UCMP MYSTERY FOSSIL #33 is . . .
the aetosaur Neoaetosauroides engaeus

Neoaetosauroides engaeus is an aetosaur, a contemporary of the earliest dinosaurs, from the Upper Triassic of San Juan Province in Argentina . Aetosaurs like Neoaetosauroides were heavily armored and some even had spikes along the shoulders and flanks. Find out more about aetosaurs here.