Based on careful study of tooth remains, researchers have found that ancient groups of crocodyliforms--the group including living and extinct relatives of crocodiles and alligators--were not the carnivores we know today, as reported in the journal Current Biology on June 27. In fact, the evidence suggests that a veggie diet arose in the distant cousins of modern crocodylians at least three times.
"The most interesting thing we discovered was how frequently it seems extinct crocodyliforms ate plants," says Keegan Melstrom (@gulosuchus) of the University of Utah. "Complex teeth, which we infer to indicate herbivory, appear in the extinct relatives of crocodiles at least three times and maybe as many as six in our dataset alone."
All living crocodylians possess a similar general morphology and ecology to match their lifestyle as semiaquatic generalist carnivores, which includes relatively simple, conical teeth. It was clear from the start that extinct species 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," Melstrom says. "Omnivores, organisms that eat both plant and animal material, fall somewhere in between. 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."
To infer what those extinct crocodyliforms most likely ate, Melstrom and his advisor 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, they measured 146 erupted teeth from 16 different taxa of extinct crocodyliforms at a resolution of 25 data rows per tooth.
Using a combination of quantitative dental measurements and other morphological features, the researchers 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 lineage, the researchers conclude, shortly after the end-Triassic mass extinction, and persisted until the end-Cretaceous mass extinction. Their analysis suggests that herbivory arose independently a minimum of three times, and possibly six times, in Mesozoic crocodyliforms.
"Our work demonstrates that extinct crocodyliforms had an incredibly varied diet," Melstrom says. "Some were similar to living crocodylians and were primarily carnivorous, others were omnivores, and still others likely specialized in plants. The herbivores lived on different continents at different times, some alongside mammals and mammal relatives, and others did not. This suggests that an herbivorous crocodyliform was successful in a variety of environments!"
Melstrom says they are continuing to reconstruct the diets of extinct crocodyliforms, including in fossilized species that are missing teeth. He also wants to understand why the extinct relatives of crocodiles diversified so radically after one mass extinction but not another and whether dietary ecology could have played a role.
This research was supported by the US National Science Foundation, the Welles Fund from the University of California Museum of Paleontology, and the University of Utah Department of Geology & Geophysics Chapman Fund.
Current Biology, Melstrom and Irmis: "Repeated Evolution of Herbivorous Crocodyliforms during the Age of Dinosaurs" https://www.cell.com/current-biology/fulltext/S0960-9822(19)30690-6
Current Biology (@CurrentBiology), published by Cell Press, is a bimonthly journal that features papers across all areas of biology. Current Biology strives to foster communication across fields of biology, both by publishing important findings of general interest and through highly accessible front matter for non-specialists. Visit: http://www.cell.com/current-biology. To receive Cell Press media alerts, contact firstname.lastname@example.org.