[1] Narrow grooves on the surface of the skull bones called sulci show it had sensory organs that could detect vibrations and pressure under water, similar to the lateral lines on fish.

The marked reduction of the limbs, the strong tail and sensory grooves on the head called sulci show that Mastodonsaurus was an aquatic animal that rarely, if ever, ventured on land.

Mastodonsaurus may have been completely unable to leave the water, as large quantities of bones have been found that suggest individuals died en masse when pools dried up during times of drought.

Bite marks on Mastodonsaurus bones show that the large terrestrial archosaur Batrachotomus actively preyed on the giant amphibians, entering the water or attacking individuals stranded in pools during droughts.

[4] Based on the misattributed tracks and misidentified bones from other Triassic animals, early illustrations depicted the giant amphibians (often referred to as "Labyrinthodon" at the time) as big froglike creatures that supposedly crossed their legs as they walked since the outer fifth digit on the Chirotherium footprints resembled a thumb.Most of the skeleton of Mastodonsaurus, apart from skulls and jaws, remained poorly known until recently.

[10] A site discovered during road construction near the town of Kupferzell in southern Germany in 1977 provided researchers with important new fossils of Mastodonsaurus giganteus that included well preserved skulls and disarticulated bones from all parts of the body.

Thousands of individual fossils were recovered during a three-month salvage operation before road work resumed, including, in addition to Mastodonsaurus, remains of the temnospondyl Gerrothorax and the archosaur Batrachotomus, as well as of many fishes.

[1] Although no complete and fully articulated skeleton has been found to date, research since 1999 was incorporated into a composite skeletal reconstruction and a fleshed-out model displayed at the Staatliches Museum für Naturkunde Stuttgart in Germany that give Mastodonsaurus more crocodile-like proportions, with a lengthened tail for swimming, similar to some other capitosaurs.

[14] Stereospondyls lacked a true larval stage of development and Mastodonsaurus followed a slow, conservative ontogenetic pattern with relatively minor changes as it grew so that small juveniles would have resembled adults.

[6] The German paleontologist Georg Friedrich von Jaeger gave the name Mastodonsaurus in 1828 to a single large conical fang with vertical striations and a worn-off tip, found in the Triassic Lettenkeuper deposits near Gaildorf in Baden-Württemberg in southern Germany.

Also in 1828, Jaeger identified the back part or occiput of a large skull found in the same area as coming from a giant amphibian-like animal as indicated by the double articulation of the occipital condyles.

The maze-like inner tooth structure in Mastodonsaurus is found in multiple types of extinct amphibians, and Richard Owen created the formal taxonomic category Labyrinthodontia (published in 1860) as a supposed order of "Reptilia" to unite them.

However, the "order" turned out to contain multiple types of animals that not are not closely related and the category Labyrinthodontia no longer has recognized scientific status, although the general form "labyrinthodont" is still used as a descriptive term.

In recent work, German paleontologist Rainer R. Schoch has recognized Mastodonsaurus giganteus (Jaeger 1828) as the official type species, with the occiput (GPIT Am 678) as the holotype specimen, considered diagnostic.

[17] Holl had treated Mastodonsaurus and Salamandroides as distinct and unrelated animals (a reptile (from the tooth) and an amphibian (from the back of a skull) respectively), in line with Jaeger’s first descriptions of the fossils in 1828.

The species M. andriani, M. indicus, M. laniarius, M. lavisi, M. meyeri, M. pachygnathus and M. silesiacus, when reexamined by Moser and Schoch, were not deemed assignable to the genus Mastodonsaurus due to the fragmentary nature of the type specimens and as such are considered nomen dubium.