Dissertation/Thesis Abstract

New cranial and postcranial remains of late Paleocene Plesiadapidae (“Plesiadapiformes,” Mammalia) from North America and Europe: Description and evolutionary implications
by Boyer, Douglas Martin, Ph.D., State University of New York at Stony Brook, 2009, 597; 3393638
Abstract (Summary)

Plesiadapidae are a diverse monophyletic family of primate-like mammals that existed during the Paleogene in Europe and North America. They are thought to be nested within the Plesiadapoidea, which are in turn a member of a larger group of fossil mammals called the Plesiadapiformes. An understanding of plesiadapid evolutionary history is important for investigations into the origin of extant primates (including humans, apes, monkeys, lemurs, etc.) plus anatomically modern, extinct primates (euprimates). Specifically, this dissertation accumulates evidence bearing on the hypothesis that the ancestor of euprimates inherited a big toe that (1) is divergent from the lateral digits, (2) exhibits lateral torsion, and (3) has a nail, from its common ancestor with the Plesiadapoidea, in which these features were adaptations to foraging for angiosperm products in a fine branche niche. New information on plesiadapid skeletons may alter views on the phylogenetic relationship of various plesiadapiforms to euprimates. Altered phylogenetic arrangements of plesiadapids, other plesiadapiforms, and euprimates, are likely to indicate that the unusual features of the feet of euprimates were not inherited from plesiadapiforms and evolved at roughly the same time as forward facing orbits and other euprimate novelties. This would re-open the possibility that euprimate features are adaptations for a behavior besides foraging among small branches for angiosperm products.

I describe the first known crania of Pronothodectes gaoi, a species of the most basal plesiadapid genus known; a skull of Nannodectes intermedius, the oldest known for a plesiadapid; and the first skull and postcranial skeleton of the large, late-occurring North American species Plesiadapis cookei. I provide new morphological observations for a cranial specimen of Nannodectes gidleyi and several specimens of Plesiadapis tricuspidens. These new data document basic characteristics of plesiadapid cranial and postcranial anatomy more accurately and comprehensively than previously possible. I use quantitative comparative and cladistic methodologies to (1) reconstruct cranial characteristics for the ancestral node of the family Plesiadapidae, (2) generate hypotheses regarding phylogenetic relationships of various plesiadapid species to each other, and (3) test hypotheses regarding the phylogenetic relationship of Plesiadapidae and other plesiadapiforms to extant euarchontan mammals (Primates, Scandentia, and Dermoptera). I use quantitative comparative morphology to reconstruct functional and behavioral features of various plesiadapid species.

The ancestral node of the family Plesiadapidae is reconstructed as having an enlarged premaxilla that contacts the frontal; a laterally-positioned, intratympanic, transpromontorial route for the internal carotid plexus; a non-functional internal carotid artery; a non-tubular external auditory meatus; and an auditory bulla, the composition of which is uncertain, but best interpreted as petrosal. P. cookei and P. tricuspidens appear derived in having external auditory meati that are tubular and in having a maxilla that lacks extensive dorsal exposure of the molar tooth roots within the orbit. P. tricuspidens is apparently autapomorphic among plesiadapids in having an increased relative size and posterior projection of the premaxilla, consequent narrowing of the frontal contact with the nasal, an increased relative size of the glenoid fossae of the squamosal and decreased prominence and posterior projection of the nuchal crests. N. gidleyi is autapomorphic in having an increased relative size of the glenoids, and in the apparent lack of an intratympanic route for the internal carotid plexus.

P. cookei and P. tricuspidens have been traditionally thought to be close relatives. P. tricuspidens is also thought to be closely related to Platychoerops. The skull of P. cookei differs from that of P. tricuspidens in ways that make it similar to earlier-occurring North American plesiadapids. Unlike P. tricuspidens or other North American plesiadapids, P. cookei has a dentition indicating it had a diet focused on leaves. These features and a cladistic analysis of other dental characters suggest that P. cookei is a closer relative of Platychoerops than is P. tricuspidens.

The postcranial skeleton of Plesiadapis cookei, while generally similar to that of other plesiadapids, has more elongate limbs, possibly reflecting a more specialized arboreal habitus.

Finally, new information from plesiadapid skulls and postcrania increases cladistic support for previously proposed higher-level clades. Specifically, the new data support the hypotheses that plesiadapids, carpolestids, saxonellids and Chronolestes comprise the Plesiadapoidea, and that Plesiadapoidea is the sister taxon to Euprimates. Therefore the results of this dissertation corroborate the hypothesis that euprimates evolved their grasping features from a fine-branch foraging, common ancestor with plesiadapoids.

Indexing (document details)
Advisor: O'Leary, Maureen A.
School: State University of New York at Stony Brook
School Location: United States -- New York
Source: DAI-B 71/02, Dissertation Abstracts International
Subjects: Evolution and Development, Paleontology, Paleoecology
Keywords: Cranial remains, Diet reconstruction, Plesiadapidae, Plesiadapiform, Postcranial remains
Publication Number: 3393638
ISBN: 978-1-109-61725-2
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