Priacodon: How to tell a crown mammal from a mammal mimic

Jäger et al. 2020 discuss ‘molar’ occlusion
in a tiny taxon, Priacodon fruitaensis (LACM 120451, Fig. 1), they said was a crown mammal (a clade with living relatives). Priacodon is principally represented by a mandible with teeth and a maxilla with teeth. Triconodont ‘molar’ cusps are three in number and aligned like a row of three knives distinct from basal cynodonts and basal mammals.

Figure 1. Priacodon µCT scans from Jäger et al. 2020. Colors and restoration added. This looks like a mammal jaw. The LRT nests it with mammal mimics. That's an odd sort of canine with more than one cusp.

Figure 1. Priacodon µCT scans from Jäger et al. 2020. Colors and restoration added. This looks like a mammal jaw. The LRT nests it with mammal mimics. That’s an odd sort of canine with more than one cusp.

The authors wrote: 
“Triconodontids are a clade of the eutriconodontans which is a clade of early crown mammals with a fossil record from the Late Jurassic through the Late Cretaceous.”

So this clade had plenty of time to develop their unique teeth and convergent jaw joints alongside crown mammals (= monotremes + marsupials + placentals).

By contrast 
the large reptile tree (LRT, 1786+ taxa, subset Fig. 4) nested Priacodon and kin like Sinocodon (Fig. 2), within a clade of mammal mimics arising from the cynodont,  Pachygenelus, and preceding the Last Common Ancestor of all living mammals, Megazostrodon (Fig. 5). That LCA status makes Megazostrodon the most primitive of crown mammals. Any taxa preceding Megazostrodon are excluded from crown mammals. A valid cladogram is needed to place taxa within a crown clade or outside it. Jäger et al. did not provide a cladogram.

Wang et al. 2001 provided a traditional cladogram of mammals and pre-mammals. That was invalidated in 2016 by the addition of taxa to the LRT.

The single replacement of milk teeth with adult teeth
also marks Megazostrodon as a mammal because toothless hatchlings are initially feeding on their mother’s mammary glands, but that’s beside the point. That’s a trait, not a phylogenetic nesting  node.

Figure 1. Sinoconodon growth series including jaws and teeth, here colorized from Zhang et al. 1992.

Figure 2. Sinoconodon growth series including jaws and teeth, here colorized from Zhang et al. 1992. Note the lack of tiny post-dentary bones in this mammal-mimic.

Unfortunately,
this is a continuing problem in mammal paleontology going back before Repenomamus (Fig. 3), an Early Cretaceous mammal-mimic, typically considered the largest mammal in the Cretaceous. According to Wikipedia, “Repenomamus is a genus of triconodonts, a group of early mammals with no modern relatives.” According to the LRT, they have no living relatives because they are pre-mammals or mammal-mimic cynodonts.

Tiny post-dentary bones
This is a classic case of “Pulling a Larry Martin” because both Repenomamus and Priacodon have a certain trait shared with mammals by convergence. They lack the small post-dentary bones thought to be lost only in mammals. As a result they also have a dentary-squamosal jaw joint. The authors put all their money on this single trait and did not recognize the possibility of convergence. They didn’t provide a phylogenetic analysis that included all pertinent taxa.

In counterpoint, 
Megazostrodon (Fig. 5) retains tiny post-dentary bones. These ultimately migrate to help form the middle ear bones of higher mammals.

A few years ago
I had a chat with co-author R Cifelli in Oklahoma with regard to the nesting of multituberculates in Glires in the LRT. Multis redevelop tiny post-dentary bones by reversal according to the LRT, which tests a suite of 235 traits from head to tail. Cifelli wasn’t ready to consider non-traditional solutions based on an expanded taxon list and the possibility of a reversal.

Figure 4. Repenomamus reconstructed using DGS methods. The manus and feet are loose figments at present. Despite its predatory nature, note the reduction in canines, a clade trait.

Figure 3. Repenomamus reconstructed using DGS methods. The manus and feet are loose figments at present. Despite its predatory nature, note the reduction in canines, a clade trait.

Relatives of Sinoconodon replace their teeth multiple times,
(Fig. 2) as in cynodonts and reptiles in general. But even if they had single tooth replacement, their nesting on the LRT apart from crown mammals indicates they are not crown mammals, but mammal-mimics. Like Repenomamus (Fig. 3) and Priacodon (Fig. 1), Sinoconodon also lacked tiny post-dentary bones and had a dentary-squamosal jaw joint.

In their conclusion, the Jäger et al. note:
“Triconodontidae exhibit a molar series that is unique among mammals and is not directly comparable to any extant counterpart.” That’s because triconodonts are not related to extant counterparts, aka: crown mammals. These esteemed authors “Pulled a Larry Martin” by putting a few traits ahead of a suite of hundreds of traits in a phylogenetic analysis.

Convergence runs rampant in the LRT.
The LRT weeds out convergence. That’s why you need to run your own analysis and expand your own taxon list. Don’t rely on a few traditional traits.

Figure 2. Subset of the LRT highlighting the anomodontia and dicynodontia closer to the origin of the Therapsida.

Figure 4. Subset of the LRT from 2019 focusing on the Therapsida. Red taxa were tested separately due to too few characters known for a permanent place in the LRT.

Whatever Jäger et al. discovered
by closely examining the occlusal pattern in Priacodon, their study was hobbled by their invalid assignment of Priacodon to the clade crown Mammalia. Despite years in this profession, they had no idea that triconodonts were mammal mimics. To avoid problems like this, get a wide-angle view before setting up your microscopic views.

Figure 1. Megazostrodon skull in several views. Drawings from Gow 1986. Colors applied here.

Figure 5. Megazostrodon skull in several views. Drawings from Gow 1986. Colors applied here.

Taxon exclusion continues to be the number one problem in paleontology,
as you can see dozens of times if you click here: keyword: taxon+exclusion.


References
Jäger KRK, Cifelli RC and Martin T 2020. Molar occlusion and jaw roll in early
crown mammals. Scientific Reports (2020) 10:22378 https://doi.org/10.1038/s41598-020-79159-4
Wang Y-Q, Hu Y-M, Meng J and Li C-K 2001. An ossified Meckel’s cartilage in two Cretaceous mammals and origin of the mammalian middle ear. Science 294:357–361.

wiki/Crown_group
wiki/Repenomamus
wiki/Priacodon

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