| Herrerasaurus | |
|---|---|
 | |
| Reconstructed skeleton in Japan | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Family: | †Herrerasauridae |
| Genus: | †Herrerasaurus Reig, 1963 |
| Species: | †H. ischigualastensis |
| Binomial name | |
| †Herrerasaurus ischigualastensis Reig, 1963 | |
| Synonyms | |
| |
Herrerasaurus is likely a genus ofsaurischian dinosaur from theLate Triassic period. Measuring 6 m (20 ft) long and weighing around 350 kg (770 lb), this genus was one of the earliestdinosaurs from the fossil record. Its name means "Herrera's lizard", after the rancher who discovered the first specimen in 1958 inSouth America. All knownfossils of this carnivore have been discovered in theIschigualasto Formation ofCarnian age (lateTriassic according to theICS, dated to 231.4 million years ago) in northwesternArgentina.[1] Thetype species,Herrerasaurus ischigualastensis, was described byOsvaldo Reig in 1963[2] and is the onlyspecies assigned to thegenus.Ischisaurus andFrenguellisaurus aresynonyms.
For many years, the classification ofHerrerasaurus was unclear because it was known from very fragmentary remains. It washypothesized to be abasaltheropod, a basalsauropodomorph, a basalsaurischian, or not a dinosaur at all but another type ofarchosaur. However, with the discovery of an almost complete skeleton and skull in 1988,[3][4]Herrerasaurus has been classified as an early saurischian in most of the phylogenies on the origin and early evolution of dinosaurs.[5][6][7][8][9][10][11][12] It is a member of theHerrerasauridae, a family of similar genera that were among the earliest of thedinosaurianevolutionary radiation.[13][14]
Herrerasaurus was named bypaleontologist Osvaldo Reig after Victorino Herrera, an Andeangoatherd who first noticed itsfossils in outcrops near the city ofSan Juan, Argentina, in 1959.[2] These rocks, which later yieldedEoraptor,[15] are part of theIschigualasto Formation and date from the lateCarnian stage of the LateTriassic period.[16] Reig named a second dinosaur from these rocks in the same publication asHerrerasaurus;[2] this dinosaur,Ischisaurus cattoi, is now considered ajunior synonym and ajuvenile ofHerrerasaurus.[17]
Reig believedHerrerasaurus was an early example of acarnosaur,[2] but this was the subject of much debate over the next 30 years, and the genus was variously classified during that time. In 1970, Steel classifiedHerrerasaurus as aprosauropod.[18] In 1972,Peter Galton classified the genus as not diagnosable beyondSaurischia.[19] Later, usingcladistic analysis, some researchers putHerrerasaurus andStaurikosaurus at the base of the dinosaur tree before the separation between ornithischians and saurischians.[20][21][22][23] Several researchers classified the remains as non-dinosaurian.[24]
Two other partial skeletons with skull material were namedFrenguellisaurus ischigualastensis byFernando Novas in 1986,[25] but this species, too, is now thought to be a synonym.[17]Frenguellisaurus ischigualastensis was discovered in 1975 and was described by Novas (1986), who considered it a primitive saurischian and possibly atheropod. Novas (1992) and Sereno and Novas (1992) examined theFrenguellisaurus remains and found them referable toHerrerasaurus.[26]Ischisaurus cattoi was discovered in 1960 and described by Reig in 1963. Novas (1992) and Sereno and Novas (1992) reviewed its remains and found them also to be referable toHerrerasaurus.[26]
A completeHerrerasaurus skull was found in 1988 by a team of paleontologists led byPaul Sereno.[4] Based on the new fossils, authors such asThomas Holtz[27] andJosé Bonaparte[28] classifiedHerrerasaurus at the base of the saurischian tree before the divergence between prosauropods and theropods. However, Sereno favored classifyingHerrerasaurus (and the Herrerasauridae) as primitive theropods. These two classifications have become the most persistent, with Rauhut (2003)[29] and Bittencourt and Kellner (2004)[30] favoring the early theropodhypothesis, and Max Langer (2004),[10] Langer andBenton (2006),[31] and Randall Irmis and his coauthors (2007)[32] favoring the basal saurischian hypothesis. IfHerrerasaurus were indeed a theropod, it would indicate that theropods,sauropodomorphs, andornithischians diverged even earlier than herrerasaurids, before the middleCarnian, and that "all three lineages independently evolved several dinosaurian features, such as a more advanced ankle joint or an open acetabulum".[33] This view is further supported byichnological records showing large tridactyl (three-toed) footprints that can be attributed only to a theropod dinosaur. These footprints date from the early CarnianLos Rastros Formation in Argentina, which predatesHerrerasaurus by several million years.[34][35]
