A new look at Jidapterus (basal azhdarchid pterosaur)

Wu, Zhou and Andres 2017
bring us long anticipated details on Jidapterus (Early Cretaceous, Dong, Sun and Wu 2003) which was previously presented as a small in situ photograph lacking details. Even so a reconstruction could be made (Fig. 1). Coeval larger tracks (Elgin and Frey 2011) have been matched to that reconstruction.

Figure 2. Jidapterus matched to the Gansu, Early Cretaceous pterosaur tracks. The trackmaker was one-third larger than the Jidapterus skeleton.

Figure 1. Jidapterus matched to the Gansu, Early Cretaceous pterosaur tracks. The trackmaker was one-third larger than the Jidapterus skeleton.

Of interest today
is the fact that Jidapterus was originally and, so far, universally considered toothless. Its specific name, J. edentatus, refers to that condition. Wu, Zhou and Andres 2017 produced tracings (Figs. 2, 3) of the rostrum that are also toothless. However, they are crude and appear to miss the premaxilla and maxilla sutures, the palatal elements… and maybe some teeth. Those jaw rims are not slippery smooth like those of Pteranodon. Outgroups in the large pterosaur tree (LPT), all have tiny teeth.

Figure 2. Rostrum of Jidapterus (RCPS-030366CY) and traced according to Wu et al. and colorized using DGS to reveal skull sutures and possible teeth.

Figure 2. Rostrum of Jidapterus (RCPS-030366CY) and traced according to Wu et al. and colorized using DGS to reveal skull sutures and possible teeth. See figure 3 for details. What Wu, Zhou and Andres label the  “low ridge of rostrum” is here identified as the rostral margin above the palatal portion. 

The cladogram of Wu, Zhou and Andres
lacks dozens of key taxa found in the LPT that separate azhdarchids from convergent tapejarids and shenzhoupterids. In the LPT giant azhdarchids arise from tiny toothy azhdarchids once considered Pterodactylus specimens… and these, in turn are derived from tiny and mid-sized dorygnathids in the Middle Jurassic.

What Wu, Zhou and Andres label the  “low ridge of rostrum”
is here identified as the rostral margin rim at the edge of the palate.

Figure 3. Focus on the rostral tip of Jidapterus shown in figure 2. Are these teeth?

Figure 3. Focus on the rostral tip of Jidapterus shown in figure 2. Are these teeth? You decide. I present the data. 

As in all pterosaurs
each premaxilla of Jidapterus has four teeth according to this data.

Are these tiny teeth?
Or are they tiny occlusions and/or chisel marks. Let’s get even better closeups to figure this out. Phylogenetic bracketing indicates either tiny teeth or edentulous jaws could be present here.

References
Dong Z, Sun Y and Wu S 2003. On a new pterosaur from the Lower Cretaceous of Chaoyang Basin, Western Liaoning, China. Global Geology 22(1): 1-7.
Elgin and Frey 2011. A new azhdarchoid pterosaur from the Cenomian (Late Cretaceous) of Lebanon. Swiss Journal of Geoscience. DOI 10.1007/s00015-011-0081-1
Wu W-H, Zhou C-F and Andres B 2017. The toothless pterosaur Jidapterus edentus (Pterodactyloidea: Azhdarchoidea) from the Early Cretaceous Jehol Biota and its paleoecological implications. PLoS ONE 12(9): e0185486.

wiki/Jidapterus

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There’s nothing special about Henosferus

The incisors are not too big
or weird or crowded (Fig. 1), the canine just rises above the rest of the teeth, there are only 5 premolars all standard-shaped, and only three molars, all standard-shaped. The dentary definitely formed the main jaw joint and the post-dentary bones must have been tiny.

Figure 1. Henosferus mandible restored by Rougier et al. 2005 from several broken specimens.

Figure 1. Henosferus mandible restored by Rougier et al. 2005 from several broken specimens.

…and that’s why
Henosferus ( Rougier et al. 2007; Middle Jurassic) makes a good candidate for basalmost mammal. There are too few traits here to add it to the large reptile tree (LRT). Frankly, I’m eyeballing this restoration. It compares well with Juramaia (Fig. 2) without the odd molars and incisors. 

