Wikipedia

Herrerasaurus

Herrerasaurusis likely a genus ofsaurischiandinosaur from theLate Triassicperiod. Measuring 6 m (20 ft) long and weighing around 350 kg (770 lb), this genus was one of the earliestdinosaursfrom the fossil record. Its name means "Herrera's lizard", after the rancher who discovered the first specimen in 1958 in South America. All knownfossilsof this carnivore have been discovered in theIschigualasto FormationofCarnianage (lateTriassicaccording to theICS,dated to 231.4 million years ago) in northwestern Argentina.[1]Thetype species,Herrerasaurus ischigualastensis,was described byOsvaldo Reigin 1963[2]and is the onlyspeciesassigned to thegenus.IschisaurusandFrenguellisaurusaresynonyms.

Herrerasaurus
Temporal range:Late Triassic(Carnian),231.4–228.91Ma
↓
Reconstructed skeleton in Japan
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Family: †Herrerasauridae
Genus: †Herrerasaurus
Reig,1963
Species:
†H. ischigualastensis
Binomial name
†Herrerasaurus ischigualastensis
Reig, 1963
Synonyms
  • Ischisaurus cattoi
    Reig, 1963
  • Frenguellisaurus ischigualastensis
    Novas,1986

For many years, the classification ofHerrerasauruswas unclear because it was known from very fragmentary remains. It washypothesizedto 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]Herrerasaurushas 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]

Discovery

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The most complete skull, specimen PVSJ 407, and left maxilla PVSJ 053

Herrerasauruswas named bypaleontologistOsvaldo Reig after Victorino Herrera, an Andeangoatherdwho first noticed itsfossilsin outcrops near the city ofSan Juan, Argentinain 1959.[2]These rocks, which later yieldedEoraptor,[15]are part of theIschigualasto Formationand date from the lateCarnianstage of the LateTriassicperiod.[16]Reig named a second dinosaur from these rocks in the same publication asHerrerasaurus;[2]this dinosaur,Ischisaurus cattoi,is now considered ajunior synonymand ajuvenileofHerrerasaurus.[17]

Reig believedHerrerasauruswas 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 classifiedHerrerasaurusas aprosauropod.[18]In 1972,Peter Galtonclassified the genus as not diagnosable beyondSaurischia.[19]Later, usingcladisticanalysis, some researchers putHerrerasaurusandStaurikosaurusat 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 ischigualastensisbyFernando Novasin 1986,[25]but this species too is now thought to be a synonym.[17]Frenguellisaurus ischigualastensiswas 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 theFrenguellisaurusremains and found them referable toHerrerasaurus.[26]Ischisaurus cattoiwas 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 completeHerrerasaurusskull 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]classifiedHerrerasaurusat 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. IfHerrerasauruswere indeed a theropod, it would indicate that theropods,sauropodomorphs,andornithischiansdiverged 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 byichnologicalrecords showing large tridactyl (three-toed) footprints that can be attributed only to a theropod dinosaur. These footprints date from the early CarnianLos Rastros Formationin Argentina, which predatesHerrerasaurusby several million years.[34][35]

The study of early dinosaurs such asHerrerasaurusandEoraptortherefore has important implications for the concept of dinosaurs as amonophyleticgroup (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 ofHerrerasaurusby 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 ofHerrerasaurusalso led him to propose several new dinosaurian synapomorphies.[4]

Description

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Scale diagram showing the holotype specimen (red) and the largest-known specimen (gray), compared in size with a human

Herrerasauruswas 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]

Herrerasauruswas fully bipedal. It had strong hind limbs with shortthighsand 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 ofHerrerasauruswere less than half the length of its hind limbs. Theupper armandforearmwere 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]

Life restoration

Herrerasaurusdisplays 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 thefemurmeets thepelvis) that was only partially open. Theilium,the main hip bone, is supported by only twosacrals,abasaltrait.[10]However, thepubispoints backwards, aderivedtrait as seen indromaeosauridsandbirds.Additionally, the end of the pubis has a booted shape, like those inavetheropods;and thevertebralcentrahave anhourglassshape as found inAllosaurus.[38]

Herrerasaurushad a long, narrowskullthat lacked nearly all the specializations that characterized later dinosaurs,[41]and more closely resembled those of more primitivearchosaurssuch 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 fenestraeand a pair of tiny, 1-centimeter-long (0.39 in) slit-like holes called promaxillary fenestrae.[42]

Reconstructed skeleton inNaturmuseum Senckenberg

Herrerasaurushad 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),Herrerasauruscan be distinguished based on the following features:[44]the presence of apremaxilla-maxillafenestra, 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 thejugalbone, and a deeply incisedsupratemporal fossathat extends across the medial postorbital process; the subquadrate ventral squamosal process has a lateral depression, and thequadratojugalbone overlaps the posterodorsalquadrateface; thepterygoidprocess of the quadrate has an inturned, trough-shaped ventral margin, and the presence of a slender ribbed posterodorsaldentaryprocess; thesurangularbone has a forked anterior process for articulation with the posterodorsal dentary process; thehumerus' internaltuberosityis 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 theilialpeduncleforms a right angle with the dorsal border of the shaft on theischium.

