Master's Theses

Date of Award

Spring 1999

Degree Name

Master of Science (MS)

Department

Geosciences

Advisor

Richard Zawerksi

Abstract

The Ptychodontidae historically have been classified within the Hybondontoidea, a group of chondrichthyans at an intermediate stage of evolution. As both the most primitive and most advanced forms of Ptychodus possess skeletal and histological features previously described only in the most advanced chondrichtyhyans, the generally accepted systematic position of Ptychodus may be incorrect. Ptychodus was a duronphagous shark, and workers have correlated its distribution with the presence of shelf facies that contain inoceramids. Observations of the Kansas and Texas distributions of ptychodonts, grouped into crow-height classes, reveals a pattern of distribution that suggests the taxon underwent species succession. Complex, three-layered enameloid consisting of a shiny layer of enameloid (SLE) on the outer surface, a middle layer of “parallel-fibred” enameloid (PFE), and an inner layer of “tangled fibred” enameloid (TFE) is considered a synapomoyphy for the neoselachian sharks. Scanning electron microscopy shows PFE and SLE exist in the teeth of Ptychodus decurrens. P. mortoni possesses three distinct layers of enameled-TFE, SLE, and an intermediate layer that remains to be named. The PFE of Ptychodus is similar to the PFE of neoselachian sharks. The presence of complex layers enameloid in Ptychodus supports suggestions of other works who believe the Ptychodontidae are neoselachian sharks. Ptychodus may have utilized three-point bending stresses to fracture the shells of its prey (mollusks). Bending stresses occurred at a large scale (relative to the entire dentition) and a small scale (relative to the ridges of the teeth). The multi-layered histology of the teeth prevents cracks form migrating through the teeth. In addition, the domed shape and arched transverse ridges of the teeth serve to increase the structural strength of the dentition.

Rights

Copyright 1999 Michell L. David

Comments

Notice: This material may be protected by copyright law (Title 17 U.S. Code).

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