The study of early dinosaurs such asHerrerasaurus andEoraptor, therefore, has important implications for the concept of dinosaurs as amonophyletic group (a group descended from a common ancestor). The monophyly of dinosaurs was explicitly proposed in the 1970s by Galton andRobert T. Bakker,[36][37] who compiled a list of cranial and postcranialsynapomorphies (common anatomical traits derived from the common ancestor). Later authors proposed additional synapomorphies.[20][21] An extensive study ofHerrerasaurus 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 toconvergent evolution. Sereno's analysis ofHerrerasaurus also led him to propose several new dinosaurian synapomorphies.[4]
Herrerasaurus was a lightly built bipedal carnivore with a long tail and a relatively small head. Adults had skulls up to 56 cm (22 in) long and were up to 6 m (20 ft) in total length[4] and 350 kg (770 lb) in weight.[38] Smaller specimens were about 4.5 m (15 ft) long and weighed about 200 kg (440 lb).[39]
Herrerasaurus was fully bipedal. It had strong hind limbs with shortthighs and rather long feet, indicating that it was likely a swift runner. The foot had five toes, but only the middle three (digits II, III, and IV) bore weight. The outer toes (I and V) were small; the first toe had a small claw. The tail, partially stiffened by overlapping vertebral projections, balanced the body and was also an adaptation for speed.[10] The forelimbs ofHerrerasaurus were less than half the length of its hind limbs. Theupper arm andforearm were rather short, while themanus (hand) was elongated. The first two fingers and the thumb ended in curved, sharp claws for grasping prey. The fourth and fifth digits were small stubs without claws.[4][40]
Herrerasaurus displays traits that are found in different groups of dinosaurs, and several traits found in non-dinosaurian archosaurs. Although it shares most of the characteristics of dinosaurs, there are a few differences, particularly in the shape of its hip and leg bones. Its pelvis is like that of saurischian dinosaurs, but it has a bonyacetabulum (where thefemur meets thepelvis) that was only partially open. Theilium, the main hip bone, is supported by only twosacrals, abasal trait.[10] However, thepubis points backwards, aderived trait as seen indromaeosaurids andbirds. Additionally, the end of the pubis has a booted shape, like those inavetheropods; and thevertebralcentra have anhourglass shape as found inAllosaurus.[38]
Herrerasaurus had a long, narrowskull that lacked nearly all the specializations that characterized later dinosaurs,[41] and more closely resembled those of more primitivearchosaurs such asEuparkeria. It had five pairs offenestrae (skull openings) in its skull, two pairs of which were for the eyes and nostrils. Between the eyes and the nostrils were twoantorbital fenestrae and a pair of tiny, 1-centimeter-long (0.39 in) slit-like holes called promaxillary fenestrae.[42]
Herrerasaurus had a flexible joint in the lower jaw that could slide back and forth to deliver a grasping bite.[41] This cranial specialization is unusual among dinosaurs but has evolved independently in somelizards.[43] The rear of the lower jaw also had fenestrae. The jaws were equipped with large serrated teeth for biting and eating flesh, and the neck was slender and flexible.[17][41]
According to Novas (1993),Herrerasaurus can be distinguished based on the following features:[44] the presence of apremaxilla-maxilla fenestra, and the dorsal part of laterotemporal fenestra is less than a third as wide as the ventral part; the presence of a ridge on the lateral surface of thejugal bone, and a deeply incisedsupratemporal fossa that extends across the medial postorbital process; the subquadrate ventral squamosal process has a lateral depression, and thequadratojugal bone overlaps the posterodorsalquadrate face; thepterygoid process of the quadrate has an inturned, trough-shaped ventral margin, and the presence of a slender ribbed posterodorsaldentary process; thesurangular bone has a forked anterior process for articulation with the posterodorsal dentary process; thehumerus' internaltuberosity is proximally projected and separated from the humeral head by a deep groove (also present in coelophysoids); possesses enlarged hands, which are 60% of the size of the humerus+radius, and the humeral entepicondyle is ridge-like with anterior and posterior depressions; and the posterior border of theilialpeduncle forms a right angle with the dorsal border of the shaft on theischium.
