Ischigualastiajenseni

Overview: Some 231 million years ago in Argentina, dinosaurs had just appeared, but they were not the dominant creatures. The largest animal here was the herbivorous dicynodont Ischigualastia, discovered in the mid-20th Century. This strange distant relative of mammals was one of the last and largest of its kind and would soon be replaced by the dinosaurs which were beginning to proliferate around it.

Discovery: Ischigualastia jenseni was discovered in the 20th Century and named in 1962 by C. Barry Cox. It was found in the Ischigualasto Valley in San Juan Province, Argentina. Its genus name references the Ischigualasto Formation and Ischigualasto Valley, meaning “Ischigualasto One”, and its species name honors James Jensen who prepared the specimen. James Jensen, or Jim, as he was often known, went on to found Brigham Young University’s paleontology program where he became famous for his extremely prolific and successful fossil collecting ventures and his revolutionary way of mounting dinosaur skeletons. The holotype of Ischigualastia consists of a skull and partial skeleton.

Evolution and Description: Ischigualastia was a type of therapsid or “mammal-like reptile”. Therapsids in turn belong to the clade Synapsida, which includes mammals and all animals closer to mammals than to sauropsids (reptiles). Synapsids are typically identified by having a single temporal fenestra (a hole in the skull behind the eye socket). Ischigualastia was a member of the Dicynodonta, which was an herbivorous group within the therapsids. Dicynodonts were the dominant herbivores on Earth after the Permian-Triassic Extinction (or “Great Dying”). They were stout creatures with barrel-shaped bodies, thick semi-sprawling limbs, short tails, a beaked mouth, similar to a turtle or ceratopsian dinosaur, and most sported a pair of tusks or tusk-like structures. Ischigualstia was in the family Stahleckeriidae and was thus a close relative of the earlier Brazilian Stahleckeria and the more famous North American Placerias. Ischigualastia was among the largest of the dicynodonts, probably weighing up to 2 tonnes (2.2 tons) and measuring 3.5 m (11.48 ft) in length. It lacked teeth, and like its relative Stahleckeria, it lacked true tusks, instead having tusk-like extensions on the sides of the beak.

Dicynodonts may have been endothermic (warm-blooded) and may have even had primitive fur, though this is unconfirmed. Nevertheless, endothermy seems likely, while fur remains uncertain as skin impressions from the dicynodont Lystrosaurus show that their skin texture consisted of raised bumps, though this does not necessarily preclude the presence of sparse fur that simply didn’t get preserved. Coprolites (fossil feces) containing dicynodont bones and possible fur may suggest that it was present on dicynodonts to some degree. So Ischigualastia was likely warm-blooded, and may have had a covering of fur, though if it did, it was probably a sparse covering, like that seen in modern elephants and rhinos, which would make sense for such a large endothermic animal living in a warm climate.

Ecology: Ischigualastia lived in the Ischigualasto Formation which dates from 231.7 and 225 million years ago. We can get somewhat precise dates for the formation thanks to the abundant volcanic ash deposits within it. The environment of the Ischigualasto Formation was a floodplain environment with abundant rivers and plants. Plants included trees such as Ginkgoites, a relative of today’s ginkgoes, Araucarioxylon, a relative of Araucaria trees, and the especially large conifer Protojuniperoxylon, which could grow to be over 40 m (130 ft) tall. Other plants included ferns, horsetails, and cycads. The environment evidently sufficiently lush to support such vegetation, though rainfall was probably strongly seasonal. Ischigualastia would have eaten plants closer to the ground, and likely ate a lot of ferns and horsetails. During the dry season it may have used the tusk-like extensions on its beak to help it dig for roots, though these features may have been more used for display, with males competing with each other and showing off to females. Ischigualastia shared its environment with the slightly smaller dicynodont, Jachaleria, the large aetosaur (an armored herbivorous relative of crocodiles) Aetosauroides, the small herbivorous Hyperodapedon (a type of rhynchosaur), and the large herbivorous cyndont Exaeretodon. The early small sauropodomorph dinosaurs Panphagia and Chromogisaurus, and the enigmatic dinosaur-like silesaurids Ignotosaurus and Pisanosaurus also filled herbivore niches. Patrolling the rivers were the large temnospondyl amphibians Promastodonsaurus and Pelorocephalus, as well as the crocodile-like proterochampsid reptile Proterochampsa. Terrestrial predators included small cynodonts, the early crocodylomorph Trialestes, and early dinosaurs such as Eoraptor, Eodromaeus, and Herrerasaurus. The largest predator however was no dinosaur, but a terrestrial crocodile relative called Saurosuchus which was likely Ischigualastia’s primary predator.

Extinction and Legacy: Ischigualastia lived during a time of great change. While dicynodonts dominated herbivore niches during the Early and Middle Triassic, they began to decline in the Late Triassic. In their place, large herbivorous dinosaurs rose to dominance. By the end of the Triassic, dicynodonts had all gone extinct. While Ischigualastia wasn’t the very last of them (Placerias and Lisowicia lasted longer), it nevertheless lived at the beginning of the end for these odd herbivores. The holotype of Ischigualastia resides in the Bernardino Rivadavia Natural Sciences Argentine Museum in Buenos Aires, Argentina. A skeletal mount of Ischigualastia can be seen in the Museo de Ciencias Naturales de Ischigualasto, in Ischigualasto Provincial Park in San Juan, Argentina. A skull can be seen in the National Museum of Natural History (Smithsonian) in Washington DC, USA, and another skull can be seen at the BYU Museum of Paleontology in Provo, UT, USA. Placerias, a close relative of Ischigualastia was featured in the 1999 BBC documentary Walking with Dinosaurs.