| Staurikosaurus | |
|---|---|
 | |
| Reconstructed skeleton | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Family: | †Herrerasauridae |
| Genus: | †Staurikosaurus Colbert,1970 |
| Species: | †S. pricei |
| Binomial name | |
| †Staurikosaurus pricei Colbert, 1970 | |
| Synonyms | |
| |
Staurikosaurus (Pronounced/ˌstɔɹ̠ikoʊˈsɔɹ̠ʌs/,STOR-ree-koh-SOR-ruhs; "Southern Cross lizard") is agenus ofherrerasaurid[1]dinosaur from theLate Triassic ofBrazil, found in theSanta Maria Formation.
Colbert (1970) describedStaurikosaurus as a small and agile,bipedal predator.[2]Staurikosaurus lived during the late-Carnian and early-Norian stage, of theLate Triassic, approximately 225 million years ago—which makes it one of the earliest dinosaurs known. Its length is measured at 2.2–2.25 metres (7 ft 3 in – 7 ft 5 in) long,[3] butGregory S. Paul presented a lower length estimate of 2.1 metres (6 ft 11 in) and a body mass estimate of 12 kilograms (26 lb).[4]Staurikosaurus was small in comparison to later theropods likeMegalosaurus. The type specimen has long but relatively slender limb bones.
There exists a very incomplete fossil record ofStaurikosaurus, 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 ofStaurikosaurus' other features as being primitive also can be reconstructed. For example,Staurikosaurus is usually depicted with five toes and five fingers[5]—very simple features of an unspecialized dinosaur. However, since the skeletal structure of the legs is known, it can be seen thatStaurikosaurus was a quick runner for its size. It also had just twovertebrae joining thepelvis to thespine, a distinctly primitive condition.
The available teeth forStaurikosaurus 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).[6] This dentition suggests thatStaurikosaurus could catch and hold prey, as well as slice and tear flesh to aid in mechanical digestion.[7]
The tail ofStaurikosaurus 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 ofStaurikosaurus'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.[8]
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) amandible almost as long as thefemur, suggesting a proportionately large head; (ii) a fairly deep but thindentary with 13 to 14 teeth and with a well-developed retroarticular process; (iii) a vertebral column with 9 to 10 cervical, 15 dorsal, 2sacral, 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-likecoracoid; (ix) ahumerus featuring a prominent deltopectoral crest (represents a primitive condition) as well having distinctly expanded articular ends; (x) anilium with an extensively developed medial wall of a semiperforateacetabulum (likeHerrerasaurus, but unlike any other dinosaur); (xi) a longpubis, two-thirds the length of thefemur; (xii) hollow limb bones that feature fairly thick walls; (xiii) a robust femur with an S-shaped shaft: and (xiv) atibia andfibula slightly longer than the femur.[9] Novas (1993) added thatStaurikosaurus is distinguished from other dinosaurs based on the presence of a distal bevel on anterior margin of its pubis.[10] Langer and Benton (2006) noted thatStaurikosaurus can be distinguished based on the anterior trochanter being reduced to a scar.[11] Bittencourt and Kellner (2009) also noted that the proximal fibula has a medial sulcus, which is unique toStaurikosaurus pricei.[6]
Staurikosaurus is named after the "Southern Cross" the star constellation visible from the Southern Hemisphere (from the Greek "Stauros" meaning cross and "saurus" meaning lizard), thus "Cross Lizard".[12][13] The species namepricei is in the honor of paleontologistLlewellyn Ivor Price.[12]
The first known specimen ofStaurikosaurus (MCZ 1669) was recovered from thePaleontological Site Jazigo Cinco of theSanta Maria Formation,[5]Rio Grande do Sul, southernBrazil.Staurikosaurus was found in mid-Carnian sediments. The genus name refers to thestarconstellation "TheSouthern Cross", pictured in thecoat of arms of Brazil and only visible in theSouthern Hemisphere—whenStaurikosaurus was described in 1970,[2] it was unusual to find dinosaurs in the Southern Hemisphere. Thespecific name honors the BrazilianpaleontologistLlewellyn Ivor Price, who discovered it in 1936. It was described byEdwin Harris Colbert, working at theAmerican Museum of Natural History. The rarity ofStaurikosaurus 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.[5] However recent evidence indicates that the ecosystem was probably more dry.[14] Nonetheless, Garcia et al. (2019) referred theholotype ofTeyuwasu barberenai as a second specimen ofStaurikosaurus pricei (see Classification).[15]
