96-413: 1st row (early saurischians): Herrerasaurus ischigualastensis ( herrerasaur ), Eodromaeus murphi ( basal theropod ); 2nd row ( theropods ): Pelecanus occidentalis , Tyrannosaurus rex ; 3rd row ( sauropodomorphs ): Apatosaurus louisae , Plateosaurus trossingensis . Saurischia ( / s ɔː ˈ r ɪ s k i ə / saw- RIS -kee-ə , meaning "reptile-hipped" from
192-458: A Herrerasaurus ischigualastensis , had a pit in a skull bone attributed by Paul Sereno and Novas to a bite. Two additional pits occurred on the splenial . 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 another Herrerasaurus . The holotype of Herrerasaurus (PVL 2566)
288-494: A crown group that includes the most recent common ancestor of living birds and crocodilians, and all of its descendants. The base of Archosauria splits into two clades: Pseudosuchia , which includes crocodilians and their extinct relatives; and Avemetatarsalia , which includes birds and their extinct relatives (such as non-avian dinosaurs and pterosaurs). Older definitions of the group Archosauria rely on shared morphological characteristics, such as an antorbital fenestra in
384-545: A monophyletic grouping, thus forming a true clade. One of the first studies of archosaur phylogeny was authored by French paleontologist Jacques Gauthier in 1986. Gauthier split Archosauria into Pseudosuchia , the crocodilian line, and Ornithosuchia , the dinosaur and pterosaur line. Pseudosuchia was defined as all archosaurs more closely related to crocodiles, while Ornithosuchia was defined as all archosaurs more closely related to birds. Proterochampsids, erythrosuchids, and proterosuchids fell successively outside Archosauria in
480-1049: A pseudosuchian ) and the unnamed theropod from the Dockum Group of Texas (now assigned to the rauisuchian Postosuchus ). In 1985, Charig noted that Herrerasaurus was of uncertain classification, showing similarities to both " prosauropods " and "carnosaurians". Romer (1966), simply noted that Herrerasaurus was a prosauropod possibly within Plateosauridae. In the description of Staurikosaurus , Colbert noted that there were many similarities between his taxon and Herrerasaurus , but classified them in separate families, with Herrerasaurus in Teratosauridae . In 1970, Bonaparte also proposed similarities between Herrerasaurus and Staurikosaurus , and while classifying them both clearly as in Saurischia , he stated that they appeared as though they could not be placed in
576-1637: A clade (Herrerasauria) outside Dinosauria. Other recent studies support a view closer to the traditional Saurischia hypothesis, with theropods closer to sauropodomorphs than to ornithischians. Novas et al . (2021) support Cau's herrerasaur phylogeny but place this clade in Saurischia. † Ornithischia (incl. " Silesauridae ") [REDACTED] † Herrerasauridae [REDACTED] † Daemonosaurus [REDACTED] † Chindesaurus [REDACTED] † Tawa [REDACTED] † Saltopus [REDACTED] † Eodromaeus [REDACTED] † Sauropodomorpha [REDACTED] Theropoda [REDACTED] [REDACTED] † Silesauridae † Ornithischia [REDACTED] † Herrerasauridae [REDACTED] † Daemonosaurus [REDACTED] † Chindesaurus [REDACTED] † Tawa [REDACTED] † Sauropodomorpha [REDACTED] † Eodromaeus [REDACTED] Neotheropoda [REDACTED] [REDACTED] † Silesauridae † Herrerasauridae [REDACTED] † Sauropodomorpha [REDACTED] † Ornithischia [REDACTED] Theropoda [REDACTED] [REDACTED] † Silesauridae (incl. Pisanosaurus ) † Herrerasauridae [REDACTED] † Daemonosaurus [REDACTED] † Tawa [REDACTED] † Sauropodomorpha [REDACTED] † Eodromaeus [REDACTED] † Ornithischia [REDACTED] Theropoda [REDACTED] [REDACTED] [REDACTED] [REDACTED] [REDACTED] Herrerasaurus ischigualastensis Herrerasaurus
672-525: A clade called Eu saurischia . Langer (2004) conducted a phylogenetic analysis, and found that it was much more likely that Herrerasaurus was a basal saurischian, than either a theropod or a non-dinosaurian. 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. Unlike Nesbitt, Ezcurra (2010) conducted
768-525: A clade containing only the Sauropodomorpha and Herrerasauridae . Thomas Holtz (2017) recommended using the name Sauropodomorpha to refer to a possible clade that includes traditional sauropodomorphs and herrerasaurids; alternatively, he proposed redefining the long-disused taxon Pachypodosauria to include Sauropodomorpha and Herrerasauridae as subclades. Cau (2018) also supported Ornithoscelida but placed herrerasaurids, Tawa and Daemonosaurus in
864-557: A common ancestor, and that the others were attributable to convergent evolution . Sereno's analysis of Herrerasaurus also led him to propose several new dinosaurian synapomorphies. 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 and 350 kg (770 lb) in weight. Smaller specimens were about 4.5 m (15 ft) long and weighed about 200 kg (440 lb). Herrerasaurus
960-700: A current family. This was further supported by Benedetto in 1973, who named for the taxa the new family Herrerasauridae , which he classified as saurischians, possibly within Theropoda but not in Sauropodomorpha . However, in 1977 Galton proposed that Herrerasauridae only included Herrerasaurus , and found it to be Saurischian incertae sedis . Proposed in 1987 by Brinkman and Sues, Herrerasaurus has at times been considered basal to Ornithischia and Saurischia, although Brinkmann and Sues still considered it to be inside Dinosauria . They supported this on
