Sunday, October 6, 2019

Placodus: Beast of the Week

This week we will be checking out a really cool, specialized reptile from the Triassic.  Get a load of Placodus gigas!

Placodus was a marine reptile that lived in what is now Germany, France, Poland, and China, during the middle Triassic period, about 240 million years ago.  From snout to tail, it measured about six feet long and it would have eaten shellfish when alive.  The genus name, Placodus, translates to "flat tooth" because as we will learn, it's teeth were indeed quite flat!

Placodus gigas life reconstruction by Christopher DiPiazza.

Placodus had a very wide and barrel-shaped midsection, which was flatter on the bottom.  It's vertebra overlapped and its belly ribs angled upwards to almost connect with the top ribs, so this creature would not have had a very flexible body.  It likely used its more flexible tail as its main source of propulsion in the water, with aid from its proportionally short arms and legs, of which the fingers and toes were likely webbed.  Placodus could have probably hauled out on land and clumsily walked around if it needed to.  They likely hatched from eggs like many reptiles do, and had to come out of the ocean at least to lay eggs.

Placodus skeleton on display at the American Museum of Natural History in New York.  This beast always reminded me of a giant newt...or a reptilian walrus.

Placodus also had a row of small bony plates down the middle of its back.  They may have been to help break up its shape as a form of camouflage, or possibly taken part in some kind of intraspecies display to impress mates and intimidate rivals.  Modern lizards with ridges down their backs use them in similar ways today.  These small plates also may have aided Placodus in thermoregulation, expanding its surface area and acting as small solar panels to absorb more warmth from the sun, as it was likely ectothermic like many of today's reptiles and would have needed the sun in order to stay warm.  Living in and near the ocean, it would have been easier to lose heat due to the water, so staying warm enough have the energy to to swim would have been important to an animal like Placodus.  If you've ever been to the beach, even on the hottest days, you will notice being near or in the water is always much cooler.

Close up of Placodus' skull at the American Museum of Natural History in New York.  Note the chisel teeth in the front of its jaws and the extremely wide, flat teeth in the back of its mouth.  It is very likely Placodus was a specialist at eating shellfish.

Placodus' head is perhaps its most interesting feature.  It had a short skull, with its eyes and nostrils on the top, implying it was spending time in the water and therefore wouldn't need to surface much in order to breathe and see.  Modern swimming animals like frogs, alligators, and beavers, to name a few, have similar facial arrangements for this very purpose.  In the front of its mouth, it had forward-facing teeth that were chisel-like.  Inside its jaws, even on the roof of its mouth, it had wide, platform-like teeth.  It is almost certain that Placodus specialized in eating shellfish, like mussels and clams.  The front teeth would have been ideal for raking them out of the seafloor, or plucking them off of rocks, and the back teeth were perfect for crushing shells once they were in the mouth.  Modern day walruses, which are not related to placodus, but also specialize in eating bivalves, have similar teeth in the backs of their jaws.  In fact, in many ways Placodus likely filled an overall similar ecological niche as walruses do today.  Placodus belonged to the now extinct family of reptiles that flourished during the Triassic, called placodontidae.  All members of this group had similar flat teeth for crushing shelled mollusks.

Underside of Placodus' skull.  Check out how the entire roof of the mouth is covered in those wide, flat teeth!  Cast on display at the Naturalis Museum in the Netherlands.  Photo credit: Ghedogedo from Wikipedia.

That is all for this week!  As always please feel free to comment below!


Naish, D. 2004. Fossils explained 48. Placodonts. Geology Today 20 (4), 153-158.

Rieppel, O. (2002). Feeding mechanisms in Triassic stem-group sauropterygians: the anatomy of a successful invasion of Mesozoic seas Zoological Journal of the Linnean Society, 135, 33-63