Eocasea martini, a carnivorous animal that lived in what is today Kansas during Carboniferous period, about 300 million years ago, is the oldest known ancestor of terrestrial herbivores, say paleontologists from the University of Toronto Mississauga.

This is an artist’s reconstruction of Eocasea martini. Image credit: Danielle Dufault.

“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,” explained Prof Robert Reisz, the first author of the paper published in the open-access journal PLoS ONE.

A partial skull and postcranial skeleton of a juvenile Eocasea martini were collected by Dr Larry Martin from the Dyke Museum of Natural History University of Kansas, at Hamilton Quarry near Hamilton, Greenwood County, Kansas.

Eocasea martini was about 20 cm long and weighed less than 2 kg as an adult. It belonged to the caseid branch of Synapsida, a group representing an ancient lineage leading by stages to the mammals.

The genus name, Eocasea, refers to the fact that this is the oldest known member of the clade. The species name, martini, honors Dr Larry Martin, the original collector of the specimen.

“Eocasea martini 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. This shows that caseid synapsids were much more ancient than previously documented in the fossil record,” said second author Dr Jörg Fröbisch from the Museum für Naturkunde and Humboldt-University in Berlin.

“Eocasea martini is also the most primitive member and was carnivorous, feeding on insects and other small animals.”

“Younger members were herbivorous, clear evidence that large terrestrial herbivores evolved from the group’s small, non-herbivorous members, such as Eocasea martini,” Prof Reisz said.

Photograph and interpretive drawing of the Eocasea martini fossil. Preserved parts of the skeleton include the posterior 1/3rd of the skull and mandible, the vertebral column with ribs, pelvic girdle and right hind limb. Abbreviations: as – astragalus; ca – calcaneum; f – femur; fi – fibula; il – ilium; is – ischium; pu – pubis; s1 – first sacral rib; s2 – second sacral rib; t – tibia; roman numerals indicate the digit number in the pes. Image credit: Reisz RR, Fröbisch J.

“Eocasea martini 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 paleontologists also found that herbivory was established not just in the lineage that includes Eocasea martini. It arose independently at least 5 times, including twice in reptiles.

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

The five groups developed the novel ability to live off plants in staggered bursts with synapsids such as Eocasea martini preceding reptiles by nearly 30 million years.

This shows that herbivory as a feeding strategy evolved first among distant relatives of mammals, instead of ancient reptiles – the branch that eventually gave rise to dinosaurs, birds, and modern reptiles.

When Prof Reisz and Dr Fröbisch mapped the animals on an evolutionary tree, they found that 4 of the groups showed a tremendous increase in size during the Permian Period.

Caseids were the most extreme example of this size increase. The oldest member of the group, Eocasea martini, was very small, while the youngest, last member exceeded 500 kg.

Prof Reisz said that the discovery of Eocasea martini 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.”

Source: sci.news