Mastodonsaurus was a large-headed temnospondyl that belonged to a group of advanced, mostly Triassic amphibians called capitosaurs. It was a giant among the stegocephalians and the largest animal of its time (late Triassic, 200 MYA). It looked like a huge frog, but instead of being semicircular, as in frogs, its head was triangular and reached 1.25 m. in length, the total length of the animal would be about 4-5 meters. The large, oval eye sockets were midway along the skull. The jaws were armed with conical teeth. The body was relatively small in proportion to the large head, and the tail was very short. The greatly reduced limbs had cartilaginous carpal and tarsal joints. The marked reduction of the limbs and the sinus lines on the head show that Mastodonsaurus was an aquatic animal which hardly ever left water. It inhabited swampy pools and lived mainly on fish, whose remains have been found in its fossilized excrete (coprolites).