Rauisuchia
Encyclopedia
Rauisuchia is a group of predatory and mostly large (often 4 to 6 meters) Triassic
archosaur
s. As a clade
, Rauisuchia includes these Triassic forms and all crocodylomorphs, which are descendants of Triassic rauisuchians. The group in its traditional sense is paraphyletic, because it does not include crocodylomorph descendants. To designate it as an informal group in scientific literature, the name in its traditional sense is often enclosed in quotation marks.
Both José Bonaparte
and Michael Benton
argue that rauisuchians such as Saurosuchus
developed an erect stance independently of, and different from that of dinosaur
s, by means of having the femur
vertical and angling the acetabulum
ventrally, rather than having an angled neck or curve in the femur. This has been referred to as the pillar-erect posture.
The erect gait indicates that these animals were clearly active, agile predators, with locomotor superiority over the kannemeyeriid dicynodonts
and abundant rhynchosaur
s on which they fed. They were successful animals, the largest with skulls up to a meter or more in length, and continued right until the end of the Triassic, when, along with many other large archosaurs, they were killed off by the end Triassic extinction event
. With their demise, theropod dinosaurs were able to emerge as the sole large terrestrial predators. Meat-eating dinosaur footprints suddenly increase in size at the start of the Jurassic
, when rauisuchians are absent.
, but it is now known that they are crurotarsans
. Three families are generally recognised: Prestosuchidae
, Rauisuchidae
, and Poposauridae
, as well as a number of forms (e.g. those from the Olenekian
of Russia) that are too primitive and/or poorly known to fit in any of these groups. There has been considerable suggestion that the group as currently defined is paraphyletic, representing a number of related lineages independently evolving and filling the same ecological niche of medium to top terrestrial predator. For example, Parrish and Juul found poposaurid rauisuchians to be more closely related to Crocodilia
than to prestosuchids. In a more recent study, Nesbitt presented a different phylogeny with a monophyletic Rauisuchia. The group may even be something of a "wastebasket taxon". Determining exact phylogenetic relationships is difficult because of the scrappy nature of a lot of the material. However, recent discoveries and studies such as those of Batrachotomus
and restudies of other forms such as Erpetosuchus are shedding light on the evolutionary relationships of this poorly known group.
In a study of the ctenosauriscid Arizonasaurus
, paleontologist Sterling Nesbitt
defined a clade
of rauisuchians called "Group X." This group includes Arizonasuchus, Lotosaurus (another ctenosauriscid), and Sillosuchus
, Shuvosaurus
, and Effigia (all poposaurids). One distinguishing feature of Group X is their lack of osteoderms, which are common among many other crurotarsans. Many more features are found in the pelvis
, including fully fused sacral vertebrae and a long, thin crest on the illium called the supra-acetabular crest. Additionally, many members of Group X have smooth frontal
and nasal bone
s, which make up the upper portion of the rostrum
. In other rauisuchians and many other crurotarsans, this area has bumps and ridges.
Nesbitt later erected another clade, "Group Y", in 2007. Group Y falls within Group X to include Sillosuchus, Shuvosaurus, and Effigia. Group Y is diagnosed by the presence of four or more sacral vertebrae with fully fused neural arches, which is also seen in theropod dinosaur
s (a case of evolutionary convergence). In addition, the cervical vertebrae that make up the neck are strongly amphicoelus, meaning that they are concave at both ends. The fourth trochanter
, a ridge of bone on the femur
for muscle attachment seen in nearly all archosaurs, is absent in Group Y.
Although not placed within Group Y, Lotosaurus shares many similarities with members of the clade, foremost of which is edentulous
, or toothless, jaws. Edentulism is also seen in Shuvosaurus and Effigia, which have beak-like jaws. Nesbitt suggested that the derived characters of Lotosaurus may indicate that it is a transitional form between basal members of Group X and members of Group Y.
