Master's Theses

Document Type

Thesis - campus only access

Date of Award

Spring 1988

Degree Name

Master of Science (MS)

Department

Geosciences

Advisor

Michael E. Nelson

Abstract

Where present, the basal Buckhorn Member of the Cedar Mountain Formation (Aptian-Albian) separates the overlying main member from the mudrocks of the Tithonian Brushy Basin Member of the Morrison Formation. Without the Buckhorn, the mudrocks of the Cedar Mountain Formation appear to grade into those of the Morrison, so one is apt to call the entire sequence Morrison. Where the Cedar Mountain Formation appears to grade in to the Morrison, use of lower vertebrate teeth can provide an additional means to separate the Morrison from the Cedar Mountain Formation. Lower vertebrate teeth and associated microvertebrate remains (eggshell and bone fragments) occur in variable concentrations throughout the exposure of the Cedar Mountain Formation main member. Most of the lithologies exhibit varying degrees of paleosol development with siliceous tubules and calcareous tubules and nodules. The greatest concentration of microvertebrate remains occur in calcareous paleosols containing spindle nodules developed in natural levee siltstones at the Robinson Eggshell Quarry (REQ). Lesser concentrations occur in paleosols developed in overbank mudstones and claystones that contain chalcedony tubules at REQ and contain calcareous tubules and nodules at the Dave Hunter BS Quarry (DHBS) sections. The DHBS Quarry is found in an overbank carbonaceous mudstone lacking also occur in a lens of carbonaceous shale thought to represent a back swamp at the Rough Road Quarry. Until recently, the microvertebrate-carbonaceous shale association was thought to be the typical mode of occurrence within the Cedar Mountain Formation. Thus, microvertebrate remains may occur with sufficient frequency to have a stratigraphic application in the separation of the Morrison and Cedar Mountain formations. Lower vertebrates found in the DHBS and REQ sections of the Cedar Mountain Formation include teeth of the fish Lepidotes (Semiontiformes, Actinopterygii), a stingray (Dasyatidae, Batoidea), immature crocodilians: Bernissartia (Bernissartidae), Goniopholis (Goniopholididae), Pholidosauridae, Polydectes (?Goniopholididae), Machimosaurus (Teleosauridae), and Theriosuchus (Atoposauridae); and a theropod dinosaur (Coeluridae). The dimensions of the crocodilian teeth rarely exceeded 3.50 mm and probably belonged to nestlings and immature individuals. Furthermore, the association of dinosaur eggshell and tiny crocodilian teeth at all three locales suggests the proximity of reptilian nesting grounds to the channels that deposited the Cedar Mountain Formation. Periodic floods encroached on the nesting grounds and the teeth became sedimentary particles subject to transportation. However, fluvial transportation fluvial did not overtly damage the teeth. Most of the cracking and chipping of tooth enamel occurred during predation, or the teeth developed vertical cracks because of drying. The reptiles and fish of the Cedar Mountain Formation occupied various niches. Bernissartids, Lepidotes, and the dasyatid ray had low crowned teeth for eating mollusks. Polydectes had sharp, piercing teeth for eating fish. Goniopholids, pholidosaurids, and teleosaurids crunching teeth for eating crustaceans and ganoid fish. Finally, atoposaurids had a specialized, shearing dentition and probably ate fish or acted as scavengers. Finally, the association of theropod teeth with the crocodilian teeth may indicate a predator -prey relationship. Such diversity in lower vertebrates, especially crocodilians, depositional contrasts nature, that with have two units, presumed of similar shortages of crocodilians: the Cloverly Formation (Lower Cretaceous) and the Morrison. The scar city in crocodilians in these units may result from the lack of a concerted effort to recover tiny crocodilian teeth. Once such an effort is mounted, I predict the Cedar Mountain and Cloverly formations to have greater faunal similarity, whereas the Cedar Mountain fauna will contrast with that of the Morrison Formation in the types of crocodilians found. Two such crocodilians, bernissartids and pholidosaurids, are at present limited to the Cretaceous in North America and lack counterparts in the Late Jurassic. The above distinction is made because atopossaurids, goniopholids, and teleosaurids are known from the Jurassic of North America. During the Jurassic and Early Cretaceous, North America and Europe were joined and crocodilians dispersed between these continents. Because of the favorable plate configuration, pholidosaurs, Machimosaurus, and Theriosuchus display a pattern of Late Jurassic occurrence in Europe and Early Cretaceous appearance in North America. In addition, the crocodilian Bernissartia, known in the Barremian-Aptian beds at Galve (Province of Teruel), Spain, reached Texas in the lower Albian and arrived in Utah late in the Albian.

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Rights

© 1988 Michael L. Pomes

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