Master's Theses

Document Type

Thesis

Date of Award

Spring 2020

Degree Name

Master of Science (MS)

Department

Geosciences

Advisor

Dr. Laura Wilson

Abstract

Convergence is an evolutionary phenomenon wherein distantly related organisms independently develop features or functional adaptations to overcome similar environmental constraints. Historically, convergence among organisms has been speculated or asserted with little rigorous or quantitative investigation. More recent advancements in systematics has allowed for the detection and study of convergence in a phylogenetic context, but this does little to elucidate convergent anatomical features in extinct taxa with poorly understood evolutionary histories. The purpose of this study is to investigate one potentially convergent system—the feeding structure of Xiphactinus audax (Teleostei: Ichthyodectiformes) and Megalops atlanticus (Teleostei: Elopiformes)—using a comparative anatomical approach to assess the degree of shared morphospace occupation. X. audax was a large, predatory fish that inhabited the Western Interior Seaway (WIS) during the Late Cretaceous and went extinct 66 mya. M. atlanticus—the Atlantic tarpon—is a large elopiform fish that inhabits the Gulf and Atlantic coasts. Because of structural similarities in their crania and post-crania, M. atlanticus is often used formally and informally as a modern analog for X. audax. Landmark-based geometric morphometrics (GM) was applied to determine the structural similarity in the feeding morphology of these two fish species. Six X. audax and six M. atlanticus specimens were 3D scanned and reconstructed as 3D models, and the GM procedure was conducted on in both 2D and 3D treatments. Principal components analysis (PCA), discriminant function analysis (DFA), sequential agglomerative hierarchical non-overlapping (SAHN) cluster analysis, and a multiiv response permutation procedure (MRPP) were all performed to quantify the shape difference between the 12 specimens. All analyses produce comparable results. X. audax and M. atlanticus differ significantly in the structure of their feeding morphology and do not overlap considerably in morphospace, casting doubt on the idea that X. audax and M. atlanticus are structurally convergent in their feeding morphology. Most notably, there are substantial differences in the size and shape of the premaxilla, the length of the maxilla, and the inflection of the anterior dentary. The differences in these structures likely relate to the preferred feeding habits of each fish, with X. audax preferring large individual prey, and M. atlanticus relying on suction feeding to consume smaller schooling prey. These results suggest M. atlanticus is a poor modern analog for X. audax with respect to feeding morphology.

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Rights

© 2020 Edward Chase Holt Shelburne

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