Massetognathus

Massetognathus Temporal range: Middle Triassic PreЄ Є O S D C P T J K Pg N
Restoration
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Order:	Therapsida
Suborder:	Cynodontia
Family:	†Traversodontidae
Subfamily:	†Massetognathinae
Genus:	†Massetognathus
Type species
†Massetognathus pascuali
Species
† Massetognathus teruggi † Massetognathus ochagaviae † Massetognathus major

Massetognathus (pron.: MASS-eh-TOG-nah-thuss; Greek for “chewing muscle jaw”) is a genus of extinct plant-eating cynodont belonging to the Traversodontid family existing during the Middle Triassic period, 235 million years ago, in the Chañares Formation in Argentina and the Santa Maria Formation in Brazil.

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.