Crurotarsi

Crurotarsi is a group of archosauriform reptiles that includes the archosaurs (represented today by birds and crocodilians) and the extinct, crocodile-like phytosaurs.[1] The name is derived from the Latin word crus and the Greek word tarsos; it refers to the specialized articulation between crus and tarsus—specifically between fibula and calcaneum—present in the skeletons of suchians and phytosaurs, with a hemicylindrical condyle on the calcaneum articulating against fibula.[2][3]

Crurotarsans
Temporal range:
Early TriassicPresent, 250–0 Ma
Protome batalaria
Life restoration of Protome batalaria
Ornithosuchus BW
Life restoration of Ornithosuchus woodwardi
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Archosauriformes
Clade: Crurotarsi
Sereno & Arcucci, 1990
Subgroups

Taxonomic history

The name Crurotarsi was erected as a node-based clade by Paul Sereno and A. B. Arcucci in 1990 to supplant the old term Pseudosuchia, but with a different definition.[2] Crurotarsi includes, by most published definitions, all descendants of the common ancestor of modern crocodiles, ornithosuchids, aetosaurs, and phytosaurs; Nesbitt (2011) provided a shorter definition, defining Crurotarsi as "the least inclusive clade containing Rutiodon carolinensis Emmons, 1856, and Crocodylus niloticus Laurenti, 1768".[1] According to two studies published in 2011 by Nesbitt and coworkers, using either of these definitions leads to the inclusion of all other true archosaurs in Crurotarsi, due to the possibly basal phylogenetic position of the phytosaurs. This means that grouping the phytosaurs and crocodilians into a clade while excluding the avemetatarsalians (pterosaurs, dinosaurs, and birds) would result in a paraphyletic grouping. A more definitive group is Pseudosuchia, which is defined as all archosaurs closer to crocodiles than to birds (matching the traditional content of Crurotarsi).[1][4]

Phylogeny

Paul Sereno and A. B. Arcucci named Crurotarsi in 1990, defining it as "Parasuchia [phytosaurs], Ornithosuchidae, Prestosuchus, Suchia, and all descendants of their common ancestor".[2] The groups in this definition were considered crocodile-line archosaurs, as opposed to the bird-line archosaurs. Ornithosuchids were once considered bird-line archosaurs (as implied by their name, which means "bird crocodiles" in Greek), but were later recognized as crocodile-line archosaurs. This reclassification may have inspired Sereno's Crurotarsi, a node-based clade defined by the inclusion of ornithosuchids and other early archosaurs.

Two names were proposed for crocodile-line archosaurs before Crurotarsi was erected. The first, Pseudosuchia, was established as a stem-based clade in 1985.[5] It includes crocodiles and all archosaurs more closely related to crocodiles than to birds. The second, Crocodylotarsi, was named in 1988, possibly as a replacement for Pseudosuchia.[6] The name Pseudosuchia, meaning "false crocodiles", has been used for over a century, and traditionally included aetosaurs. As a clade, Pseudosuchia includes the group Eusuchia, or "true crocodiles". Crocodylotarsi may have been named to remove confusion, but as a stem-based clade it is synonymous with Pseudosuchia. Because Pseudosuchia was named first, it has precedence. Crurotarsi traditionally contains the same archosaurs as Pseudosuchia, but as a node-based clade it is not synonymous.[7]

In 2011, Sterling J. Nesbitt found phytosaurs to be the sister taxon of Archosauria, and therefore not crocodile-line archosaurs. Because phytosaurs are included in the definition of Crurotarsi, this change in their phylogenetic placement expanded the scope of Crurotarsi, which therefore now includes phytosaurs, crocodiles, pterosaurs and dinosaurs. However, Pseudosuchia still contains only crocodile-line archosaurs.

Below is a cladogram modified from Nesbitt (2011) showing the new changes:[1]

Archosauriformes 

ProterosuchidaeProterosuchusDB flipped

ErythrosuchidaeErythrosuchus africanus

VancleaveaVancleavea white background

ProterochampsiaPseudochampsa life restoration white background

EuparkeriaEuparkeria white background

 Crurotarsi 

PhytosauriaSmilosuchus adamanensis flipped

 Archosauria 

Avemetatarsalia (bird-line archosaurs)Meyers grosses Konversations-Lexikon - ein Nachschlagewerk des allgemeinen Wissens (1908) (Antwerpener Breiftaube).jpg

