the symmetrodont mammal, Maotherium sinensis (NGMC 97415, Rougier et al. 2013, Early Cretaceous 125 mya; Fig. 1) to the large reptile tree (Fig. 2; LRT) nests it at the base of the primates. This nesting has not been recovered before.
Not sure yet
about M. asiaticus (Ji et al. 2009). It has not yet been added to the LRT.
Wikipedia reports: “Maotherium is a genus extinct symmetrodont mammal. Though little is known about this group, the symmetrodonts have several similarities – specifically their teeth. They have tall pointed, but simple molars in a triangular arrangement.”
As you know,
the large reptile tree has no characters based on variation in molar shape. Given that, Maotherium is known from a complete and articulated fossil. It has a complete postorbital ring, The tail is relatively short and without chevrons. The ilium is long and at least four sacrals were present, indicating both a strong hind limb and a reinforced sacrum, both possible leaping structures. What look like epipubes are likely displaced pubes, which are otherwise buried. Both humeri are broken at mid shaft in the fossil. No retro process is present on the lower posterior dentary. The hand and foot of Maotherium is quite primate-like, even at this early stage, 125 million years ago, and not as lemur-like as in Notharctus.
I didn’t expect this nesting
but once I scored about twenty traits, the trend became apparent and finally overwhelming. This puts primate origins back about twice as far a previously figured, from 65 mya to 125 mya. This is a trend we looked at earlier here.
not necessarily basal primates, include Zhangheotherium and Akidolestes. At this point, symmetrodonts might just be a primitive grade, rather than a clade of mammals. Only testing will tell.
And once again,
mammal phylogeny is proving to be much more simple than conventional wisdom and paradigm seem to indicate.
Rougier GW, Qiang J, Novacek MJ 2003. A New Symmetrodont Mammal with Fur Impressions from the Mesozoic of China. Acta Geologica Sinica 77 (1): 7–14. doi:10.1111/j.1755-6724.2003.tb00104.x.
Ji Q, Luo Z-X, Zhang X-L, Yuan C-X and Xu L 2009. Evolutionary development of the middle ear in Mesozoic therian mammals. Science 326 (5950): 278–281. doi:10.1126/science.1178501. PMID 19815774.
Kielan-Jaworowska Z 2013. In pursuit of early mammals. Indiana University Press 272 pp.