1056-499: A monophyletic Saurischia, according to its traditional definition. Instead, the group was found to be paraphyletic . As a solution, Theropoda was removed from the group and placed as the sister group to the Ornithischia in the newly defined clade Ornithoscelida . As another result, the authors redefined Saurischia as "the most inclusive clade that contains D[iplodocus] carnegii , but not T[riceratops] horridus ", resulting in
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#17327838523731152-534: A new tree in a phylogenetic study of basal archosaurs. As in Gauthier's tree, Benton and Clark's revealed a basal split within Archosauria. They referred to the two groups as Crocodylotarsi and Ornithosuchia. Crocodylotarsi was defined as an apomorphy -based taxon based on the presence of a "crocodile-normal" ankle joint (considered to be the defining apomorphy of the clade). Gauthier's Pseudosuchia, by contrast,
1248-421: A phylogenetic analysis to place his new taxon Chromogisaurus , and found that Herrerasauridae was basal to Eusaurischia. In 2010, Alcocer and Martinez described a new taxon of herrerasaurid, Sanjuansaurus . It could be distinguished from Herrerasaurus based on multiple features. In the phylogenetic analysis, Herrerasaurus , Sanjuansaurus and Staurikosaurus all were in a polytomy , and Herrerasauridae
1344-528: A polytomy with Theropoda and Sauropodomorpha, with Eoraptor also being in an unresolved position. This cladogram is shown below. Ornithischia Eoraptor Sauropodomorpha Staurikosaurus Herrerasaurus Sanjuansaurus Eodromaeus Tawa Neotheropoda Other members of the clade may include Chindesaurus from the Upper Petrified Forest ( Chinle Formation ) of Arizona, and possibly Caseosaurus from
1440-470: 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);
1536-510: A similar hip anatomy independently of each other, possibly as an adaptation to their herbivorous or omnivorous diets. In his paper naming the two groups, Seeley reviewed previous classification schemes put forth by other paleontologists to divide up the traditional order Dinosauria. He preferred one that had been put forward by Othniel Charles Marsh in 1878, which divided dinosaurs into four orders: Sauropoda , Theropoda , Ornithopoda , and Stegosauria (these names are still used today in much
1632-538: A simple hinge. This arrangement, which was only suitable for animals with erect limbs, provided more stability when the animals were running. The earliest avemetatarsalians, such as Teleocrater and Asilisaurus, retained "primitive mesotarsal" ankles. The ornithodirans differed from other archosaurs in other ways: they were lightly built and usually small, their necks were long and had an S-shaped curve, their skulls were much more lightly built, and many ornithodirans were completely bipedal . The archosaurian fourth trochanter on
1728-410: A slender ribbed posterodorsal dentary process; the surangular bone has a forked anterior process for articulation with the posterodorsal dentary process; the humerus ' internal tuberosity 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
1824-479: A theropod dinosaur. These footprints date from the early Carnian Los Rastros Formation in Argentina, which predates Herrerasaurus by several million years. The study of early dinosaurs such as Herrerasaurus and Eoraptor therefore has important implications for the concept of dinosaurs as a monophyletic group (a group descended from a common ancestor). The monophyly of dinosaurs was explicitly proposed in
1920-475: A valid grouping. Because they are considered a "basal stock", thecodonts are paraphyletic , meaning that they form a group that does not include all descendants of its last common ancestor: in this case, the more derived crocodilians and birds are excluded from "Thecodontia" as it was formerly understood. The description of the basal ornithodires Lagerpeton and Lagosuchus in the 1970s provided evidence that linked thecodonts with dinosaurs, and contributed to
2016-406: A wide range of taxa including dinosaurs , crocodilians , thecodonts , sauropterygians (which may be related to turtles), rhynchocephalians (a group that according to Cope included rhynchosaurs , which nowadays are considered to be more basal archosauromorphs , and tuataras , which are lepidosaurs ), and anomodonts , which are now considered synapsids. It was not until 1986 that Archosauria
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#17327838523732112-484: Is a clade of diapsid sauropsid tetrapods , with birds and crocodilians being the only extant representatives. Although broadly classified as reptiles , which traditionally exclude birds, the cladistic sense of the term includes all living and extinct relatives of birds and crocodilians such as non-avian dinosaurs , pterosaurs , phytosaurs , aetosaurs and rauisuchians as well as many Mesozoic marine reptiles . Modern paleontologists define Archosauria as
