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

both cases the distal humeral anatomy of these

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

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

during locomotion on small diameter arboreal

supports or during single-handed manual

manipulation of food items. In this poster, we consider

the best phyletic position for the Microsyopidae

Acknowledgments

We thank Dr. Luo Zhe-Xi and April Neander at the

University of Chicago for microCT scanning these

fossils. Rendering and images for this poster was

done by Joshua Schwartz (Geology Department,

Northern Illinois University). Financial support was

provided by the David B. Jones Foundation.

Figure 2. Forelimb elements associated

with the maxilla of Microsyops

latidens, USGS 16647

References

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Beard KC. 1993b Phylogenetic systematics of the Primatomorpha, with

special reference to Dermoptera. In Mammal phylogeny, volume 2,

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(Microsyopidae, Primates) and the evolution of the brain in primitive

primates. J Hum Evol 58:505-521.

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Discussion

The phylogenetic position of microsyopids has been

controversial for decades. Recent phylogenetic

analyses of euarchontan relationships reconstructs

microsyopids as relatively basal members of the

plesiadapiform radiation, all of which are interpreted as

comprising a paraphyletic stem group of primates

(Bloch et al., 2007, 2016; Silcox et al., 2017). However,

disagreement persists with regard to the precise

phylogenetic placement of microsyopids in these

analyses, with some tree topologies reconstructing

microsyopids as the sister group of micromomyids

(Bloch et al., 2016), while others reconstruct

microsyopids as being more closely related to

paromomyids, plesiadapoids, and crown clade primates

(Bloch et al., 2007; Silcox et al., 2017). In contrast, the

large-scale phylogenetic analysis of morphological and

molecular data reported by Ni et al. (2016)

reconstructed microsyopids and other plesiadapiforms

either as stem dermopterans or stem primatomorphs.

One approach advocated before has been Beard’s

(1993a,b) Primatomorpha, a phyletic grouping that links

both Orders, Dermoptera and Primates, with several

clades of plesiadapiforms contained within Dermoptera.

Microsyopid elbow anatomy adds a new anatomical

complex to this phyletic debate and this character

complex supports a closer evolutionary connection to

primates for the Microsyopidae -- in contrast to an

interpretation as a basal member among the clades of

plesiadapiforms. When we assess additional

anatomical features that might also signal a phyletic link

between the Microsyopidae with primates or

“euprimates” as sister taxa, the list of features is short.

The arterial supply to the brain (MacPhee et al., 1983)

or the larger encephalization quotient for Microsyops

(Silcox et al., 2010) are additional features worthy of

consideration. In contrast, the dental evidence for

microsyopids has largely been interpreted as “primitive”

among comparisons with other plesiadapiforms. If

microsyopids are a sister taxon to primates than a

sister link with Plesiadapodiea is not correct In the

end, the phyletic position of the Microsyopidae

continues to be controversial.

Conclusion

The forelimb anatomy of the

Microsyopidae closely resembles crown

clade primates, especially relative to any

of the distal humeri known for the other

plesiadapiforms. If the elbow

morphology among microsyopids makes

a sister taxon link to early Eocene

primates, a position we advocate here,

then the Plesiadapoidea are not. The

plesiadapoid clades, the Saxonellidae,

the Carpolestidae, and the

Plesiadapidae, must be re-evaluated as

a close phyletic link to “euprimates” in

the Bloch et al. (2007) or Silcox et al.

(2007) sense. A formal cladistic analysis

will be needed to support this claim.

Materials

Two specimens from the early Eocene Willwood

Formation, Bighorn Basin, Wyoming, provide

information on the forelimb anatomy of Microsyopidae.

USGS 16647, consisting of associated maxillary

fragments (Fig. 1) and left forelimb elements (Fig. 2),

was collected by Dr. Chris Beard at locality D-1647,

which occurs stratigraphically approximately 591 m

above the base of the Willwood Formation (Bown et

al., 1994). Based on the size and morphology of its

dentition, USGS 16647 pertains to Microsyops

latidens, which is the only microsyopine known to

occur at this stratigraphic level in the Willwood

Formation (Gunnell, 1989).

USNM 639759 is a tiny right distal humerus (Fig.

3) from the same freshwater limestone that yielded the

sample of micromomyid plesiadapiforms reported by

Beard and Houde (1989). The provenance of this

fossil is very near University of Michigan locality SC-4,

which occurs stratigraphically in the early Wasatchian

Wa-1 faunal zone. Based on its size, morphology, and

provenance, we allocate this isolated distal humerus

to the uintasoricine microsyopid Niptomomys

doreenae.

Figure 1. USGS 16647,

maxilla of Microsyops

latidens

Figure 3. Microsyopid elbow anatomy. Left: USNM 639759,

Niptomomys doreenae; Right: USGS 16647, Microsyops

latidens. At the top, (a) and (e) compare distal humeri at scale.

Below, (b) through (d) are enlarged and reversed humeral comparisons of USNM 639759 at relatively the same bicondylar

width of USGS 16647 (anterior (b) and (f), distal edge (c) and (g),

and posterior (d) and (h) views.

Figure 4. Distal humeral comparisons. A through d are early Eocene fossil primates: a) Cantius, USGS 6759; b)

asiadapine, GU 713, reversed; c) Teilhardina, IRSNB M2160; d) Archicebus, IVPP V18618. e) Plesiadapis,

MNH BR 3L and B4L, redrawn from Szalay et al., 1975) and f ) Niptomomys, USNM 639759) are

plesiadapiforms. Note the less rounded capitulum and the capitulaum-trochlear connection (shaded joints) in

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).

Enlarged & Reversed

Niptomomys Microsyops