Graduation Semester and Year

2013

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Earth and Environmental Science

Department

Earth and Environmental Sciences

First Advisor

Merlynd K Nestell

Abstract

The Arlington Archosaur Site (AAS) is a North Texas fossil locality that preserves a Mid- Cretaceous ecosystem along a low lying coastal plain. The site lies within the Cenomanian (95 Mya) rocks of the Woodbine Formation in North Arlington, Tarrant County, TX. The depositional environment of the AAS is a coastal delta plain, principally a peat bog. The vertebrates recovered from the AAS include dinosauria (ornithopod and theropod), crocodyliform, chelonian and dipnoan. A large, nearly complete crocodyliform was excavated in the summer of 2009, and proved to be a new taxon; Deltasuchus motherali. The name of the new taxon was based upon an adult crocodyliform skeleton that was recovered, along with the remains of numerous juveniles. Of the AAS dinosaurs, ornithopods are more common and theropod quite rare, represented only by scant isolated teeth and claws. The ornithopod material is predominantly hadrosauroid post crania from a single adult individual and includes; an axis, cervical, dorsal and caudal vertebra, a scapula, coracoid, a left femur and a near complete left pelvis (ilium, ischium and prepubis). Of equal importance is the first juvenile ornithopod (hadrosauroid) material recovered from the Woodbine Formation at the AAS. The AAS juvenile hadrosauroid material is principally post crania and represents multiple individuals of variable growth stages.The AAS ornithopod (hadrosauroid) material represents the most recovered from the Woodbine Formation to date and was used to describe the post cranial anatomy of a primitive hadrosauroid; Protohadros. Other components of the coastal plain ecosystem include a new species of lungfish. The new dipnoan is represented by one pterygopalatine and five prearticular tooth plates. The AAS lungfish is significant as it represents a new species that is unique to Texas, as well as it extends the biostratigraphic range of Ceratodus into the Cenomanian .The new species was named Ceratodus carteri in honor of the fossil collector that discovered it, Brad Carter. Numerous trace fossils were documented and recovered from the AAS, of particular interest were coprolites that preserved data on the regional ecology. Over 150 coprolites were recovered that demonstrate a variety of morphologies suggestive of multiple taxa. The morphologies are Cylindrical, Spiral and Ovoid. The cylindrical coprolites were interpreted as crocodyliform intestinal tract material (Thulborn, 1991). Spiral coprolites are typically indicative of marine taxa; shark and fish (Coy, 1995). Ovoid coprolites are indicative of reptiles; dinosaurs and crocodyliforms. The most common coprolite found at the AAS were attributed to crocodyliforms. The mapped locations of the coprolites show that most of the crocodyliform coprolites were found with broken and bitten turtle remains along with numerous crocodyliform teeth. The coprolites were used to propose a paleobiological model for the AAS as being a coastal crocodyliform feeding ground. Examples of paleobiological behavior in extinct organisms are rare. This study presents fossils from the Woodbine Formation that exhibit tooth marks consistent with predation by large crocodyliforms (Deltasuchus). The feeding traces consist of pits, scores, and punctures that occur on multiple turtle shell fragments and two dinosaur limb bones. The pattern of marks and breakage on turtle carapaces and plastra suggests they were crushed, whereas marks on the dinosaur bones indicate possible dismemberment. Marks are consistent with those produced by living crocodylians exhibiting similar behavior. The morphology of the new crocodyliform, Deltasuchus and distribution of bite marks indicates it was likely a generalist; an opportunistic predator that fed on a variety of prey, including turtles and dinosaurs. Given this evidence and the paleoenvironmental setting, the ecology of large crocodyliforms from the Woodbine Formation was likely most similar to that of fossil and living crocodylians inhabiting delta-plain environments. Not only were these crocodyliforms likely apex predators in the AAS ecosystem, they also played an important taphonomic role in the assembly of vertebrate remains from the coastal AAS community. This analysis of the Woodbine Formation at the AAS brings forth more data on the dynamics of Cenomanian coastal ecosystems in North America with regard to Cretaceous paleogeography, as the AAS represents the most complete ecosystem discovered in southwest Appalachia.

Disciplines

Earth Sciences | Physical Sciences and Mathematics

Comments

Degree granted by The University of Texas at Arlington

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