Thesis
A reassessment of the Eocene crocodyloid Asiatosuchus grangeri Mook,1940 and its implications on the phylogenetic relationships and evolution of Paleogene crocodiles
University of Iowa
Master of Science (MS), University of Iowa
Spring 2024
DOI: 10.25820/etd.007351
Abstract
The genus Asiatosuchus has historically been used as a wastebasket taxon, referring to a basal crocodile from the Paleogene and has been attributed to species from across North America and Eurasia. These species represent some of the closest extinct relatives to all modern crocodiles, possessing a combination of ancestral and derived traits to the crocodile lineage. Many of these species are represented by only fragmentary remains and are poorly described in the literature, leading to a lack of resolution in the phylogenetic relationships of these species. This study sought to reassess the type species for this genus, Asiatosuchus grangeri Mook, 1940, which was recovered from the early to middle Eocene Arshanto Formation in the Erlian Basin of Inner Mongolia, China. All known fossil specimens were reexamined to construct a composite model of the species and used to create a comprehensive morphologic description. From this description, a maximum parsimony analysis was conducted to ascertain the nature of its evolutionary relationships with other basal crocodiles and to compare it to other species which have been previously referred to Asiatosuchus.
Asiatosuchus grangeri largely resembles a modern crocodile, with a dorsoventrally compressed skull and triangular snout, but with a more lingual tooth occlusal pattern, flatter orbits, and longer dentary symphysis. Compared to previous analyses, A. grangeri was found to be somewhat more derived and less closely related to other species that have been previously referred to the genus. The phylogenetic trees generated from this study demonstrate a transition of traits from the ancestral crocodilian to the modern crocodile. The results of this analysis potentially point towards an Asian lineage of early crocodile which may have persisted from the Late Cretaceous up to the Eocene, with the Asian species Asiatosuchus nanlingensis Young, 1964 being returned in some trees as closely related to A. grangeri, though this remains uncertain due to A. nanlingensis’s lability in this study. Further studies are required among basal crocodyloids to both resolve the remaining degree of uncertainty in their phylogenetic relationships and reclassify species that should no longer be diagnosed to Asiatosuchus, as well incorporate these findings with other crocodilian groups to assess larger scale evolutionary patterns.
Details
- Title: Subtitle
- A reassessment of the Eocene crocodyloid Asiatosuchus grangeri Mook,1940 and its implications on the phylogenetic relationships and evolution of Paleogene crocodiles
- Creators
- John Joseph Nhan Nguyen
- Contributors
- Christopher A Brochu (Advisor)Jonathan M Adrain (Committee Member)Massimo Delfino (Committee Member)
- Resource Type
- Thesis
- Degree Awarded
- Master of Science (MS), University of Iowa
- Degree in
- Geoscience
- Date degree season
- Spring 2024
- DOI
- 10.25820/etd.007351
- Publisher
- University of Iowa
- Number of pages
- vii, 99 pages
- Copyright
- Copyright 2024 John Joseph Nhan Nguyen
- Grant note
- Thank you to the Max and Lorraine Littlefield Fund for funding this study. (ii)
- Language
- English
- Date submitted
- 04/18/2024
- Description illustrations
- color illustrations
- Description bibliographic
- Includes bibliographical references (page 95-99).
- Public Abstract (ETD)
The genus Asiatosuchus has historically been used to refer to any early crocodile species from the Paleogene that possessed some mixture of ancestral and derived characteristics. Due to the fragmentary nature and incomplete description of these species, phylogenetic analyses involving them often return a high degree of uncertainty regarding the evolutionary relationships between them and other basal crocodiles.
This study examined the species Asiatosuchus grangeri Mook 1940, from the Eocene of northern China. In previous studies, it had only been briefly and incompletely described, leading to a lack of resolution regarding its morphology and phylogenetic relationships. Here, all known fossil specimens of the species were used to develop a description that could begin to address this uncertainty. These findings were then input into a phylogenetic analysis that generated a model of A. grangeri’s evolutionary relationships.
The results of this study place A. grangeri as more closely related to modern crocodiles than previously thought and less so to other species that have been referred to Asiatosuchus. Some trees also place it as closely related to the species Asiatosuchus nanlingensis Young, 1964, though A. nanlingensis remains labile in this analysis and the exact nature of this relationship remains uncertain. Further studies may be built from this that help to resolve the remaining evolutionary uncertainty among early crocodiles and provide useful insight into questions regarding morphologic and molecular relationships within the wider context of all crocodilians.
- Academic Unit
- Earth and Environmental Sciences
- Record Identifier
- 9984647455102771
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