is unknown. The high scapulae indicate great strength in the pectoral region, likely for arboreal locomotion in a taxon of this size. Zhangheotherium nests as a basal pangolin. It was preserved in ventral view. Here the epipubes are identified as pubes, which is otherwise not shown.

Keep in mind, for the moment,
that neither bats nor pangolins travel in a parasagittal posture. Pangolins are bipedal (video here). Bats are inverted bipeds. Both fold their fingers posteriorly when not using them. That’s why the forelimbs are lifted here (Fig.1) even though neither Zhangheotherium nor Manis would be reconstructed as a biped if not known from in vivo behavior.

Figure 2. Hu et al. nested Zhangheotherium basal to the Placental/Marsupial split, contra the results of the large reptile tree.

Figure 2. Hu et al. nested Zhangheotherium basal to the Placental/Marsupial split, contra the results of the large reptile tree.

Unfortunately, 
Hu et al. thought Zhangheotherium radiated before the divergence of living marsupials and placentals. Here, in the large reptile tree (LRT) Zhangheotherium nests at the base of the Ernanodon + pangolins clade, close to Chriacus and the bats and not far from the dermopterans. Clearly this was originally or principally an arboreal clade. Ernanodon is the exception that got big as it left the trees for the ground, something other pangolins did, too.

Figure 3. Subset of the large reptile tree showing the nesting Zhangheotherium basal to pangolins.

Figure 3. Subset of the large reptile tree showing the nesting Zhangheotherium basal to pangolins.

Hu et al note:
“A mobile clavicle–interclavicle joint that allows a wide range of movement of the forelimb has an ancient origin in the mammalian phylogeny.” This is quite visible in the fossil and interesting with regard to Zhangheotherium’s relation to bat ancestors. Bats and Didelphis likewise have a large floating pectoral girdle. Pangolins have a large scapula more closely associated with the rib cage.

In Zhangheotherium, Hu et all note [my remarks follow in brackets]:

  1. The cervical ribs were unfused.[have not gotten close enough to pangolins to check this out]
  2. The caudal transverse processes were wide [as in pangolins].
  3. Three or four sacrals were present [suggesting stress in this area, perhaps for balance].
  4. The pisiform is very large [as in Ptilocercus and Manis].
  5. Only the dorsal acetabulum is preserved [open ventrally as in pangolins]
  6. Zhangheotherium has an external pedal spur, as in Ornithorhynchus [not sure about this disarticulated bone, perhaps not a spur, but a simple spindle-shaped ankle bone similar to one seen in Manis, see Fig. 4]
  7. The interclavicle is triangular and the sternal manubria are only three in number. [Could not find com parables here]
  8. It is more primitive than Henkelotherium and Vincelestes in retaining the interclavicle in its pectoral girdle/sternal manubrium [no comparables found]
  9. These new data suggest that the mobility of the clavicle and scapula has a more ancient origin than the more parasagittal posture of the forelimbs [or… this type of arboreal locomotion loosens the girdles]
  10. The mobile and pivotal clavicle evolved before the divergence of multituberculates and therians. [in the LRT multituberculates are therians and placentals, and so is Zhangheotherium].
Figure 4. The pes of Zhangheotherium with spine in orange. The same bone shrinks in Cryptomanis and further in Manis.

Figure 4. The pes of Zhangheotherium with identified spine in orange. The same bone shrinks in Cryptomanis and further in Manis.

Hu et al. report, “It has been argued that dental characters are as homoplasic as non-dental characters and the reliability of dental characters for inferring the relationships of major lineages of mammals has been questioned. Zhangheotherium has provided more extensive basicranial and postcranial evidence to corroborate the traditional hypothesis that symmetrodonts represent a part of the basal therian radiation.” [Zhangheotherium nests in a basal placental position in the LRT.

Figure 4. Henkelotherium in situ and colorized using DGS and Photoshop.

Figure 5. Henkelotherium in situ and colorized using DGS and Photoshop. Note how tiny it is.

 

Hu et al. link
Zhangheotherium to Henkelotherium (Krebs 1991; Late Jurassic, Kimmeridgian; Figs. 5, 6). So let’s look at Henkelotherium while we’re here.

Figure 6. Henkelotherium reconstructed using DGS methods and Photoshop. The epipubes may in fact be displaced pubes. Note the shrinking canine here. Later taxa don't have it. Note the larger incisors here. We don't have the premaxilla, but the anterior dentary incisors are larger, as in later taxa. 

Figure 6. Henkelotherium reconstructed using DGS methods and Photoshop. The epipubes may in fact be displaced pubes. Note the shrinking canine here. Later taxa don’t have it. Note the larger incisors here. We don’t have the premaxilla, but the anterior dentary incisors are larger, as in later taxa.

