Imagine you’re a small mammal of the Mesozoic. Snuffling around one day, you run into a cat-size, scaly, big-eyed reptile that looks not unlike a crocodile found later in the 21st century. Spotting you, he opens his mouth wide to reveal … tiny, intricate teeth. Then, he turns his head and munches on leaves.
Such encounters may have been common in prehistory. Research published Thursday in Current Biology suggests that vegetarianism evolved at least three separate times in ancient crocs — a conclusion reached after scientists studied the unusual teeth sported by many species, including the Simosuchus described above.
Crocodiles and their relatives, among them alligators, caimans and gharials, can be found across the Southern hemisphere. They have many things in common, including meat-heavy diets, a penchant for swimming and their teeth. Ask them to smile for a family reunion photo, and each mouth would bristle with simple, blunt-tipped cones.
But the Mesozoic was a different story. About 250,000 years ago, scores of crocodyliform species could be found across the globe, some on land and some in seas and rivers. A particular species might eat only plants, only animals, or both. To support these varied diets, many had “unique, interesting teeth,” said Keegan Melstrom, a geobiology graduate student at the University of Utah and lead author of the new study.
A tooth can reveal a lot about its owner. Carnivores tend to have sharp, smooth teeth, used for biting and tearing. Herbivores must break down food in their mouths before they swallow, so their teeth are more complex, with many bumps and ridges. Omnivores, like us, are in the middle.
Melstrom has been keen on crocodile teeth since 2011, when he saw a presentation on an extinct crocodyliform called Pakasuchus. Pakasuchus had canines in the front of its mouth and molars in the back. When its jaw closed, the teeth would neatly slot together — more like a mammal’s mouth than the akimbo grin of modern crocodiles. “It just blew my mind,” Melstrom said.
For the new study, Melstrom and his co-author, Randall Irmis, analyzed 146 teeth from 16 extinct crocodyliform species. They used a method called orientation patch count rotated. From a scan of an object, the method generates a numerical score indicating the complexity of the object’s shape. “It allows us to compare teeth that have no landmarks in common,” Melstrom said.
This proved especially useful for studying prehistoric crocs, he said, whose teeth often have “no modern-day analogues.” One of the few Chimaerasuchus teeth known to science has “at least two rows of seven cusps, each of those cusps varying in size,” like a miniature mountain range in enamel. One species, Iharkutosuchus, had big square teeth cut through with deep grooves.
The researchers gathered the complexity scores of the teeth and compared them to those of living reptiles and mammals with known diets. Half of the ancient species seemed to have been on the plant-eating end of the spectrum — “a genuine surprise,” Melstrom said.
What’s more, several of them were from distinct lineages. This suggests to Melstrom that vegetarianism was not an anomaly in crocodyliform history but evolved at least three times. He hopes to find out why vegetarian crocodyliforms didn’t last past the end of the Cretaceous; the answer may relate to cooling climates, changes in plant life or competition with mammals, he said.
Scientists have speculated that herbivorous crocodyliforms may once have existed, but the study provides “the first quantitative support” for the idea, said Attila Ősi, a vertebrate paleontologist at Eötvös Loránd University in Budapest, Hungary, who was not involved in the study. These early crocodiles “could have been significant members of Mesozoic herbivore communities.”