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We think of crocodiles as fearsome predators, but it wasn’t always so. During the dinosaur era, many crocodile-like reptiles were peaceful plant-eaters. The strategy evolved on at least three separate occasions and seems to have been both common and successful.

Modern crocodiles and alligators belong to a larger group called the crocodyliforms, which has existed since the early days of the dinosaurs over 200 million years ago. Many extinct crocodyliforms are known from fossils.

“People had previously hypothesised, just by looking at their teeth, that some of these animals had been herbivores,” says Keegan Melstrom at the Natural History Museum of Utah in Salt Lake City. But these were only educated guesses, because the ancient crocodyliforms’ teeth don’t match those of any modern animals.

Complex teeth

With his Utah colleague Randall Irmis, Melstrom has taken a systematic look by measuring how complex the shapes of the teeth are. Herbivore teeth are more complex than carnivore teeth, so Melstrom examined the teeth of 16 extinct crocodyliforms to figure out what they ate.

Eight were herbivores. One example was Pakasuchus, which had back teeth that slotted neatly together like those of a mammal and probably chewed its food rather than swallowing it whole. At least one of the others ate a mixed “omnivorous” diet.

When Melstrom slotted the herbivorous crocodyliforms into the family tree, he found that herbivory popped up in several branches. Herbivory evolved “anywhere from three to six times”, he says. Far from a rare oddity, “this is a really successful dietary strategy”.

In contrast, since the end of the dinosaur era all known crocodyliforms have been carnivores. Recent studies have shown that modern crocodyliforms do occasionally eat fruit and other plant matter, but meat is their main food. It isn’t clear why the group has abandoned herbivory, says Melstrom. “There’s a lot of things that have changed in the last 66 million years,” he says.

Carnivorous animals do sometimes evolve to eat plants instead. Giant pandas are the most famous example, but there is also a vegetarian spider.