All land-dwelling herbivores descended from this critter smaller than your foot

A new study from the University of Toronto Mississauga reports that the earliest ancestor of plant-eating, land-dwelling animals was an adorable little critter no bigger than your foot. And through this creature, carnivores gradually evolved into large, terrestrial herbivores.
 
“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,” stated paleontologist and Department of Biology professor, Robert Reisz. “These herbivores in turn became a major food resource for large land predators.”
 
The previously unknown Eocasea martini is only around 20cm long. It lived about 300 million years ago, about 80 million years before the reign of the dinosaurs.
 
A fossilized juvenile skeleton of the creature was discovered in what is now modern-day Kansas. Only a partial skull was recovered, but most of the specimen’s verterbral column remains intact. A hind limb and the pelvis have also been unearthed.
 
The ancestors of today’s modern mammals
 
With the help of his colleague Jörg Fröbisch of the Museum für Naturkunde and Humboldt-University in Berlin, Reisz compared the anatomy of the Eocasea specimen to the skeletons of known related animals. This resulted in the discovery that the Eocasea belongs to the caseid branch of the synapsid group of animals. This group, which consisted of early land-dwelling herbivores as well as large apex predators, eventually evolved into our modern-day 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,” Fröbisch explained. “This shows that caseid synapsids were much more ancient than previously documented in the fossil record.”
 
Eocasea was discovered to be the most primitive of the caseid synapsids. It was carnivorous, preying on insects and other small creatures. Later members of this group, however, have been observed not only to be herbivorous, but also comparatively large, with some weighing over 1100 pounds. This shows that these small, meat-eating creatures ultimately evolved into large, terrestrial plant-lovers.
 
“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 Reisz.
 
Digesting and processing the high-fiber content of plants – or herbivory – is quite a difficult feat. This is why the evolution of the Eocasea into a plant eater took millions of years.
 
Herbivory also did not arise from the lineage of Eocasea alone. In fact, it arose independently in other animals at least five times. Two of those five times were in reptiles.
 
“When the ability to feed on plants occurred after Eocasea, it seems as though a threshold was passed,” said Reisz. “Multiple groups kept re-evolving the same herbivorous traits.”
 
These five groups developed herbivory side-by-side with Eocasea before reptiles came prominently into the picture some 30 million years later. What this demonstrates is that this special plant-centric feeding strategy first evolved in ancient mammalian relatives, and not in the ancient reptiles that eventually turned into dinosaurs, birds, and the reptiles we know of today.
 
Plant-eating also caused a dramatic increase in the size of the animals. Creatures belonging to four of the groups grew to tremendous sizes, particularly during the Permian Period, at the closing of the Paleozoic Era.
 
According to Reisz, however, the discovery of Eocasea has given rise to even more questions. “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 study was recently published in the journal PLoS ONE. - TJD, GMA News