2208-466: Is a member of the Herrerasauridae , a family of similar genera that were among the earliest of the dinosaurian evolutionary radiation . 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. These rocks, which later yielded Eoraptor , are part of
2304-531: Is likely a genus of saurischian dinosaur from the Late Triassic period. Measuring 6 m (20 ft) long and weighing around 350 kg (770 lb), this genus was one of the earliest dinosaurs from the fossil record. Its name means "Herrera's lizard", after the rancher who discovered the first specimen in 1958 in South America. All known fossils of this carnivore have been discovered in
2400-487: Is ridge-like with anterior and posterior depressions; and the posterior border of the ilial peduncle forms a right angle with the dorsal border of the shaft on the ischium . According to Sereno (1993), Herrerasaurus can be distinguished based on the following features, all of which are unknown in other herrerasaurids: a circular pit is present on the humeral ectepicondyle, a feature also present in Saturnalia ;
2496-499: Is within the larger clade Archosauriformes , which includes some close relatives of archosaurs, such as proterochampsids and euparkeriids . These relatives are often referred to as archosaurs despite being placed outside of the crown group Archosauria in a more basal position within Archosauriformes. Historically, many archosauriforms were described as archosaurs, including proterosuchids and erythrosuchids , based on
2592-540: The Anisian stage (247–242 Ma) of Tanzania , and include Asilisaurus (an early silesaurid ), Teleocrater (an aphanosaur ), and Nyasasaurus (a possible early dinosaur). Synapsids are a clade that includes mammals and their extinct ancestors . The latter group are often referred to as mammal-like reptiles, but should be termed protomammals, stem mammals, or basal synapsids, because they are not true reptiles by modern cladistic classification. They were
2688-592: The Early Triassic period, though the first archosauriforms and archosauromorphs (reptilians closer to archosaurs than to lizards or other lepidosaurs ) appeared in the Permian . Archosaurs quickly diversified in the aftermath of the Permian-Triassic mass extinction (~252 Ma ), which wiped out most of the then- dominant therapsid competitors such as the gorgonopsians and anomodonts , and
2784-552: The Frenguellisaurus remains and found them referable to Herrerasaurus . 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 to Herrerasaurus . A complete Herrerasaurus skull was found in 1988, by a team of paleontologists led by Paul Sereno . Based on the new fossils, authors such as Thomas Holtz and José Bonaparte classified Herrerasaurus at
2880-582: The Greek sauros ( σαῦρος ) meaning 'lizard' and ischion ( ἴσχιον ) meaning 'hip joint') is one of the two basic divisions of dinosaurs (the other being Ornithischia ), classified by their hip structure. Saurischia and Ornithischia were originally called orders by Harry Seeley in 1888 though today most paleontologists classify Saurischia as an unranked clade rather than an order. All carnivorous dinosaurs (certain types of theropods ) are traditionally classified as saurischians, as are all of
2976-558: The Ischigualasto Formation and date from the late Carnian stage of the Late Triassic period. Reig named a second dinosaur from these rocks in the same publication as Herrerasaurus ; this dinosaur, Ischisaurus cattoi , is now considered a junior synonym and a juvenile of Herrerasaurus . Reig believed Herrerasaurus was an early example of a carnosaur , but this was the subject of much debate over
Saurischia - Misplaced Pages Continue
3072-454: 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
3168-473: The K-Pg extinction, rediversifying in the subsequent Cenozoic era. Birds in particular have become among the most species-rich groups of terrestrial vertebrates in the present day. Archosaurs can traditionally be distinguished from other tetrapods on the basis of several synapomorphies , or shared characteristics, which were present in their last common ancestor . Many of these characteristics appeared prior to
3264-587: The Norian stage of the Triassic period, approximately 228 to 208 million years ago. However, these specimens are no longer regarded as pertaining to Herrerasaurus . In 1960, Scaglia collected specimen MACN 18.060, originally the holotype of Ischisaurus cattoi , in sediments deposited in the Carnian stage. In 1961, Scaglia collected Herrerasaurus specimen PVL 2558, in the Carnian beds of this formation. In 1990,
3360-586: The Olenekian stage (247–251 Ma) of the Early Triassic . A few fragmentary fossils of large carnivorous crocodilian-line archosaurs (informally termed " rauisuchians ") are known from this stage. These include Scythosuchus and Tsylmosuchus (both of which have been found in Russia ), as well as the Xilousuchus , a ctenosauriscid from China . The oldest known fossils of bird-line archosaurs are from
