1Department of Anthropology, Northern Illinois University, 2Department of Ecology and Evolutionary Biology, University of Kansas
April 17, 2020 , Platinum Ballroom
Although historical attempts to reconstruct the phylogenetic relationships of the extinct family Microsyopidae have yielded variable and often conflicting results, all recent analyses have interpreted these mammals as stem Primates or members of the crown clade Primatomorpha. Here we describe the forelimb anatomy of Microsyops and Niptomomys, two microsyopids from the Eocene of North America. In both cases the distal humeral anatomy of these two microsyopids mirrors that of crown clade primates from the early Eocene in having a spherical capitulum associated with a wide and distinct spatial gutter separating the capitulum from the trochlea. This euprimate-like distal humeral anatomy contrasts with that of other known plesiadapiforms, which have a subspherical capitulum and lack a distinct separation between the capitulum and trochlea. Microsyopid elbow morphology allows for an extensive range of forearm movements that are often utilized during arboreal locomotion on small diameter supports or during single-handed manual manipulation of food items.
Microsyopid elbow anatomy adds a new anatomical complex to the plesiadapiform-primate phyletic debate. Microsyopid elbow anatomy supports a closer evolutionary connection to “euprimates”. Additional anatomical features that might also signal a phyletic link between Microsyopidae and “euprimates” are the arterial supply to the brain and the larger encephalization quotient of Microsyops. In contrast, the dental evidence for microsyopids has largely been interpreted as “primitive” when compared to other plesiadapiforms. If microsyopids are the sister taxon of primates then the hypothesis of a Euprimates-Plesiadapoidea clade is incorrect. The phyletic position of the Microsyopidae continues to be controversial.