Eunotosaurus

eunotosaurus
Eunotosaurus (Wikimedia Commons).

Name:

Eunotosaurus (Greek for "original noded lizard"); pronounced you-NO-toe-SORE-us

Habitat:

Swamps of southern Africa

Historical Period:

Late Permian (260-255 million years ago)

Size and Weight:

About one foot long and a few pounds

Diet:

Unknown; possibly omnivorous

Distinguishing Characteristics:

Small size; wide, shell-like ribs

 

About Eunotosaurus

The ultimate origin of turtles and tortoises is still shrouded in mystery, but many paleontologists believe that these shelled reptiles can trace their ancestry all the way back to the late Permian Eunotosaurus.

The striking thing about this prehistoric reptile is that it possessed wide, elongated ribs that curved around its back, a kind of "proto-shell" that one can easily imagine evolving (over the course of tens of millions of years) into the giant carapaces of Protostega and Meiolania. As to what kind of animal Eunotosaurus itself was, that's a matter of debate; some experts think it was a "pareiasaur," a family of ancient reptiles best represented by Scutosaurus.

Recently, researchers at Yale University made a major discovery that cements Eunotosaurus at the root of the testudine family tree. Technically, modern turtles and tortoises are "anapsid" reptiles, meaning they lack characteristic structural holes on the sides of their skulls. Investigating the fossilized skull of a juvenile Eunotosaurus, the Yale scientists identified small openings characteristic of diapsid reptiles (the vast family that includes crocodiles, dinosaurs and modern birds) that closed up later in life.

What this means is that anapsid testudines almost certainly evolved from diapsid reptiles some time during the Permian period, which would rule out the proposed pareiasaur origin mentioned above.

Given the hypothesis that Eunotosaurus was ancestral to modern turtles, what was the reason for this reptile's elongated ribs?

The most likely explanation is that its slightly rounded and expanded ribcage would have made Eunotosaurus harder to bite through and swallow; otherwise, this foot-long reptile would have been easy pickings for the large, predatory therapsids of ifs southern African ecosystem. If this anatomical bulge gave Eunotosaurus even a slight edge in survival, it makes sense that future turtles and tortoises would improve on this body plan--to the extent that the giant turtles of the later Mesozoic Era were virtually immune to predation as adults (though hatchlings, of course, could easily be gobbled up as they emerged from their eggs).