Figure 2. Juramaia (Late Jurassic, 160 mya) is more completely known and nests between monotremes and therians (marsupials + placentals).

Figure 2. Juramaia (Late Jurassic, 160 mya) is more completely known and nests between monotremes and therians (marsupials + placentals).

Henosferus is traditionally considered
a member of the Australosphenida, a group of mammals that include monotremes, and other taxa known chiefly from scraps. Vincelestes sometimes makes this list, but in the LRT it nests as a carnivorous marsupial.

References
Luo Z-X, Yuan C-X, Men Q-J and JiQ 2011. A Jurassic eutherian mammal and divergence of marsupials and placentals. Nature 476: 442–445. doi:10.1038/nature10291.
Rougier, GW, Martinelli AG, Forasiepi AM and Novacek M J 2007. New Jurassic mammals from Patagonia, Argentina : a reappraisal of australosphenidan morphology and interrelationships. American Museum novitates, no. 3566. online here.

wiki/Juramaia
wiki/Henosferus

New Como Bluff (Latest Jurassic) pterosaurs

Bits and pieces
of new Latest Jurassic pterosaurs are coming out of aquatic deposits in western North America according to McLain and Bakker 2017. The material is 3D and not very mineralized, so it is extremely fragile.

Specimen(s) #1 – HMNS/BB 5027, 5028 and 5029
“One proximal and two distal femora match a complete femur (BYU 17214) referred to Mesadactylus. Unexpectedly, both of the BBF distal femora possess a large intercondylar pneumatopore. BYU 17214 also possesses an intercondylar pneumatopore, but it is smaller than in the BBF femora. Distal femoral pnuematicity is previously recognized only in Cretaceous azhdarchoids and pteranodontids.”

The Mesadactylus holotype and referred specimens reconstructed to match the flightless pterosaur, Sos2428.

Figure 1. The Mesadactylus holotype (Jensen and Padian 1989) nests with the North American anurognathids. Several referred specimens (Smith et al. 2004), when reconstructed nest at the base of the azhdarchidae, with Huanhepterus and the flightless pterosaur SOS 2428.  The new BYU 17214 femur is essentially identical to the femur shown here.

Earlier we looked at two specimens referred to Mesadactylus. One is an anurognathid (Fig. 1). The other is a basal azhadarchid close to Huanhepterus, not far removed from its Dorygnathus ancestors in the large pterosaur tree. Instead McLain and Bakker compare the femora with unrelated and Early Cretaceous Dsungaripterus, which convergently has a similar femur. The better match is to the basal azhdarchid, so distal femoral pneumaticity does not stray outside of this clade. By the way, it is possible that Mesadactylus was flightless.

Specimen(s) #2 – HMNS/BB 5032 (formerly JHU Paleon C Pt 5)
“A peculiar BBF jaw fragment shows strongly labiolingually compressed, incurved crowns with their upper half bent backwards; associated are anterior fangs. We suspect this specimen is a previously undiagnosed pterosaur.”

These toothy specimens were compared to two Early Cretaceous ornithocheirids, one Middle Jurassic dorygnathid, and one Latest Jurassic bird, Archaeopteryx. None are a good match. A better, but not perfect,match can be made to the Early Jurassic pre-ctenochasmatid, Angustinaripterus (Fig. 2) which has relatively larger posterior teeth than does any Dorygnathus specimen.

The HMNS BB 5032 specimen(s) probably belong to a new species of Angustinaripterus or its kin based on the relatively large posterior teeth not seen among most Dorygnathus specimens.

The HMNS BB 5032 specimen(s) probably belong to a new species of Angustinaripterus or its kin based on the relatively large posterior teeth not seen among most Dorygnathus specimens.