3456-530: The Ornithosuchidae had "reversed crurotarsal" ankles, with a peg on the calcaneum and socket on the astragalus. The earliest fossils of Avemetatarsalia ("bird ankles") appear in the Anisian age of the Middle Triassic . Most Ornithodirans had "advanced mesotarsal" ankles. This form of ankle incorporated a very large astragalus and very small calcaneum, and could only move in one plane, like
3552-736: The Tecovas Formation of the Dockum Group in Texas, although the relationships of these animals are not fully understood, and not all paleontologists agree. Other possible basal theropods, Alwalkeria from the Late Triassic Lower Maleri Formation of India , and Teyuwasu , known from very fragmentary remains from the Late Triassic of Brazil, might be related. Paul (1988) noted that it had been incorrectly suggested that Staurikosaurus pricei
3648-470: The Triassic . In their ankles, the astragalus was joined to the tibia by a suture and the joint rotated round a peg on the astragalus which fitted into a socket in the calcaneum. Early "crurotarsans" still walked with sprawling limbs, but some later crurotarsans developed fully erect limbs. Modern crocodilians are crurotarsans that can employ a diverse range of gaits depending on speed. Euparkeria and
3744-645: The birds and one of the two primary lineages of herbivorous dinosaurs, the sauropodomorphs . At the end of the Cretaceous Period , all saurischians except birds became extinct in the course of the Cretaceous–Paleogene extinction event . Birds, as a group of maniraptoran theropod dinosaurs, are a sub- clade of saurischian dinosaurs in phylogenetic classification . Saurischian dinosaurs are traditionally distinguished from ornithischian dinosaurs by their three-pronged pelvic structure, with
3840-547: The last common ancestor of two or more taxa and all of its descendants. Ornithodira includes the last common ancestor of pterosaurs and dinosaurs (which include birds), while Crurotarsi includes the last common ancestor of living crocodilians and three groups of Triassic archosaurs: ornithosuchids , aetosaurs , and phytosaurs . These clades are not equivalent to "bird-line" and "crocodile-line" archosaurs, which would be branch-based clades defined as all taxa more closely related to one living group (either birds or crocodiles) than
3936-470: The pubis pointed forward. The ornithischians' pelvis is arranged with the pubis rotated backward, parallel with the ischium , often also with a forward-pointing process, giving a four-pronged structure. The saurischian hip structure led Seeley to name them " lizard -hipped" dinosaurs, because they retained the ancestral hip anatomy also found in modern lizards and other reptiles. He named ornithischians "bird-hipped" dinosaurs because their hip arrangement
Saurischia - Misplaced Pages Continue
4032-402: The 1970s by Galton and Robert T. Bakker , who compiled a list of cranial and postcranial synapomorphies (common anatomical traits derived from the common ancestor). Later authors proposed additional synapomorphies. 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
4128-438: The 20th century. Thecodonts were considered the "basal stock" from which the more advanced archosaurs descended. They did not possess features seen in later avian and crocodilian lines, and therefore were considered more primitive and ancestral to the two groups. With the cladistic revolution of the 1980s and 90s, in which cladistics became the most widely used method of classifying organisms, thecodonts were no longer considered
4224-481: The Cancha de Bochas Member produced more Herrerasaurus specimens, also from its Carnian beds. 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. Although Herrerasaurus shared the body shape of the large carnivorous dinosaurs, it lived during a time when dinosaurs were small and few. It
4320-535: The Stegosauria and Ornithopoda in the Ornithischia, and the Theropoda and Sauropoda in the Saurischia. Furthermore, Seeley used this major difference in the hip bones, along with many other noted differences between the two groups, to argue that "dinosaurs" were not a natural grouping at all, but rather two distinct orders that had arisen independently from more primitive archosaurs . This concept that "dinosaur"
4416-694: The Triassic Period, dinosaurs were becoming the dominant large land animals, and the other archosaurs and synapsids declined in variety and number. Studies suggest that the paleoenvironment of the Ischigualasto Formation was a volcanically active floodplain covered by forests and subject to strong seasonal rainfalls. The climate was moist and warm, though subject to seasonal variations. Vegetation consisted of ferns ( Cladophlebis ), horsetails , and giant conifers ( Protojuniperoxylon ). These plants formed lowland forests along
4512-802: The banks of rivers. Herrerasaurus remains appear to have been the most common among the carnivores of the Ischigualasto Formation. 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 ). [REDACTED] [REDACTED] [REDACTED] Archosaur Archosauria ( lit. ' ruling reptiles ' ) or archosaurs ( / ˈ ɑːr k ə ˌ s ɔːr / )
