45-635: Arizonasaurus was a ctenosauriscid archosaur from the Middle Triassic (243 million years ago). Arizonasaurus is found in the Middle Triassic Moenkopi Formation of northern Arizona. A fairly complete skeleton was found in 2002 by Sterling Nesbitt . The taxon has a large sailback formed by elongated neural spines of the vertebrae. The type species , Arizonasaurus babbitti , was named by Samuel Paul Welles in 1947. The type species , Arizonasaurus babbitti ,
90-494: 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
135-545: 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
180-477: A 2011 analysis by Butler et al. , the first based on Brusatte et al. 2010 and the second based on Nesbitt 2011. Lotosauridae Arizonasaurus Bromsgroveia Ctenosauriscus Hypselorhachis Waldhaus ctenosauriscid Poposaurus Sillosuchus Effigia Shuvosaurus Qianosuchus Arizonasaurus Archosaur Archosauria ( lit. ' ruling reptiles ' ) or archosaurs ( / ˈ ɑːr k ə ˌ s ɔːr / )
225-534: 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,
270-538: 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
315-659: A stage transitional fauna between the faunas of older and younger age. [REDACTED] [REDACTED] [REDACTED] [REDACTED] Ctenosauriscidae Ctenosauriscidae is an extinct family of pseudosuchian archosaurs within the clade Poposauroidea . Ctenosauriscids existed in Africa , Asia , Europe and North America during the Early Triassic to the Middle Triassic period (latest Olenekian to Anisian stages). All species had large "sails" on their backs. Ctenosauriscids are among some of
360-475: 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
405-406: 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
450-484: 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
495-422: Is a phylogenetic cladogram simplified from Butler et al. in 2011 showing the cladistics of Archosauriformes , focusing mostly on Pseudosuchia : Qianosuchus Arizonasaurus Xilousuchus Hypselorhachis Ctenosauriscus Bromsgroveia Waldhaus Taxon Poposaurus gracilis H Poposaurus gracilis Y Lotosaurus Sillosuchus Shuvosaurus Effigia Arizonasaurus
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#1732798655269540-403: Is described from two braincase specimens. Some ancestral features of these braincases are plesiomorphic for crurotarsans . Below is a list of characteristics found by Nesbitt in 2005 that distinguish Arizonasaurus : Arizonasaurus was closely related to Ctenosauriscus ; and, together with a few other genera, they make up Ctenosauriscidae . The ctenosauriscids were closely related to
585-612: Is from the middle Triassic Moenkopi Formation of northern Arizona . The presence of a poposauroid in the early Middle Triassic suggests that the divergence of birds and crocodiles occurred earlier than previously thought. Ctenosauriscids from the Middle Triassic allow the distribution of Triassic faunas to be more widespread, now in Europe, Asia, North America and Africa. The fauna of the Moenkopi Formation represents
630-415: Is one of the oldest archosaurs known to date. Hypselorhachis Valid Anisian [REDACTED] Tanzania Manda Formation "Waldhaus" ctenosauriscid Valid Anisian [REDACTED] Germany Xilousuchus Valid Olenekian [REDACTED] China Heshanggou Formation Xilousuchus is one of the oldest archosaurs known to date. Ctenosauriscidae
675-499: 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
720-540: 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
765-529: 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
810-473: 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
855-586: 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
900-466: 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
945-470: 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
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#1732798655269990-547: 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
1035-494: The poposaurids , as shown by a few shared derived characteristics. The pelvic girdle in Arizonasaurus unites this taxon with Ctenosauriscus , Lotosaurus , Bromsgroveia , and Hypselorhachus . Together, newly identified pseudosuchian features act as evidence that poposaurids, such as Poposaurus , Sillosuchus , and Chatterjeea , and ctenosauriscids form a monophyletic group that is derived rauisuchians . Below
1080-438: 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
1125-400: 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
1170-444: 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
1215-493: 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
1260-499: 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
1305-505: The earliest archosaurs and represent the first global radiation of the group. Arizonasaurus Valid Anisian [REDACTED] USA Moenkopi Formation Bromsgroveia Valid Anisian [REDACTED] United Kingdom Bromsgrove Sandstone Formation Bystrowisuchus Valid Olenekian [REDACTED] Russia Lipovskaya Formation Ctenosauriscus Olenekian [REDACTED] Germany Solling Formation Ctenosauriscus
1350-452: 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
1395-404: 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
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1440-664: 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
1485-783: 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
1530-425: 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,
1575-399: 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
1620-451: 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
1665-519: 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
1710-459: 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
1755-469: 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
1800-493: 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
1845-446: 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
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1890-514: 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
1935-528: 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
1980-500: Was named by Oskar Kuhn in 1964 to include the genus Ctenosauriscus . It is a stem-based taxon defined by Richard J. Butler, Stephen L. Brusatte, Mike Reich, Sterling J. Nesbitt, Rainer R. Schoch and Jahn J. Hornung in 2011 as "the most inclusive clade containing Ctenosauriscus koeneni but not Poposaurus gracilis , Effigia okeeffeae , Postosuchus kirkpatricki , Crocodylus niloticus , Ornithosuchus longidens , or Aetosaurus ferratus ". The cladograms below follows
2025-420: Was named by Samuel Paul Welles in 1947 on the basis of a few teeth and a maxilla , labelled as specimen UCMP 36232. A fairly complete skeleton was found in 2002 by Sterling Nesbitt . Arizonasaurus had a sail made of tall neural spines. This sail was similar to those of other basal archosaurs , such as other ctenosauriscids like Ctenosauriscus , Bromsgroveia , and Hypselorhachis . Arizonasaurus
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