Thesis
A new alligatorine from the Middle Eocene of Utah and the origins of modern Alligator
University of Iowa
Master of Science (MS), University of Iowa
Spring 2020
DOI: 10.17077/etd.005413
Abstract
The Uinta Basin of Utah has many exposed layers from the middle Eocene, and nearly all 500 m of the Uinta Formation are exposed. Although this area is known for its Cenozoic mammals it also contains early crocodylids, including an unidentified alligatorine. The Uinta Formation and the alligatorine material discovered therein would fit perfectly into the ~30 mya gap between the earliest species of Alligator, A. prenasalis, and the sister taxon to Alligator, Wannaganosuchus brachymanus. We reviewed material that could be assigned to the new taxon and coded the Uinta material into an updated matrix.
The Uinta material was found to have characteristics of both the more basal alligatorines while also sharing characteristics with the earliest species of Alligator. Like most basal alligatorines, the Uinta material has a shorter snout compared to modern Alligator and extremely robust posterior teeth. However, its snout is longer than those basal alligatorines and less pointed, and the morphology of the external naris is closer to that of A. prenasalis and the other early alligators.
In this paper I describe the Uinta material as a new genus and species, Uintachampsa elseyae, and describe its defining characteristics. Phylogenetic analyses placed Uintachampsa as sister to the genus Alligator, but support for the nodes of the Wannaganosuchus + Uintachampsa + Alligator clade is relatively weak until we get to the alligator genus. Despite this, if Uintachampsa is indeed on the Alligator lineage then it leads to two interesting conclusions: first, that throughout its evolution Alligator was moving away from a more specialized morphology towards a more generalist morphology in seeming violation of “Cope’s Law,” and second, that the reason for this shift in morphology is not related to the extinction of other large generalist crocodylians of the North American Western Interior, as Uintachampsa shows that this trend was beginning before the extinction took place.
Details
- Title: Subtitle
- A new alligatorine from the Middle Eocene of Utah and the origins of modern Alligator
- Creators
- Margaret Rubin
- Contributors
- Christopher A Brochu (Advisor)Bradley Cramer (Committee Member)Maurine Neiman (Committee Member)
- Resource Type
- Thesis
- Degree Awarded
- Master of Science (MS), University of Iowa
- Degree in
- Geoscience
- Date degree season
- Spring 2020
- DOI
- 10.17077/etd.005413
- Publisher
- University of Iowa
- Number of pages
- vii, 65 pages
- Copyright
- Copyright 2020 Margaret Rubin
- Language
- English
- Description illustrations
- illustrations (some color)
- Description bibliographic
- Includes bibliographical references (pages 35-41).
- Public Abstract (ETD)
The Uinta Basin is in the northeastern corner of Utah, stretching almost to the border with Wyoming. The area has been very well studied in terms of fossil mammals, but only recently has there been serious examination of the extinct crocodylians found there. I am focusing on the layer the Uinta Formation from the mid-Eocene, around 45 mya. This time period fits almost perfectly in the middle of the “gator gap” the approximately 30 million year time span between the earliest member of the genus Alligator and the next closest relative. In this study, I review and describe fossils that I am assigning to a new genus and species and examining its placement in the alligator family tree.
The ancestors of modern alligators did not look very much like the American and Chinese alligators that we see today. The American alligator especially is known for its long, broad, rounded snout and its ability and tendency to eat just about anything that it can fit in its mouth; in comparison, early alligator relatives generally had short, pointed snouts and large back teeth that are thought to be suited for eating hard-shelled prey. The material that I am looking at is an intermediate between the two morphologies, with a snout that is proportionally longer than the those of earlier relatives and less pointed, but still retaining the blunt, robust crushing teeth.
Phylogenetic analyses place the Uinta material as sister to the genus Alligator. This gives us clues as to how the alligator lineage evolved, and why we see the unusual trend in morphology of changing from something specialized to something more general. A better understanding of alligator evolution will help us to protect the modern species that remain and other crocodylians around the world.
- Academic Unit
- Earth and Environmental Sciences
- Record Identifier
- 9983949691102771
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