Publication Date

2020

Document Type

Dissertation

Committee Members

Christopher Barton, Ph.D. (Advisor); Charles Ciampaglio, Ph.D. (Committee Member); Sarah Tebbens, Ph.D. (Committee Member); Stephen Jacquemin, Ph.D. (Committee Member); Margaret Yacobucci, Ph.D. (Committee Member)

Degree Name

Doctor of Philosophy (PhD)

Abstract

Ammonoid cephalopods have chambered shells that regulated buoyancy. The morphology of their shells strongly influenced the physical properties acting on these animals during life. Heteromorph ammonoids, which undergo changes in coiling throughout ontogeny, are the focus of this dissertation. The biomechanics of these cephalopods are investigated in a framework involving functional morphology, paleoecology, and possible modes of life. Constructional constraints were investigated for the marginally-corrugated septal walls within the chambered ammonoid shell. These constraints governed the positive relationship between septal complexity and terminal size. Furthermore, increased septal complexity facilitated liquid retention via surface tension. More complex septa would have increased liquid retention at larger scales, which could have been used as liquid ballasts, reserves for buoyancy adjustment, or to prevent disruptive sloshing of cameral liquid. New methods for the virtual reconstruction of cephalopod shells are described. The shell constrains the volume and shape of each material of unique density that influenced organismal mass and its distribution. Therefore, these virtual models can be used to compute the conditions for neutral buoyancy, hydrostatic stability, syn vivo orientation, and the directional efficiency of movement. The rigid shell also constrains how the living ammonoid would have interacted with fluids in a dynamic setting. A new method for the construction of neutrally-buoyant, physical models is described, which can be used to compute hydrodynamic properties such as drag and swimming velocity. The biomechanics of three heteromorph ammonoid morphotypes from the North American Western Interior Seaway are discussed. These morphotypes represent the families Baculitidae, Nostoceratidae, and Scaphitidae. These investigations provide a better understanding of the hydrostatic and hydrodynamic properties that constrained the modes of life for the majority of prominent ammonoid taxa from the Late Cretaceous of the U.S. Western Interior. Novel modeling techniques provide data that suggests heteromorph ammonoids had selective pressures imposed on them for specific biological functions or life habits, rather than pointless morphological experimentation or liberation from such evolutionary pressures. These syn vivo physical properties also influenced the biogeographic dispersal and paleoecology for these enigmatic creatures that were once vital components of marine ecosystems.

Page Count

232

Year Degree Awarded

2020


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