Dissertations, Theses, and Capstone Projects
Date of Degree
6-2025
Document Type
Doctoral Dissertation
Degree Name
Doctor of Philosophy
Program
Anthropology
Advisor
Stephen G. B. Chester
Committee Members
Eric Delson
Christopher C. Gilbert
John R. Wible
Subject Categories
Biological and Physical Anthropology
Keywords
Plesiadapiforms, Euarchonta, microCT, cranial morphology, evolution
Abstract
Plesiadapiforms are central to questions regarding the origin and early evolution of primates, and plesiadapiform cranial morphology has long played an important role in evaluating their position within Euarchonta (primates + colugos + treeshrews). The main objective of this dissertation is to use high resolution microCT scan data to document the cranial anatomy of plesiadapiforms. These data provide clarity on previously described basicranial structures, unveil entirely new anatomy, and help document ways in which plesiadapiforms and/or crown primates are distinct from other mammals. This dissertation has three major components: 1) documentation of the first known basicranial anatomy of a picrodontid mammal and a phylogenetic analysis to assess the evolutionary relationship of picrodontids to primates; 2) investigation of the oldest known plesiadapiform cranium; and 3) an evaluation of auditory bulla composition within the plesiadapiform families Microsyopidae, Micromomyidae, Paromomyidae, Carpolestidae, and Plesiadapidae.
Picrodontids are primarily known from their highly autapomorphic dentitions, which has made it difficult to assess their phylogenetic position within Mammalia, but they have generally been considered members of the plesiadapiform radiation and close fossil relatives of primates for the last 50 years based on dental similarities. The partial cranium of Zanycteris paleocenus, the first known picrodontid specimen to preserve basicranial anatomy, was microCT scanned here for the first time. MicroCT scan data reveal that picrodontids possess distinct basicranial morphology from that of other plesiadapiforms and instead resemble non-euarchontan mammals, such as nyctitheriids and apatemyids. Results of a phylogenetic analysis incorporating these new data generally support these morphological interpretations and do not recover picrodontids as closely related to plesiadapiforms nor even within Euarchonta. Instead picrodontids are recovered as closely related to non-euarchontan placental groups such as the apatemyids. These results emphasize the need to scrutinize proposed synapomorphies, especially of highly autapomorphic taxa with poor fossil records.
Palaechthonids are a primitive family of plesiadapiforms that are primarily known by their relatively plesiomorphic dentitions. One relatively complete partial cranium of Plesiolestes nacimienti is the geologically oldest plesiadapiform cranial fossil known. This specimen was studied over 50 years ago, which led to the inference that the ancestral primate was terrestrial, insectivorous, and reliant on tactile and olfactory information over vision. This specimen was reanalyzed to better understand the cranial morphology of palaechthonids and that of primitive plesiadapiforms. MicroCT scan data unveiled new anatomy (e.g., foramen ovale, caudally extensive ectopterygoid process), and contrary to previous inferences, showed that Plesiolestes nacimienti did not possess a postorbital process. Comparisons underscore similarities between Plesiolestes and non-microsyopid plesiadapiforms but fail to uncover unambiguous synapomorphies with an extant euarchontan clade. Based on paleobiological inferences made here and within the context of the last 50 years of research, Plesiolestes and, by extension, the ancestral plesiadapiform is reconstructed as broadly like the extant arboreal treeshrew, Ptilocercus lowii, albeit more frugivorous.
While primates are often differentiated from other euarchontans by a suite of characteristics, the presence of a petrosal bulla is thought to represent a truly diagnostic feature. Close Paleogene relatives of primates, the plesiadapiforms, have been proposed to possess a variety of bullar compositions ranging from the inferred absence of an ossified bulla (microsyopids), the presence of an entotympanic bulla (paromomyids), and the presence of a petrosal bulla (micromomyids, carpolestids, and plesiadapids). However, these observations relied on physical observations of specimens or relatively low resolution microCT scan data. Here, I microCT scanned specimens belonging to the Microsyopidae, Micromomyidae, Paromomyidae, Carpolestidae, and Plesiadapidae at high resolution. Results demonstrate that the bulla of microsyopids was ossified and likely constructed of the ectotympanic or ectotympanic/entotympanic; the bulla of micromomyids and carpolestids is a composite of the petrosal, basisphenoid/alisphenoid, and ectotympanic; and the bulla of paromomyids and possibly plesiadapids is derived from the petrosal with contributions from the ectotympanic.
My analysis revealed that the bulla of microsyopids displays some similarities to extant colugos, supporting previous hypotheses of these affinities. Results also documented distinct similarities among non-microsyopid plesiadapiforms to the exclusion of microsyopids and all other euarchontan mammals, which might suggest that non-microsyopid plesiadapiforms form a clade to the exclusion of other euarchontans. If non-microsyopid plesiadapiforms are monophyletic, the inferred petrosal bulla of paromomyids and possibly plesiadapids would be convergent with primates. Regardless of whether non-microsyopid plesiadapiforms are stem primates or stem primatomorphans, the likely independent evolution of a petrosal bulla in some non-microsyopid plesiadapiforms provides a model to understand how this structure evolved within primates.
Recommended Citation
Crowell, Jordan, "Micro-Computed Tomography Unveils New Plesiadapiform Cranial Morphology: Implications for Early Primate Evolution" (2025). CUNY Academic Works.
https://academicworks.cuny.edu/gc_etds/6258