As before,
we paleontologists don’t always have to go to our ‘go to’ taxon list of familiar fossils. Expand your horizons and take a fresh look at some of the less famous taxa to make your comparisons. You’ll find a good place to start at ReptileEvolution.com

References
McLain MA and RT Bakker 2017. Pterosaur material from the uppermost Jurassic of the uppermost Morrison Formation, Breakfast Bench Facies, Como Bluff,
Wyoming, including a pterosaur with pneumatized femora.

Ornithischian incubation longer and relatively longer than bird incubation

A new paper by Erickson et al. 2017 reports:
“Birds stand out from other egg-laying amniotes by producing relatively small numbers of large eggs with very short incubation periods (average 11–85 d). Here, nonavian dinosaurian incubation periods in both small and large ornithischian taxa (Protoceratops and Hypacrosaurus) are empirically determined through growth-line counts in embryonic teeth. Our results show unexpectedly slow incubation (2.8 and 5.8 mo) like those of outgroup reptiles.”

Now let’s do the math:
2.8 mo @ 30 days/month = 84 d. Hey! That’s one less than the upper limit in brds! 5.8 mo = 178 days (a few 31 day months added). Actual figures are 83 d for Protoceratops. 172 d for the much larger Hypacrosaurus. At this point, let’s remind ourselves that larger mammals have larger gestation/incubation times, too. And it’s also important to note that no theropod eggs were tested. Oviraptor embryos have no teeth.

Now let’s see some details
Comparison of Protoceratops incubation period relative to that typical for birds with same-sized eggs shows greater than twofold slower values (83.16 vs. 39.72 d). Relative to that typical for reptiles Protoceratops was modestly faster values (∼17%, 83.16 vs. 100.40 d) than predicted for typical reptiles.

Comparison of Hypacrosaurus incubation period relative to that typical for birds with same-sized eggs shows greater than twofold slower values (171.47 vs. 81.54 d). Relative to that typical for reptiles Hypacrosaurus was modestly faster values (∼12%, 171.47 vs. 153.72 d) than predicted for typical reptiles.

Phylogenetically 
all phytodinosaurs, including Ornithischia, are about as distant from birds as are the crocs, which are also proximal outgroups to the Theropoda in the LRT, contra many other studies that nest crocs much more distantly.

Figure 1 Full chart from Erickson et al. 2017. See figure 2 for details.

Figure 1 Full chart from Erickson et al. 2017. See figure 2 for details.

If you’re curious
The ostrich (Struthio) egg is not listed in the Erickson chart. Ostrich eggs are the largest of all birds, but the smallest bird eggs in relation to the adult bird’s size. Their incubation range is well within the Ercikson bird cloud.

Figure 2. Closeup of figure 1. In both cases Struthio was added.

Figure 2. Closeup of figure 1. In both cases Struthio was added. Two ornithischian dinos are shown. No theropods are shown. Cd – crocs.

The slowest incubation period among birds,
is among the Procellariformes, a clade of seabirds including the albatrosses and petrels. Not the chart above it’s blue and labeled Pr. See how closely it comes to the Protoceratops icon?

Oddly, turtles
have a relatively faster incubation time than do lizards and crocs.

Apparently no data yet on theropod dinosaur embryo teeth.
I’m sure that’s where it gets even more interesting (i.e. closer to birds).

References
Erickson GM, Zelenitsky DK, Kay DI, and Norell MA 2017. Dinosaur incubation periods directly determined from growth-line counts in embryonic teeth show reptilian-grade development. Proceedings of the National Academy of Sciences (advance online publication).doi: 10.1073/pnas.1613716114  PDF

All toothless whales are baleen whales

Updated November 20, 2016 based on input from Dr. RW Boessenecker who corrected a mistake I made reconstructing Tokarahia by noting the mandible was axially rotated in situ 180º. Correcting that error produces the bowed out Gothic arch set of mandibles, typical of mysticetes. 

Preamble
Several of the mistakes discussed below are based on the long-standing tradition that whales are monophyletic. That is, odontocetes (toothed whales) and mysticetes (baleen whales) were long thought to have a common ancestor with flukes and fins. That paradigm  was overturned by the large reptile tree (LRT) earlier here. Workers thought that common ancestor would be an archaeocete, but, so far, all tested archaeocetes nest basal to extant odontocetes and were derived from tenrecs in the LRT.