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 thehumeral ectepicondyle, a feature also present inSaturnalia; a saddle-shaped ulnar condyle of thehumerus, and the articular surface for theulnare on theulna is convex; the articular surface of the ulnare is smaller than that of the ulna, a feature unknown inStaurikosaurus andSanjuansaurus; 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 theoccipital condyle are subequal in width (noted by Sereno and Novas, 1993).[46]
Herrerasaurus was originally considered to be a genus withinCarnosauria, which then included forms similar toMegalosaurus andAntrodemus (the latter is probably equivalent toAllosaurus[47]), even thoughHerrerasaurus lived many millions of years before them and therefore would have retained multipleprimitive features. This carnosaurian classification was amended upon by Rozhdestvensky and Tatarinov in 1964, who classifiedHerrerasaurus within the familyGryponichidae inside Carnosauria. The same year, Walker published a differing opinion thatHerrerasaurus instead was allied withPlateosauridae, although it differed in possessing a pubic boot. Walker also proposed thatHerrerasaurus may instead be close toPoposaurus (now considered apseudosuchian[48]) and the unnamed theropod from theDockum Group ofTexas (now assigned to therauisuchianPostosuchus[49]). In 1985, Charig noted thatHerrerasaurus was of uncertain classification, showing similarities to both "prosauropods" and "carnosaurians". Romer (1966), simply noted thatHerrerasaurus was a prosauropod possibly within Plateosauridae. In the description ofStaurikosaurus, Colbert noted that there were many similarities between his taxon andHerrerasaurus, but classified them in separate families, withHerrerasaurus inTeratosauridae. In 1970, Bonaparte also proposed similarities betweenHerrerasaurus andStaurikosaurus, and while classifying them both clearly as inSaurischia, he stated that they appeared as though they could not be placed in a current family. This was further supported by Benedetto in 1973, who named for the taxa the new familyHerrerasauridae, which he classified as saurischians, possibly withinTheropoda but not inSauropodomorpha.[50] However, in 1977 Galton proposed that Herrerasauridae only includedHerrerasaurus, and found it to be Saurischianincertae sedis.[51]
Proposed in 1987 by Brinkman and Sues,Herrerasaurus has at times been considered basal toOrnithischia and Saurischia, although Brinkmann and Sues still considered it to be insideDinosauria. They supported this on the basis thatHerrerasaurus has a large pedal digit V, and has a well developed medial wall on the acetabulum. Brinkmann and Sues consideredStaurikosaurus andHerrerasaurus to not form a true group called Herrerasauridae, and that instead they were successively more primitive forms. Also, they considered the characters used by Benedetto to be invalid, instead representing only theplesiomorphic state that was found in both taxa.[20] This was disagreed with in 1992 by Novas, who stated many derivedsynapomorphies of Herrerasauridae, such as a distinct pubic boot, but still classified them as basal to Ornithischia and Saurischia. Novas defined the family as the least common ancestor ofHerrerasaurus andStaurikosaurus and all its descendants.[21] A differing definition of Herrerasauridae as the most inclusive clade includingHerrerasaurus but notPasser domesticus was first suggested by Sereno (1998), and more closely follows the original inclusion proposed by Benedetto.[52] Another group,Herrerasauria was named by Galton in 1985, and defined asHerrerasaurus but notLiliensternus orPlateosaurus by Langer (2004), who used the node-based definition for Herrerasauridae.[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 ofTriceratops andbirds. They also discussed what this definition would do to the most basal taxa, such as Herrerasauridae, andEoraptor. Padian and May considered that since both Herrerasauridae andEoraptor lack many diagnostic features of Saurischia or Ornithischia, that they could not be considered inside Dinosauria.[54]
