Some large herbivores living today may have had smaller carnivores as ancestors. 

Eocasea martini, a lizard-like creature that lived 300 million years ago, may have been a distant ancestor to all plant-eating animals on land today. The ancient creature was about eight inches long and was discovered in Kansas. They lived 80 million years before the first dinosaurs evolved on Earth. 

This partial fossil consists of part of a skull, large parts of the vertebral column, pelvis and one hind limb. 

Synapsids were early mammal-like lizards and E. martini is classified as a member of the caseid branch of that class of animals. These animals looked more like reptiles than mammals. The best-known of these are dimetrodons, marked by large fins along their backs. 

Unlike other caseids, E. martini did not possess as barrel-shaped rib cage. A relatively wide rib enclosure is needed by herbivores to enclose long intestines, required to digest plants. 

The ancient animals were an important transitional species between carnivores to herbivores. It is likely that E. martini ate insects. 

"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. These herbivores in turn became a major food resource for large land predators," said Robert Reisz, a professor in the Department of Biology at the University of Toronto.

The ability to digest high-cellulose fibers evolved at least five times during the age of E. martini. Once animals were able to consume large quantities of plants and grasses, species became larger. 

This growth came during the Permian Period, which lasted from roughly 300 to 250 million years ago.

"When the ability to feed on plants occurred after Eocasea, it seems as though a threshold was passed. Multiple groups kept re-evolving the same herbivorous traits," Reisz added.

Paleontologist Larry Martin of the University of Kansas discovered the fossil in Hamilton Quarry in southeast Kansas more than 20 years ago. This ancient lagoon is well-known well known for fish and plant fossils. The artifact was kept in the Dyke Museum of Natural History for years, before being examined by Reisz. 

Investigation of the fossil was detailed in the online journal PloS One.