Earliest Ancestor of Plant-Eaters on Land Discovered in Kansas Fossil

Eocasea

The fossil remains of an unusual animal unearthed in Kansas represent the oldest known ancestor of the world’s first land-dwelling plant-eaters, paleontologists say.

A seemingly paradoxical creature, the small specimen — no larger than a modern lizard — likely ate insects and small animals, but it appears to be the forebear of the big, barrel-bellied herbivores that grazed their way through the Permian period.

Scientists have identified the diminutive quadruped as a new genus, named 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,” said paleontologist Robert Reisz, lead author a new paper describing the find, in a press statement.

“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,” Reisz added.

“These herbivores in turn became a major food resource for large land predators.”

Eocasea
An artist’s rendering depicts Eocasea, the earliest known ancestor of land-dwelling plant-eaters. (Danielle Dufault)

The Kansas fossil dates back 300 million years — about 70 million years before the appearance of dinosaurs — and includes a backbone, pelvis, hind limb and partial skull.

After analyzing its skeletal structure, Reisz and his colleague found that the small meat-eater was actually a kind of caseid, a group of basal animals typically known — though not terribly well understood — from the fossil record  as large, lumbering plant-eaters.

And although they may have looked to modern eyes like reptiles, caseids belonged to a class of intriguing early animals called synapsids, which are the ancestors of modern mammals.

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 the study’s co-author Jörg Fröbisch.

“This shows that caseid synapsids were much more ancient than previously documented in the fossil record.” [Related: “‘Bizarre’ Porpoise With Giant Underbite Is First Mammal of Its Kind in Fossil Record“]

The ability to eat and digest plants apparently led to an exponential increase in the animals’ sizes, the duo notes.

Eocasea, as the earliest known caseid, probably weighed no more than 2 kilograms, but its herbivorous descendants ballooned to 500 kilograms or more.

And, the scientists point out, while Eocasea may be the ancestor of the earliest known plant-eaters, herbivory seems to have arisen separately, several times, in the course of animals’ evolution.

As some lineages like the caseids went extinct, new ones appeared that arrived at the same strategy for survival, in a process known as convergent evolution.

Casea broilii
A descendant of Eocasea, a kind of caseid known as Casea broilii, grew up to 1.2 meters (Nobu Tomura)

“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.”

“One of the great mysteries to my mind is: Why did herbivory not happen before, and why did it happen independently in several lineages?” he posited.

“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.”

Reisz and Fröbisch publish their findings in the open-access online journal PLOS ONE.


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