Temporal range: Late Triassic, 228–216.5Ma
|Display at the Royal Ontario Museum|
|Species:||† P. mertii|
Pisanosaurus (pron.:"PIE-san-uh-SAWR-us") is a genus of primitive ornithischian dinosaur from the Late Triassic of what is now South America. It was a bipedal herbivore described by Argentine paleontologist Rodolfo Casamiquela in 1967. Only one species, the type, Pisanosaurus mertii, is known, based on a single partial skeleton. The fossils were discovered in Argentina's Late Triassic Ischigualasto Formation, which dates to about 228 to 216.5 million years ago.
The exact classification of Pisanosaurus has been the topic of debate by scientists for over 40 years; the current consensus is that Pisanosaurus is the oldest known ornithischian, part of a diverse group of dinosaurs which lived during nearly the entire span of the Mesozoic Era.
Based on the known fossil elements from a partial skeleton, Pisanosaurus was a small, lightly built dinosaur approximately 1 m (3 ft 3 in) in length. Its weight was between 2.27–9.1 kg (5–20 lb). These estimates vary due to the incompleteness of the holotype specimen PVL 2577. This specimen consists of a partial skull with a fragmentary right maxilla, the right lower jaw, six cervical vertebrae, seven dorsal vertebrae, a fragmentary scapula, a coracoid, molds of five sacral vertebrae, fragmentary ilium, ischium and pubis, an impression of 3 metacarpals, complete femora, tibia, fibula, astragalus, calcaneum, metatarsals III and IV, three phalanges from metatarsal III and four metatarsals and the ungual from metatarsal IV. 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. The tail of Pisanosaurus has been reconstructed as being as long as the rest of the body, based on other early ornithischians, but as a tail has not been recovered, this is speculative. It was bipedal and, like all other known ornithischians, was probably herbivorous.
Discovery and naming 
The name Pisanosaurus is derived from the name of Juan A. Pisano, an Argentine paleontologist, and the Greek word "sauros" (σαυρος), meaning "lizard"; thus, "Pisano's lizard". The specific name, "mertii honors late Araucanian naturalist Carlos Merti. Pisanosaurus mertii was described and named by Argentine paleontologist Rodolfo Casamiquela in 1967.
|Cladogram of basal Ornithischia after Butler et al. (2008), showing the position of Pisanosaurus as the earliest example of an ornithischian.|
Pisanosaurus is very basal within Ornithischia; the postcrania seem to lack any good ornithischian synapomorphy and it was even suggested by Paul Sereno in 1991 that the fossil is a chimera. 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.
Pisanosaurus is the type genus of the Pisanosauridae, a family erected by Casamiquela in the same paper which named Pisanosaurus. The pisanosauridae family has fallen into disuse, as a 1976 study considered the group synonymous with the already named Heterodontosauridae.
Provenance and occurrence 
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. This specimen was collected by José Bonaparte, Herbst and the preparators Vince and Scaglia in 1962, and is housed in the collection of the Laboratorio de Paleontologia de Vertebrados, Instituto Miguel Lillo, in San Miguel de Tucumán, Argentina.
Fauna and habitat 
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 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 it its paleoenvironment. 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 
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