Mammal tooth evolution toward complexity and then simplicity

Maybe the very last traits to learn in vertebrate paleontology,
are those ever-evolving mammal tooth cusps (Fig. 1), unless you are a mammal expert. Then you learn those first. Unfortunately, too often tooth cusps build false positive phylogenies when tested with more skeletal traits. That’s because they tend to devolve, (return to primitive shapes) especially in marine taxa (Fig. 1) and pre-toothless taxa.

Figure 3. Mammal tooth evolution alongside odontocete tooth evolution, reversing the earlier addition of cusps.

Figure 1. Mammal tooth evolution alongside odontocete tooth evolution, reversing the earlier addition of cusps. Colors added.

Hopson and Rougier 1993 were focusing
on the braincase of Vincelestes (Early Cretaceous, Fig. 1), which they considered “a therian mammal of pre-tribosphenic dental grade.” As we learned earlier with odontocetes (Fig. 1) and other mammals, sometimes mammal teeth loose their tribosphenic (= three cusped) morphology. The same can be said of Vincelestes (Fig. 2), which nests in the large reptile tree  LRT, 1366 taxa) as a derived marsupial (metathere) close to the much more derived sabertooth, Thylacosmilus. Hopson and Rougier, along with many later paleontologists, did not realize this phylogeny due to taxon exclusion.

Figure 2. Vincelestes drawing compared to Digimorph.org image. Colors applied here. The LRT indicates this is a sister taxon to the highly derived marsupial sabertooth, Thylacosmilus.

Figure 2. Vincelestes drawing compared to Digimorph.org image. Colors applied here. The LRT indicates this is a sister taxon to the highly derived marsupial sabertooth, Thylacosmilus.

The tribosphenic molar
Hopson and Rougier describe, “The molars of Vincelestes have the characteristic ‘reversed triangles’ pattern of therian mammals but differ from molars of tribosphenic therians in having a very small talonid without a true basin and in possessing a very small, low protocone. Therefore, Vincelestes may be the sister taxon of the Tribosphenida of McKenna (1975).”

Figure 5. Megazostrodon molars display a tribosphenic cusp arrangement. Therefore all mammals are derived from this cusp pattern.

Figure 3. Megazostrodon upper molars display a symmetrodont cusp arrangement. The lower molars have a tribosphenic cusp arrangement. This taxon is basal to all tested mammals in the LRT.

Don’t confuse
teeth evolving toward the tribosphenic morphology with those evolving away from the tribosphenic morphology.

In similar fashion, in traditional cladograms
Zhangheotherium (Early Cretaceous) nests with other so-called ‘symmetrodonts’ (Fig. 1) based on their simplified teeth, by only due to taxon exclusion. When you add in a few pangolins and pangolin ancestors, Zhangheotherium shifts to that clade as a basal member.

While it’s already January 2 in Australia,
it’s January 1, 2019 here in the USA. I wanted to take a moment to thank you for your readership. Essentially, this has been my diary, letting you know what I am learning as I learn it, while letting you know which taxa are added to the LRT as I add them. I’ve spent the last few days repairing mistakes I made in the clade Glires, guided by the low Bremer scores at certain nodes, alerting me to those errors. It’s a tricky clade.

References
Hopson JA and Rougier G 1993. Braincase structure in the oldest known skull of the therian mammal: Implications for mammalian systematics and cranial evolution. American Journal of Science 293-A-A:268–299.

Smith AJ website on dental morphology

wiki/molar

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