An odd sort of critter, now on permanent display at the Petrified Forest National Park visitor’s center in Arizona:
Placerias hesternus was not a dinosaur — it was a dicynodont therapsid that lived in the late Triassic Period, some 221-210 million years ago. This is a bit of a mouthful, so let’s break it down:
- Placerias — “broad body”
- dicynodont — “two-dog tooth,” it had toothless beak-like jaws with two canine-like downward-pointing tusks
- therapsid — a class of mammal-like reptiles
Placerias was the largest herbivore of its time, and is thought to have filled an ecological niche similar to that of the modern-day hippo.
From the placard:
Therapsids were large reptiles that possessed many mammalian characters including a “cheek” bone, enlarged canine teeth, and a specialized attachment of the skull to the spine. This massive plant-eater was up to 9 feet (2.7 m) long and might have weighed as much as two tons.
Like other dycynodonts, Placerias had a short neck, barrel-shaped body, small tail, and large tusk-like bones protruding from its upper jaw. The beak-like jaws helped to pull up and tear tough plants and roots. A large number of Placerias fossils were found in a single quarry near St. Johns, just southeast of the park.
The quarry mentioned on the placard contained the remains of forty Placerias, and was found in 1930. Based on sediment surrounding the fossils, it’s thought that they died in a flood. Placerias fossils have also been found in North Carolina and Wyoming.