A small lizard-like creature that lived more than 300 million years ago is believed to be the earliest known ancestor of plant-eating mammals.
Eocasea martini, which measured less than eight inches, marked a pivotal transition from carnivores to herbivores on the land.
Scientists showed that a partial skeleton of a juvenile specimen found in Kansas, US, belonged to the caseid branch of a family of early mammal-like reptiles called synapsids.
Although a carnivore, feeding on insects, its larger descendants were herbivores.
"Eocasea is one of the oldest relatives of modern mammals and closes a gap of about 20 million years to the next youngest members of the caseid family," said Dr Jorg Froebisch, from Humboldt university in Berlin, Germany. "This shows that caseid synapsids were much more ancient than previously documented in the fossil record."
Canadian colleague Professor Robert Reisz, from the University of Toronto, said: "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.
"Eocasea is the first animal to start the process that has resulted in a terrestrial ecosystem with many plant eaters supporting fewer and fewer top predators."
The research is published in the online journal Public Library of Science ONE.
It suggests plant-eating first evolved in the distant relatives of mammals some 30 million years before the ancestors of dinosaurs, birds, and modern reptiles adopted the same behaviour.