''Riojasuchus'' is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch ...

(

Norian The Norian is a division of the Triassic Period. It has the rank of an age (geochronology) or stage (chronostratigraphy). It lasted from ~227 to million years ago. It was preceded by the Carnian and succeeded by the Rhaetian. Stratigraphic defi ...

) quadrupedal

archosaur Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avian d ...

. ''Riojasuchus'' is a member of Ornithosuchidae, a

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

of facultatively

bipedal Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' 'double' ...

carnivores that were geographically widespread during the Late Triassic. Two other genera, ''

Ornithosuchus ''Ornithosuchus'' (meaning "bird crocodile") is an extinct genus of pseudosuchians from the Late Triassic (Carnian) Lossiemouth Sandstone of Scotland. It was originally thought to be the ancestor to the carnosaurian dinosaurs (such as ''Allosau ...

'' and ''

Venaticosuchus ''Venaticosuchus'' is a genus of pseudosuchian archosaurs from the family Ornithosuchidae. Known from a single species, ''Venaticosuchus rusconii,'' this genus is described based on an incomplete skull and jaw (as well as a lost partial forelimb ...

'', are currently known. The

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

specimen is PVL 3827. It was found in the

Los Colorados Formation The Los Colorados Formation is a sedimentary rock formation of the Ischigualasto-Villa Unión Basin, found in the provinces of San Juan and La Rioja in Argentina. The formation dates back to the Norian age of the Late Triassic. The up to thick f ...

of the

Ischigualasto-Villa Unión Basin The Ischigualasto-Villa Unión Basin ( es, Cuenca de Ischigualasto-Villa Unión) is a small sedimentary basin located in the Argentine Northwest, Argentina. It is located in the southwestern part of La Rioja Province and the northeastern part of ...

in northwestern Argentina.

Discovery and naming

The holotype specimen is PVL 3827, consisting of a mostly complete crania and a partial postcrania. A majority of the postcranial material was preserved. The skeleton was found with others in the upper section of the

. Other than the holotype, it is known from three skeletons.

Description

The skull of the type specimen is long, and has a large, curved snout and short mandibles.

Distinguishing characteristics

Many characteristics were identified by Bonaparte in 1969. They are listed below: *a pre

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...

al vacuity well bordered by protruding edges of the smaller preorbital opening, found in ''

''; *an outgoing lateral lacrimal edge; *an infratemporal fenestra present in the shortest adult skull with mandibular fenestra; *the top of the surangular laterally pointed, and with a small prearticular process; *short atlas and cervical vertebrae, all with a ventral keel; *an ilium, pubis and femur, with the talus and calcaneus of the type of ''Ornithosuchus'' *and median orbits with a higher bottom than in ''Ornithosuchus''.

Classification

''Riojasuchus'' is a member of Ornithosuchidae, a

of facultatively

carnivores that were geographically widespread during the Late Triassic. Two other genera, ''

'' and ''

'', are currently known. The group was originally considered to be related to dinosaurs, before many phylogenetical analysises. Below is a phylogenetic cladogram by Butler ''et al.'' in 2011 showing the cladistics of

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 Latest Permian (roughly 252 million years ago). It was defined by Jacques Gauthi ...

, focusing mostly on

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 ...

: Clade names follow Nesbitt 2011.

Paleoecology

Geological map of Los Colorados Formation La Rioja

Fossils of ''Riojasuchas'' are found in the Los Colorados Formation, a Late Triassic unit in

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...

, and date to approximately 217 to 215 million years ago. At that age, it is the youngest ornithosuchid known. It lived alongside the

protosuchid Protosuchidae was a family of crocodylomorph reptiles from the Late Triassic, Jurassic, and Cretaceous time periods. They were closely related to the Gobiosuchidae. Distribution Triassic protosuchids are known from Lesotho, Argentina, and Arizon ...

Hemiprotosuchus ''Hemiprotosuchus'' is an extinct genus of protosuchid from the Late Triassic (Norian stage) Los Colorados Formation of the Ischigualasto-Villa Unión Basin in northwestern Argentina, South America. It was named by Argentinian paleontologist Jos� ...

''; the sphenosuchid '' Pseudohesperosuchus''; the

stagonolepidid Aetosaurs () are heavily armored reptiles belonging to the extinct order Aetosauria (; from Greek, (aetos, "eagle") and (, "lizard")). They were medium- to large-sized omnivorous or herbivorous pseudosuchians, part of the branch of archosaurs ...

Neoaetosauroides ''Neoaetosauroides'' is an extinct genus of primitive aetosaur. Its type and only species is ''N. engaeus''. Fossils have been found in Los Colorados Formation outcropping along the Sierra Morada River in the Ischigualasto-Villa Unión Basin in ...

''; the melanorosaurid ''

Strenusaurus ''Riojasaurus'' (meaning "Rioja lizard") was a herbivorous sauropodomorph dinosaur named after La Rioja Province in Argentina where it was found in the Los Colorados Formation in the Ischigualasto-Villa Unión Basin by José Bonaparte. It lived ...

''; and the riojasaurid '' Riojasaurus''.

References

{{Portal bar, Paleontology, Reptiles, Argentina Ornithosuchids Triassic archosaurs Norian life Late Triassic reptiles of South America Triassic Argentina Fossils of Argentina Los Colorados Formation Fossil taxa described in 1969 Taxa named by José Bonaparte Prehistoric pseudosuchian genera