Ischigualastia
Ischigualastia is an extinct genus of large dicynodont therapsids from the Late Triassic of Argentina. It is named after its place of discovery, the Ischigualasto Formation in the Ischigualasto-Villa [Unión Basin] of northwestern Argentina. Like other Late Triassic dicynodonts, Ischigualastia is a member of the family Stahleckeriidae.
Discovery
Ischigualastia was briefly named by C. Barry Cox in 1962, with a larger description published in 1965. It is one of the better-known South American dicynodonts in terms of its variety of fossils, including several nearly complete skulls. Cox named the genus after its location of discovery, the Ischigualasto Valley, while the species honors James Jensen, who prepared some of its fossils.The holotype, MACN 18.055, is a skull stored at the Bernardino Rivadavia [Natural Sciences Argentine Museum|Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"] in Buenos Aires. Though partially "restored" with plaster, it still closely resembles unaltered skulls. Cox also discussed Ischigualastia fossils collected by the Museum of Comparative Zoology in Cambridge, Massachusetts. Additional fossils are housed at the Instituto Miguel Lillo in Tucumán and Universidad Nacional de San Juan in San Juan. However, they have yet to be fully described.
There are a few reports of Ischigualastia fossils from New Mexico, Texas, and India: a femur, a snout fragment, and a poorly-preserved skull, respectively. These claims have not held up to scrutiny, since the fossils in question do not closely resemble Ischigualastia in particular. Supposed Ischigualastia fossils from Brazil are more likely to be Jachaleria, Sangusaurus, or a new unnamed species.
Description
Ischigualastia is a very large dicynodont, with a deep skull bearing pointed projections on each side of the snout, rather than tusks. The skull could reach a length of. The back of the skull is steeply sloped and has particularly large spaces for vertical jaw muscles, suitable for a strong bite. In contrast, many earlier tusked dicynodonts had low skulls which were better suited for propalinal jaw movement during a bite.In many regards Ischigualastia is similar to a nearby relative, Jachaleria, though there are a few traits entirely unique to the skull of Ischigualastia. The top of the snout has a narrow depression near where the frontal bones contact the nasal bones. The pineal foramen, a hole further back on the forehead, has swollen rims. Finally, the zygomatic arch is deep and strongly bent upwards.
Ischigualastia is similar in stature to the North American stahleckeriid Placerias, which measured around long and weighed up to. Ischigualastia, Stahleckeria, and Placerias all show evidence of sexual dimorphism, meaning that there are strong differences between the skulls of proposed males and females. Robust Ischigualastia skulls, interpreted as males, have a distinctly broader snout and intertemporal area, as well as rough patches on the nasal bones.
Paleoecology
Ischigualastia was a large quadrupedal herbivore found in late Carnian-age strata of the Ischigualasto Formation. It was a relatively common component of the local fauna, although not as abundant as the medium-sized herbivores Hyperodapedon and Exaeretodon. It was one of the two dicynodonts which lived in the Ischigualasto Formation, the other being Jachaleria, a close relative which is only found in the youngest layers of the formation. The largest carnivores in the formation were the pseudosuchian Saurosuchus and the common predatory dinosaur Herrerasaurus.Ischigualastia and Placerias were each among the last and largest dicynodonts of the Late Triassic, though they were surpassed in both regards by Lisowicia, a huge late-surviving stahleckeriid from Poland.