Tritylodontids. They were almost mammals.

If any taxa
were going to attract multituberculates to the stem-mammal side of the large reptile tree, it would likely be the tritylodontids (Fig. 1), which have similar rodent-like incisors and a diastema (space) separating the incisors from the molars (losing the canines and premolars).

When tested here, that did not happen.
The rodent-like tritylodontids, Oligokyphus (Henning 1922) and Kayentatherium (Kermack 1982, Fig. 1), nested with Pachygenelus (Watson 1913), the proximal outgroup taxon to the Mammalia. The multituberculates remained with the rodents and rabbits. So these two clades were similar by convergence.

Figure 1. The Tritylodontids, Oligokyphus and Kayentatherium nested with the proximal outgroup taxon to the Mammalia, Pachygenelus. Here you can see the loss of teeth along with the unique premaxilla that tips up at the tip.

Figure 1. The Tritylodontids, Oligokyphus and Kayentatherium nested with the proximal outgroup taxon to the Mammalia, Pachygenelus. Here you can see the loss of teeth along with the unique premaxilla that tips up at the tip. The braincase in these specimens was extremely narrow, hardly wider than the crest that topped it.

This comes as no surprise 
to paleontologists who for decades have nested Tritylodontids close to, but not within, the Mammalia. The surprise remains that multituberculates don’t occupy a nearby pre-mammal node on the large reptile tree (subset Fig. 2, darker yellow). They are just too similar to rodents and too different from all the pre-mammals so far tested.

Figure 2. Subset of the large reptile tree focusing on mammals and their ancestors. Here Kayentatherium and Oligokyphus join Pachygenelus just outside the Mammalia.

Figure 2. Subset of the large reptile tree focusing on mammals and their ancestors. Here Kayentatherium and Oligokyphus join Pachygenelus just outside the Mammalia.

Here with the addition
of Probainognathus, the reported aquatic cynodont, Castorocauda now nests more closer to Chiniquodon and the only mammaliaformes are the tritylodontids.

When there are two names
for the same clade, the older one has precedence. No doubt there are some intervening taxa I have not tested or have yet to be reported that will fulfill the role of non-tritylodontid mammaliaform.

Herbivorous mammal clades
with loss of canine and/or addition of a diastema extend to the pre-mammals. Among mammals we see this in condylarths and their descendants, certain marsupials, rodents + rabbits (including Plesiadapis) + multituberculates and certain edentates. In other words, this evolutionary path is common and convergent along multiple clades.

References
Hennig E 1922. Die Säugerzähne des württembergischen Rhät-Lias-Bonebeds. Neues Jahrbuch für Mineralogie, Geologie und Paläontologie 46:181-267.
Kermack DM 1982. A new tritylodontid from the Kayenta formation of Arizona. Zoological Journal of the Linnean Society 76:1-17.
Watson DMS 1913. On a new cynodont from the Stormberg. Geological Magazine, new series, decade 5 10(4):145-148

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