Massetognathus was about 50 centimetres (1.6 ft) long. It had cheek teeth specially adapted to chewing on vegetation. It still had the distinctive long snout of its cynodont relatives, with nipping incisors and fang-like canines, but its cheek teeth were not pointed. Instead they were flat-topped and were covered with a number of low ridges, which made them good for grinding any stems, roots and other plant materials. Massetognathus had clawed feet and a long dog-like tail. Like most of its cynodont relatives it may have had hair, but not a full fur.
Massetognathus species measured to about 46 centimeters (1.5 ft) in length with an estimated weight between 1-1.5 kilograms (2-3 lbs). Massetognathus was the smaller, plant-eating equivalent of the better-known Cynognathus with incisors, fang-like canines and flat-topped molars covered by low ridges, an adaptation for grinding tough plant stems, roots, and other plant materials. It had a low and flattened skull indicating that the specimen had a relatively shorter and broader snout than larger specimens. It was a fox sized animal with claws on its feet and a long dog-like tail. Like most cynodonts, there is some evidence that they laid eggs, were warm blooded, as indicated by the detailed structure of the bones, and had a body covered by hair.
Massetognathus is a medium-sized cynodont, which document different ontogenetic stages. It had the largest size of any cynodont in the Chañares assemblage with an approximate skull length ranging from the smallest being 72mm to the largest 204 mm. The Middle Triassic Probainognathus and Massetognathus are the earliest non-mammalian cynodonts in the fossil record that show the initial steps of several phylogenetic transformations of the quadrate and can be characterized by several features: The rotation of the dorsal plate relative to the trochlea exhibits a progressively greater rotation more closely related to mammals, squamosal contact and medial expansion of the squamosal were crucial factors in the transforming the quadrate and the articulation of the cranium. The maxillae extend far out dorsally (with a downward slope) to a point about opposite the lower margins of the orbits, then curving downward and inward, present a broad ventral surface lateral to the tooth rows. The skull is low and the orbits face more dorsally than laterally with the nasals and frontals laying flat on top of the skull. In contrast to other cynodonts, the squamosal descends ventrally.
There are four upper and three lower triangular incisors of modest size with canines that are relatively less developed. There are two rows of cheek teeth that are close together and diverge posteriorly. A short diastema separates the cheek teeth and canines. There is no significant contrast between the premolars and molars. There are generally 12 maxillary teeth. It has been established that Massetognathus with multi-cuspate post canines adapted to herbivory, moved the lower jaw posteriorly and dorsally during the power stroke of the occlusion. Massetognathus is the only cynodont from the Chañares Formation with clear adaptations for herbivory, with basined, labiolingually expanded upper and lower post-canines, ensuring a rudimentary dental occlusion, feeding on ground level vegetation or on the lower branches of taller plants and shrubs.
Medium-sized faunivores likely fed preferably on juvenile Massetognathus and dicynodonts. Additionally, the slightly larger Chanaresuchus and Pseudolagosuchus also preyed on individual Massetognathus that were not fully grown. Luperosuchus and the unnamed paracrocodylomorph represent the top predators in the Chañares Formation. They preyed on all the other members in the fauna, including fully grown dicynodonts and Massetognathus. Considering the abundance of the herbivorous cynodont Massetognathus, it is clear that this taxon represents the main food resource in the Chañares assemblage. Only a few forms were capable of preying on fully grown Massetognathus; therefore, a high predation pressure on infant, juvenile and sub-adults is expected, and this, together with a high reproductive rate, may explain the overwhelming abundance of Massetognathus bones preserved.
At least 4 different species of Massetognathus has been discovered so far.
The type species, M. pascuali, is the best-known species of the genus and is arguably considered to be the only valid species for Chañares gomphodonts while the others are considered junior synonyms. Named by Alfred Romer, the specific name is in honor of Dr. Rosendo Pascual, Professor of Paleontology in the Universidad de la Plata, who accompanied his expedition during his stay in Western Argentina.
M. teruggi is known to be the most common species of Chañares reptiles. Named after the scientist and writer Dr. Mario Teruggi by Romer. M. teruggi skulls on average are approximately 45 percent larger than the skulls of M. pascuali and had a more defined sagittal crest. The dentary is less sharp and had 15 maxillaries compared to the 12 M. pascuali had.
M. ochagaviae is know to be the most common species from the Santa Maria Formation, Rio Grande do Sul, Brazil. Named after Mário Costa Barberena. It had a higher skull and mandible, slightly dorsally pointed dentary ventral border under the coronoid process, and less post canines compared to the other three species.
M. major is known for having the largest skull out of the 4 species that can reach up to 204 mm. It had a distinctively narrow snout with the teeth being less curved compared to its sister taxa. There is a complete loss of the parietal foramen. Megagomphodon oligodens may have been derived from this species.