By DANIEL L. GEBO and K. CHRISTOPHER BEARD

Forelimb Anatomy of the Microsyopidae (Mammalia: Primatomorpha): implications for primate origins

By DANIEL L. GEBO1 and K. CHRISTOPHER BEARD2 1Department of Anthropology, Northern Illinois University, 2Department of Ecology & Evolutionary Biology, University of Kansas

Introduction

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 animals 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 Figure 1. USGS 16647, both cases the distal humeral anatomy of these maxilla of Microsyops microsyopids mirrors that of crown clade primates latidens from the early Eocene in having a spherical capitulum associated with a wide and distinct spatial gutter separating the capitulum from the trochlea. This primate-like distal humeral anatomy contrasts with that of other known plesiadapiforms, which have a subspherical capitulum and lack the distinct separation between the capitulum and trochlea found in microsyopids. Microsyopid elbow morphology allows for an extensive range of forearm pronation and supination, movements that are often utilized Figure 4. Distal humeral comparisons. A through d are early Eocene fossil primates: a) Cantius, USGS 6759; b) during locomotion on small diameter arboreal asiadapine, GU 713, reversed; c) Teilhardina, IRSNB M2160; d) Archicebus, IVPP V18618. e) Plesiadapis, supports or during single-handed manual MNH BR 3L and B4L, redrawn from Szalay et al., 1975) and f ) Niptomomys, USNM 639759) are manipulation of food items. In this poster, we consider plesiadapiforms. Note the less rounded capitulum and the capitulaum-trochlear connection (shaded joints) in the best phyletic position for the Microsyopidae the elbow joint of Plesiadapis (e) relative to early Eocene fossil primates (a-d). In contrast to Plesiadapis, the microsyopid elbow of Niptomomys (f) is similar to that of early Eocene fossil primates (a-d).

