1Department of Anthropology, Yale University, 2Florida Museum of Natural History, University of Florida
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Recent studies suggest that micromomyid plesiadapiforms are among the most primitive stem primates. Unlike other primitive plesiadapiforms that are known only from fragmentary dentitions, such as Purgatorius, micromomyids are known from complete dentitions, partial crania, and elements of the postcranium. Micromomyids are small-bodied (15-50g) arborealists represented by at least ten species that span approximately five million years, overlapping with the first appearance of euprimates. Though several new taxa have recently been described, questions concerning interrelationships among micromomyids have received little treatment over the last two decades. Here we report new fossils of late Paleocene and early Eocene micromomyids recovered from freshwater limestones of the Clarks Fork Basin, Wyoming, that include previously unknown tooth positions of Chalicomomys antelucanus, an isolated tooth of Dryomomys, and the earliest occurrence and first substantial Paleocene sample (n=23) of Tinimomys graybulliensis. A new cladistic analysis of dental and gnathic characteristics (n=28) for all known micromomyid species resulted in a fairly well resolved hypothesis of relationships, strongly suggesting that most species previously classified in the genus Micromomys do not form a natural grouping. Specifically, a basal clade is recovered including Micromomys fremdi, whereas the type species, M. silvercouleei, is more closely related to other micromomyid species. Early Eocene C. antelucanus is quite primitive, which implies a fairly long ghost lineage. The Dryomomys and Tinimomys clades, which are uniquely derived in possessing relatively large premolars and more bunodont molar cusps, respectively, appear to have diverged from more basal micromomyids during or before the late Paleocene.
This study was funded by a National Science Foundation Dissertation Improvement Grant (SBE-1028505 to Eric Sargis and S.G.B.C.), a Leakey Foundation General Research Grant (S.G.B.C.), and a Theodore Roosevelt Memorial Grant (S.G.B.C.).