In the large reptile tree
Henkelotherium and Zhangheotherium do not nest together. Henkelotherium nests between Asioryctes and the Tupaia clade, not far from Plesiadapis. Wikipedia considers Henkelotherium a paurodontid dryolestid (formerly considered eupantotheres) and similar in locomotion patterns to tree shrews and opossums. Key to Henkelotherium are the enlarged dentary incisors (premaxilla remains unknown). This represents the first step toward the larger incisors found in plesiadapiformes, Tupaia-like tree shrews, apatemyids and rodents + multituberculates.

Back to Zhangheotherium, you’ll note
the dentary condylar process curves dorsally and no post-dentary bones are present (all had become middle ear bones enclosed within the petrosal). That dorsal curve removes most of the ability to resist jaw dislocation often caused by struggling large prey and or small pieces of even larger prey are working against large canines, which were also not present in Zhangheotherium. These traits point to a tiny prey diet, likely of insects, just like pangolins.

References
Hu Y-M, Wang YQ, Luo Z and Li CK 1997. A new symmetrodont mammal from China and its implications for mammalian evolution. Nature 390:137-142.
Krebs B 1991. Skelett von Henkelotherium guimarotae gen. et sp. nov. (Eupantotheria, Mammalia) aus dem Oberen Jura von Portugal. Berl Geowiss Abh A.: 133:1–110.

wiki/Zhangheotherium

Teeth, more teeth and toothlessness in basal mammals and whales

The traditional labeling of mammal teeth
may need to be revised based on the primitive condition found in Monodelphis (Fig. 1) and the need to homologize every derived taxon tooth with those more primitive counterparts in Monodelphis. Currently we don’t do that with teeth.

We already do (or should do) this with mammal phalanges
which are missing the m3.2, m4.2 and m4.3 phalanges found in basal therapsids and pelycosaurus. Thus, in the human manual digit 4 you have the homologous phalanges m4.1, m4.4 and m4.5, the ungual.

We also do this with tetrapod digits
which came in handy with the theropod Limusaurus, which redeveloped the 0 digit medial to digits 1-3. That digit was last seen in basal tetrapods like Acanthostega, which have more than five manual digits, one or more extra medially. Traditional paleontologists mislabeled digit 0 as digit 1 in Limusaurus, and that caused a flurry of papers about “phase-shifting” listed here.

You can’t simply count the teeth from front to back
when teeth are sometimes added between the canine and premolars (Fig.1), but this is what traditional paleontologists too often do (Fig. 1 in gray).

Figure 1. The addition of teeth in Kuehneosaurus and Akidolestes led to the loss of teeth in Ornithorhynchus.

Figure 1. The addition of teeth in Kuehneosaurus and Akidolestes led to the loss of teeth in Ornithorhynchus. Here the basal tooth pattern is shown by Monodelphis. The addition of teeth distally labels teeth in a negative direction. More teeth proximally increases the tooth number.

Like digits
I propose that we label extra mammal teeth (based on the tooth pattern in the basalmost mammal, Monodelphis) on the pattern shown here (Fig. 1). Since we already label tooth numbers from front to back (distally to proximally) additional distal teeth should be labeled negatively: 0, -1, -2, like a thermometer. Additional proximal teeth should continue to be labeled with higher numbers. You can actually see the homologies in size and shape when you follow this new paradigm. But those sizes and shapes are lost on traditional paleontologists who simply number the teeth as they appear behind or in front of the canine without regard to novel tooth eruptions.

This new system becomes necessary
in only a few mammal clades. Most mammals have fewer teeth than those found in Monodelphys. However, in Kuehneotherium (Late Triassic) and Akidolestes, both basal to the living toothless (as an adult) Ornithorhynchus, three new small premolars erupt between the canine and the traditional larger premolars (Fig. 1). One new molar appears to erupt between the premolars and the molars while yet another erupts behind (proximal to) the molar row.

The sperm whale question
Physeter macrocephalus, the extant sperm whale, has no upper teeth and 27 completely identical lower teeth. How does one identify them? Is it even necessary?

Figure 2. A first guess at the identify of sperm whale teeth based on the premaxilla/maxilla suture for placement of the canine. The other teeth are guesses based on patterns in more primitive whales.

Figure 2. A first guess at the identify of sperm whale teeth based on the premaxilla/maxilla suture for placement of the canine. The other teeth are guesses based on patterns in more primitive whales.

If we follow the patterns
of other mammals and other whales, we can reduce the amount of guesswork applied to the sperm whale tooth question. We can place the dentary canine below the premaxilla/maxilla suture, where the upper canine would have been. That means six incisors precede the canine. Based on their angle to the tip of the jaws, it appears that incisors 1- 2 are absent, so the remaining incisors are 3-8. The teeth posterior to the canine cannot be divided by shape into premolars and molars. So, here (Fig. 2) you may  retain the primitive number of premolar teeth likely present in Leviathan, and imagine that the remaining molars erupted posteriorly as the dentary elongated. It is also possible that additional simple-cone-shaped premolars and molars developed anew between the division between the two tooth types, or a long series of premolars developed behind the canine. Since new teeth are typically small teeth, I presume they erupt at the back where they are not critical and can enter the tooth row gradually and over the generations become critical.

What do you think?