A later 1994 study by Novas instead classifiedHerrerasaurus within Dinosauria, and strongly supported its position within Saurischia, as well as provided synapomorphies that it shared with Theropoda. Novas found that the primitive features of lacking a brevis fossa and having only two sacral vertebrae were simply reversals found in the genus.[55] In 1996, Novas went further by supporting a theropod position forHerrerasaurus with aphylogenetic analysis, which placed it closer toNeotheropoda thanEoraptor or Sauropodomorpha.[56] Langer (2004) mentioned that this hypothesis was widely accepted, but that more later authors instead preferred to placeHerrerasaurus as well asEoraptor basal to Theropoda and Sauropodomorpha, a clade called Eusaurischia. Langer (2004) conducted a phylogenetic analysis, and found that it was much more likely thatHerrerasaurus was a basal saurischian, than either a theropod or a non-dinosaurian.[53] Langer's proposal was supported by multiple studies until the discovery ofTawa, when Nesbittet al. conducted a more inclusive analysis, and the resultingcladogram placed Herrerasauridae basal toEoraptor, but closer toDilophosaurus than Sauropodomorpha.[57][58] Unlike Nesbitt, Ezcurra (2010) conducted a phylogenetic analysis to place his new taxonChromogisaurus, and found that Herrerasauridae was basal to Eusaurischia.[59]
In 2010, Alcocer and Martinez described a new taxon of herrerasaurid,Sanjuansaurus. It could be distinguished fromHerrerasaurus based on multiple features. In the phylogenetic analysis,Herrerasaurus,Sanjuansaurus andStaurikosaurus all were in apolytomy, and Herrerasauridae was the most primitive group of saurischian, outside Eusaurischia,Eoraptor andGuaibasaurus.[1] In 2011, Martinezet al. describedEodromaeus, a basal theropod from the same formation asHerrerasaurus. In a phylogenetic analysis,Eoraptor was placed within Sauropodomorpha, Herrerasauridae was placed as the most basal theropods, andEodromaeus was placed as the next most basal.[60] A more recent analysis, by Bittencourtet al. (2014), placed Herrerasauridae in a polytomy with Theropoda and Sauropodomorpha, withEoraptor also being in an unresolved position. This cladogram is shown below.[61]
Other members of theclade[5] may includeChindesaurus from the UpperPetrified Forest (Chinle Formation) of Arizona,[62] and possiblyCaseosaurus from theTecovas Formation of theDockum Group in Texas,[63] although the relationships of these animals are not fully understood, and not all paleontologists agree. Other possible basal theropods,Alwalkeria from the Late TriassicLower Maleri Formation ofIndia,[64] andTeyuwasu, known from very fragmentary remains from the Late Triassic of Brazil, might be related.[65] Paul (1988) noted that it had been incorrectly suggested thatStaurikosaurus pricei was a juvenileHerrerasaurus. This claim was refuted when pelvic bones from a juvenileHerrerasaurus were discovered, which upon examination did not resemble the pelvic bones ofStaurikosaurus.[38]
The teeth ofHerrerasaurus indicate that it was acarnivore; its size indicates it would have preyed upon small and medium-sized plant eaters. These might have included other dinosaurs, such asPisanosaurus, as well as the more plentifulrhynchosaurs andsynapsids.[66] The similarity of itsdental microwear patterns to those of modern slender-snouted crocodylians and small-bodied broad-snouted crocodylians, however, indicates thatHerrerasaurus may have mainly consumed small prey that could have been swallowed whole with very minimal or no oral processing, although the authors of the study concluding this caution that this may be due to the small sample size ofHerrerasaurus teeth that underwent dental microwear texture analysis in the study.[67]Herrerasaurus itself may have been preyed upon by giant "rauisuchians" (loricatans) likeSaurosuchus; puncture wounds were found in one skull.[41]
Coprolites (fossilized dung) containing small bones but no trace of plant fragments, discovered in the Ischigualasto Formation, have been assigned toHerrerasaurus based on fossil abundance. Mineralogical and chemical analysis of these coprolites indicates that if the referral toHerrerasaurus was correct, this carnivore could digest bone.[68]