 Pseudosuchia (crocodile-line archosaurs) 

OrnithosuchidaeOrnithosuchus BW white background

SuchiaDescription des reptiles nouveaux, ou, Imparfaitement connus de la collection du Muséum d'histoire naturelle et remarques sur la classification et les caractères des reptiles (1852) (Crocodylus moreletii)

Below is a cladogram after Nesbitt & Norell (2006) and Nesbitt (2007) with Crurotarsi in its traditional sense encompassing just crocodile-line archosaurs:[8][9]

Archosauriformes 

EuparkeriaEuparkeria white background

ProterochampsidaePseudochampsa life restoration white background

 Archosauria 

to AvemetatarsaliaMeyers grosses Konversations-Lexikon - ein Nachschlagewerk des allgemeinen Wissens (1908) (Antwerpener Breiftaube).jpg

Crurotarsi 

PhytosauriaSmilosuchus adamanensis flipped

 Suchia 

AetosauriaDesmatosuchus spurensis flipped

CrocodylomorphaDescription des reptiles nouveaux, ou, Imparfaitement connus de la collection du Muséum d'histoire naturelle et remarques sur la classification et les caractères des reptiles (1852) (Crocodylus moreletii)

OrnithosuchidaeOrnithosuchus BW white background

 Rauisuchia 

RauisuchidaePostosuchus kirkpatricki flipped

PrestosuchidaePrestosuchus-chiniquensis (2)

 "Group X" 

ArizonasaurusArizonasaurus BW white background

LotosaurusLotosaurus BW white background

 "Group Y" or Shuvosaurinae 

Sillosuchus

ShuvosaurusShuvosaurus BW flipped

EffigiaEffigia BW white background

Cladogram after Brusatte, Benton, Desojo and Langer (2010) [10]

Archosauriformes 

ErythrosuchusErythrosuchus africanus

EuparkeriaEuparkeria white background

ProterochampsidaePseudochampsa life restoration white background

 Archosauria 

to AvemetatarsaliaMeyers grosses Konversations-Lexikon - ein Nachschlagewerk des allgemeinen Wissens (1908) (Antwerpener Breiftaube).jpg

Crurotarsi 

PhytosauriaSmilosuchus adamanensis flipped

 Suchia 

AetosauriaDesmatosuchus spurensis flipped

 Paracrocodylomorpha 

GracilisuchusGracilisuchus BW white background

 Bathyotica 

ErpetosuchusErpetosuchus BW white background

CrocodylomorphaDescription des reptiles nouveaux, ou, Imparfaitement connus de la collection du Muséum d'histoire naturelle et remarques sur la classification et les caractères des reptiles (1852) (Crocodylus moreletii)

Revueltosaurus

OrnithosuchidaeOrnithosuchus BW white background

 Rauisuchia 
 †Rauisuchoidea 

Arganasuchus

Fasolasuchus

Stagonosuchus

TicinosuchusTicinosuchus BW white background

 Prestosuchidae 

SaurosuchusSaurosuchus BW white background

Batrachotomus

PrestosuchusPrestosuchus-chiniquensis (2)