4608-500: The basal saurischian hypothesis. If Herrerasaurus were indeed a theropod, it would indicate that theropods, sauropodomorphs , and ornithischians diverged even earlier than herrerasaurids, before the middle Carnian , and that "all three lineages independently evolved several dinosaurian features, such as a more advanced ankle joint or an open acetabulum". This view is further supported by ichnological records showing large tridactyl (three-toed) footprints that can be attributed only to
4704-400: The basal split and thought that the crurotarsan ankle developed independently in these two groups, but in opposite ways. Cruickshank also thought that the development of these ankle types progressed in each group to allow advanced members to have semi-erect (in the case of crocodilians) or erect (in the case of dinosaurs) gaits. In many phylogenetic analyses, archosaurs have been shown to be
4800-536: The basal tuber and the occipital condyle are subequal in width (noted by Sereno and Novas, 1993). Herrerasaurus was originally considered to be a genus within Carnosauria , which then included forms similar to Megalosaurus and Antrodemus (the latter is probably equivalent to Allosaurus ), even though Herrerasaurus lived many millions of years before them and therefore would have retained multiple primitive features. This carnosaurian classification
4896-520: The base of the saurischian tree before the divergence between prosauropods and theropods. However, Sereno favored classifying Herrerasaurus (and the Herrerasauridae) as primitive theropods. These two classifications have become the most persistent, with Rauhut (2003) and Bittencourt and Kellner (2004) favoring the early theropod hypothesis , and Max Langer (2004), Langer and Benton (2006), and Randall Irmis and his coauthors (2007) favoring
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#17327838523734992-418: The basis that Herrerasaurus has a large pedal digit V, and has a well developed medial wall on the acetabulum. Brinkmann and Sues considered Staurikosaurus and Herrerasaurus 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 the plesiomorphic state that
5088-444: The catastrophic Permian-Triassic extinction event . Unlike their close living relatives, the lepidosaurs, archosaurs lost the vomeronasal organ . Archosaurs are a subgroup of archosauriforms , which themselves are a subgroup of archosauromorphs . Both the oldest archosauromorph ( Protorosaurus speneri ) and the oldest archosauriform ( Archosaurus rossicus ) lived in the late Permian. The oldest true archosaurs appeared during
5184-462: 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 bony acetabulum (where the femur meets the pelvis ) that was only partially open. The ilium , the main hip bone, is supported by only two sacrals , a basal trait. However, the pubis points backwards, a derived trait as seen in dromaeosaurids and birds . Additionally,
5280-675: The clade, and redefined it as the latest common ancestor of Triceratops and birds . They also discussed what this definition would do to the most basal taxa, such as Herrerasauridae, and Eoraptor . Padian and May considered that since both Herrerasauridae and Eoraptor lack many diagnostic features of Saurischia or Ornithischia, that they could not be considered inside Dinosauria. 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. Novas found that
5376-493: The disuse of the term "Thecodontia", which many cladists consider an artificial grouping. With the identification of "crocodilian normal" and "crocodilian reversed" ankles by Sankar Chatterjee in 1978, a basal split in Archosauria was identified. Chatterjee considered these two groups to be Pseudosuchia with the "normal" ankle and Ornithosuchidae with the "reversed" ankle. Ornithosuchids were thought to be ancestral to dinosaurs at this time. In 1979, A.R.I. Cruickshank identified
5472-499: The dominant land vertebrates throughout the Permian , but most perished in the Permian–;Triassic extinction event . Very few large synapsids survived the event, but one form, Lystrosaurus (a herbivorous dicynodont ), attained a widespread distribution soon after the extinction. Following this, archosaurs and other archosauriforms quickly became the dominant land vertebrates in the early Triassic . Fossils from before
5568-516: The end of the pubis has a booted shape, like those in avetheropods ; and the vertebral centra have an hourglass shape as found in Allosaurus . Herrerasaurus had a long, narrow skull that lacked nearly all the specializations that characterized later dinosaurs, and more closely resembled those of more primitive archosaurs such as Euparkeria . It had five pairs of fenestrae (skull openings) in its skull, two pairs of which were for
5664-423: The eyes and nostrils. Between the eyes and the nostrils were two antorbital fenestrae and a pair of tiny, 1-centimeter-long (0.39 in) slit-like holes called promaxillary fenestrae. Herrerasaurus had a flexible joint in the lower jaw that could slide back and forth to deliver a grasping bite. This cranial specialization is unusual among dinosaurs but has evolved independently in some lizards . The rear of
