UCMP MYSTERY FOSSIL #52 is . . .
the dicynodont Placerias gigas

Placerias was a dicynodont, a therapsid that lived during the Middle to Upper Triassic period. During the Late Permian and Triassic, dicynodonts were the most successful and wide-ranging group of herbivores around. Dicynodonts had a unique dentition — the front of the skull and the lower jaw were narrow and beak-like, with the upper and lower jaws probably having horny beaks like those in turtles. The body was short and broad, supported by strong, thick limbs. To see a reconstruction of Placerias, try this page, or, to find out more about dicynodonts in general, look here.