Later research by Sues et al. (2011) supports thatStaurikosaurus and the related genusHerrerasaurus are theropods and evolved after the sauropod line had split from the Theropoda.[16] Mortimer points out that Benedetto (1973) and Galton (1985) were the first to recognize thatStaurikosaurus andHerrerasauruswere more closely related to each other than to sauropodomorphs or avepods, placing them both in the Herrerasauridae and Herrerasauria.[17][18][19]Staurikosaurus differs fromHerrerasaurus because of its considerably smaller size (femur length of 23 centimetres (9.1 in) vs. 47 centimetres (19 in)). Sereno et al. (1993) concluded thatStaurikosaurus was not a theropod and considered it a basal saurischian outside Theropoda and Sauropodomorpha.[20]Staurikosaurus was originally incorrectly assigned by Colbert to Palaeosauriscidae, a defunct family based largely onEfraasia, a prosauropod dinosaur. All major phylogenetic analyses since 1994 have assignedStaurikosaurus to the cladeHerrerasauridae, which is the current scientific consensus on classification of this genus. Bittencourt and Kellner (2009) stated that the phylogenetic position ofStaurikosaurus is constrained by its close relationship withHerrerasaurus ischigualastensis, which is more complete and well known.[6] Below is acladogram based on the phylogenetic analysis conducted by Sues et al. in 2011, showing the relationships ofStaurikosaurus:[16]
Staurikosaurus was placed in the cladeHerrerasauridae by Benedetto in 1973. Herrerasauridae also includesHerrerasaurus ischigualastensis, both small predatory animals that were either dinosaurs or precursors to dinosaurs.[21] These three dinosaurs lived during the Carnian stage of the Triassic period. Most phylogenetic analyses excludedEoraptor from the Herrerasauridae.[22] Phylogenetic analysis by Sues, Nesbitt, Berman and Henrici, in 2011, excludeEoraptor, and includeChindesaurus along withHerrerasaurus as more derived thanStaurikosaurus.[23]Sanjuansaurus was assigned to Herrerasauridae by Alcober and Martínez (2010).[24] Sues (1990) assignedIschisaurus to Herrerasauridae.[25] Other proposed members of theclade have includedSanjuansaurus[26] from the sameIschigualasto Formation of Argentina asHerrerasaurus, and possiblyCaseosaurus from theDockum Formation of Texas,[27] although the relationships of these animals are not fully understood, and not all paleontologists agree. Alcober and Martinez (2010) concluded thatStaurikosaurus andSanjuansaurus are closely related based on similarities in theirpubis andtibia.[28]
The controversialdinosauriform"Teyuwasu barberenai" was recently considered asynonym ofStaurikosaurus pricei.[15] 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 toStaurikosaurus within almost 50 years of its naming.[2][15] 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 does not exceeds the lateral margin of the bone; (v) and rounded distal end of the tibia.
The synonymy is commented in two subsequent papers, which cast doubt in the association of "Teyuwasu" withStaurikosaurus. In the first paper, the authors only mention that the holotype of "Teyuwasu" is not well preserved, and thus cannot be attributed toStaurikosaurus.[29] In the second, the authors argue that several of the five character states cited to unite the taxa are present in immature specimens of other dinosauriforms.[30] However, the combination (that is, the simultaneous presence) of the five characters listed by Garcia et al.[15] is not present in any of the aforementioned dinosauriforms, and therefore remains unique between "Teyuwasu" andStaurikosaurus.[citation needed]
Therefore, further investigations are needed in order to whether confirm or not the synonymy between "Teyuwasu barberenai" andStaurikosaurus pricei.
Staurikosaurus was a small but active bipedal predator, that preyed on small and medium-sized terrestrial vertebrates such ascynodonts,rhynchosaurs, and herbivoroussynapsids. The mandible ofStaurikosaurus suggests that a sliding joint in the jaw allowed it to move backwards and forwards, as well as up and down. However, some authors questioned the presence of an intramandibular joint inStaurikosaurus, due to the poor preservation of the holotype.[6] Smaller prey could be worked backwards towardsStaurikosaurus's throat, aided along by its small, backwards-curving teeth.[7] 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 largerauisuchianarchosaurs likeRauisuchus, which were the top carnivores in their ecosystem[31]Staurikosaurus's paleocommunity included medium- to large-sized herbivorousrhynchosaurs anddicynodonts. Medium-sized omnivorousaetosaurs andcynodonts were also present. Dinosaurs were represented by the Herrerasaurids, which includeStaurikosaurus, and the basalsauropodomorphSaturnalia. The contemporaneous occurrence of basal theropodsStaurikosaurus,Herrerasaurus, andEoraptor with the ornithischianPisanosaurus suggests that the main carnivorous and herbivorous lineages were established during the middle part of the Carnian stage.[21] 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 theIschigualasto Formation, and making the two formations approximately equal as the earliest dinosaur localities.[32]
The region whereStaurikosaurus lived was arid to semi-arid,[14] characterized by a mix ofseed ferns,ferns,cycads, andconifers.[33] The area was subjected to a major degree of uplift.[14]
Media related toStaurikosaurus at Wikimedia Commons
Data related toStaurikosaurus at Wikispecies