5760-452: The femur may have made it easier for ornithodirans to become bipeds, because it provided more leverage for the thigh muscles. In the late Triassic, the ornithodirans diversified to produce dinosaurs and pterosaurs . Archosauria is normally defined as a crown group , which means that it only includes descendants of the last common ancestors of its living representatives. In the case of archosaurs, these are birds and crocodilians. Archosauria
5856-519: The idea that the former had evolved directly from the latter, possibly by way of an enigmatic family that seemed to possess characters of both groups, the segnosaurs . However, it was later found that segnosaurs were an unusual type of herbivorous theropod saurischian closely related to birds , and the Phytodinosauria hypothesis fell out of favor. A 2017 study by Matthew Grant Baron, David B. Norman and Paul M. Barrett did not find support for
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#17327838523735952-430: The length of its hind limbs. The upper arm and forearm were rather short, while the manus (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. Herrerasaurus displays traits that are found in different groups of dinosaurs, and several traits found in non-dinosaurian archosaurs. Although it shares most of
6048-400: 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. According to Novas (1993), Herrerasaurus can be distinguished based on the following features: the presence of a premaxilla - maxilla fenestra, and the dorsal part of laterotemporal fenestra is less than a third as wide as the ventral part;
6144-404: The main explanation for Mesozoic mammals being small. Since the 1970s, scientists have classified archosaurs mainly on the basis of their ankles. The earliest archosaurs had "primitive mesotarsal" ankles: the astragalus and calcaneum were fixed to the tibia and fibula by sutures and the joint bent about the contact between these bones and the foot. The Pseudosuchia appeared early in
6240-664: The mass extinction have only been found around the Equator, but after the event fossils can be found all over the world. Suggested explanations for this include: However, this theory has been questioned, since it implies synapsids were necessarily less advantaged in water retention, that synapsid decline coincides with climate changes or archosaur diversity (neither of which tested) and the fact that desert dwelling mammals are as well adapted in this department as archosaurs, and some cynodonts like Trucidocynodon were large sized predators. A study favors competition amidst mammaliaforms as
6336-783: The monophyly of both of these clades were questioned. Sereno and Arcucci incorporated archosaur features other than ankle types in their analyses, which resulted in a different tree than previous analyses. Below is a cladogram based on Sereno (1991), which is similar to the one produced by Sereno and Arcucci: † Proterosuchidae [REDACTED] † Erythrosuchidae [REDACTED] † Euparkeria [REDACTED] † Proterochampsidae [REDACTED] † Parasuchia [REDACTED] † Ornithosuchidae [REDACTED] Suchia [REDACTED] † ? Scleromochlus † Pterosauria [REDACTED] Dinosauromorpha [REDACTED] Ornithodira and Crurotarsi are both node-based clades, meaning that they are defined to include
6432-476: The more plentiful rhynchosaurs and synapsids . Herrerasaurus itself may have been preyed upon by giant " rauisuchians " ( loricatans ) like Saurosuchus ; puncture wounds were found in one skull. Coprolites (fossilized dung) containing small bones but no trace of plant fragments, discovered in the Ischigualasto Formation, have been assigned to Herrerasaurus based on fossil abundance. Mineralogical and chemical analysis of these coprolites indicates that if
6528-448: The next 30 years, and the genus was variously classified during that time. In 1970, Steel classified Herrerasaurus as a prosauropod . In 1972, Peter Galton classified the genus as not diagnosable beyond Saurischia . Later, using cladistic analysis, some researchers put Herrerasaurus and Staurikosaurus at the base of the dinosaur tree before the separation between ornithischians and saurischians. Several researchers classified
6624-425: The origin of the clade Archosauria, as they were present in archosauriforms such as Proterosuchus and Euparkeria , which were outside the crown group . The most obvious features include teeth set in deep sockets, antorbital and mandibular fenestrae (openings in front of the eyes and in the jaw, respectively), and a pronounced fourth trochanter (a prominent ridge on the femur ). Being set in sockets,
6720-399: The other. Benton proposed the name Avemetatarsalia in 1999 to include all bird-line archosaurs (under his definition, all archosaurs more closely related to dinosaurs than to crocodilians). His analysis of the small Triassic archosaur Scleromochlus placed it within bird-line archosaurs but outside Ornithodira, meaning that Ornithodira was no longer equivalent to bird-line archosaurs. Below
6816-410: The presence of a ridge on the lateral surface of the jugal bone, and a deeply incised supratemporal fossa that extends across the medial postorbital process; the subquadrate ventral squamosal process has a lateral depression, and the quadratojugal bone overlaps the posterodorsal quadrate face; the pterygoid process of the quadrate has an inturned, trough-shaped ventral margin, and the presence of
