UCMP MYSTERY FOSSIL #42 is . . .
the dicynodont Dicynodon grimbeeki

Dicynodonts were therapsids, descendants of pelycosaurs like Dimetrodon, and ancestors of modern mammals. Therapsids varied greatly in size and came in both carnivorous and herbivorous “models.” Dicynodonts were strictly herbivorous, having lost their cheek teeth and developing turtle-like jaws. Some specimens of Dicynodon (like this one) had short tusks in the upper jaw—fossils indicate that some individuals had tusks and some did not, suggesting sexual dimorphism. The UCMP site has a gap in its information on dicynodonts, but you can find out more about therapsids here.