Eocasea martini
April 17, 2014

Fossil Reveals Transition From Carnivore To Herbivore

Brett Smith for redOrbit.com - Your Universe Online

The fossil of a newly-discovered species called Eocasea martini is being described as a sort of missing link in the transition from carnivorous to herbivorous behavior in land animals.

According to a report in the open access journal PLOS ONE, the fossil could eventually serve as evidence that caseids, the largest land animals around 300 million years ago, slowly evolved from being carnivores to subsisting on a diet solely of 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," said study author Robert Reisz, a paleontologist at the University of Toronto Mississauga. "These herbivores in turn became a major food resource for large land predators."

The fossil of E. martini was discovered in Kansas and consists of a partial skull, a section of the spine, the pelvis and a hind limb. In their analysis, the study team examined the skeletal physiology of associated animals. They found that E. martini fit to the caseid branch of the group Synapsid. This group, consisting of ancient terrestrial herbivores and big apex predators, eventually developed into modern-day mammals and lived about 80 million years before the dinosaurs.

"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 study author Jörg Fröbisch of the Museum für Naturkunde and Humboldt-University in Berlin. "This shows that caseid synapsids were much more ancient than previously documented in the fossil record."

Reisz said this animal was carnivorous, preying on insects and other small animals. However, younger members were herbivorous, supporting the idea that large land-dwelling herbivores evolved from the group's smaller carnivorous members, such as Eocasea.

"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," he said.

The researchers also found evidence of this evolution into herbivory “mirrored by three other clades, documenting multiple, independent, but temporally staggered origins of herbivory and increase in body size” among early land animals, according to their report. The capacity to break down high-fiber plant material was not exclusive to the Eocasea lineage. It came about separately at least five times, including twice in reptiles, the researchers said.

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

The five groups established the ability to subsist on plants in periodic bursts with synapsids such as Eocasea doing so before reptiles by around 30 million years. This indicates that plant eating as a survival strategy developed first among distant relatives of mammals, as opposed to ancient reptiles.

The adoption of herbivory also caused dramatic shifts in the size of early herbivores. When the team charted the animals on an evolutionary tree, they discovered that four of the groups had a massive boost in size during the Permian Period.

Reisz added that the finding of Eocasea generates questions even while 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,” he said.