Colbert (1970) described Staurikosaurus as a small and agile, bipedal predator. Staurikosaurus lived during the late-Carnian and early-Norian stage, of the Late Triassic, approximately 225 million years ago—which makes it one of the earliest dinosaurs known. At 2.25 metres (7 ft 5 in) long, 80 centimetres (31 in) tall, and weighing 30 kilograms (66 lb), Staurikosaurus was small in comparison to later theropods like Megalosaurus. The type specimen has long but relatively slender limb bones.
There exists a very incomplete fossil record of Staurikosaurus, consisting of most of the spine, the legs and the large lower jaw. However, dating from such an early period in the dinosaurs' history and being otherwise so primitive, most of Staurikosaurus' other features as being primitive also can be reconstructed. For example, Staurikosaurus is usually depicted with five toes and five fingers—very simple features of an unspecialized dinosaur. However, since the skeletal structure of the legs is known, it can be seen that Staurikosaurus was a quick runner for its size. It also had just two vertebrae joining the pelvis to the spine, a distinctly primitive condition.
The available teeth for Staurikosaurus bear a morphology that strongly suggests a carnivorous diet. The teeth are all serrated, laterally compressed, and caudally curved (i.e. the top of each tooth is curved back toward the throat). This dentition suggests that Staurikosaurus could catch and hold prey, as well as slice and tear flesh to aid in mechanical digestion.
The tail of Staurikosaurus was relatively long (with more than 40 vertebrae) compared to the rest of its body and was held straight and off the ground as it ran. The rear part of Staurikosaurus's tail is stiffened by features of the tail vertebrae. Ostrom (1969a) considered this adaptation to serve as a dynamic stabilizer facilitating the animal's leaping and running.
A diagnosis is a statement of the anatomical features of an organism (or group) that collectively distinguish it from all other organisms. Some, but not all, of the features in a diagnosis are also autapomorphies. An autapomorphy is a distinctive anatomical feature that is unique to a given organism or group. According to Sues (1990), Staurikosaurus can be distinguished based on the following 14 features: (i) a mandible almost as long as the femur, suggesting a proportionately large head; (ii) a fairly deep but thin dentary with 13 to 14 teeth and with a well-developed retroarticular process; (iii) a vertebral column with 9 to 10 cervical, 15 dorsal, 2 sacral, and more than 40 caudal vertebrae. Staurikosaurus is considered to be more primitive than any other dinosaur because only two sacral vertebrae are present; (iv) an elongated 3rd, 4th, and 5th cervical vertebrae, which represents a primitive condition; (v) cranial cervical vertebrae that lack epipophyses; (vi) the absence of accessory intervertebral articulations; (vii) a slender scapular blade that is not expanded proximally; (viii) a large and plate-like coracoid; (ix) a humerus featuring a prominent deltopectoral crest (represents a primitive condition)as well having distinctly expanded articular ends; (x) an ilium with an extensively developed medial wall of a semiperforate acetabulum (like Herrerasaurus, but unlike any other dinosaur); (xi) a long pubis, two-thirds the length of the femur; (xii) hollow limb bones that feature fairly thick walls; (xiii) a robust femur with an S-shaped shaft: and (xiv) a tibia and fibula slightly longer than the femur. Novas (1993) added that Staurikosaurus is distinguished from other dinosaurs based on the presence of a distal bevel on anterior margin of its pubis. Langer and Benton (2006) noted that Staurikosaurus can be distinguished based on the anterior trochanter being reduced to a scar.
Staurikosaurus means "Southern Cross" (after the star constellation visible from the Southern Hemisphere) and "Lizard" (from the Greek work "saurus" meaning lizard), thus "Southern Cross Lizard." The species name pricei is in the honor of Colbert's fellow paleontologist Llewellyn Ivor Price.
The first known specimen of Staurikosaurus was recovered from the Paleontological Site Jazigo Cinco of the Santa Maria Formation , Rio Grande do Sul, southern Brazil. Staurikosaurus was found in mid-Carnian sediments. The genus name refers to the star constellation "The Southern Cross", pictured in the coat of arms of Brazil and only visible in the Southern Hemisphere—when Staurikosaurus was described in 1970 , it was unusual to find dinosaurs in the Southern Hemisphere. The specific name honors the Brazilian paleontologist Llewellyn Ivor Price, who discovered it in 1936. It was described by Edwin Harris Colbert, working at the American Museum of Natural History. The rarity of Staurikosaurus remains may be a result of it being uncommon while alive, or because it lived in an environment like a forest, where fossils rarely form. Nonetheless, Garcia et al. (2019) referred the holotype of Teyuwasu barberenai as a second specimen of Staurikosaurus pricei (see Classification). 
