Thesis
The systematics and paleoecological significance of a new generalized eusuchian from the early Paleocene of North Dakota
University of Iowa
Master of Science (MS), University of Iowa
Spring 2023
DOI: 10.25820/etd.007215
Abstract
A new eusuchian from the early Paleocene Fort Union Formation of North Dakota is to be described based on a partial skull and jaws. The new form bears a striking outward resemblance to two ostensibly unrelated eusuchians: the basal crocodylian Borealosuchus sternbergii (late Maastrichtian, Montana) and the alligatoroid Diplocynodon remensis (late Paleocene, France) and preserves a confusing combination of plesiomorphic and derived states, calling into question the monophyly of Borealosuchus, as currently construed. In particular, the quadrate foramen aëreum of the Fort Union form is located on the dorsal rather than the dorsomedial surface of the quadrate ramus, suggesting an alligatoroid affinity. Maximum parsimony analyses recover trees supporting three distinct positions for the new form – within Diplocynodon, basal within Borealosuchus, or as an allodaposuchid. A re-evaluation of the morphology of early members of Borealosuchus from the Western Interior suggests they might be basal alligatoroids. However, later western forms (e.g. Borealosuchus wilsoni) and fossils referred to Borealosuchus from the Atlantic Coastal Plain (e.g. B. threeensis) may not actually fall within Borealosuchus and instead be eusuchians close to the ancestry of Crocodylia. A full reanalysis of species currently described as Borealosuchus is suggested. These results bear not only on our understanding of the morphology of the ancestral crocodylian, but also on the historical biogeography of early alligatoroids in North America and Asia.
Details
- Title: Subtitle
- The systematics and paleoecological significance of a new generalized eusuchian from the early Paleocene of North Dakota
- Creators
- Cameron J. Parrelly
- Contributors
- Christopher A Brochu (Advisor)Jonathan M Adrain (Committee Member)Eric Wilberg (Committee Member)
- Resource Type
- Thesis
- Degree Awarded
- Master of Science (MS), University of Iowa
- Degree in
- Geoscience
- Date degree season
- Spring 2023
- Publisher
- University of Iowa
- DOI
- 10.25820/etd.007215
- Number of pages
- vi, 52 pages
- Copyright
- Copyright 2022 Cameron J. Parrelly
- Language
- English
- Date submitted
- 01/04/2023
- Date approved
- 06/30/2023
- Description illustrations
- illustrations (some color)
- Description bibliographic
- Includes bibliographical references (pages 48-52).
- Public Abstract (ETD)
Crocodylia represents the lineage that includes modern day crocodiles, alligators, and gharials. The group first appears in the Late Cretaceous of North America, but its early history is still ambiguous. The genus Borealosuchus is of modern significance to understanding the early divergence of Crocodylia, however its placement phylogenetically has been inconsistent in scientific analyses. Borealosuchus species are known from the Late Cretaceous to the middle Eocene (80-45 million years ago) of North America, surviving through the Cretaceous-Paleogene (K-Pg) mass extinction (65 million years ago) that resulted in the end of many groups of animals, including other crocodyliforms and dinosaurs.
Borealosuchus, despite controversies over its placement, has been found to remain monophyletic. The new form described here displays a mixture of ancestral and derived traits, including a quadrate foramen aëreum on the dorsal surface, that call this monophyly into question. A revision of previously described species is suggested to determine if some borealosuchids are more closely related to alligators than others.
This study describes a new species of Borealosuchus from just above the K-Pg boundary of North Dakota. Two specimens were coded with a matrix of 189 morphological characters along with 76 other Eusuchian species and subjected to phylogenetic analysis. The majority of scored characters were cranial due to a lack of postcranial material from the specimen being described, but some postcranial material was preserved in referred material. Phylogenetic analysis indicates Borealosuchus is a monophyletic group within Crocodyia, sister to Planocraniidae+Brevirostres (Alligators, Crocodiles, and closely related species). A new species of Paleogene Borealosuchus is resolved as sister to B. sternbergii at the base of the clade.
- Academic Unit
- Earth and Environmental Sciences
- Record Identifier
- 9984425392902771
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