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Herrerasaurids are among the oldest known dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

s, appearing in the fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 record 231.4 million years ago (Late Triassic
Late Triassic
The Late Triassic is in the geologic timescale the third and final of three epochs of the Triassic period. The corresponding series is known as the Upper Triassic. In the past it was sometimes called the Keuper, after a German lithostratigraphic group that has a roughly corresponding age...

). They became extinct by the end of the Triassic period. Herrerasaurids were small-sized (not more than 4 metres (13.1 ft) long) carnivorous
A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...

 theropods or basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...

Saurischia meaning 'lizard' and ischion meaning 'hip joint') is one of the two orders, or basic divisions, of dinosaurs. In 1888, Harry Seeley classified dinosaurs into two orders, based on their hip structure...

ns. The best known representatives of this group are from South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...

Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

, Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...

), where they were first discovered in the 1960s. A nearly complete skeleton of Herrerasaurus ischigulastensis was discovered in the Ischigualasto
Ischigualasto is a geological formation and a natural park associated with it in the province of San Juan, north-western Argentina, near the border with Chile...

 Formation in San Juan
San Juan Province (Argentina)
San Juan is a province of Argentina, located in the western part of the country. Neighbouring provinces are, moving clockwise from the north, La Rioja, San Luis and Mendoza. It borders with Chile at the west....

, Argentina, in 1988. Less complete herrerasaurids have been found in North America, and they may have inhabited other continents as well.

Herrerasaurid anatomy is unusual and specialized, and they are not considered to be ancestral to any later dinosaur group. They often present a mixture of very primitive and derived traits. The acetabulum
The acetabulum is a concave surface of the pelvis. The head of the femur meets with the pelvis at the acetabulum, forming the hip joint.-Structure:...

 is only partly open, and there are only two sacral vertebrae, the lowest number among dinosaurs. The pubic bone has a derived structure, being rotated somewhat posteriorly and folded to create a superficially tetanuran-like terminal expansion, especially prominent in H. ischigulastensis. The hand is primitive in having five metacarpals and the third finger longer than the second, but clearly theropod in having only three long fingers, with curved claws. Herrerasaurids also have a hinged mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...

 like all theropods.


Where herrerasaurids lie on the early dinosaur evolutionary tree is unclear. They are possibly basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...

 theropods or basal saurischians but may in fact predate the saurischian-ornithischian split. Early researchers even proposed that they represented an early lineage of sauropodomorphs. Some analyses, such as Nesbitt et al. 2009, have found Herrerasaurus and its relatives in Herrerasauridae to be very basal theropods, while others (such as Ezcurra 2010) have found them to be basal to the clade Eusaurischia, that is, closer to the base of the saurischian tree than either theropods or sauropodomorphs, but not true members of either. The situation is further complicated by uncertainties in correlating the ages of late Triassic beds bearing land animals.

Other proposed members of the clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 have included Sanjuansaurus
Sanjuansaurus is a genus of herrerasaurid dinosaur from the Late Triassic-age Ischigualasto Formation of northwestern Argentina...

from the same Ischigualasto Formation
Ischigualasto is a geological formation and a natural park associated with it in the province of San Juan, north-western Argentina, near the border with Chile...

 of Argentina as Herrerasaurus, Staurikosaurus
Staurikosaurus is a genus of herrerasaurid dinosaur from the Late Triassic of Brazil.-Discovery:The first known specimen of Staurikosaurus was recovered from the Paleontological Site Jazigo Cinco of the Santa Maria Formation in the geopark of paleorrota , Rio Grande do Sul, southern Brazil...

from the Santa Maria Formation
Santa Maria Formation
The Santa Maria Formation is a sedimentary rock formation found in Rio Grande do Sul, Brazil. It has a late Ladinian – early Carnian age , and is notable for its fossils of early dinosaurs, including the herrerasaur Staurikosaurus, the basal saurischian Teyuwasu, and the basal sauropodomorph...

 of southern Brazil, Chindesaurus
Chindesaurus is a genus of saurischian dinosaur named after Chinde Point , near where the genoholotype specimen was discovered in the Chinle Formation of Petrified Forest National Park, Arizona, by Bryan Small in 1985...

from the Upper Petrified Forest
Petrified Forest National Park
Petrified Forest National Park is a United States national park in Navajo and Apache counties in northeastern Arizona. The park's headquarters are about east of Holbrook along Interstate 40 , which parallels a railroad line, the Puerco River, and historic U.S. Route 66, all crossing the park...

 (Chinle Formation
Chinle Formation
The Chinle is a geologic formation that is spread across the U.S. states of northern Arizona, Nevada, Utah, western New Mexico, and western Colorado. The Chinle is controversially considered to be synonymous to the Dockum Group in eastern Colorado, eastern New Mexico, southwestern Kansas, the...

) of Arizona, and possibly Caseosaurus
Caseosaurus is a genus of dinosaur from the Late Triassic of North America. The type species, Caseosaurus crosbyensis, was formally described by Hunt, Lucas, Heckert, Sullivan and Lockley in 1998. It was found in the Dockum Formation of Texas, in strata dating to the Late Triassic...

from the Dockum Formation
Dockum Group
The Dockum is a Late Triassic geologic group found primarily on the Llano Estacado of western Texas and eastern New Mexico with minor exposures in southwestern Kansas, eastern Colorado, and Oklahoma panhandle. The Dockum reaches a maximum thickness of slightly over 650 m but is usually much...

 of Texas, although the relationships of these animals are not fully understood, and not all paleontologists agree. Other possible basal theropods, Alwalkeria
Alwalkeria is a genus of basal saurischian dinosaur from the Late Triassic of India. It was a small bipedal omnivore.This dinosaur was originally named Walkeria maleriensis by Sankar Chatterjee in 1987, in honor of famous British paleontologist Alick Walker and the Maleri Formation, in which its...

from the Late Triassic Maleri Formation of India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

, and Teyuwasu
Teyuwasu is a dubious genus of dinosaur from the Late Triassic. Named by Edio-Ernst Kischlat in 1999, little is currently known about this genus; the type species, T. barbarenai, was named from leg bones discovered in geopark of paleorrota, Brazil. The specific name honors Dr. Mário Costa...

, known from very fragmentary remains from the Late Triassic of Brazil, might be related. Novas (1992) defined Herrerasauridae as Herrerasaurus, Staurikosaurus, and their most recent common ancestor. Sereno (1998) defined the group as the most inclusive clade including H. ischigualastensis but not Passer domesticus
House Sparrow
The House Sparrow is a bird of the sparrow family Passeridae, found in most parts of the world. One of about 25 species in the genus Passer, the House Sparrow occurs naturally in most of Europe, the Mediterranean region, and much of Asia...

. Langer (2004) provided first phylogenetic definition of a higher level taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...

, infraorder Herrerasauria.


The first cladogram
A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...

 presented here follows one proposed analysis by Fernando E. Novas, Martin D. Ezcurra, Sankar Chatterjee and T. S. Kutty in 2011. In this review, Herrerasaurus is a primitive saurischian, but not a theropod. The second cladogram is based on an analysis by Hans-Dieter Sues, Sterling J. Nesbitt, David S Berman and Amy C. Henrici, in 2011/12. This review indicated Herrerasaurus is a basal theropod.