There have been three major groups of dogs: the hesperocyonines, the borophagines, and the canines. The former two groups both diversified, dominated a niche as hunters of large prey, and then declined to extinction in turn. This radiation and subsequent decline of dog clades mirrors a wider pattern in faunal succession of large predators throughout the Tertiary. Each predatory ecomorphological role seems to be filled by a given family for 10 million years or so before that group declines and is replaced by another. Since this pattern of extinction repeats predictably in major taxa of mammalian predators, its possible causes are worth investigating. Furthermore, results may inform conservation methods for modern analogues.
The purpose of this research project was to test the hypothesis made by previous researchers that larger body size and hypercarnivory evolved together in hesperocyonines. This was done by seeing if their predictions were supported in specimens from the White River Group, a region with an excellent fossil record from a time when hesperocyonines were dominant. This would help determine whether changes in size and diet were universal or varied by region in hesperocyonines. Since the White River covers the Eocene-Oligocene boundary, this study also aimed to discern if the climate shift that occurred across that boundary had any influence on hesperocyonine evolution.
Data collection methods involved taking measurements of craniodental features strongly correlated with body size and carnivory, primarily the length of the first lower molar (m1) and the length of its trigonid. The sample tested at the Field Museum totaled 93 specimens, all Hesperocyon (some identified to species as Hesperocyon gregarius). Data analysis was performed by averaging measurements for all specimens from the same period or North American land mammal age, and then graphing the average lengths versus times.
The resulting data display an increase in both m1 length and relative blade length over the Eocene-Oligocene boundary. The increase in relative blade length is proportionally larger than the increase in m1 length: average m1 length increased by about 2.7% from the Eocene to the Oligocene, while relative blade length increased by 5.86%. Furthermore, the change in relative blade length is significant within a 90% confidence interval, while the change in m1 length is not. This suggests that changes in hesperocyonine diet over the Eocene-Oligocene boundary were more significant than changes in body size, and thus may have been the greater driving force in hesperocyonine evolution at that time.
Scott Kottkamp, ’15
Sponsor: John Orcutt