Almost fifty years ago whale monophyly was questioned
by Van Valen (1968), who listed a number of traits that distinguish Odontoceti from Mysticeti. Unfortunately this was before computer-assisted phylogenetic analysis and neither desmostylians nor tenrecs were offered as basal taxa with legs. This was also long before any whales with legs had been discovered. Gotta give Van Valen credit for his insight way back then.

Yesterday we looked at the desmostylian ancestors of today’s mysticete (baleen) whales. Less than 24 hours ago I encountered for the first time Aetiocetus (Emlong 1966; Figs. 1, 2). I learned it has long been considered a toothed basal mysticete. Evidently some of the back teeth are leaf-shaped and all of the teeth are small and widely spaced. Most whale workers are happy with this hypothesis or relationships, but the LRT finds otherwise based on an expanded taxon list.

Figure 1. Aetiocetus skull in several views.

Figure 1. Aetiocetus skull in several views. Most whale workers today consider this taxon close to the origin  of baleen whales. The transversely crested cranium is a trait found in living odontocetes, not mysticetes.

I added Aetiocetus
to the large reptile tree and, while given the opportunity to nest with mysticetes, Aetiocetus nested instead between Zygorhiza and Orcinus + Physeter, all members of the Odontoceti. As recently as 2015, Ekdale et al. (Fig. 2) were trying to use Aetiocetus to explain the origin of baleen in modern whales.

Figure 2. Palate and teeth of the odontocete Aetiocetus alongside palates of juvenile gray whale and embryo fin whale, members of the Mysticeti. Aetiocetus was long thought to be a basal mysticete.

Figure 2. Palate and teeth of the odontocete Aetiocetus alongside palates of juvenile gray whale and embryo fin whale, members of the Mysticeti. Aetiocetus was long thought to be a basal mysticete.

18 hours ago I encountered another mysticete,
Tokarahia kauaeroa (Boessenecker and Fordyce 2015; Late Oligocene; OU 2235), which has no teeth. Their reconstruction (Fig. 2) is

Figure 2. Tokarahia, a toothless odontocete long thought to be a basal mysticete. Original interpretation of materials is presented alongside a new interpretation, closer to the bones in situ. See figure 4.

Figure 3. Tokarahia, a toothless odontocete long thought to be a basal mysticete. Original interpretation of materials is presented alongside a new interpretation, closer to the bones in situ. See figure 4. The humerus is rotated so the ball joint fits into the ventral socket of the scapula.

Unfortunately
I thought Boessenecker and Fordyce changed the curve of the mandible in their reconstruction and did not follow the very narrow mandibles in restoring the largely missing or buried rostrum. They also moved the orbit anteriorly. I attempted a mistaken correction (Fig. 3) that was based on a narrow mandible interpretation. Dr Boessenecker reported the mandible was axially rotated in situ (Fig. 4). Those corrections was applied shortly thereafter. A good lesson in keeping an open mind.

Figure 1. Tokarahia in situ and as originally reconstructed (on right). Flipping the right mandible and reconstructing the skull anew (at left).

Figure 4. Tokarahia in situ and as originally reconstructed (on right). Flipping the right mandible and reconstructing the skull anew (at left).

12 hours ago
I also learned about Isanacetus laticephalus (Kimura and Ozawa 2002; early Miocene, 18 mya; MFM 28501; Fig. 5, 6). This fossil whale is indeed a mysticete. In the LRT it nests between the desmostylian Behemotops (presumably with at least front legs) and extant baleen whales.

Figure 5. Isanacetus skull in several views. I also present skull tracings in DGS that differ in some respects from the published drawings.

Figure 5. Isanacetus skull in several views. I also present skull tracings in DGS that differ in some respects from the published drawings. Isanacetus is a basal mysticete, derived from a sister to Behemotops.

Like all mysticetes and derived desmostylians
Isanacetus has a ventrally concave rostrum, a wide flat skull and other traits that distinguish mysticetes from odontocetes + tenrecs. Isanacetus is also one of the smallest known mysticetes, about twice the size of its current desmostylian sister, Behemotops (Fig. 6) and half again longer in the skull than the more completely known Desmostylus.

