Master's Theses

Document Type

Thesis

Date of Award

Fall 2018

Degree Name

Master of Science (MS)

Department

Geosciences

Advisor

Dr. Laura Wilson

Abstract

Clidastes was a large marine reptile from the Late Cretaceous Western Interior Seaway of North America. Though the remains of Clidastes have been found in the Cretaceous chalks and shales for over 150 years, little is known about their growth rates.

Osteohistology is a well-documented technique used to investigate growth in extinct animals. Previous histological studies of Clidastes have hypothesized higher growth rates in basal mosasaurids than varanids due to higher vascularity. These studies focused on adult specimens but did not look at ontogenetic changes in growth rates. Isotopic studies of Clidastes indicate high metabolic temperatures, leading to speculation these animals had either gigantothermic or endothermic metabolisms. Whether the growth rates in Clidastes are comparable to endothermic or ectothermic animals has not been studied. This study uses osteohistology and skeletochronology to determine age at the time of death, investigate ontogenetic changes in internal bone microstructure, and estimate growth rates through Clidastes ontogeny.

Four humeri representing a size gradient in Clidastes were histologically analyzed. Skeletochronological ages of the four specimens are yearling (year), juvenile (3-4 years), sub-adult (6-7 years), and possible adult (13-14 years). All humeri show parallel-fibered bone as the main tissue type. Vascularity and growth rates decrease as size and age increase through Clidastes ontogeny. Primary osteons and radial canals seen in early ontogeny decrease until only longitudinal simple canals remain in late ontogeny. Though no growth marks are visible in the yearling, vascularity is much higher than the other specimens, suggesting a higher growth rate during the first year of life. In the juvenile, growth rates in the second year of life are 1.70 µm(day)ˉ¹. In the sub-adult, growth rates in years four and five are 1.65 and 1.75 µm(day)ˉ¹ respectively. In the largest humerus, growth rates in years eight through eleven are variable, but all are less than 0.80 µm(day)ˉ¹. No humeri show evidence of skeletal maturity, though the slowing of growth in the largest humerus could represent sexual maturity or the onset of skeletal maturity.

Clidastes grew fastest during the first year of life. Growth slowed during the second year but continued at this same rate until after the sixth or seventh year when it slowed again. While this study finds that vascularity in Clidastes is greater than modern varanids, growth rates are more comparable to ectothermic than endothermic animals. Growth rates determined in this study support the gigantothermic rather than endothermic metabolic hypothesis for Clidastes.

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Rights

© 2018 Cyrus C. Green

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