6912-451: The presence of an antorbital fenestra. While many researchers prefer to treat Archosauria as an unranked clade , some continue to assign it a traditional biological rank. Traditionally, Archosauria has been treated as a Superorder, though a few 21st century researchers have assigned it to different ranks including Division and Class. Archosauria as a term was first coined by American paleontologist Edward Drinker Cope in 1869, and included
7008-519: The primitive features of lacking a brevis fossa and having only two sacral vertebrae were simply reversals found in the genus. In 1996, Novas went further by supporting a theropod position for Herrerasaurus with a phylogenetic analysis, which placed it closer to Neotheropoda than Eoraptor or Sauropodomorpha. 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,
7104-476: The referral to Herrerasaurus was correct, this carnivore could digest bone. 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. In a 2001 study conducted by Bruce Rothschild and other paleontologists, 12 hand bones and 20 foot bones referred to Herrerasaurus were examined for signs of stress fracture , but none were found. PVSJ 407,
7200-431: The remains as non-dinosaurian. Two other partial skeletons, with skull material, were named Frenguellisaurus ischigualastensis by Fernando Novas in 1986, but this species too is now thought to be a synonym. Frenguellisaurus ischigualastensis was discovered in 1975, and was described by Novas (1986) who considered it a primitive saurischian, and possibly a theropod . Novas (1992) and Sereno and Novas (1992) examined
7296-519: The resulting tree. Below is the cladogram from Gauthier (1986): † Proterosuchidae [REDACTED] † Erythrosuchidae [REDACTED] † Proterochampsidae [REDACTED] † Parasuchia [REDACTED] † Aetosauria [REDACTED] † Rauisuchia [REDACTED] Crocodylomorpha [REDACTED] † Euparkeria [REDACTED] † Ornithosuchidae [REDACTED] Ornithodira [REDACTED] In 1988, paleontologists Michael Benton and J. M. Clark produced
7392-480: The same way to refer to suborders or clades within Saurischia and Ornithischia). Seeley, however, wanted to formulate a classification that would take into account a single primary difference between major dinosaurian groups based on a characteristic that also differentiated them from other reptiles. He found this in the configuration of the hip bones, and found that all four of Marsh's orders could be divided neatly into two major groups based on this feature. He placed
7488-459: The skull, serrated teeth, and an upright stance. Some extinct reptiles, such as proterosuchids and euparkeriids , also possessed these features yet originated prior to the split between the crocodilian and bird lineages. The older morphological definition of Archosauria nowadays roughly corresponds to Archosauriformes , a group named to encompass crown-group archosaurs and their close relatives. The oldest true archosaur fossils are known from
7584-407: The subsequent arid Triassic climate allowed the more drought -resilient archosaurs (largely due to their uric acid -based urinary system ) to eventually become the largest and most ecologically dominant terrestrial vertebrates from the Middle Triassic period up until the Cretaceous–Paleogene extinction event (~66 Ma). Birds and several crocodyliform lineages were the only archosaurs to survive
7680-493: The teeth were less likely to be torn loose during feeding. This feature is responsible for the name " thecodont " (meaning "socket teeth"), which early paleontologists applied to many Triassic archosaurs. Additionally, non-muscular cheek and lip tissue appear in various forms throughout the clade, with all living archosaurs lacking non-muscular lips, unlike most non-avian saurischian dinosaurs. Some archosaurs, such as birds, are secondarily toothless. Antorbital fenestrae reduced
7776-499: The two dinosaurian groups has stood the test of time, and has been supported by modern cladistic analysis of relationships among dinosaurs. A node-base clade, Eusaurischia , was named for the least inclusive group containing sauropodomorphs (represented by Cetiosaurus ) and theropods (represented by Neornithes ). Any saurischian that diverged before the theropod-sauropodomorph split is therefore outside clade Eusaurischia. One alternative hypothesis challenging Seeley's classification
7872-446: The weight of the skull, which was relatively large in early archosaurs, rather like that of modern crocodilians . Mandibular fenestrae may also have reduced the weight of the jaw in some forms. The fourth trochanter provides a large site for the attachment of muscles on the femur. Stronger muscles allowed for erect gaits in early archosaurs, and may also be connected with the ability of the archosaurs or their immediate ancestors to survive
7968-514: Was a stem-based taxon . Unlike Gauthier's tree, Benton and Clark's places Euparkeria outside Ornithosuchia and outside the crown group Archosauria altogether. The clades Crurotarsi and Ornithodira were first used together in 1990 by paleontologist Paul Sereno and A. B. Arcucci in their phylogenetic study of archosaurs. They were the first to erect the clade Crurotarsi, while Ornithodira was named by Gauthier in 1986. Crurotarsi and Ornithodira replaced Pseudosuchia and Ornithosuchia, respectively, as
