Sunday, May 19, 2013

Mock-turtles and Reptilian Otters: The pioneering placodonts

Throughout the Mesozoic, marine life was dominated by reptiles of all shapes and sizes. Some of the most bizarre and striking species lived at the very beginning of the era, during the Triassic period. Among ridiculously long-necked protorosaurs and ichthyosaurs as large as sperm whales lived a family of very bizarre, and extremely specialized, animals known as the placodonts.

Placodonts, meaning "flat teeth," were a family of reptiles which lived across the globe and only survived during the Triassic. They were not giants like so many others; the largest species grew to 3m (10ft) long. They were not superpredators, nor were they diminuitive prey items. In fact, at first glance, there doesn't appear to be anything too exciting about them. But to see what makes placodonts so special, we need to look in their mouths.

The placodont Placodus, a bizarre chimera of marine iguana and sea otter. Reconstruction by Dan Varner.
The placodonts weren't named for nothin'. They were armed with almost stone-like, crushing teeth which were perfect for crushing shellfish. These were the first - and last, for a very long time - reptiles to specialize in eating hard-shelled invertebrates. For a long time, the origins of these very specialized reptiles was unknown. Recently, however, fossils of the most basal placodont known were discovered. This specimen, dubbed Palatodonta bleekeri, lacked the crushing teeth of its later relatives - instead, it had small, sharp, peg-like teeth, perfect for gripping much softer prey (Neenan et. al, 2013). These teeth, in comparison, are nothing special in the world of reptiles.

The basal placodontiform Palatodonta bleekeri. Reconstruction by Jaime Chirinos.
The Triassic, as discussed in a previous post, was an age of extremely rapid radiation, with reptiles of all sorts quickly evolving to fill any and all niches left open by the Permo-Triassic Extinction. In oceans filled with species adapted to feed on fish and cephalopods, the placodonts evolved a niche avoided by all other reptiles: scouring the sea floor for bivalves and other hard-shelled goodies.

The "mock-turtle" Henodus. Despite all similarities, it was a placodont, not even closely related to true turtles. Reconstruction by Jim Robbins.
In their day, the largest placodonts were too large to be preyed upon by most animals in their environment - smaller sharks and fish-eating reptiles were the only others to share the shallow seas in which they swam. However, as marine predators began to evolve, the placodonts had to adapt: many species evolved bony armor. This armor may cause some confusion, as some armored species were so well-protected that, at first, they look just like turtles, which had yet to evolve. Some species, such as the 2m (6ft) Psephoderma, took this armor to even greater lengths, with articulated "suits" of armor which allowed for greater mobility in the water.

Another mock-turtle, Psephoderma, and a nothosaurid. Illustration by Kahless28 on deviantArt.
The placodonts were true pioneers in the world of reptiles. They were the first reptiles to evolve a bony shell, and some of the first animals to exploit a niche which would later be filled by animals such as sea otters. Unfortunately, like so many unique species of the Triassic, they disappeared at the end of the period, paving the way for shellfish-eating sharks, turtles, and a myriad of other sea-going oddities.

James M. Neenan, Nicole Klein, Torsten M. Scheyer. 2013. European origin of placodont marine

reptiles and the evolution of crushing dentition in Placodontia. Nature Communications. March 27, 2013. doi:10.1038/ncomms2633