Materials Discussion Conclusion

Two specimens from the early Eocene Willwood The forelimb anatomy of the The phylogenetic position of microsyopids has been Formation, Bighorn Basin, Wyoming, provide Microsyopidae closely resembles crown controversial for decades. Recent phylogenetic information on the forelimb anatomy of Microsyopidae. clade primates, especially relative to any analyses of euarchontan relationships reconstructs USGS 16647, consisting of associated maxillary of the distal humeri known for the other microsyopids as relatively basal members of the fragments (Fig. 1) and left forelimb elements (Fig. 2), plesiadapiforms. If the elbow plesiadapiform radiation, all of which are interpreted as was collected by Dr. Chris Beard at locality D-1647, morphology among microsyopids makes comprising a paraphyletic stem group of primates which occurs stratigraphically approximately 591 m a sister taxon link to early Eocene (Bloch et al., 2007, 2016; Silcox et al., 2017). However, above the base of the Willwood Formation (Bown et primates, a position we advocate here, disagreement persists with regard to the precise al., 1994). Based on the size and morphology of its then the Plesiadapoidea are not. The phylogenetic placement of microsyopids in these dentition, USGS 16647 pertains to Microsyops plesiadapoid clades, the Saxonellidae, analyses, with some tree topologies reconstructing latidens, which is the only microsyopine known to the Carpolestidae, and the microsyopids as the sister group of micromomyids occur at this stratigraphic level in the Willwood Figure 2. Forelimb elements associated Plesiadapidae, must be re-evaluated as (Bloch et al., 2016), while others reconstruct Formation (Gunnell, 1989). with the maxilla of Microsyops a close phyletic link to “euprimates” in microsyopids as being more closely related to USNM 639759 is a tiny right distal humerus (Fig. latidens, USGS 16647 the Bloch et al. (2007) or Silcox et al. paromomyids, plesiadapoids, and crown clade primates 3) from the same freshwater limestone that yielded the (2007) sense. A formal cladistic analysis (Bloch et al., 2007; Silcox et al., 2017). In contrast, the sample of micromomyid plesiadapiforms reported by Niptomomys Microsyops will be needed to support this claim. Beard and Houde (1989). The provenance of this large-scale phylogenetic analysis of morphological and fossil is very near University of Michigan locality SC-4, molecular data reported by Ni et al. (2016) which occurs stratigraphically in the early Wasatchian reconstructed microsyopids and other plesiadapiforms References either as stem dermopterans or stem primatomorphs. Wa-1 faunal zone. Based on its size, morphology, and Beard KC. 1993a Origin and evolution of gliding in early Cenozoic provenance, we allocate this isolated distal humerus One approach advocated before has been Beard’s Dermoptera (Mammalia, Primatomorpha). In Primates and their relatives in (1993a,b) Primatomorpha, a phyletic grouping that links phylogenetic perspective (ed RDE MacPhee), pp. 63-90. New York: Plenum to the uintasoricine microsyopid Niptomomys Press. both Orders, Dermoptera and Primates, with several doreenae. Beard KC. 1993b Phylogenetic systematics of the Primatomorpha, with clades of plesiadapiforms contained within Dermoptera. special reference to Dermoptera. In Mammal phylogeny, volume 2, placentals (eds FS Szalay, MJ Novacek, MC McKenna), pp. 129-150. New Enlarged & Reversed Microsyopid elbow anatomy adds a new anatomical York: Springer-Verlag. Beard KC, Houde P. 1989 An unusual assemblage of diminutive complex to this phyletic debate and this character plesiadapiforms (Mammalia, ?Primates) from the early Eocene of the Clark’s complex supports a closer evolutionary connection to Fork Basin, Wyoming. J. Vert. Paleontol. 9, 388-399. Figure 3. Microsyopid elbow anatomy. Left: USNM 639759, Bloch JI, Silcox MT, Boyer DM, Sargis EJ. 2007 New Paleocene skeletons Niptomomys doreenae; Right: USGS 16647, Microsyops primates for the Microsyopidae -- in contrast to an and the relationship of plesiadapiforms to crown-clade primates. Proc. Natl. interpretation as a basal member among the clades of Acad. Sci. USA 104, 1159-1164. latidens. At the top, (a) and (e) compare distal humeri at scale. Bloch JI, Chester, SG, Silcox MT. 2016 Cranial anatomy of Paleogene Below, (b) through (d) are enlarged and reversed humeral plesiadapiforms. When we assess additional Micromomyidae and implications for early primate evolution. J. Hum. Evol. 96, 58-81. comparisons of USNM 639759 at relatively the same bicondylar anatomical features that might also signal a phyletic link Bown TM, Rose KD, Simons EL, Wing SL. 1994 Distribution and width of USGS 16647 (anterior (b) and (f), distal edge (c) and (g), between the Microsyopidae with primates or stratigraphic correlation of upper Paleocene and lower Eocene fossil and posterior (d) and (h) views. mammal and plant localities of the Fort Union, Willwood, and Tatman “euprimates” as sister taxa, the list of features is short. formations, southern Bighorn Basin, Wyoming. U.S. Geol. Surv. Prof. Pap. The arterial supply to the brain (MacPhee et al., 1983) 1540, 1-103. Gunnell GF. 1989 Evolutionary history of Microsyopoidea (Mammalia, or the larger encephalization quotient for Microsyops ?Primates) and the relationship between Plesiadapiformes and Primates. (Silcox et al., 2010) are additional features worthy of Univ. Michigan Pap. Paleontol. 27, 1-157. MacPhee, RDE, Cartmill M, Gingerich PD 1983. New Paleogene primate Acknowledgments consideration. In contrast, the dental evidence for basicrania and the definition of the order Primates. Nature 301:309-311. microsyopids has largely been interpreted as “primitive” Silcox, M.T., Benham, A.E., and Bloch, J.I. 2010. Endocasts of Microsyops We thank Dr. Luo Zhe-Xi and April Neander at the among comparisons with other plesiadapiforms. If (Microsyopidae, Primates) and the evolution of the brain in primitive primates. J Hum Evol 58:505-521. University of Chicago for microCT scanning these microsyopids are a sister taxon to primates than a fossils. Rendering and images for this poster was Silcox MT, Bloch JI, Boyer DM, Chester SGB, López-Torres S. 2017 The sister link with Plesiadapodiea is not correct In the evolutionary radiation of plesiadapiforms. Evol. Anthropol. 26, 74-94. done by Joshua Schwartz (Geology Department, end, the phyletic position of the Microsyopidae Szalay, F.S., Tattersall, I., and Decker, R.L. 1975. Phylogenetic Relationships Northern Illinois University). Financial support was of Plesiadapis – Postcranial Evidence. In F.S. Szalay, ed., Approaches to continues to be controversial. Primate Paleontology, Contrib. Primates, Karger, Basel, Vol. 5:136-166. provided by the David B. Jones Foundation.