Massetognathus ( MASS-ə-TOG-nə-thəss; Greek for "chewing muscle jaw") is an extinct genus of plant-eating traversodontid cynodonts. They lived during the Triassic Period about 235 million years ago, and are known from the Chañares Formation in Argentina and the Santa Maria Formation in Brazil.
Description Massetognathus species measured to about 46 centimeters (1.51 feet) in length with an estimated weight between 1 and 1.5 kilograms (2.2 and 3.3 lb). 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.
Skull Massetognathus was a medium-sized cynodont, which documents 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 72 millimetres (2.8 in) to the largest 204 millimetres (8.0 in). 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.
Postcranium The humerus of M. ochagaviae had a rounded lesser tuberosity and a bulbous greater tuberosity. The ulnar condyle was also highly bulbous. The iliac blade was thin and had a spoon-shaped anterior portion and lanceolate posterior portion. The femur was characterised by a robust and protuberant greater trochanter, along with a bulbous, hemispherical and dorsomedially inclined femoral head. The femoral shaft is subrectangular in cross-section at the midpoint of the femur.
Species 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 cynodonts. 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 known 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.
Discovery In 1967, American paleontologist Alfred Romer named three new species under the genus: Massetognathus pascuali, M. teruggi, and M. major during his expedition in western Argentina. In 1981, Brazilian paleontologist Mário Costa Barberena named the fourth species, Massetognathus ochagaviae on the basis of a specimen from the Santa Maria Formation in Brazil. After the discovery of M. pascuali and M. teruggi, Romer first believed that these skulls were representing growth stages due to the larger size of M. teruggi. However, after collecting multiple skull samples, they clearly sorted into two size groups, rather than showing growth stages of a single species. The size difference is also considerably greater between the two than that expected in sex differences which gave Romer another reason to believe that they were dealing with two species of a single genus.
Classification Massetognathus is classified as a cynodont in the family Traversodontidae. The cladogram below shows the position of Massetognathus following the phylogenetic results of Pavanatto et al., 2016 in their description of postcranial remains assigned to M. ochagaviae.
Paleobiology
Diet 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.
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