Figure 6. Isanacetus compared to sisters recovered in the LRT. Balaeonoptera is much reduced.

Figure 6. Isanacetus compared to sisters recovered in the LRT. Balaeonoptera is much reduced. The loss of teeth actually occurred when mysticetes had legs!

More backstory for those keenly interested
Below are some earlier and traditional reports on Aetiocetus and aetiocetids, which was considered by these authors to be related to mysticetes and cetotheres (extinct mysticetes).

Whitmore and Sanders 1976 report,
“The cheek teeth [of Aetiocetus] are leaf shaped, similar to those of Patriocetus, but smaller and with the roots coalesced. The triangular rostrum, reduced dentition, and the conformation of the posterior ends of the maxillae, premaxillae, and nasals (Ernlong, 1966:s) are characters that would be expected in the ancestor of the mysticetes. Thenius (1969:489) stated: “Even if Aetiocetus, because of its geologic age (upper Oligocene) cannot be a direct stem form of the cetotheres, yet this genus documents that a specific family (Aetiocetidae) must be classified as ancestor, the link between ancient and baleen whales.

“Among the few Cetacea known from deposits of middle Oligocene age are two occurrences of unmistakable Mysticeti. One of these, Mauicetus Benham, 1939, from New Zealafid, has long nasals embraced by premaxillae and maxillae which extend posteriorly to the level of the supraorbital process of the frontal, together with an anteriorly thrusting triangular supraoccipital. The Oligocene Mysticeti, had already evolved the elongated, edentulous rostrum, constituting 3/4 to 4/5 of total skull length, that typifies the modern baleen whales. The mandible of Oligocene Mysticeti was also edentulous and, like those of modem baleen whales, was long and slim.”

Berta and Demere 2005 reported,
“Aetiocetids are the most taxonomically and morphologically diverse clade of toothed mysticetes known from the late Oligocene of the eastern and western North Pacific. Aetiocetids can be distinguished from other toothed mysticetes by the following unequivocal synapomorphies: lobate or triangular parietal-frontal suture; zygomatic process of squamosal expanded near anterior end; “window” in the palate exposing vomer; short, broad extension of the palatine that overlaps the pterygoid; and exoccipital developed ventrally as an anteriorly directed posterior sinus.

The presence of palatal nutrient foramina associated with the upper teeth in all aetiocetids suggests that these toothed mysticetes had already evolved some type of baleen. The form and function of this rudimentary baleen is currently unknown, but the fact that these archaic mysticetes also possessed procumbent anterior teeth, broad diastemata, and posterior teeth with sharply pointed cusps, accessory denticles, and longitudinal enamel ridges suggests development of a specialized type of filter feeding differing from that of other toothed and edentulous mysticetes.”

Unfortunately
these authors had not expanded their taxon set to include desmostylians, which pull mysticetes away from odontocetes + tenrecs.

Eckland et al. 2015 report
“The origin of baleen in mysticetes heralded a major transition during cetacean evolution. Extant mysticetes are edentulous in adulthood, but rudimentary teeth develop in utero within open maxillary and mandibular alveolar grooves. The teeth are resorbed prenatally and the alveolar grooves close as baleen germ develops.”

That’s all well and good, but you really need a wide gamut taxon inclusion set that includes whales, tenrecs and desmostylians to find the diphyletic origins of extant whales and in desmostylians one should look for the origin of baleen, as discussed earlier here.

