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Pisanosaurus ( ) is an extinct genus of early dinosauriformes, likely an Ornithischia or Silesauridae, 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 species, 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.
The genus is holotype 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 bone, ischium and pubic bone, an impression of three metacarpals, the complete femur, the right tibia, the right fibula, with an articulated talus bone 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.
The genus name Pisanosaurus means "Pisano's lizard" and combines "Pisano" in honor of Argentina paleontologist Juan Arnaldo Pisano of La Plata Museum, with a Latin "saurus" from the Greek Language (σαύρα) meaning "lizard".Liddell, Henry George and Robert Scott (1980). 9780199102075, Oxford University Press. ISBN 9780199102075 Pisanosaurus was described and named by Argentine paleontologist Rodolfo Casamiquela in 1967. The type species and only valid species known today is Pisanosaurus mertii. The specific name honors the late Araucanian naturalist Carlos Merti.
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 exact classification of Pisanosaurus has been the topic of debate by scientists for over 40 years; until 2017, the consensus was that Pisanosaurus is the oldest known , part of a diverse group of dinosaurs which lived during nearly the entire span of the Mesozoic. More recently, some authors have begun to consider it a non-dinosaurian silesauridae, though this hypothesis has not reached a consensus either.
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. This assignment is also supported by Norman et al. (2004), Langer et al. (2009) and the controversial Ornithoscelida hypothesis of Baron, Norman & Barrett (2017).D. B. Norman, L. M. Witmer, and D. B. Weishampel. 2004. Basal Ornithischia. In D. B. Weishampel, H. Osmolska, and P. Dodson (eds.), The Dinosauria (2nd edition). University of California Press, Berkeley 325-334 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.
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 Monophyly sister group of dinosaurs, some studies consider it a Paraphyly grade ancestral to ornithischian dinosaurs in particular. One such study is Müller & Garcia (2020). Although they regarded Pisanosaurus as the basal-most , 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 Ischigualasto Formation was a volcanically active floodplain covered by forests, with a warm and humid climate, though subject to seasonal variations including strong rainfalls. Vegetation consisted of , , and giant , 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, , 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 such as Hyperodapedon (a beaked reptile); (spiny armored reptiles); kannemeyeriidae dicynodonts (stocky, front-heavy beaked quadrupedal animals) such as Ischigualastia; and traversodontidae (somewhat similar in overall form to dicynodonts, but lacking beaks) such as Exaeretodon. These non-dinosaurian herbivores were much more abundant than early dinosaurs.
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