"Rauisuchia" is a group of mostly large) Triassic archosaurs. It belongs to a larger clade called
Pseudosuchia. "Rauisuchia" is currently considered an evolutionary grade, or even a wastebin
taxon. It includes most pseudosuchians that lived during the Triassic Period save for
Crocodylomorpha. Since crocodylomorphs likely originated from an ancestor that would have
been a rauisuchian, Rauisuchia in its traditional sense is considered paraphyletic as it does
not include crocodylomorphs. To designate it as an informal group in scientific literature,
the name in its traditional sense is often enclosed in quotation marks.
Rauisuchians had an erect gait with legs oriented beneath the body rather than sprawling outward.
This type of gait is also seen in dinosaurs but evolved independently in the two groups.
In dinosaurs the hip socket faces outward and the femur connects to the side of the
hip, while in rauisuchians the hip socket faces downward to form a shelf of bone under
which the femur connects. This has been referred to as the pillar-erect posture.
Rauisuchians lived throughout most of the Triassic. Along with many other large archosaurs,
the group died out in the Triassic-Jurassic extinction event. After their extinction,
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.
Fossil record Well-known rauisuchians include Ticinosuchus
of the Middle Triassic of Europe, Saurosuchus of the late Triassic of South America, and
Postosuchus of the late Triassic of North America. One rauisuchian, Teratosaurus, was
for a long time even considered an early theropod dinosaur, but was later shown to be non-dinosaurian.
History of classification Rauisuchians were originally thought to be
related to erythrosuchids, 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 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. 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,
paleontologist Sterling Nesbitt defined a clade of rauisuchians called "Group X." This
group includes Arizonasuchus, Lotosaurus, and Sillosuchus, Shuvosaurus, and Effigia.
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 bones, 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 dinosaurs. 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: In their phylogenetic study of archosaurs,
Brusatte et al. 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.:
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.
Notes
References
Carroll, R. L.. Vertebrate Paleontology and Evolution. New York: WH Freeman & Co. ISBN 0-7167-1822-7.
"Archosauromorpha: Rauisuchiformes". Palaeos. Retrieved January 2010.
Poling, Jeff; Creisler, Ben. "Rauisuchia Translation and Pronunciation Guide". Dinosauria On-Line.
Retrieved January 2010.