Pisanosaurus

Extinct genus of dinosauriforms

Pisanosaurus (/p//I//s//æ//n//@//'//s//ɔː//r//@//s/) is an extinct genus of early dinosauriform, likely an ornithischian or silesaurid, from the Late Triassic of Argentina. It was a small, lightly built, ground dwelling herbivore, that could grow up to an estimated long. Only one species, the type, Pisanosaurus mertii, is known, based on a single partial skeleton discovered in the Ischigualasto Formation of the Ischigualasto Villa Unión Basin in northwestern Argentina. This part of the formation has been dated to the late Carnian, approximately 229 million years ago.

Based on the known fossil elements from a partial skeleton, Pisanosaurus was a small, lightly built dinosauriform, reaching in length and in body mass. These estimates vary due to the incompleteness of the holotype specimen PVL 2577. The orientation of the pubis is uncertain, with some skeletal reconstructions having it projecting down and forward (the propubic condition) similar to that of the majority of saurischian dinosaurs. According to a redescription by José Bonaparte in 1976, Pisanosaurus has some distinctive characteristics. The acetabulum (hip socket) is open. The peduncles of the ilium are short, resulting in a low and axially elongated acetabulum. The upper region of the ischium is wide, larger than that of the pubic bone. The metacarpals of the hand are apparently elongated, measuring about fifteen millimeters. The family Pisanosauridae has fallen into disuse; a 1976 study considered the group synonymous with the already named Heterodontosauridae, though this hypothesis has not reached a consensus either. However, recent studies suggest that the fossils belong to a single specimen. Over the years, Pisanosaurus has been classified as a heterodontosaurid, a fabrosaurid, a hypsilophodont and has also been considered the earliest known ornithischian. A 2008 study placed Pisanosaurus outside of (and more basal than) Heterodontosauridae. In this study, Pisanosaurus is the earliest and most primitive ornithischian. Other primitive ornithischians include Eocursor, Trimucrodon, and possibly Fabrosaurus. The hypothesis of ornithischian affinities for Pisanosaurus has not fallen out of favor despite competition from alternative hypothesis. Silesaurid like traits, for example, may be dinosaurian plesiomorphies (ancestral conditions) rather than unique characteristics of silesaurids. In 2017, two studies independently came to the conclusion that Pisanosaurus was a silesaurid: one was an expansive redescription by Agnolin and Rozadilla, and the other was a re analyzed Ornithoscelida matrix by Baron, Norman, & Barrett. The placement of Pisanosaurus is reliant on the placement of silesaurids as a whole, a situation which has invited much debate. While Silesauridae is often considered a monophyletic sister group of dinosaurs, some studies consider it a paraphyletic grade ancestral to ornithischian dinosaurs in particular. One such study is Müller & Garcia (2020). Although they regarded Pisanosaurus as the basal most ornithischian, taxa often considered members of Silesauridae form a step wise arrangement up to Pisanosaurus. It acts a transitional form positioned on a rung between the "silesaurid" grade (Asilisaurus, Sacisaurus, Silesaurus, etc.) and traditional ornithischians (Eocursor, Scutellosaurus, Heterodontosaurus, etc.). This phylogenetic position may explain why some authors consider Pisanosaurus a silesaur and others consider it an ornithischian, as following Müller & Garcia, Pisanosaurus has traits of both groups.

The fossils of Pisanosaurus were discovered in the "Agua de las Catas" locality at the Ischigualasto Formation in La Rioja, Argentina. Originally dated to the Middle Triassic, this formation is now believed to belong to the Late Triassic Carnian stage, deposited approximately 228 to 216.5 million years ago. though subject to seasonal variations including strong rainfalls. Vegetation consisted of ferns, horsetails, and giant conifers, which formed highland forests along the banks of rivers. Herrerasaurus remains appear to have been the most common among the carnivores of the Ischigualasto Formation. Sereno (1993) noted that Pisanosaurus was found in "close association" with therapsids, rauisuchians, archosaurs, Saurosuchus and the dinosaurs Herrerasaurus and Eoraptor, all of whom lived in its paleoenvironment. Bonaparte (1976) postulated that Pisanosaurus played a role in a fauna dominated by therapsids. The large carnivore Herrerasaurus may have fed upon Pisanosaurus. Herbivores were represented by rhynchosaurs such as Hyperodapedon (a beaked reptile); aetosaurs (spiny armored reptiles); kannemeyeriid dicynodonts (stocky, front heavy beaked quadrupedal animals) such as Ischigualastia; and traversodontids (somewhat similar in overall form to dicynodonts, but lacking beaks) such as Exaeretodon. These non dinosaurian herbivores were much more abundant than early dinosaurs.