In the animal kingdom, there are generally two options when it comes to sustenance: You either eat the food that grows out of the ground, or you eat the animals that eat the food (omnivores, like bears and humans, do a little of both). The former is easy, but puts you at risk for predation; the latter is harder but allows for a relatively worry-free life. Thanks to a 300 million year-old fossil, we now know how some carnivorous mammals evolved into the sedan-sized herbivores we know today.
Eocasea martini, a tiny lizard-like animal, represents the oldest known example of a caseid. Caseids were themselves primitive synapsids, which include mammals and their close relatives. Prehistoric synapsids, though they looked reptilian, represent an entirely different branch of evolution from reptiles or birds.
“It’s within this side of vertebrate evolution that we have the first plant-eating animals,” said study leader Robert Reisz, a paleontologist at the University of Toronto.
The find is significant for a simple reason: Though E. martini is thought to have lived on small animals (like insects), all other known caseids are herbivores. At some point – between E. martini and the modern rhinoceros – things changed drastically. The specimen was actually discovered 20 years ago in Kansas, left unnoticed because it was such a small, seemingly unremarkable animal.
The giveaway was the ribcage. Once Reisz and his team determined the animal was a caseid, they also noticed that it lacked caseids’ characteristic broad, barrel-shaped ribcage. Herbivores, due to the volume of plant material they must digest, typically need large guts. Not so with E. martini. With such a svelte torso, it was in all likelihood carnivorous.
E. martini will also help scientists determine how other animals transformed their diets as well, whether going from animals to plants, or from insects to genuine meat.
“What’s really interesting is that this is not the only group that it happened to,” Reisz said . “Other groups seem to have been doing this roughly at the time.”