Pisanosaurus

Pisanosaurus is known from a single fragmented skeleton discovered in 1962 by Galileo Juan Scaglia at the Hoyada del Cerro Las Lajas locality The genus is based on a specimen given the designation PVL 2577, which consists of a partial skull including a fragmentary right maxilla with teeth, and incomplete right mandibular ramus (lower jaw), six incomplete cervical vertebrae, seven incomplete dorsal vertebrae, molds of five sacral vertebrae, a rib and several rib fragments, a fragmentary right scapula, a coracoid, molds of a fragmentary ilium, ischium and pubic bone, an impression of three metacarpals, the complete femora, the right tibia, the right fibula, with an articulated astragalus and calcaneum, a tarsal element with a metatarsal, metatarsals III and IV, three phalanges from the third toe and five phalanges (including the ungual) from the fourth toe, and an indeterminate long bone fragment. Pisanosaurus was described and named by Argentine paleontologist Rodolfo Casamiquela in 1967. The type and only valid species known today is Pisanosaurus mertii. The specific name honors the late Araucanian naturalist Carlos Merti. == Description ==

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. == Classification ==

}} }} Pisanosaurus is the type genus of the Pisanosauridae, a family erected by Casamiquela in the same paper which named Pisanosaurus. 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. == Paleoecology ==

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. == References ==