Below is the cladogram
from Nesbitt (2007):
In their phylogenetic study of archosaurs, Brusatte et al. (2010) found only weak support for Rauisuchia as a monophyletic grouping. As a result of their analysis, two clades were found to be within Rauisuchia, which they named Rauisuchoidea and Poposauroidea. Rauisuchoidea included Rauisuchidae and Prestosuchidae, as well as several basal taxa that were once assigned to the families, including Fasolasuchus
and Ticinosuchus
. Poposauroidea included poposaurids and ctenosauriscids, but the phylogeny had a large polytomy
of genera in both groups that was difficult to resolve, which included Arizonasaurus, Poposaurus
, and Sillosuchus. However, the characters linking these two groups were weak, and the question of whether or not "Rauisuchia" forms a natural group remained unresolved.
Below is the cladogram from Brusatte et al. (2010):
In a more thorough test of archosaurian relationships published in 2011 by Sterling Nesbitt, "rauisuchians" were found to be paraphyletic, with Poposauroidea
at the base of the clade Paracrocodylomorpha
, and the rest of the "rauisuchians" forming a grade within the clade Loricata
. Nesbitt noted that no previous study of "rauisuchian" relationships had ever included a wide variety of supposed "rauisuchians" as well as a large number of non-"rauisuchian" taxa as controls.
of the Middle Triassic of Europe (Switzerland and Northern Italy), Saurosuchus
of the late Triassic (Late Carnian
) of South America (Argentina), and Postosuchus
of the late Triassic (Late Carnian
to Early Norian
) of North America (SW USA). One rauisuchian, Teratosaurus
, was for a long time even considered an early theropod dinosaur, but was later shown to be non-dinosaurian.
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
archosaur
Archosaur
Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...
s. As a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
, Rauisuchia includes these Triassic forms and all crocodylomorphs, which are descendants of Triassic rauisuchians. The group in its traditional sense is paraphyletic, because it does not include crocodylomorph descendants. To designate it as an informal group in scientific literature, the name in its traditional sense is often enclosed in quotation marks.
José Bonaparte
José Fernando Bonaparte, Ph.D. , is an Argentine paleontologist who discovered a plethora of South American dinosaurs and mentored a new generation of Argentine paleontologists like Rodolfo Coria...
and Michael Benton
Michael J. Benton
Michael J. Benton is a British paleontologist, a Fellow of the Royal Society of Edinburgh, and professor of vertebrate palaeontology in the Department of Earth Sciences at the University of Bristol....
argue that rauisuchians such as Saurosuchus
Saurosuchus
Saurosuchus is an extinct genus of rauisuchian archosaur in the family Prestosuchidae. With a length of around 7 m , it was the largest rauisuchian, except perhaps for the less well known Fasolasuchus. Like other rauisuchians, Saurosuchus walked on four fully erect limbs...
developed an erect stance independently of, and different from that of dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s, by means of having the femur
Femur
The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...
vertical and angling the acetabulum
Acetabulum
The acetabulum is a concave surface of the pelvis. The head of the femur meets with the pelvis at the acetabulum, forming the hip joint.-Structure:...
ventrally, rather than having an angled neck or curve in the femur. This has been referred to as the pillar-erect posture.
The erect gait indicates that these animals were clearly active, agile predators, with locomotor superiority over the kannemeyeriid dicynodonts
Kannemeyeriidae
Kannemeyeriidae is a family of large, stocky, beaked and sometimes tusked dicynodonts. They were the dominant large terrestrial herbivores through most of the Triassic period...
and abundant rhynchosaur
Rhynchosaur
Rhynchosaurs were a group of Triassic diapsid reptiles related to the archosaurs.-Description:Rhynchosaurs were herbivores, and at times abundant , with stocky bodies and a powerful beak...
s on which they fed. They were successful animals, the largest with skulls up to a meter or more in length, and continued right until the end of the Triassic, when, along with many other large archosaurs, they were killed off by the end Triassic extinction event
Triassic-Jurassic extinction event
The Triassic–Jurassic extinction event marks the boundary between the Triassic and Jurassic periods, , and is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. In the seas a whole class and twenty percent of all marine families...
