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).
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?
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?