Animals

Plant-eating crocodiles thrived in dinosaur times

New analysis of fossil teeth suggests that the dino-killing asteroid also wiped out the vegetarians of the crocodile family.Sunday, June 30, 2019

By Tim Vernimmen
The extinct crocodilian <i>Pakasuchus</i>, seen here in an illustration, was an herbivore, according to analysis of its fossilised teeth.

In children’s books and cartoons, crocodiles and their kin tend to have an impressive array of identical teeth, each shaped like a sharp, pointed weapon primed for tearing through flesh. In reality, there is often a bit more variation, says palaeontologist Keegan Melstrom of the University of Utah.

“But that is nothing compared to the staggering diversity in the teeth of extinct crocodile-like reptiles, or Crocodyliformes,” he says. “Some of those extinct crocs had really weird teeth.”

Now, an analysis of 146 fossilised teeth belonging to 16 extinct crocodile relatives has revealed something surprising: At least three times in their history, ancient croc cousins became vegetarians.

“This shows that this was a succesful dietary strategy,” says Melstrom, whose team presents the results today in the journal Current Biology. “And I think that as we find more teeth in the future, we are likely to find even more groups that independently became herbivores.”

Dedicated chomping

For their analysis, Melstrom and coauthor Randall Irmis, also of the University of Utah, adopted a method that was especially developed to compare dissimilar teeth, borrowed from earlier work by palaeontologists studying ancient mammals.

“What it comes down to is that we count how many separate surface areas there are on every tooth,” Melstrom says. “We consider them to be separate if they are tilted in a different direction.”

Based on research in mammals as well as reptiles alive today, scientists know that carnivores tend to have simple teeth with very few separate surfaces. The komodo dragon, for instance, is a predator with teeth that look like steak knives—thin and sharp, straight and simple, with no frills. These teeth are great for catching prey and slicing it into chunks that the lizard can then swallow without chewing. At the other extreme are animals with teeth full of nooks and crannies that increase their surface area, creating more space and different tools to grind away at various hardy plant parts.

A curious Nile crocodile examines a remote camera in Kenya's Lake Turkana.

“These teeth almost invariably belong to animals that feed on plants, the leafs, branches, and stems of which often require a lot of chewing before they can be digested,” Melstrom says.

As such, the teeth of the almost entirely carnivorous crocodilians alive today are usually rather simple, Melstrom explains, but some of the extinct species had teeth with up to 20 separate surfaces. This suggests that they probably engaged in some very dedicated chomping or possibly other behaviours that allowed them to nibble away on nutritious plants that were difficult to access.

“One of the most complex teeth in the set we’ve studied are those of Simosuchus, a small crocodyliform with an almost rectangular snout, as if someone hit a crocodile in the head with a shovel,” Melstrom says. These teeth look remarkably like those of the marine iguana from the Galápagos, which grazes algae from the rocks. “Simosuchus wasn’t aquatic but probably did live near the water, so one might imagine it did something similar,” Melstrom says.

Surprisingly, Melstrom’s study has now firmly established it was not just one wayward group that went vegetarian. It turns out there were at least three independent groups sporting a variety of more complex chompers, suggesting that a move to plant-based diets happened multiple times throughout evolution.

Patrick O’Connor, a palaeontologist at Ohio University who was not involved in the study, is enthusiastic about the team’s approach.

“This method can be replicated and expanded with the discovery of new fossils, which should allow us to test different ideas for why herbivory repeatedly evolved in crocs,” he says. His colleague Diego Pol, currently at the Museo Paleontológico Egidio Feruglio in Argentina, agrees, but he also warns that scientists shouldn’t take the diets suggested by tooth complexity for granted and should look for other lines of evidence to back up their conclusions.

Select survivors

Succesful as they may once have been, herbivorous crocodyliforms didn’t make it past the mass extinction event that eliminated roughly three quarters of all species on Earth about 66 million years ago—despite the fact that the crocodilians alive today were among the very few large four-limbed animals that did survive. And no herbivorous crocs have evolved since, maybe because mammals took their place in that ecological niche.

“Becoming a herbivore always involves some kind of specialisation,” says Attila Ősi, a Hungarian paleontologist who discovered a fair number of the teeth used in the study, but who was not involved in the work. That might be a disadvantage when the plants you like disappear. Another clue may lie in the fact that not just the herbivores, but all fully terrestrial crocs have gone extinct. The two dozen species alive today inhabit lakes, rivers, and occasionally seashores, where they mostly feed on meat and fish.

Still, even modern crocs are not strict carnivores. Many species have been found to occasionally eat fruit, sometimes straight from the tree. And American alligators fed largely plant-based diets for a few months did not appear to suffer any negative health consequences. Clearly, crocodilians are more flexible than they are usually given credit for, and today’s crocs are far better adapted than the common misnomer “living fossil” might imply.

Mikael Fortelius of the University of Helsinki, Finland, did not work on this study but has used the method in mammals, and he agrees the label is often unhelpful.

“Just like many extinct crocodyliforms were not carnivores, most ancient hyaenas weren't bone crushers and most rhinos didn't have horns,” he says. “Many animals alive today may not be typical of the groups they originate from.”

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