Apr 17, 2014 12:04 PM EDT By Russell Westerholm, UniversityHerald Reporter

A new discovery of a 300-million-year-old carnivorous mammal in all likelihood holds the key to learning how meat-eaters evolved into herbivores.

According to LiveScience, the fossil is of a caseid named Eocasea martini. It was part of an ancient group of synapsids, creatures that looked like reptiles but had some key differences.

"It's within this side of vertebrate evolution that we have the first plant-eating animals," study leader Robert Reisz, a paleontologist at the University of Toronto, told LiveScience.

The researchers published their work in the journal PLOS One.

Reisz and her team determined the animal behind the fossil was 4.4 pounds and ate mostly insects. Caseids are typically herbivores, making E. martini's diet stand out.

"All other members of this group, the caseids, are plant eaters," Reisz said. "This one, the oldest, isn't. We see a transformation within the group from an insectivorous animal to a plant-eating animal."

For his team's study, Reisz examined a fossil discovered about 20 years ago and left in storage at the University of Kansas' Dyke Museum of Natural History since.

"Nobody paid much attention to it," he told LiveScience. "It's a tiny little animal.

"There are very few terrestrial vertebrates coming out of that locality, but each one has turned out to be very important scientifically."

When analyzing the animals' anatomy, they found it did not have a broad rib cage, a known indicator of a plant diet. Instead, they found E. martini to have a rib cage more similar to what a herbivore's would look like. Reisz said since this fossil is among the older casieds, its marks the evolution of carnivores into animals that preferred to eat plants.

"The evolution of herbivory was revolutionary to life on land because it meant terrestrial vertebrates could directly access the vast resources provided by terrestrial plants," Reisz said in a press release. "These herbivores in turn became a major food resource for large land predators."

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