 Rauisuchidae 

Tikisuchus

Rauisuchus

PostosuchusPostosuchus kirkpatricki flipped

Teratosaurus

 †Poposauroidea 

Yarasuchus

Qianosuchus

ArizonasaurusArizonasaurus BW white background

Bromsgroveia

LotosaurusLotosaurus BW white background

PoposaurusPoposaurus gracilis (1) flipped

Sillosuchus

 Shuvosauridae 

ShuvosaurusShuvosaurus BW flipped

EffigiaEffigia BW white background

References

  1. ^ a b c d Nesbitt, S.J. (2011). "The early evolution of archosaurs: relationships and the origin of major clades" (PDF). Bulletin of the American Museum of Natural History. 352: 1–292. doi:10.1206/352.1.
  2. ^ a b c Sereno, P.C.; Arcucci, A.B. (1990). "The monophyly of crurotarsal archosaurs and the origin of bird and crocodile ankle joints". Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen. 180: 21–52.
  3. ^ Sereno, Paul (1991). "Basal archosaurs: phylogenetic relationships and functional implications". Journal of Vertebrate Paleontology. 11 (Suppl. 4): 1–51. doi:10.1080/02724634.1991.10011426.
  4. ^ Gauthier, J. A.; Nesbitt, S. J.; Schachner, E. R.; Bever, G. S.; Joyce, W. G. (2011). "The bipedal stem-crocodilian Poposaurus gracilis: inferring function in fossils and innovation in archosaur locomotion". Bulletin of the Peabody Museum of Natural History. 52: 107–126. doi:10.3374/014.052.0102.
  5. ^ Gauthier, J.A.; Padian, K. (1985). "Phylogenetic, functional, and aerodynamic analyses of the origin of birds and their flight". In Hecht, M.K.; Ostrom, J.H.; Viohl, G.; Wellnhofer, P. (eds.). The Beginnings of Birds. Eichstatt: Freunde des Jura-Museums. pp. 185–197.
  6. ^ Benton, M.J.; Clark, J.M. (1988). "Archosaur phylogeny and the relationships of the Crocodylia". In Benton, M.J. (ed.). Phylogeny and Classification of the Tetrapods. 1. Oxford: Clarendon Press. pp. 295–338.
  7. ^ Brochu, C.A. (1997). "Synonymy, redundancy, and the name of the crocodile stem-group". Journal of Vertebrate Paleontology. 17 (2): 448–449. doi:10.1080/02724634.1997.10010992.
  8. ^ Nesbitt, SJ; Norell, MA. (2006). "Extreme convergence in the body plans of an early suchian (Archosauria) and ornithomimid dinosaurs (Theropoda)". Proceedings of the Royal Society of London B: Biological Sciences. 273 (1590): 1045–1048. doi:10.1098/rspb.2005.3426. PMC 1560254. PMID 16600879.
  9. ^ Nesbitt, S. (2007). "The anatomy of Effigia okeeffeae (Archosauria, Suchia), theropod-like convergence, and the distribution of related taxa" (PDF). Bulletin of the American Museum of Natural History. 302: 84. doi:10.1206/0003-0090(2007)302[1:taoeoa]2.0.co;2.
  10. ^ Brusatte, Stephen L.; Benton, Michael J.; Desojo, Julia B.; Langer, Max C. (2010). "The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida)". Journal of Systematic Palaeontology. 8 (1): 3–47. doi:10.1080/14772010903537732.

External links

Archosaur

Archosaurs are a group of diapsid amniotes whose living representatives consist of birds and crocodilians. This group also includes all extinct dinosaurs, extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most recent common ancestor of living birds and crocodilians and all of its descendants. It includes two main 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).

Archosauriformes

Archosauriformes (Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles that developed from archosauromorph ancestors some time in the Late Permian (roughly 250 million years ago). It was defined by Jacques Gauthier (1994) as the clade stemming from the last common ancestor of Proterosuchidae and Archosauria (the group that contains crocodiles, pterosaurs, dinosaurs, and birds); Phil Senter (2005) defined it as the most exclusive clade containing Proterosuchus and Archosauria. These reptiles, which include members of the family Proterosuchidae and more advanced forms, were originally superficially crocodile-like predatory semi-aquatic animals about 1.5 meters (5 ft) long, with a sprawling elbows-out stance and long snouts. Unlike the bulk of their therapsid contemporaries, the proterosuchids survived the catastrophe at the end of the Permian, perhaps because they were opportunistic scavengers or because they could retreat into water to find respite from an overheated climate. Any such scenarios are hypothetical; what is clearer is that these animals were highly successful in their new environment, and evolved quickly. Within a few million years at the opening of the Triassic, the proterosuchids had given rise to the Erythrosuchidae (the first sauropsids to totally dominate their environment), which in turn were the ancestors of the small agile Euparkeriidae, from which a number of successfully more advanced families – the archosaurs proper – evolved rapidly to fill empty ecological niches in the devastated global system. The Archosauria includes crocodylians, birds, and their extinct relatives. The archosaurs were the only members of the Archosauriformes which survived the late Triassic extinction.Pre-Euparkeria Archosauriformes have previously been included in the suborder Proterosuchia of the order Thecodontia. Under the cladistic methodology, Proterosuchia has been rejected as a paraphyletic assemblage, and the pre-archosaurian taxa are simply considered as basal Archosauriformes.

Archosaurus

Archosaurus is an extinct genus of carnivorous archosauriform reptile. From the latest Permian of Russia and Poland, it is one of the earliest known archosauriformes.

