Mastodonsaurus (meaning "breast tooth lizard") was a large-headed temnospondyl that belonged to a group of advanced, mostly Triassic amphibians called capitosaurids. 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).It probably also ate land-living animals, such as small archosaurs. The fossils of some smaller temnospondyls bear tooth marks made by Mastodonsaurus-like animals.
According to some scientists, Mastodonsaurus was completely unable to leave the water, and this option is borne out by finds of large quantities of bones showing that in times of drought, when the pools died up, these creatures died en masse.
Mastodonsaurus was once thought to be responsible for the footprints found in Triassic sandstones and described as Chirotherium, but more recent research had found that the tracks belong to lizard-like reptiles of the Pseudosuchia group.
Recent studies have shown, however, that its body was less compact and the tail much longer, giving it an overall-appearance much like a crocodile. Two triangular tusks pointed up from near the tip of its lower jaw. When the jaws closed, these slotted through openings on the palate and projected through the top of the skull.