Date of Award

2021

Degree Name

Biological Sciences

College

College of Science

Type of Degree

M.S.

Document Type

Thesis

First Advisor

Dr. F. Robin O’Keefe, Committee Chairperson

Second Advisor

Dr. Herman Mays

Third Advisor

Dr. Habiba Chirchir

Abstract

Polycotylidae is a family of plesiosaurian marine reptiles that evolved during the Early Cretaceous and radiated into multiple genera during the Late Cretaceous, achieving a worldwide distribution. Derived polycotylids of the subclade Polycotylinae have a gracile and elongated rostrum, homodont dentition, an extended mandibular symphysis, and foreshortened temporal fenestrae. In this thesis, I describe a small and highly derived new polycotylid taxon based on three specimens from the Campanian of the Western Interior Seaway in North America. A high number of maxillary teeth, fused neural arches, propodials with well-defined facets, and heavily remodeled cortical bone indicate the specimens are adults, making this taxon the smallest known adult polycotylid. The new taxon has a large orbit with two autapomorphic features: a triangular supraorbital ledge that projects laterally over its anterodorsal margin and a broad supraorbital ridge that borders its posterodorsal rim. Other diagnostic features include a parasphenoid that posteriorly doubles in width, an unusually short postorbital region, a uniquely wide occiput, a retroarticular process consisting of the angular in nearly its entire dorsal margin, and femora that are distally narrower than those of other polycotylids. A phylogenetic analysis reveals the new taxon to be deeply nested within Polycotylinae and differentiated from Dolichorhynchops osborni, despite the previous referral of one of the specimens to this species. The analysis places D. osborni and D. herschelensis in a subclade sister to a subclade containing the new taxon and D. bonneri, suggesting that the generic name of the latter should also be revised. Morphometric analyses confirm the similarity between the two known skulls and quantify their morphological distinction from other polycotylid taxa, including ontogenetically young polycotyline specimens. Morphological similarity between the supraorbital ledge of the new taxon and the supraorbital processes of raptorial birds suggests a diurnal lifestyle and shallow-water habitat for this taxon and has ecological implications for other taxa.

Subject(s)

Plesiosauria -- Research.

Convergence (Biology) -- Research.

Paleontology -- Cretaceous -- Research.

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