300-Million Year-Old Ancestor of Land Herbivore Unearthed in Kansas

Paleontologists have unearthed what may be the earliest ancestor of land-dwelling herbivores, offering clues about the emergence of plant eaters on land, according to a new study.

University of Toronto researchers have found evidence of how carnivores transitioned into herbivores. Fossilized remains of a 300-million-year-old  juvenile Eocasea martini, were unearthed in Kansas. It is believed to be the oldest ancestor of the modern plant-eating mammals.

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

The team got their hands on the partial skull, pelvis, hind limb and most of the vertebral column.  The researchers then compared the anatomy of the skeleton to other related animals to trace their origin. They found that the juvenile ancestor belonged to the caseid branch of the group Synapsid that includes terrestrial herbivore and other large top predators that eventually evolved into modern living mammals.

This study also included researcher Jorg Frobisch of the Museum fur Naturkunde and Humboldt-University in Berlin. This juvenile ancestor thrived some 80 million years before the age of dinosaurs.  This carnivorous ancestor is said to be the most primitive member and fed mostly on insects and small animals.  The later members of this group were plant lovers clearly indicating that the large terrestrial herbivores came from this group consisting of small-non herbivorous members.

"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," says Frobisch. "This shows that caseid synapsids were much more ancient than previously documented in the fossil record."

The findings reveal that Eocasea is the first animal that brought a change that resulted in a terrestrial ecosystem.

They were surprised to see that the ability to digest high-fiber plant material (herbivory) was not present in only Eocasea lineage, but it evolved over five times independently on earth and has been seen twice in reptiles.

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

The study reveals that the herbivory type of feeding started among distant relatives of mammals and not reptiles also the shift towards plant-eating triggered changes in the size of ancient herbivores.

Reisz concludes saying, "the discovery of Eocasea creates questions even as it answers them. One of the great mysteries to my mind is: why did herbivory not happen before and why did it happen independently in several lineages? That's what's fascinating about this event. It's the first such occurrence, and it resulted in a colossal change in our terrestrial ecosystem."

The finding was documented in the journal PLOS One.