Comparisons between thescleral rings ofHerrerasaurus and modern birds and reptiles suggest that it may have beencathemeral, active throughout the day at short intervals.[69]
In a 2001 study conducted by Bruce Rothschild and other paleontologists, 12 hand bones and 20 foot bones referred toHerrerasaurus were examined for signs ofstress fracture, but none were found.[70]
PVSJ 407, aHerrerasaurus ischigualastensis, had a pit in a skull bone attributed byPaul Sereno and Novas to a bite. Two additional pits occurred on thesplenial. The areas around these pits are swollen and porous, suggesting the wounds were afflicted by a short-lived non-lethal infection. Because of the size and angles of the wound, it is likely that they were obtained in a fight with anotherHerrerasaurus.[71]
Theholotype ofHerrerasaurus (PVL 2566) was discovered in the Cancha de Bochas Member of theIschigualasto Formation in San Juan, Argentina. It was collected in 1961 by Victorino Herrera, in sediments that were deposited in theCarnian stage of theTriassic period, approximately 231 to 229 million years ago.[72] Over the years, the Ischigualasto Formation produced other fossils ultimately referred toHerrerasaurus. In 1958, A.S. Romer discovered specimen MCZ 7063, originally referred toStaurikosaurus in Carnian sediments.Herrerasaurus specimens PVL 2045 and MLP(4)61, were collected in 1959 and 1960, respectively, in sediments that were deposited in theNorian stage of the Triassic period, approximately 228 to 208 million years ago. However, these specimens are no longer regarded as pertaining toHerrerasaurus.[5][73][74] In 1960, Scaglia collected specimen MACN 18.060, originally the holotype ofIschisaurus cattoi, in sediments deposited in the Carnian stage. In 1961, Scaglia collectedHerrerasaurus specimen PVL 2558, in the Carnian beds of this formation. In 1990, the Cancha de Bochas Member produced moreHerrerasaurus specimens, also from its Carnian beds.[75] Specimen PVSJ 53, originally the holotype ofFrenguellisaurus ischigualastensis, was collected by Gargiulo & Oñate in 1975 in sediments that were deposited in the Carnian stage.[10]
AlthoughHerrerasaurus shared the body shape of the large carnivorous dinosaurs, it lived during a time when dinosaurs were small and few. It was the time of non-dinosaurian reptiles, not dinosaurs, and a major turning point in the Earth's ecology. The vertebrate fauna of the Ischigualasto Formation and the slightly laterLos Colorados Formation consisted mainly of a variety ofcrurotarsalarchosaurs andsynapsids.[73] In the Ischigualasto Formation, dinosaurs constituted only about 10% of the total number of fossils,[60][73] but by the end of the Triassic Period, dinosaurs were becoming the dominant large land animals, and the other archosaurs and synapsids declined in variety and number.[76]
Studies suggest that thepaleoenvironment of the Ischigualasto Formation was a volcanically activefloodplain covered by forests and subject to strong seasonal rainfalls. The climate was moist and warm,[77] though subject to seasonal variations.[78] Vegetation consisted offerns (Cladophlebis),horsetails, and giantconifers (Protojuniperoxylon).[79] These plants formed lowland forests along the banks of rivers.[4]Herrerasaurus remains appear to have been the most common among the carnivores of the Ischigualasto Formation.[16] It lived in the jungles of Late TriassicSouth America alongside other early dinosaurs, such asSanjuansaurus,Eoraptor,Panphagia, andChromogisaurus, as well as rhynchosaurs (Scaphonyx), cynodonts (e.g.,Exaeretodon,Ecteninion andChiniquodon), dicynodonts (Ischigualastia), pseudosuchians (e.g.,Saurosuchus,Sillosuchus andAetosauroides), proterochampsids (e.g.,Proterochampsa) and temnospondyls (Pelorocephalus).[5][73]
Wikijunior Dinosaurs/Herrerasaurus at Wikibooks
Media related toHerrerasaurus at Wikimedia Commons
Data related toHerrerasaurus at Wikispecies