References
Berta A and Demere TA 2005. Phylogenetic relationships among the diverse toothed mysticete clade the aetiocetidae and reconsideration of the filter feeding niche. Evolution of  aquatic tetrapods. Fourth triennial convention abstracts May 16-20 2005, Akron, OH, USA.
Boessenecker RW and Fordyce RE 2015. A new genus and species of eomysticetid (Cetacea: Mysticeti) and a reinterpretation of ‘Mauicetus’ lophocephalus Marples, 1956: Transitional baleen whales from the upper Oligocene of New Zealand. Zoological Journal of the Linnean Society. in press. doi:10.1111/zoj.12297.
Demere TA 2005. Palate vascularization in an Oligocene toothed mysticete (Cetacea: Mysticeti): Aetiocetidae); implications for the evolution of baleen. Evolution of  aquatic tetrapods. Fourth triennial convention abstracts May 16-20 2005, Akron, OH, USA.
Ekdale EG, Demere TA and Berta A 2015. Vacularization of the gray whale palate (Cetacea, Mysticeti, Eschrichtius robustus): soft tissue evidence for an alveolar source of blood to baleen. The Anatomical Record Advances in Integrative Anatomy and Evolutionary Biology 298(4) · February 2015.
Whitmore FC Jr and Sanders AE 1976. Review of the Oligocene Cetacea. US Geological Survey Staff — Published Research. Paper 237.
Van Valen L 1968. Monophyly or diphyly in the origin of whales”. Evolution. 22 (1):37–41.

wiki/Aetiocetus
wiki/Tokarahia
wiki/Isanacetus – not created yet

The number of molars in marsupials and placentals

Here
the large reptile tree (subset Fig. 1) divides mammals into pre-therians (monotremes and kin), and therians (marsupials + placentals). Note: there are no allotherians here. They all nest elsewhere.

Typical molars
are multi-rooted teeth that erupt only once in mammals as they approach adulthood. Some molars are not multi-rooted, but ever-growing and lack enamel (xenarthrans). Most molars have several cusps, but a few (i.e. Stylinodon, Vintana, Glyptotherium) do not.

Traditional paleontology
holds that marsupials have four molars while placentals have three (see below). I tested that tradition in the LRT and found that you can’t always count on this rule. Turns out there is a mix of molar numbers in marsupials and placentals (Fig. 1).

Figure 1. Molar numbers in mammals. Four molars is the basal number. A few taxa add molars. Several lose one molar for a total of three. A few have fewer than three molars. Among xenartharans the number of molars is difficult to ascertain due to the transformation of all the teeth into similar often non-molar shapes.

Figure 1. Molar numbers in mammals. Four molars is the basal number. A few taxa add molars. Several lose one molar for a total of three. A few have fewer than three molars. Among xenartharans the number of molars is difficult to ascertain due to the transformation of all the teeth into similar often non-molar shapes.

According to Wikipedia: “The early marsupials have… three premolars and four molars. In other groups [derived marsupials] the number of teeth is reduced. Marsupials in many cases have 40 to 50 teeth, significantly more than placental mammals … and they have more molars than premolars.”

In the LRT
the situation is a little different. Four molars is the basal number for all mammals. You’ll find four molars in Sinoconodon. By contrast, you’ll find that Kuehneotherium, Amphitherium and Akidolestes increase that number to six while another sister, highly derived Ornithorynchus sheds all molars as an adult. Juramaia had three molars. In tiny, but adult, Hadrocodium only two molars were present.

Among marsupials, the creodonts beginning with Hyaenodon had only three molars. The odd wombats Vintana and Zalambdalestes also had three molars. In Vintana a tooth anterior to the molar row is vestigial and non-working.

In placentals, four molars remain the basal number. The fourth molar is lost by convergence in several clades (Fig. 1), but retained in Asioryctes, flying lemurs, Henkelotherium + Nambaroo, tenrecs + whales and Onychodectes at the base of the Condylarthra. Glyptotherium appears to have had six molars. Bradypus appears to have had four. Only the anterior one of five teeth in Bradypus is distinct from the others and they are all ventral to the jugal, hence the molar designation. By contrast, in the related  Peltephilus, the molars are vestiges and three premolars appear to be present.

Among pretherians
With maturation, the anterior premolars of Morganucodon are shed and not replaced, and a diastema forms behind the upper canine that is elongated over time as premolars are lost.

All this is very interesting
and points to the importance of establishing a cladogram of relationships before establishing phylogenetic ‘rules.’ for various traits.

 

 

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.

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?