According to Sereno (1993),Herrerasauruscan be distinguished based on the following features, all of which are unknown in other herrerasaurids:[45]a circular pit is present on thehumeralectepicondyle, a feature also present inSaturnalia;a saddle-shaped ulnar condyle of thehumerus,and the articular surface for theulnareon theulnais convex; the articular surface of the ulnare is smaller than that of the ulna, a feature unknown inStaurikosaurusandSanjuansaurus;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 condyleare subequal in width (noted by Sereno and Novas, 1993).[46]

Classification

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Skeletal diagram

Herrerasauruswas originally considered to be a genus withinCarnosauria,which then included forms similar toMegalosaurusandAntrodemus(the latter is probably equivalent toAllosaurus[47]), even thoughHerrerasauruslived many millions of years before them and therefore would have retained multipleprimitivefeatures. This carnosaurian classification was amended upon by Rozhdestvensky and Tatarinov in 1964, who classifiedHerrerasauruswithin the familyGryponichidaeinside Carnosauria. The same year, Walker published a differing opinion thatHerrerasaurusinstead was allied withPlateosauridae,although it differed in possessing a pubic boot. Walker also proposed thatHerrerasaurusmay instead be close toPoposaurus(now considered apseudosuchian[48]) and the unnamed theropod from theDockum GroupofTexas(now assigned to therauisuchianPostosuchus[49]). In 1985, Charig noted thatHerrerasauruswas of uncertain classification, showing similarities to both "prosauropods"and" carnosaurians ". Romer (1966), simply noted thatHerrerasauruswas 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, withHerrerasaurusinTeratosauridae.In 1970, Bonaparte also proposed similarities betweenHerrerasaurusandStaurikosaurus,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 withinTheropodabut not inSauropodomorpha.[50]However, in 1977 Galton proposed that Herrerasauridae only includedHerrerasaurus,and found it to be Saurischianincertae sedis.[51]

Reconstructed skull in theNatural History MuseuminMilan

Proposed in 1987 by Brinkman and Sues,Herrerasaurushas at times been considered basal toOrnithischiaand Saurischia, although Brinkmann and Sues still considered it to be insideDinosauria.They supported this on the basis thatHerrerasaurushas a large pedal digit V, and has a well developed medial wall on the acetabulum. Brinkmann and Sues consideredStaurikosaurusandHerrerasaurusto 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 theplesiomorphicstate that was found in both taxa.[20]This was disagreed with in 1992 by Novas, who stated many derivedsynapomorphiesof 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 ofHerrerasaurusandStaurikosaurusand all its descendants.[21]A differing definition of Herrerasauridae as the most inclusive clade includingHerrerasaurusbut notPasser domesticuswas first suggested by Sereno (1998), and more closely follows the original inclusion proposed by Benedetto.[52]Another group,Herrerasauriawas named by Galton in 1985, and defined asHerrerasaurusbut notLiliensternusorPlateosaurusby Langer (2004), who used the node-based definition for Herrerasauridae.[53]

Life restoration

In a revision of basal Dinosauria, Padian and May (1993) discussed the definition of the clade, and redefined it as the latest common ancestor ofTriceratopsandbirds.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 andEoraptorlack many diagnostic features of Saurischia or Ornithischia, that they could not be considered inside Dinosauria.[54]

A later 1994 study by Novas instead classifiedHerrerasauruswithin 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 forHerrerasauruswith aphylogeneticanalysis, which placed it closer toNeotheropodathanEoraptoror Sauropodomorpha.[56]Langer (2004) mentioned that this hypothesis was widely accepted, but that more later authors instead preferred to placeHerrerasaurusas well asEoraptorbasal to Theropoda and Sauropodomorpha, a clade called Eusaurischia.Langer (2004) conducted a phylogenetic analysis, and found that it was much more likely thatHerrerasauruswas 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 resultingcladogramplaced Herrerasauridae basal toEoraptor,but closer toDilophosaurusthan 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 fromHerrerasaurusbased on multiple features. In the phylogenetic analysis,Herrerasaurus,SanjuansaurusandStaurikosaurusall were in apolytomy,and Herrerasauridae was the most primitive group of saurischian, outside Eusaurischia,EoraptorandGuaibasaurus.[1]In 2011, Martinezet al.describedEodromaeus,a basal theropod from the same formation asHerrerasaurus.In a phylogenetic analysis,Eoraptorwas placed within Sauropodomorpha, Herrerasauridae was placed as the most basal theropods, andEodromaeuswas placed as the next most basal.[60]A more recent analysis, by Bittencourtet al.(2014), placed Herrerasauridae in a polytomy with Theropoda and Sauropodomorpha, withEoraptoralso being in an unresolved position. This cladogram is shown below.[61]