Arganasuchus

Arganasuchus is a genus of rauisuchid archosaur. Its fossils have been found in Upper Triassic rocks of the Argana Basin, Morocco. It is based on remains found in the 1970s and initially thought to belong to the rauisuchian Ticinosuchus. Bones recovered include part of the upper jaws, neck vertebrae, and hip bones. Arganasuchus was described in 2007 by Nour-Eddine Jalil and Karin Peyer. The type species is A. dutuiti. Arganasuchus had several anatomical details in common with the rauisuchids Batrachotomus, Fasolasuchus, and Postosuchus. It is thought Arganasuchus was a carnivore, due to dentary characteristics.

Ayllusuchus

Ayllusuchus is an extinct genus of sebecid mesoeucrocodylian. Fossils have been found in the Lumbrera Formation of Argentina (Eocene age, Casamayoran).

Baharijodon

Baharijodon is an extinct genus of trematochampsid crocodylomorph. Fossils have been found that date back to the Cenomanian stage of the Late Cretaceous.

Caririsuchus

Caririsuchus is an extinct genus of peirosaurid crocodylomorph. Fossils have been found from the Romualdo Formation of the Santana Group in the Araripe Basin in northeastern Brazil, dating back to the Albian stage of the Early Cretaceous. It was about 1.5 metres (4.9 ft) in length.

Crurotarsal

A crurotarsal joint is one that’s situated between the bones of crus, i.e. shin (tibia and fibula) and the proximal tarsal bones, i.e. astragalus and calcaneum.

The ankle joint of therian mammals (marsupials and placentals) is a crurotarsal joint, with the main joint of ankle bending between the tibia and the astragalus; the calcaneum has no contact with the tibia but forms a heel to which muscles can attach.

A group of archosauriform diapsids, Crurotarsi (including living crocodilians and their extinct relatives) is named after specialized crurotarsal joint in the skeletons of the members of this group, located between their fibula and calcaneum, with a hemicylindrical condyle on the calcaneum articulating against fibula. This joint is present in the skeletons of suchians (including crocodilians) and phytosaurs, and was cited as one of the characters supporting uniting these two groups in a clade to the exclusion of avemetatarsalian archosaurs (birds and their extinct relatives). However, according to a study published in 2011, suchians are more closely related to Avemetatarsalia than to phytosaurs; there is, however, not enough information to find out whether the aforementioned crurotarsal joint evolved independently in suchians and in phytosaurs, or whether it was already present in the skeleton of their most recent common ancestor (and secondarily lost in avemetatarsalians).The ankle joint of pseudosuchians (including crocodilians) and phytosaurs, passing between the astragalus and calcaneum, is also called crurotarsal joint in the literature. In the skeletons of the phytosaurs and most of the pseudosuchians this joint bends around a peg on the astragalus which fits into a socket in the calcaneum (the “crocodile normal” tarsus); only in the skeletons of the ornithosuchid pseudosuchians a peg on the calcaneum fits into a socket in the astragalus (the “crocodile reversed” tarsus). Strictly speaking this ankle is not a crurotarsal joint in the previously discussed sense, as it's situated between the two proximal tarsal bones. However, while calcaneum is not fixed to the fibula, the astragalus is fixed to the tibia by a suture and thus in practice it functions as an extension of the crus.

Desmatosuchinae

Desmatosuchinae is one of the two subfamilies of aetosaurs, the other being Aetosaurinae. It is a stem-based taxon defined as all aetosaurs more closely related to Desmatosuchus than the last common ancestor of Desmatosuchus and Stagonolepis. All synapomorphies that diagnose the clade can be found in the osteoderms. These include tongue-and-groove articulations for lateral plates present in dorsal presacral paramedian plates and large spikes on the lateral cervical, dorsal, and caudal plates.

Energosuchus

Energosuchus (meaning "active crocodile" in Greek) is an extinct genus of rauisuchian. Fossils are present from the upper Karyomayol and lower Synya Formations outcropping along the banks of the Bolshaya Synya River in the Timan-North Urals region in northern European Russia, as well as from the Bukobay Formation in the southern part of Bashkortostan in the southern Urals of European Russia. Both localities date back to the Ladinian stage of the Middle Triassic.

Gracilisuchus

Gracilisuchus (meaning "slender crocodile") is an extinct genus of tiny pseudosuchian (a group which includes the ancestors of crocodilians) from the Middle Triassic of Argentina. It contains a single species, G. stipanicicorum, which is placed in the clade Suchia, close to the ancestry of crocodylomorphs.

