Marine Reptile’s Weird Body Armor a Sign of Life’s Great Recovery

Nothing opens up the possibility for evolutionary oddballs to emerge quite like a mass extinction. The worst such catastrophe – the Permian-Triassic mass extinction, about 252 million years ago – so dramatically cut back the world’s biodiversity that the relatively few survivors stepped into an open world where entirely novel forms of life could evolve.

Triassic reptiles, in particular, underwent an evolutionary explosion in the aftermath of the disaster, and within that proliferation multiple lineages of the scaly vertebrates started to slide into the sea. Fish-like ichthyosaurs, shell-crushing placodonts, and the quad-flippered ancestors of plesioaurs were among the most famous lineages to take the plunge, but paleontologists have recently started to turn up even odder marine reptiles from the early days of the Triassic. The zipper-mouthed Atopodentatus, named earlier this year, was once such Triassic weirdo, and just last month Xiao-hong Chen and colleagues added another to the list – Parahupehsuchus longus, an aquatic reptile whose body was encased in a tube of bone.

There’s nothing quite like this highly-armored reptile alive today. With elongated, toothless jaws, stumpy paddles for limbs, a thick torso, and an elongated tail, Parahupehsuchus longus was a bizarre critter belonging to an ephemeral group of Triassic reptiles found in eastern China called the Hupesuchia. And within this group, the 248 million year old Parahupehsuchus longus stands out for having such well-developed anti-predator defenses.

At about twenty inches long and two and a half inches thick, the “body tube” of this Triassic enigma was made of modified ribs and gastralia, or “belly ribs.” The bones expanded so much, in fact, that there’s no space remaining between the ribs, much like a turtle’s shell but evolved totally independently. And this was a stiff structure. Each rib of Parahupehsuchus longus articulated with the spine at two sites, severely limiting how much the ribs could move.

Stout ribs were not the reptile’s only defense. Running along the back of Parahupehsuchus longus was a triple layer of dermal ossicles, or bone armor embedded in the skin. The lower two layers were interlocking triangles, overlain by large, flat bones that span two or three vertebrae each. This extensive armor determined how the reptile could swim. Up and down or side-to-side, the trunk of Parahupehsuchus longus probably wasn’t very flexible, Chen and coauthors point out, so this stiff reptile probably swam by swishing its long tail back-and-forth like modern crocodiles do.

This Triassic curiosity is more than just another strange reptile that sculled through the ancient seas, though. At about 248 million years old, Parahupehsuchus longus was swimming around a relatively scant four million years after the close of the world’s worst mass extinction. That not only points to the rapid invasion of the seas by reptiles, but the specialized defenses of Parahupehsuchus longus means that there were already fearsome superpredators capable of biting sizable chunks out of their aquatic neighbors in the Early Triassic oceans.

The same deposits that have yielded Parahupehsuchus longus, Chen and colleagues point out, also contain an as-yet-unnamed marine predator that stretched nine to twelve feet long. Parahupehsuchus and the other marine reptiles only got to be about three feet long, making them possible prey for the larger animal. And it wasn’t very long before even larger predators menaced the marine realm. By about 245 million years ago – only seven million years into the Triassic – there were enormous, macropredatory ichthyosaurs with wicked jaws. This ramping up of the classic evolutionary arms race between predators and prey means that the world’s ecosystems bounced back quickly after being so vitally damaged. The unusual armaments of Triassic marine reptiles are a sign of life’s resilience in the aftermath of near-destruction.

[For another take on Parahupehsuchus and what that “body tube” is all about, see Andy Farke’s post here.]

Reference:

Chen, X-h., Motani, R., Cheng, L., Jiang, D-y., Rieppel, O. 2014. A carapace-like bony ‘body tube’ in an Early Triassic marine reptile and the onset of marine tetrapod predation. PLoS ONE. 9, 4: e94396. doi:10.1371/journal.pone.0094396