. With their demise, theropod dinosaurs were able to emerge as the sole large terrestrial predators. Meat-eating dinosaur footprints suddenly increase in size at the start of the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
, when rauisuchians are absent.
History of classification
Originally it was believed that "rauisuchians" were related to erythrosuchidsErythrosuchidae
Erythrosuchidae are a family of large basal archosauromorph carnivores that lived from the later Early Triassic to the early Middle Triassic . Their fossil remains are known so far from South Africa , the Perm region of Russia, and China...
, but it is now known that they are crurotarsans
Crurotarsi
The Crurotarsi are a group of archosauriformes, represented today by the crocodiles,...
. Three families are generally recognised: Prestosuchidae
Prestosuchidae
Prestosuchids were a group of Triassic carnivorous archosaurs. They were large active terrestrial apex predators, ranging from around in length. They succeeded the Erythrosuchidae as the largest archosaurs of their time...
, Rauisuchidae
Rauisuchidae
Rauisuchidae is a group of large predatory Triassic archosaurs, and constitute advanced representatives of the larger group Rauisuchia. There is some disagreement over which genera should be included in the Prestosuchidae, which in Rauisuchidae, and which in the Poposauridae, and indeed whether...
, and Poposauridae
Poposauridae
The Poposauridae are a family of large carnivorous archosaurs which lived alongside dinosaurs during the Late Triassic. They are known from fossil remains from North and South America...
, as well as a number of forms (e.g. those from the Olenekian
Olenekian
In the geologic timescale, the Olenekian is an age in the Early Triassic epoch or a stage in the Lower Triassic series. It spans the time between 249.7 ± 0.7 Ma and 245 ± 0.7 Ma . The Olenekian follows the Induan and is followed by the Anisian.The Olenekian saw the deposition of a large part of the...
of Russia) that are too primitive and/or poorly known to fit in any of these groups. There has been considerable suggestion that the group as currently defined is paraphyletic, representing a number of related lineages independently evolving and filling the same ecological niche of medium to top terrestrial predator. For example, Parrish and Juul found poposaurid rauisuchians to be more closely related to Crocodilia
Crocodilia
Crocodilia is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period . They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria...
than to prestosuchids. In a more recent study, Nesbitt presented a different phylogeny with a monophyletic Rauisuchia. The group may even be something of a "wastebasket taxon". Determining exact phylogenetic relationships is difficult because of the scrappy nature of a lot of the material. However, recent discoveries and studies such as those of Batrachotomus
Batrachotomus
Batrachotomus is a genus of prehistoric archosaur. Fossils of this animal have been found in southern Germany and dated from the Ladinian stage of Middle Triassic period, around 228 to 231 million years ago. Batrachotomus was described by palaeontologist David J...
and restudies of other forms such as Erpetosuchus are shedding light on the evolutionary relationships of this poorly known group.
Cladistics
Despite its inclusion as an informal grouping in numerous phylogenetic studies, "Rauisuchia" has never received a formal definition. Most analyses in the past decade have found "Rauisuchia" to be a paraphyletic grouping, including all studies with a large sample size. Those that found the possibility that it was a natural group produced only weak support for this hypothesis. In his large 2011 analysis of archosarian relationships, Nesbitt recommended that the term "Rauisuchia" be abandoned.In a study of the ctenosauriscid Arizonasaurus
Arizonasaurus
Arizonasaurus was a ctenosauriscid archosaur from the Middle Triassic . 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 elongate neural spines of the...
, paleontologist Sterling Nesbitt
Sterling Nesbitt
Sterling Nesbitt , is an American paleontologist.-Biography:Sterling Nesbitt received his B.A. in integrative biology with a minor in geology from University of California Berkeley in 2004. He received his Ph.D from Columbia University in 2009, completing the majority of his research at the...
defined a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
of rauisuchians called "Group X." This group includes Arizonasuchus, Lotosaurus (another ctenosauriscid), and Sillosuchus
Sillosuchus
Sillosuchus is a genus of poposaurid crurotarsans from the Late Triassic of Argentina.The generic name refers to Dr. William Sill, and "suchus", from Greek meaning crocodile. A reconstruction of the fossil was presented in 2008 by the National University of San Juan in Argentina, in an homage to Dr...
