Some Extinct Relatives of Living Crocodylians Were Vegetarians

A veggie diet arose in extinct crocodyliforms — the distant cousins of living crocodylians (alligators, caimans, crocodiles, and gharials) — at least three times, according to new research.

Life reconstructions of extinct crocodyliforms. Differences in tooth shape are related to differences in diets. Image credit: Jorge Gonzalez.

“The most interesting thing we discovered was how frequently it seems extinct crocodyliforms ate plants,” said lead author Keegan Melstrom, a doctoral student in the Department of Geology and Geophysics and the Natural History Museum of Utah at the University of Utah.

“Our study indicates that complexly-shaped teeth, which we infer to indicate herbivory, appear in the extinct relatives of crocodiles at least three times and maybe as many as six.”

All living crocodylians possess a similar general body shape and ecology to match their lifestyle as semiaquatic generalist carnivores, which includes relatively simple, conical teeth.

Extinct crocodyliforms showed a different pattern, including species with many specializations not seen today. One such specialization is a feature known as heterodonty: regionalized differences in tooth size or shape.

“Carnivores possess simple teeth whereas herbivores have much more complex teeth. Omnivores fall somewhere in between,” Melstrom said.

“Part of my earlier research showed that this pattern holds in living reptiles that have teeth, such as crocodylians and lizards. So these results told us that the basic pattern between diet and teeth is found in both mammals and reptiles, despite very different tooth shapes, and is applicable to extinct reptiles.”

False color 3D images showing the range in shape of crocodyliform teeth. Carnivores (left), such as the living caiman, have simple teeth, whereas herbivores (right) have much more complex teeth. Image credit: Keegan Melstrom, Natural History Museum of Utah.

To infer what those extinct crocodyliforms most likely ate, Melstrom and his colleague, University of Utah’s Dr. Randall Irmis, compared the tooth complexity of extinct crocodyliforms to those of living animals using a method originally developed for use in living mammals.

Overall, the researchers measured 146 teeth from 16 different species of extinct crocodyliforms.

Using a combination of quantitative dental measurements and other morphological features, they reconstructed the diets of those extinct crocodyliforms.

The results show that those animals had a wider range of dental complexities and presumed dietary ecologies than had been appreciated previously.

“Plant-eating crocodyliforms appeared early in the evolutionary history of the group, shortly after the end-Triassic mass extinction, and persisted until the end-Cretaceous mass extinction that killed off all dinosaurs except birds,” the scientists said.

“Our analysis suggests that herbivory arose independently a minimum of three times, and possibly six times, in Mesozoic crocodyliforms.”

The research appears in the journal Current Biology.

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Keegan M. Melstrom Randall B. Irmis. Repeated Evolution of Herbivorous Crocodyliforms during the Age of Dinosaurs. Current Biology, published online June 27, 2019; doi: 10.1016/j.cub.2019.05.076