All known fossils of this carnivore have been discovered in the Ischigualasto Formation of Carnian age (late Triassic according to the ICS, dated to 231.4 million years ago) in northwestern Argentina.

[13][14] Herrerasaurus was named by paleontologist Osvaldo Reig after Victorino Herrera, an Andean goatherd who first noticed its fossils in outcrops near the city of San Juan, Argentina in 1959.

[17] Reig believed Herrerasaurus was an early example of a carnosaur,[2] but this was the subject of much debate over the next 30 years, and the genus was variously classified during that time.

[19] Later, using cladistic analysis, some researchers put Herrerasaurus and Staurikosaurus at the base of the dinosaur tree before the separation between ornithischians and saurischians.

[33] This view is further supported by ichnological records showing large tridactyl (three-toed) footprints that can be attributed only to a theropod dinosaur.

These footprints date from the early Carnian Los Rastros Formation in Argentina, which predates Herrerasaurus by several million years.

[20][21] An extensive study of Herrerasaurus by Sereno in 1992 suggested that of these proposed synapomorphies, only one cranial and seven postcranial features were actually derived from a common ancestor, and that the others were attributable to convergent evolution.

[38] Herrerasaurus had a long, narrow skull that lacked nearly all the specializations that characterized later dinosaurs,[41] and more closely resembled those of more primitive archosaurs such as Euparkeria.

According to Sereno (1993), Herrerasaurus can be distinguished based on the following features, all of which are unknown in other herrerasaurids:[45] a circular pit is present on the humeral ectepicondyle, a feature also present in Saturnalia; a saddle-shaped ulnar condyle of the humerus, and the articular surface for the ulnare on the ulna is convex; the articular surface of the ulnare is smaller than that of the ulna, a feature unknown in Staurikosaurus and Sanjuansaurus; the centrale is placed distal to the radiale; a broad subnarial process of the premaxilla, and a broad supratemporal depression (noted by Sereno and Novas, 1993);[46] the basal tuber and the occipital condyle are subequal in width (noted by Sereno and Novas, 1993).

Walker also proposed that Herrerasaurus may instead be close to Poposaurus (now considered a pseudosuchian[48]) and the unnamed theropod from the Dockum Group of Texas (now assigned to the rauisuchian Postosuchus[49]).

[20] This was disagreed with in 1992 by Novas, who stated many derived synapomorphies of Herrerasauridae, such as a distinct pubic boot, but still classified them as basal to Ornithischia and Saurischia.

[53] In a revision of basal Dinosauria, Padian and May (1993) discussed the definition of the clade, and redefined it as the latest common ancestor of Triceratops and birds.

[54] A later 1994 study by Novas instead classified Herrerasaurus within Dinosauria, and strongly supported its position within Saurischia, as well as provided synapomorphies that it shared with Theropoda.

[56] Langer (2004) mentioned that this hypothesis was widely accepted, but that more later authors instead preferred to place Herrerasaurus as well as Eoraptor basal to Theropoda and Sauropodomorpha, a clade called Eusaurischia.

[53] Langer's proposal was supported by multiple studies until the discovery of Tawa, when Nesbitt et al. conducted a more inclusive analysis, and the resulting cladogram placed Herrerasauridae basal to Eoraptor, but closer to Dilophosaurus than Sauropodomorpha.

[57][58] Unlike Nesbitt, Ezcurra (2010) conducted a phylogenetic analysis to place his new taxon Chromogisaurus, and found that Herrerasauridae was basal to Eusaurischia.

In the phylogenetic analysis, Herrerasaurus, Sanjuansaurus and Staurikosaurus all were in a polytomy, and Herrerasauridae was the most primitive group of saurischian, outside Eusaurischia, Eoraptor and Guaibasaurus.

[61] Ornithischia Eoraptor Sauropodomorpha Staurikosaurus Herrerasaurus Sanjuansaurus Eodromaeus Tawa Neotheropoda Other members of the clade[5] may include Chindesaurus from the Upper Petrified Forest (Chinle Formation) of Arizona,[62] and possibly Caseosaurus from the Tecovas Formation of the Dockum Group in Texas,[63] although the relationships of these animals are not fully understood, and not all paleontologists agree.

[67] Comparisons between the scleral rings of Herrerasaurus and modern birds and reptiles suggest that it may have been cathemeral, active throughout the day at short intervals.

[70] The holotype of Herrerasaurus (PVL 2566) was discovered in the Cancha de Bochas Member of the Ischigualasto Formation in San Juan, Argentina.

It was collected in 1961 by Victorino Herrera, in sediments that were deposited in the Carnian stage of the Triassic period, approximately 231 to 229 million years ago.

Herrerasaurus specimens PVL 2045 and MLP(4)61, were collected in 1959 and 1960, respectively, in sediments that were deposited in the Norian stage of the Triassic period, approximately 228 to 208 million years ago.

[5][72][73] In 1960, Scaglia collected specimen MACN 18.060, originally the holotype of Ischisaurus cattoi, in sediments deposited in the Carnian stage.

[74] Specimen PVSJ 53, originally the holotype of Frenguellisaurus ischigualastensis, was collected by Gargiulo & Oñate in 1975 in sediments that were deposited in the Carnian stage.

[75] Studies suggest that the paleoenvironment of the Ischigualasto Formation was a volcanically active floodplain covered by forests and subject to strong seasonal rainfalls.

[16] It lived in the jungles of Late Triassic South America alongside other early dinosaurs, such as Sanjuansaurus, Eoraptor, Panphagia, and Chromogisaurus, as well as rhynchosaurs (Scaphonyx), cynodonts (e.g., Exaeretodon, Ecteninion and Chiniquodon), dicynodonts (Ischigualastia), pseudosuchians (e.g., Saurosuchus, Sillosuchus and Aetosauroides), proterochampsids (e.g., Proterochampsa) and temnospondyls (Pelorocephalus).