, Shuvosaurus
Shuvosaurus
Shuvosaurus is a genus of beaked reptile from the Late Triassic of Texas. It was described by Sankar Chatterjee in 1993 after it was discovered by his son Shuvo. It was initially interpreted as a Triassic member of the Cretaceous dinosaur family Ornithomimidae...
, and Effigia (all poposaurids). One distinguishing feature of Group X is their lack of osteoderms, which are common among many other crurotarsans. Many more features are found in the pelvis
Pelvis
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...
, including fully fused sacral vertebrae and a long, thin crest on the illium called the supra-acetabular crest. Additionally, many members of Group X have smooth frontal
Frontal bone
The frontal bone is a bone in the human skull that resembles a cockleshell in form, and consists of two portions:* a vertical portion, the squama frontalis, corresponding with the region of the forehead....
and nasal bone
Nasal bone
The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face, and form, by their junction, "the bridge" of the nose.Each has two surfaces and four borders....
s, which make up the upper portion of the rostrum
Rostrum (anatomy)
The term rostrum is used for a number of unrelated structures in different groups of animals:*In crustaceans, the rostrum is the forward extension of the carapace in front of the eyes....
. In other rauisuchians and many other crurotarsans, this area has bumps and ridges.
Nesbitt later erected another clade, "Group Y", in 2007. Group Y falls within Group X to include Sillosuchus, Shuvosaurus, and Effigia. Group Y is diagnosed by the presence of four or more sacral vertebrae with fully fused neural arches, which is also seen in theropod dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s (a case of evolutionary convergence). In addition, the cervical vertebrae that make up the neck are strongly amphicoelus, meaning that they are concave at both ends. The fourth trochanter
Fourth trochanter
The fourth trochanter is a shared characteristic common to archosaurs. It is a knob-like feature on the medial side of the femur that serves as a muscle attachment....
, a ridge of bone on the femur
Femur
The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...
for muscle attachment seen in nearly all archosaurs, is absent in Group Y.
Although not placed within Group Y, Lotosaurus shares many similarities with members of the clade, foremost of which is edentulous
Edentulism
Edentulism is the condition of being toothless to at least some degree; it is the result of tooth loss. Loss of some teeth results in partial edentulism, while loss of all teeth results in complete edentulism....
, or toothless, jaws. Edentulism is also seen in Shuvosaurus and Effigia, which have beak-like jaws. Nesbitt suggested that the derived characters of Lotosaurus may indicate that it is a transitional form between basal members of Group X and members of Group Y.
Below is the cladogram
Cladogram
A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...
from Nesbitt (2007):
In their phylogenetic study of archosaurs, Brusatte et al. (2010) found only weak support for Rauisuchia as a monophyletic grouping. As a result of their analysis, two clades were found to be within Rauisuchia, which they named Rauisuchoidea and Poposauroidea. Rauisuchoidea included Rauisuchidae and Prestosuchidae, as well as several basal taxa that were once assigned to the families, including Fasolasuchus
Fasolasuchus
Fasolasuchus is an extinct genus of rauisuchid rauisuchian. Fossils have been found from Argentina that date back to the Rhaetian stage of the Late Triassic, making it one of the last rauisuchians to have existed before the order went extinct at the end of the Triassic...
and Ticinosuchus
Ticinosuchus
Ticinosuchus is an extinct genus of rauisuchian archosaur from the Middle Triassic of Switzerland and Italy.Ticinosuchus was about long, and its whole body, even the belly, was covered in thick, armoured scutes. The structure of the hips shows that its legs were placed under the body almost...
