Massetognathus

Extinct genus of cynodonts

Massetognathus (,/m//æ//s//ɪ/'/t//ɒ//ɡ//n//ə//θ//ə//s/ MASSəTOGnəthəss; 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.

Massetognathus species measured to about in length with an estimated weight between 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 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 to the largest 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.

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. 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.

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. The Chañares Formation represented an extremely thick accumulation of volcanic tuffs that were deposited in wither eolian or lacustrine settings. The lack of stratification prompted Romer to further conclude that deposition in water was unlikely. Instead, Romer envisioned the Chañares landscape “with tremendous quantities of volcanic ash in Pompeii like fashion”. The formation included deposits of muddy streams transporting abundant volcanic detritus. Ash flow sheets potentially were emplaced as mass flows on sub alluvial surfaces and in lakes that occupied the Ischigualasto Villa Únion Basin.

Tetrapod fossils have been found only in the lower lithologic unit of the Chañares Formation, where they are preserved almost exclusively within carbonate concretions. Fossilized bone preserved in concretion shows some of the best form of preservation, with dark brown bone surfaces exhibiting virtually no evidence of macroscopic weathering. The fossil accumulations of the Chañares assemblage are considered to be the product of two different taphonomic pathways: attritional accumulation associated with natural deaths of individuals by predation, disease and old age, and mass mortality of animals associated with volcanic events. In the mass mortality event, there is a clear bias towards preservation of individuals representing smaller sized to mid sized taxa such as Massetognathus. The mass mortality assemblage, with a large quantity of complete or partially articulated skeletons, shows some post mortem sorting, followed by rapid burial that prevent the carcasses from being scavenged, crushed by species or exposed to weathering. In the lower lithologic unit of the Chañares Formation, the matrix that encompasses the fossils were in a copious amount of volcanic ash indicating the possibility that lethal volcanic processes led to the demise of the fauna. Volcanic reactions could have also led to the landscape flooding and damming of local rivers also resulting in volcanism being the cause of mass mortality in the Chañares Formation.