Were dinosaurs already undergoing a long-term decline before an asteroid hit at the end of the Cretaceous about 65.5 million years ago? A new study led by Museum scientists gives a multifaceted answer.
The findings, published online today in the journal Nature Communications, suggest that in general, large-bodied, “bulk-feeding” herbivores were declining during the last 12 million years of the Cretaceous Period. But carnivorous dinosaurs and mid-sized herbivores were not.
“Few issues in the history of paleontology have fueled as much research and popular fascination as the extinction of non-avian dinosaurs,” said lead author Steve Brusatte, a Columbia University graduate student affiliated with the Museum’s Division of Paleontology. “Did sudden volcanic eruptions or an asteroid impact strike down dinosaurs during their prime? We found that it was probably much more complex than that, and maybe not the sudden catastrophe that is often portrayed.”
The research team, which includes Brusatte; Mark Norell, chair of the Museum’s Division of Paleontology; and scientists Richard Butler of Ludwig Maximilian University of Munich and Albert Prieto-Márquez from the Bavarian State Collection for Palaeontology, both in Germany, is the first to look at dinosaur extinction based on “morphological disparity”—the variability of body structure within particular groups of dinosaurs.
By looking at the change in variability within a given dinosaur group over time, researchers can create a rough snapshot of the animals’ overall well-being. This is because groups that show an increase in variability might have been evolving into more species, giving them an ecological edge. On the other hand, decreasing variability might be a warning sign of extinction in the long term.
The researchers calculated morphological disparity for seven major dinosaur groups using wide-ranging characteristics of the intricate skeletal structure of nearly 150 different species. They found that hadrosaurs and ceratopsids, two groups of large-bodied, bulk-feeding herbivores—animals that did not feed selectively—may have experienced a decline in biodiversity in the 12 million years before the dinosaurs ultimately went extinct. In contrast, small herbivores (ankylosaurs and pachycephalosaurs), carnivorous dinosaurs (tyrannosaurs and coelurosaurs), and enormous herbivores without advanced chewing abilities (sauropods) remained relatively stable or even slightly increased in biodiversity.
As a complication, hadrosaurs showed different levels of disparity in different locations. While declining in North America, the disparity of this dinosaur group seems to have been increasing in Asia during the latest Cretaceous.
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Brusatte and Norell met at the Museum for a discussion and Q&A session about these findings, moderated by Wired Science Associate Editor Brandon Keim.
Thumbnail: © AMNH/J. Brougham