Pisanosaurus (meaning "Pisano lizard") 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 one partially complete skeleton. The fossils were discovered in Argentina's Late Triassic Ischigualasto Formation, from around 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, Pisanosaurus was a small, lightly-built dinosaur approximately in length and in height. Its weight was between . These estimates vary due to the incompleteness of the fossil. 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 ornithischians, was probably exclusively herbivorous.

Discovery and naming

Pisanosaurus mertii was described by Argentine paleontologist Rodolfo Casamiquela in 1967. The name Pisanosaurus honors Juan A. Pisano, an Argentine paleontologist, while saurus is derived from the Greek σαυρος, meaning "lizard". Pisanosaurus is known from a single fragmented skeleton found in Argentina. It is based on a specimen given the designation PVL 2577, which was discovered in the Ischigualasto Formation.


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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; 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.

Pisanosaurus has been classified either as a heterodontosaurid or as 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.

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.


The fossils of Pisanosaurus were discovered in Argentina's Ischigualasto Formation. Originally dated to the Middle Triassic, this formation is now believed to belong to the Late Triassic Carnian stage, around 228 to 216.5 million years ago. Pisanosaurus shared its habitat with rhynchosaurs, cynodonts, dicynodonts, prestosuchids, ornithosuchids, aetosaurs, and primitive dinosaurs. The early carnivorous dinosaur Herrerasaurus lived in this area and at this time, and may have fed upon Pisanosaurus.


  • Casamiquela, R.M. (1967). "Un nuevo dinosaurio ornitisquio triásico (Pisanosaurus mertii; Ornithopoda) de la Formación Ischigualasto, Argentina". Ameghiniana 4(2):47–64

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