8064-429: Was a juvenile Herrerasaurus . This claim was refuted when pelvic bones from a juvenile Herrerasaurus were discovered, which upon examination did not resemble the pelvic bones of Staurikosaurus . The teeth of Herrerasaurus indicate that it was a carnivore ; its size indicates it would have preyed upon small and medium-sized plant eaters. These might have included other dinosaurs, such as Pisanosaurus , as well as
8160-403: Was amended upon by Rozhdestvensky and Tatarinov in 1964, who classified Herrerasaurus within the family Gryponichidae inside Carnosauria. The same year, Walker published a differing opinion that Herrerasaurus instead was allied with Plateosauridae , although it differed in possessing a pubic boot. Walker also proposed that Herrerasaurus may instead be close to Poposaurus (now considered
8256-434: Was an outdated term for two distinct orders lasted many decades in the scientific and popular literature, and it was not until the 1960s that scientists began to again consider the possibility that saurischians and ornithischians were more closely related to each other than they were to other archosaurs. Although his concept of a polyphyletic Dinosauria is no longer accepted by most paleontologists, Seeley's basic division of
8352-528: Was defined as a crown-clade, restricting its use to more derived taxa. Cope's term was a Greek-Latin hybrid intended to refer to the cranial arches, but has later also been understood as "leading reptiles" or "ruling reptiles" by association with Greek ἀρχός "leader, ruler". The term "thecodont", now considered an obsolete term, was first used by the English paleontologist Richard Owen in 1859 to describe Triassic archosaurs, and it became widely used in
8448-739: 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. Over the years, the Ischigualasto Formation produced other fossils ultimately referred to Herrerasaurus . In 1958, A.S. Romer discovered specimen MCZ 7063, originally referred to Staurikosaurus in Carnian sediments. Herrerasaurus specimens PVL 2045 and MLP(4)61, were collected in 1959 and 1960, respectively, in sediments that were deposited in
8544-408: Was first suggested by Sereno (1998), and more closely follows the original inclusion proposed by Benedetto. Another group, Herrerasauria was named by Galton in 1985, and defined as Herrerasaurus but not Liliensternus or Plateosaurus by Langer (2004), who used the node-based definition for Herrerasauridae. In a revision of basal Dinosauria, Padian and May (1993) discussed the definition of
8640-469: Was found in both taxa. 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. Novas defined the family as the least common ancestor of Herrerasaurus and Staurikosaurus and all its descendants. A differing definition of Herrerasauridae as the most inclusive clade including Herrerasaurus but not Passer domesticus
8736-466: Was fully bipedal. It had strong hind limbs with short thighs 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. The forelimbs of Herrerasaurus were less than half
8832-500: Was proposed by Robert T. Bakker in his 1986 book The Dinosaur Heresies . Bakker's classification separated the theropods into their own group and placed the two groups of herbivorous dinosaurs (the sauropodomorphs and ornithischians) together in a separate group he named the Phytodinosauria ("plant dinosaurs"). The Phytodinosauria hypothesis was based partly on the supposed link between ornithischians and prosauropods , and
8928-518: Was superficially similar to that of birds, though he did not propose any specific relationship between ornithischians and birds. However, in the view which has long been held, this "bird-hipped" arrangement evolved several times independently in dinosaurs, first in the ornithischians, then in the lineage of saurischians including birds ( Avialae ), and lastly in the therizinosaurians . This would then be an example of convergent evolution : avialans, therizinosaurians, and ornithischian dinosaurs all developed
9024-479: Was the most primitive group of saurischian, outside Eusaurischia, Eoraptor and Guaibasaurus . In 2011, Martinez et al. described Eodromaeus , a basal theropod from the same formation as Herrerasaurus . In a phylogenetic analysis, Eoraptor was placed within Sauropodomorpha, Herrerasauridae was placed as the most basal theropods, and Eodromaeus was placed as the next most basal. A more recent analysis, by Bittencourt et al. (2014), placed Herrerasauridae in
9120-458: 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 later Los Colorados Formation consisted mainly of a variety of crurotarsal archosaurs and synapsids . In the Ischigualasto Formation, dinosaurs constituted only about 10% of the total number of fossils, but by the end of
9216-448: 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 an early saurischian in most of the phylogenies on the origin and early evolution of dinosaurs. It
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