Fossil teeth suggest plant-eating evolved at least three times in crocs of the Mesozoic Era
Some extinct crocs may have been keen to eat greens.
An analysis of fossil teeth suggests that plant-eating relatives of modern crocodiles evolved at least three times during the Mesozoic Era, which stretched from roughly 252 million to about 66 million years ago, researchers report June 27 in Current Biology.
Today’s crocodiles are predominantly carnivorous, and have the simple, conical chompers typical of meat eaters. But in the teeth of their relatives of yore, “there is this tremendous diversity … that we don’t see today,” says study coauthor Keegan Melstrom, a paleontologist at the University of Utah and Natural History Museum of Utah, both in Salt Lake City.
Melstrom and his adviser, paleontologist Randall Irmis, studied CT scans of 146 teeth from 16 extinct types of crocodyliforms. (No living member of the group, which includes modern alligators and crocodiles, eats primarily plants.) A computer program treated the teeth like miniature mountains, analyzing their shapes and giving each tooth a score that captured its complexity.
In general, the most textured teeth belong to herbivores, while those of omnivores and carnivores are usually less complex. Elongated, sharp teeth help carnivores kill and eat their prey, but broader, bumpier teeth are more useful in tearing leaves and grinding up plants.
Among the group of reptiles that includes today’s crocodiles and alligators, the teeth of carnivores such as the modern caiman (left in these false color 3-D images) and the extinct Boverisuchus vorax (second from left) are simple and cone-shaped. The tooth of an extinct omnivore in the group (middle, Brachychampsa sp.) has more complexity. But teeth from two ancient herbivores (right two) have the most texture, an indication those ancient crocs ate mostly plants, researchers say.
Comparing the fossil teeth with teeth from modern reptiles helped the scientists get a sense of what the ancient crocodyliforms likely chewed. Some of the fossil teeth were much bumpier than those of plant-eating reptiles alive today, including iguanas, suggesting that the chompers were also from predominantly herbivorous species. Other teeth looked specialized to crush bones, tear meat or eat insects.
The teeth of the suspected plant eaters “really stand out,” says Domenic D’Amore, a herpetologist at Daemen College in Amherst, N.Y. “Few studies have quantified these differences, and this study really shows how different [the teeth] are.”
Surveying the evolutionary family tree of ancient crocodyliforms, the researchers found that veggie-munching crocs appear to have evolved at least three times and perhaps up to six times during the Mesozoic.
Ancient crocodyliforms lived in freshwater and marine environments and on land, says Patrick O’Connor, an evolutionary biologist at Ohio University in Athens. This study starts to figure out the animals’ places in their ecosystems, he says.
Since plant-eating crocodyliforms lived in different kinds of environments, herbivory was likely an important eating strategy, Melstrom and Irmis say.
K.M. Melstrom and R.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.
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