Date of Degree
PhD (Doctor of Philosophy)
Christopher A. Brochu
First Committee Member
Second Committee Member
Third Committee Member
Fourth Committee Member
This project seeks to explore, name, and describe some of the earliest known members of Alligatoroidea. Explorations of Bottosaurus harlani and Deinosuchus reveal that early in their evolutionary history alligatoroids had attained bauplans that are highly divergent from the ancestral condition in both body size and morphology.
Bottosaurus harlani preserves aspects of the skull table – including constricted supratemporal fenestrae, a linear frontoparietal suture, and a large trapezoidal dorsal supraoccipital exposure – that are similar to those of caimans. Optimal trees from phylogenetic analysis recover B. harlani in three different positions; as a sister either to the modern dwarf caimans (Paleosuchus), or either living species of Paleosuchus. That a substantial stratigraphic gap separates B. harlani from both species of Paleosuchus, which first appears in the Miocene, along with low character and nodal support raises questions about this relationship. However, should the relationships recovered here be true, Bottosaurus harlani would be the oldest known caiman. The taxon indicates that morphologies common to modern caimans date to the earliest record of the clade.
In addition to the enigmatic B. harlani, the Campanian giant Deinosuchus was re-evaluated as part of this project. Recent consensus has been that the three named species of Deinosuchus (D. hatcheri, D. riograndensis, and D. rugosus) represent a single, widely ranging species. Newly-collected material from the Big Bend region of western Texas and increased sampling of the lineage from throughout North America allowed for a review of species-level systematics of Deinosuchus and helped refine its phylogenetic placement among crocodylians. Deinosuchus from western and eastern North America can be consistently differentiated and represent different species.
As a result of the lack of diagnostic characters in the very incomplete holotype specimen, the name Deinosuchus is restricted to D. hatcheri. To encompass specimens formerly included in Deinosuchus a new genus, Deinosuchoides, is erected. In addition to naming a new genus, the holotype specimen for Deinosuchus rugosus is determined to be undiagnostic to species level and is therefore a nomen dubium. A new species, Deinosuchoides schwimmeri, is erected upon a cranial specimen from Mississippi.
The snout of Deinosuchoides is very long and wide. Almost invariably, crocodylian snouts are either long or wide, but not both. In addition to the unusually long and wide snout, the skull table of this taxon bears hallmarks found in species of other long-snouted taxa such as Tomistoma and Gavialis.
To explore the variability of the crocodylian skull table a morphometric analysis was conducted with the inclusion of fossil taxa. As the skull table is robust and likely to be recovered in the paleontological record this element was an ideal fit for a morphometric study. In addition to establishing the morphospace occupation of fossil taxa, the morphometric analysis found considerable overlap in morphospace between Alligatoroidea and Crocodyloidea – the overlap between these groups may be the product of shared ancestry. Additionally, similarities exist in the ecologies of these groups as evidenced by shared snout shape categories. This project finds association between skull table shape and snout length. As such, plotting isolated skull tables in morphospace, may indicate snout length and thus ecology of fossil taxa.
When landmarks representing the supratemporal fenestrae are included in the analysis Gavialoidea is broadly separated from the other groups in morphospace. It has been long hypothesized that the size of the supratemporal fenestrae reflect the length of the snout as a result of jaw musculature attaching to their medial margins. However, this relationship is not as straightforward as previously hypothesized; the snouts of the crocodyloids Tomistoma and Euthecodon may exceed the length of the snout in gavialoids but their supratemporal fenestrae are proportionally smaller. This study suggests that a phylogenetic constraint on the size of the supratemporal fenestrae may be present in crocodyloids.
In addition to exploring morphospace occupation, allometric trajectories of all extant taxa with available ontogenetic sequences were explored. The smallest extant taxa (O. tetraspis, P. palpebrosus, and P. trigonatus) demonstrate allometric trajectories that plot alongside the juveniles of the other taxa in this analysis. This may suggest that the small sizes of the skull tables in these species were achieved through paedomorphosis, or the maintenance of juvenile morphologies into adulthood.
Alligatoroidea, Crocodylia, Morphology, Morphometrics, Phylogenetics, Systematics
xiii, 271 pages
Includes bibliographical references (pages 257-271).
Copyright © 2018 Adam Patrick Cossette
Cossette, Adam Patrick. "The early history of character evolution in alligatoroids." PhD (Doctor of Philosophy) thesis, University of Iowa, 2018.