Later research by Sues et al. (2011) supports that Staurikosaurus and the related genus Herrerasaurus are theropods and evolved after the sauropod line had split from the Theropoda. Mortimer points out that Benedetto (1973) and Galton (1985) were the first to recognize that Staurikosaurus and Herrerasaurus were more closely related to each other than to sauropodomorphs or avepods, placing them both in the Herrerasauridae and Herrerasauria. Staurikosaurus differs from Herrerasaurus because of its considerably smaller size (femur length of 23 centimetres (9.1 in) vs. 47 centimetres (19 in)). Sereno et al. (1993) concluded that Staurikosaurus was not a theropod and considered it a basal saurischian outside Theropoda and Sauropodamorpha.Staurikosaurus was originally incorrectly assigned by Colbert to Palaeosauriscidae, a defunct family based largely on Efraasia, a prosauropod dinosaur. All major phylogenetic analyses since 1994 have assigned Staurikosaurus to the clade Herrerasauridae, which is the current scientific consensus on classification of this genus. Below is a cladogram based on the phylogenetic analysis conducted by Sues et al. in 2011, showing the relationships of Staurikosaurus:
Staurikosaurus was placed in the clade Herrerasauridae by Benedetto in 1973. Herrerasauridae also includes Herrerasaurus ischigualastensis and Eoraptor lunesis, both small predatory animals that were either dinosaurs or precursors to dinosaurs. These three dinosaurs lived during the Carnian stage of the Triassic period. Some phylogenetic analyses excluded Eoraptor from the Herrerasauridae Phylogenetic analysis by Sues, Nesbitt, Berman and Henrici, in 2011, exclude Eoraptor, and include Chindesaurus along with Herrerasaurus as more derived than Staurikosaurus. Sanjuansaurus was assigned to Herrerasauridae by Alcober and Martínez (2010). Sues (1990) assigned Ischisaurus to Herrerasauridae. Other proposed members of the clade have included Sanjuansaurus from the same Ischigualasto Formation of Argentina as Herrerasaurus, and possibly Caseosaurus from the Dockum Formation of Texas, although the relationships of these animals are not fully understood, and not all paleontologists agree. Alcober and Martinez (2010) concluded that Staurikosaurus and Sanjuansaurus are closely related based on similarities in their pubis and tibia.
The controversial dinosauriform Teyuwasu barberenai was recently considered a synonym of Staurikosaurus pricei. Both taxa are known from single incomplete and somewhat poorly preserved specimens, therefore the former holotype specimen of Teyuwasu would be the second specimen ascribed to Staurikosaurus within almost 50 years of its naming. The synonymy was based on a combination of five osteological features that are only present in both specimens among Triassic early dinosauriforms: (i) femur without a trochanteric shelf; (ii) symmetric fourth trochanter of the femur; (iii) crista tibiofibularis poorly separated from the lateral condyle at the distal end of the femur; (iv) posterolateral flange of the distal end of the tibia of does not exceeds the lateral margin of the bone; (v) and rounded distal end of the tibia.
Staurikosaurus was a small but active bipedal predator, that preyed on small and medium-sized terrestrial vertebrates such as cynodonts, rhynchosaurs, and herbivorous synapsids. The mandible of Staurikosaurus suggests that a sliding joint in the jaw allowed it to move backwards and forwards, as well as up and down. Smaller prey could be worked backwards towards Staurikosaurus's throat, aided along by its small, backwards-curving teeth. This feature was common in theropods of its time, but would disappear in later theropods.
During the Late Triassic dinosaurs played only a minor role in terrestrial life; a fact that would change by the Early Jurassic. Staurikosaurus coexisted with large rauisuchian archosaurs like Saurosuchus, which were the top carnivores in their ecosystem Staurikosaurus's paleocommunity included medium- to large-sized herbivorous rhynchosaurs and dicynodonts. Medium-sized omnivorous aetosaurs and cynodonts were also present. Dinosaurs were represented by the Herrerasaurids, which include Staurikosaurus, and the basal sauropodomorph Saturnalia. The contemporaneous occurrence of basal theropods Staurikosaurus, Herrerasaurus, and Eoraptor with the ornithischian Pisanosaurus suggests that the main carnivorous and herbivorous lineages were established during the middle part of the Carnian stage. A U-Pb (uranium decay) dating found that the Santa Maria Formation dated around 233.23 million years ago, putting it 1.5 million years older than the Ischigualasto Formation, and making the two formations approximately equal as the earliest dinosaur localities.