List of crurotarsan genera

This list of crurotarsans is a comprehensive listing of all genera that have ever been included in the clade Crurotarsi, excluding purely vernacular terms. Under some definitions Crurotarsi includes all archosaurs, but this list excludes archosaur genera that are included in Avemetatarsalia (pterosaurs, nonavian dinosaurs, and birds). The list includes all commonly accepted genera, but also genera that are now considered invalid, doubtful (nomen dubium), or were not formally published (nomen nudum), as well as junior synonyms of more established names, and genera that are no longer considered crurotarsan. Extinct taxa are denoted with a dagger (†). The list contains 570 names, of which approximately 480 are considered either valid crurotarsan genera or nomina dubia.

Miadanasuchus

Miadanasuchus is an extinct genus of trematochampsid which existed in the Maevarano Formation of Madagascar during the late Cretaceous period (Campanian age). It was first named by Erin L. Rasmusson Simons and Gregory A. Buckley in 2009 and the type species is Miadanasuchus oblita.

Mystriosuchini

Mystriosuchini is an extinct tribe of derived phytosaurs in the clade Leptosuchomorpha. As with all other phytosaurs, mystriosuchins lived during Late Triassic. The name is derived from the genus Mystriosuchus.

Genera classified in Mystriosuchini include Coburgosuchus, Machaeroprosopus, Mystriosuchus, Nicrosaurus and Redondasaurus. It includes the most ecologically divergent phytosaurs, the terrestrial Nicrosaurus and the fully aquatic Mystriosuchus.

Nundasuchus

Nundasuchus is an extinct genus of crurotarsan, possibly a suchian archosaur related to Paracrocodylomorpha. Remains of this genus are known from the Middle Triassic Manda beds of southwestern Tanzania. It contains a single species, Nundasuchus songeaensis, known from a single partially complete skeleton, including vertebrae, limb elements, osteoderms, and skull fragments.Nundasuchus lived in what is now Tanzania, Africa around 240 million years ago. Members of this genus were likely carnivores, around 2.7 to 3 meters (9 feet) long, with ziphodont (steak knife-like) teeth and rows of bony plates (osteoderms) along their back. Phylogenetic analyses consistently place this genus within the group Crurotarsi based on features of the ankle. Most studies also consider it a pseudosuchian, meaning that it was more closely related to modern crocodilians than it was to dinosaurs. However, Nundasuchus had an upright stance, with legs situated directly underneath the body, as with various other early pseudosuchians (such as "rauisuchians" and aetosaurs) but unlike modern crocodilians.

The classification of Nundasuchus relative to other pseudosuchians is somewhat controversial. Some phylogenetic analyses place it near or at the base of the group, sometimes along with phytosaurs, based on certain plesiomorphic (primitive) features such as teeth on the palate, a short pubis, and characteristics of the calcaneum (heel bone). Another hypothesis, supported by its original 2014 description, considers it to be somewhat more "advanced" than those groups, instead being closer to Ticinosuchus and paracrocodylomorphs (the group containing "rauisuchians" and the ancestors of modern crocodilians). This classification scheme is justified by the presence of "staggered" osteoderms, heart-shaped "spine tables", and a groove on the femoral head. Regardless of these hypotheses, it is clear that Nundasuchus represents a previously unknown group of reptiles with a mixture of features both plesiomorphic and derived with respect to suchian archosaurs.

Omosaurus

Omosaurus is an extinct genus of the Crurotarsi, perhaps from the late Triassic (Carnian).

In the middle of the nineteenth century, geologist Professor Ebenezer Emmons discovered several reptilian teeth in the colliery of the Chatham company in North Carolina. In 1856, the fossils in his collection were described by paleontologist Joseph Leidy. Leidy combined the teeth with some vertebrae and ribs; adding to them a osteoderm or scute found in the same strata by Professor Michael Tuomey, he named the whole Omosaurus perplexus. Leidy provided no etymology; the specific name suggests he was intrigued by the "intricate" find. The generic name might be derived from the Greek ὠμός, omos, "rough", perhaps in reference to the rough surface of the scute or to the "savage" nature of a carnivorous reptile. Today, all syntypes are lost.