. Poposauroidea included poposaurids and ctenosauriscids, but the phylogeny had a large polytomy
Polytomy
A polytomy , meaning many temporal based branches, is a section of a phylogeny in which the evolutionary relationships can not be fully resolved to dichotomies. In a phylogenetic tree, a polytomy is represented as a node which has more than two immediate descending branches...
of genera in both groups that was difficult to resolve, which included Arizonasaurus, Poposaurus
Poposaurus
Poposaurus is a genus of reptile from the Late Triassic of the southwestern United States. It was a crurotarsan archosaur which lived in what is now Wyoming, Utah, Arizona and Texas. The type species, Poposaurus gracilis, was described by M. G. Mehl in 1915. The second species, P...
, and Sillosuchus. However, the characters linking these two groups were weak, and the question of whether or not "Rauisuchia" forms a natural group remained unresolved.
Below is the cladogram from Brusatte et al. (2010):
In a more thorough test of archosaurian relationships published in 2011 by Sterling Nesbitt, "rauisuchians" were found to be paraphyletic, with Poposauroidea
Poposauroidea
Poposauroidea is a clade of rauisuchians. It includes poposaurids, shuvosaurids, and ctenosauriscids, but excludes the large predatory quadrupedal rauisuchians such as rauisuchids and prestosuchids. Although it was first formally defined in 2007, the term has been used for many years...
at the base of the clade Paracrocodylomorpha
Paracrocodylomorpha
Paracrocodylomorpha is a clade of crurotarsan archosaurs that includes crocodylomorphs and several closely related Triassic forms. Erected in 1993, the clade usually contains early crurotarsan taxa like Poposauridae and Gracilisuchus. These groups are considered sister taxa of the clade Bathyotica,...
, and the rest of the "rauisuchians" forming a grade within the clade Loricata
Loricata
Loricata is a group of reptiles that includes crocodylians and their ancestors. It was named by German naturalist Blasius Merrem in his 1820 Versuch eines Systems der Amphibien. Merrem considered it to be one of three groups of Pholidota , the other two being Testudinata and Squamata...
. Nesbitt noted that no previous study of "rauisuchian" relationships had ever included a wide variety of supposed "rauisuchians" as well as a large number of non-"rauisuchian" taxa as controls.
Fossil record
Well-known "rauisuchians" include TicinosuchusTicinosuchus
Ticinosuchus is an extinct genus of rauisuchian archosaur from the Middle Triassic of Switzerland and Italy.Ticinosuchus was about long, and its whole body, even the belly, was covered in thick, armoured scutes. The structure of the hips shows that its legs were placed under the body almost...
of the Middle Triassic of Europe (Switzerland and Northern Italy), Saurosuchus
Saurosuchus
Saurosuchus is an extinct genus of rauisuchian archosaur in the family Prestosuchidae. With a length of around 7 m , it was the largest rauisuchian, except perhaps for the less well known Fasolasuchus. Like other rauisuchians, Saurosuchus walked on four fully erect limbs...
of the late Triassic (Late Carnian
Carnian
The Carnian is the lowermost stage of the Upper Triassic series . It lasted from about 228.7 till 216.5 million years ago . The Carnian is preceded by the Ladinian and is followed by the Norian...
) of South America (Argentina), and Postosuchus
Postosuchus
Postosuchus, meaning "crocodile from Post ", was a basal archosaur that lived in what is now North America during the middle through to the late Triassic period...
of the late Triassic (Late Carnian
Carnian
The Carnian is the lowermost stage of the Upper Triassic series . It lasted from about 228.7 till 216.5 million years ago . The Carnian is preceded by the Ladinian and is followed by the Norian...
to Early Norian
Norian
The Norian is a division of the Triassic geological period. It has the rank of an age or stage . The Norian lasted from 216.5 ± 2.0 to 203.6 ± 1.5 million years ago. It was preceded by the Carnian and succeeded by the Rhaetian.-Stratigraphic definitions:The Norian was named after the Noric Alps in...
) of North America (SW USA). One rauisuchian, Teratosaurus
Teratosaurus
Teratosaurus was a genus of rauisuchian known from the Triassic Stubensandstein of Germany...
, was for a long time even considered an early theropod dinosaur, but was later shown to be non-dinosaurian.