Averostra, or "bird snouts", is a clade that includes most theropod dinosaurs that have a promaxillary fenestra (fenestra promaxillaris), an extra opening in the front outer side of the maxilla, the bone that makes up the upper jaw. Two groups of averostrans, the Ceratosauria and the Orionides, survived into the Cretaceous period. When the Cretaceous–Paleogene extinction event occurred, ceratosaurians and two groups of orionideans within the clade Coelurosauria, the Tyrannosauroidea and Maniraptoriformes, were still extant. Only one subgroup of maniraptoriformes, Aves, survived the extinction event and persisted to the present day.Avetheropoda
Avetheropoda, or "bird theropods", is a clade that includes carnosaurians and coelurosaurs to the exclusion of other dinosaurs.Carnian
The Carnian (less commonly, Karnian) is the lowermost stage of the Upper Triassic series (or earliest age of the Late Triassic epoch). It lasted from 237 to 227 million years ago (Ma). The Carnian is preceded by the Ladinian and is followed by the Norian. Its boundaries are not characterized by major extinctions or biotic turnovers, but a climatic event (known as the Carnian Pluvial Event) occurred during the Carnian and seems to be associated with important extinctions or biotic radiations.Cerapoda
Cerapoda ("ceratopsians and ornithopods") is a clade of the dinosaur order Ornithischia.Chindesaurus
Chindesaurus ( CHIN-di-SAWR-əs) is a genus of herrerasaurid dinosaur that lived approximately 235-210 million years ago during the latter part of the Triassic Period in what is now the Southwestern United States. Chindesaurus was a small, bipedal carnivore that could grow up to 2 to 2.3 m (6.6 to 7.5 ft) long.Dinosauriformes
Dinosauriformes is a clade of archosaurian reptiles that include the dinosaurs and their most immediate relatives. All dinosauriformes are distinguished by several features, such as shortened forelimbs and a partially to fully perforated acetabulum, the hole in the hip socket traditionally used to define dinosaurs. The oldest known member is Asilisaurus, dating to about 245 million years ago in the Anisian age of the middle Triassic period.Eodromaeus
Eodromaeus (meaning "dawn runner") was a genus of basal theropod dinosaur known from the Late Triassic (Carnian) Valle de la Luna Member of the Ischigualasto Formation of the Ischigualasto-Villa Unión Basin in northwestern Argentina. It has been cited by Sereno as resembling a supposed common ancestor to all dinosaurs, the "Eve" of the dinosaurs.Herrerasauridae
Herrerasauridae is a family of carnivorous basal saurischian dinosaurs. They are among the oldest known dinosaurs, first appearing in the fossil record around 233.23 million years ago (Late Triassic), before becoming extinct by the end of the Triassic period. Herrerasaurids were relatively small-sized dinosaurs, normally not more than 4 metres (13 ft) long. The best known representatives of this group are from South America (Brazil, Argentina), where they were first discovered in the 1960s. A nearly complete skeleton of Herrerasaurus ischigulastensis was discovered in the Ischigualasto Formation in San Juan, 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 only superficially resemble theropods and often present a mixture of very primitive and derived traits. The acetabulum 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 resembles those of theropods in having only three long fingers, with curved claws. Herrerasaurids also have a hinged mandible, which is also found in theropods.Herrerasaurus
Herrerasaurus was one of the earliest dinosaurs. Its name means "Herrera's lizard", after the rancher who discovered the first specimen. 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. The type species, Herrerasaurus ischigualastensis, was described by Osvaldo Reig in 1963 and is the only species assigned to the genus. Ischisaurus and Frenguellisaurus are synonyms.