The teeth were described as being rather straight, slightly curved inwards, conical and pointed with a length of up to one inch. They had two edges at the inside and a D-shaped cross-section with the convex part positioned at the outer side. The surface of the teeth was smooth with little wrinkles, running vertically at the inside and horizontally at the outside. The vertebrae were amphicoelous and constricted at the waist, with a length of about three centimetres and somewhat taller than wide in cross-section. The scute was ornamented with a fan-shaped pattern of splitting ridges.Leidy himself believed Omosaurus to be a marine reptile, probably a plesiosaur, suspecting the remains were referable to some already named genus. In 1902, Frederick Augustus Lucas recognised the fossils as "crocodilian" in nature and placed Omosaurus in the Crocodilia. At the same time he affirmed the priority over Omosaurus Owen 1875, which stegosaur was by him renamed to Dacentrurus. Today, Omosaurus is commonly listed as a nomen dubium, a possible member of the Phytosauria.

Phytosaur

Phytosaurs are an extinct group of large, mostly semiaquatic Late Triassic archosauriform reptiles. Phytosaurs belong to the family Phytosauridae and the order Phytosauria. Phytosauria and Phytosauridae are often considered to be equivalent groupings containing the same species, but some studies have identified non-phytosaurid phytosaurians. Phytosaurs were long-snouted and heavily armoured, bearing a remarkable resemblance to modern crocodilians in size, appearance, and lifestyle, as an example of convergence or parallel evolution. The name "phytosaur" means "plant reptile", as the first fossils of phytosaurs were mistakenly thought to belong to plant eaters. The name is misleading because the sharp teeth in phytosaur jaws clearly show that they were predators.

For many years, phytosaurs were considered to be the most basal group of Pseudosuchia (crocodile-line archosaurs), meaning that they were thought to be more closely related to the crocodilians than to birds (the other living group of archosaurs). Some recent studies of the evolutionary relationships of early archosauriforms suggest that phytosaurs evolved before the split between crocodile- and bird-line archosaurs and are the sister taxon of Archosauria. Others retain the older classification of phytosaurs as pseudosuchians.

Phytosaurs had a nearly global distribution during the Triassic. Fossils have been recovered from Europe, North America, India, Morocco, Thailand, Brazil, Greenland and Madagascar. Fossils attributed to phytosaurs have been found in Early Jurassic rocks, possibly extending their temporal range beyond the Triassic-Jurassic boundary.

Phytosaurus

Phytosaurus is a dubious genus of phytosaur (an extinct group of superficially crocodile-like archosaurs), and also the first phytosaur to be described, by G. Jaeger in 1828. The name Phytosaurus means "plant lizard" (though it is now known to have been a carnivore), and the type species is P. cylindricodon.

Pseudosuchia

Pseudosuchia is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely related to crocodilians than to birds. Pseudosuchians are also informally known as "crocodilian-line archosaurs". Prior to 2011, the clade Pseudosuchia was often called Crurotarsi in reference to the crurotarsal ankle found in almost all members of the group, which traditionally included phytosaurs, ornithosuchids, and suchians. However, a major 2011 study of Triassic archosaur relations proposed that phytosaurs were not closely related to other traditional "crurotarsans", at least compared to "bird-line archosaurs" (Avemetatarsalians) such as pterosaurs and dinosaurs. As a result, the possession of a crurotarsal ankle was considered a plesiomorphic ("primitive") feature retained by pseudosuchians. Crurotarsi now refers to a broader group of reptiles including Pseudosuchia, Phytosauria, and Avemetatarsalia. Despite Pseudosuchia meaning "false crocodiles", the name is a misnomer as true crocodilians are a subset of the group.

Contrary to popular belief, crocodilians differ significantly from their ancestors and distant relatives, as Pseudosuchia contains a staggering diversity of reptiles with many different lifestyles. Early pseudosuchians were successful in the Triassic period. They included giant, quadrupedal apex predators such as Saurosuchus, Prestosuchus, and Luperosuchus. Ornithosuchids were large scavengers, while erpetosuchids and gracilisuchids were small, light-footed predators. A few groups acquired herbivorous diets, such as the heavily armored aetosaurs, and several were bipedal, such as Poposaurus and Postosuchus. The bizarre, ornithomimid-like shuvosaurids were both bipedal and herbivorous, with toothless beaks.Many of these Triassic pseudosuchian groups went extinct at or before the Triassic-Jurassic extinction event. However, one group, the crocodylomorphs, survived the major extinction. Crocodylomorphs themselves evolved a diverse array of lifestyles during the Jurassic and Cretaceous periods, although only a single subset of crocodylomorphs survived to the present day. These sole remaining pseudosuchians are the crocodilians: crocodiles, alligators, caimans, and gharials.

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