Docodon and molar count

The genus Docodon (Marsh 1881; Late Jurassic, 170 mya; Fig. 1) is represented by a jaw with several more molars than typical (Fig. 1). Hard to tell the premolars from the molars in lateral view. See the dorsal view for the distinct difference. Even so, the count may be off, because molars are not molars based on shape, but on the fact that they appear once and are not replaced during growth. I cannot tell, note have I found references that say where the division is in Docodon.

Figure 1. The holotype of Docodon has 4 incisors, 4 premolars and a whopping 7 molars.

Figure 1. The holotype of Docodon has 4 incisors, 4 premolars and a whopping 7 molars. diplocynodon has 8 according to Osborn, who confirms the 7 in Docodon.

More than four molars in a mammal jaw is relatively rare.
But not rare in ancestral monotremes. Among tested taxa  Amphitherium, Kuehneotherium and Akidolestes have 6.

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.

There is also “a rule” that says
only one canine appears, but the other teeth can vary greatly in number. I’m wondering if that is true. Sometimes there will be a small, simple tooth arising between the canine and the double-rooted premolars. Is that a tiny canine? or a tiny premolar? Maybe someone out there has not only the answer, but the reason why.

And yes,
I’m aware of the convention that numbers premolars 1-4. But the anterior one, is almost always the smallest, as if it just arrived.

References
Marsh OC 1881. Notice of new Jurassic mammals. American Journal of Science. (3) xxi: 511-513.

wiki/Docodon

 

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