Herrerasaurus(large),Eoraptor(small), andPlateosaurus(skull), three earlysaurischians
Dinosauria

Other members of theclade[5]may includeChindesaurusfrom the UpperPetrified Forest(Chinle Formation) of Arizona,[62]and possiblyCaseosaurusfrom theTecovas Formationof theDockum Groupin Texas,[63]although the relationships of these animals are not fully understood, and not all paleontologists agree. Other possible basal theropods,Alwalkeriafrom the Late TriassicLower Maleri FormationofIndia,[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 priceiwas a juvenileHerrerasaurus.This claim was refuted when pelvic bones from a juvenileHerrerasauruswere discovered, which upon examination did not resemble the pelvic bones ofStaurikosaurus.[38]

Paleobiology

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An artist's impression ofHerrerasaurusfeeding on a smallcynodont

The teeth ofHerrerasaurusindicate 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 plentifulrhynchosaursandsynapsids.[66]Herrerasaurusitself 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 toHerrerasaurusbased on fossil abundance. Mineralogical and chemical analysis of these coprolites indicates that if the referral toHerrerasauruswas correct, this carnivore could digest bone.[67]

Comparisons between thescleral ringsofHerrerasaurusand modern birds and reptiles suggest that it may have beencathemeral,active throughout the day at short intervals.[68]

In a 2001 study conducted by Bruce Rothschild and other paleontologists, 12 hand bones and 20 foot bones referred toHerrerasauruswere examined for signs ofstress fracture,but none were found.[69]

PVSJ 407, aHerrerasaurus ischigualastensis,had a pit in a skull bone attributed byPaul Serenoand 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.[70]

Paleoecology

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Model depicted with prey

TheholotypeofHerrerasaurus(PVL 2566) was discovered in the Cancha de Bochas Member of theIschigualasto Formationin San Juan, Argentina. It was collected in 1961 by Victorino Herrera, in sediments that were deposited in theCarnianstage of theTriassicperiod, approximately 231 to 229 million years ago.[71]Over the years, the Ischigualasto Formation produced other fossils ultimately referred toHerrerasaurus.In 1958, A.S. Romer discovered specimen MCZ 7063, originally referred toStaurikosaurusin Carnian sediments.Herrerasaurusspecimens PVL 2045 and MLP(4)61, were collected in 1959 and 1960, respectively, in sediments that were deposited in theNorianstage of the Triassic period, approximately 228 to 208 million years ago. However, these specimens are no longer regarded as pertaining toHerrerasaurus.[5][72][73]In 1960, Scaglia collected specimen MACN 18.060, originally the holotype ofIschisaurus cattoi,in sediments deposited in the Carnian stage. In 1961, Scaglia collectedHerrerasaurusspecimen PVL 2558, in the Carnian beds of this formation. In 1990, the Cancha de Bochas Member produced moreHerrerasaurusspecimens, also from its Carnian beds.[74]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]

AlthoughHerrerasaurusshared 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 Formationconsisted mainly of a variety ofcrurotarsalarchosaursandsynapsids.[72]In the Ischigualasto Formation, dinosaurs constituted only about 10% of the total number of fossils,[60][72]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.[75]

Studies suggest that thepaleoenvironmentof the Ischigualasto Formation was a volcanically activefloodplaincovered by forests and subject to strong seasonal rainfalls. The climate was moist and warm,[76]though subject to seasonal variations.[77]Vegetation consisted offerns(Cladophlebis),horsetails,and giantconifers(Protojuniperoxylon).[78]These plants formed lowland forests along the banks of rivers.[4]Herrerasaurusremains appear to have been the most common among the carnivores of the Ischigualasto Formation.[16]It lived in the jungles of Late TriassicSouth Americaalongside other early dinosaurs, such asSanjuansaurus,Eoraptor,Panphagia,andChromogisaurus,as well as rhynchosaurs (Scaphonyx), cynodonts (e.g.,Exaeretodon,EcteninionandChiniquodon), dicynodonts (Ischigualastia), pseudosuchians (e.g.,Saurosuchus,SillosuchusandAetosauroides), proterochampsids (e.g.,Proterochampsa) and temnospondyls (Pelorocephalus).[5][72]

References

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