For many years, the classification of Herrerasaurus was unclear because it was known from very fragmentary remains. It was hypothesized to be a basal theropod, a basal sauropodomorph, a basal saurischian, or not a dinosaur at all but another type of archosaur. However, with the discovery of an almost complete skeleton and skull in 1988, Herrerasaurus has been classified as either an early theropod or an early saurischian in at least five recent reviews of theropod evolution, with many researchers treating it at least tentatively as the most primitive member of Theropoda.It is a member of the Herrerasauridae, a family of similar genera that were among the earliest of the dinosaurian evolutionary radiation.Jingshanosaurus
Jingshanosaurus (meaning "Jingshan lizard") is a genus of sauropodomorph dinosaurs from the early Jurassic period.Neotheropoda
Neotheropoda (meaning "new theropods") is a clade that includes coelophysoids and more advanced theropod dinosaurs, and the only group of theropods who survived the Triassic–Jurassic extinction event. Yet all of the neotheropods became extinct during the early Jurassic period except for Averostra.Orionides
Orionides is a clade of tetanuran theropod dinosaurs from the Middle Jurassic to the Present. The clade includes most theropod dinosaurs, including birds.Paleontological Site Jazigo Cinco
Paleontological Site Jazigo Cinco is located in the city of Santa Maria, Rio Grande do Sul, Brazil. And belongs to Santa Maria Formation. It is located in the neighborhood Kilometro 3 near Castelinho, is to 2.7 kilometers away from the Paleontological Site Arroio Cancela. It belongs to UFSM (Federal University of Santa Maria) and is a center of research. It is the place where he was collecting the Staurikosaurus, the first Brazilian dinosaur. Site belongs to the region paleorrota.Panphagia
Panphagia is a genus of sauropodomorph dinosaur described in 2009. It lived around 231 million years ago, during the Carnian age of the Late Triassic period in what is now northwestern Argentina. Fossils of the genus were found in the La Peña Member of the Ischigualasto Formation in the Ischigualasto-Villa Unión Basin. The name Panphagia comes from the Greek words pan, meaning "all", and phagein, meaning "to eat", in reference to its inferred omnivorous diet. Panphagia is one of the earliest known dinosaurs, and is an important find which may mark the transition of diet in early sauropodomorph dinosaurs.Sanga da Alemoa
The Sanga da Alemoa paleontological site is located in the city of Santa Maria, Rio Grande do Sul, in Brazil. It belongs to the Caturrita Formation and the Santa Maria Formation. It is located in the neighborhood of Castelinho. The site belongs to the paleorrota region.Sanjuansaurus
Sanjuansaurus ("San Juan Province lizard") is a genus of herrerasaurid dinosaur from the Late Triassic (Carnian) Cancha de Bochas and La Peña Members of the Ischigualasto Formation of the Ischigualasto-Villa Unión Basin in northwestern Argentina.Santa Maria Formation
The Santa Maria Formation is a sedimentary rock formation found in Rio Grande do Sul, Brazil. It has a Late Anisian to Early Norian age (Early to Late Triassic), and is notable for its fossils of early dinosaurs and other dinosauromorphs, including the herrerasaurid Staurikosaurus, the basal sauropodomorphs Buriolestes and Saturnalia, and the lagerpetid Ixalerpeton. It received this name because it was discovered first in the city of Santa Maria, on the central region of Rio Grande do Sul state.
The distinguished English paleontologist Arthur Smith Woodward determined the age of Santa Maria Formation dated Mesozoic Era, Upper Triassic period (about 220 million years). A U-Pb (Uranium decay) dating of a locality of the Upper portion of the Santa Maria Formation dated around 233.23±0.73 million years ago, putting that locality 1.5 million years older than the Ischigualasto Formation, instead being in the temporal range of the Los Chañares Formation, and making the two formations (Santa Maria and Ischigualasto) approximately equal as having the earliest dinosaur localities.The Santa Maria Formation is part of the Candelária Sequence, and is biostratigraphically subdivided into Dinodontosaurus (latest Ladinian-earliest Carnian), Santacruzodon (earliest Carnian-middle Carnian), and Hyperodapedon (middle Carnian-latest Carnian) Assemblage Zones (from the oldest to youngest). Moreover, the Hyperodapedon Assemblage Zone is subdivided into Hyperodapedon Acme Zone (most of the zone, where the rhynchosaur Hyperodapedon is widely reported) and Exaeretodon Sub-Zone (restricted to 3 known and sampled localities, where Hyperodapedon is almost absent, but the traversodontid cynodont Exaeretodon is widely reported). These subdivisions are also known as Lower and Upper Hyperodapedon Assemblage Zone, respectively.Spondylosoma
Spondylosoma (meaning "vertebra body") is a genus of avemetatarsalian archosaur belonging to the clade Aphanosauria from the late Ladinian-age Middle Triassic Lower Santa Maria Formation in Paleorrota Geopark, Brazil.Unaysaurus
Unaysaurus is a genus of unaysaurid sauropodomorph herbivore dinosaur. Discovered in southern Brazil, in the geopark of Paleorrota, in 1998, and announced in a press conference on Thursday, December 3, 2004, it is one of the oldest dinosaurs known. It is closely related to plateosaurid dinosaurs found in Germany, which indicates that it was relatively easy for species to spread across the giant landmass of the time, the supercontinent of Pangaea.The fossils of Unaysaurus are well preserved. They consist of an almost complete skull, complete with a lower jaw, and partial skeleton with many of the bones still connected to each other in their natural positions. It is one of the most complete dinosaur skeletons (including complete skull) ever recovered in Brazil.