Riojasuchus

The holotype specimen is PVL 3827, consisting of a complete skull and mandibles, as well as a nearly complete postcranial skeleton.[4] Three other specimens are known. PVL 3828 is almost as complete as the holotype, including a nearly complete skull. PVL 3826 consists of articulated vertebrae and limb fragments. PVL 3814 only involves a few fragments of vertebrae, limb bones, and osteoderms.[5]

All four specimens were found close together in the upper section of the Los Colorados Formation.[3] The formation's diverse fossil beds have been radiometrically dated to the mid-Norian stage of the Late Triassic.[6] The fossils are stored in the vertebrate paleontology collection (PVL) at Instituto Miguel Lillo in San Miguel de Tucumán.[4][5]

Riojasuchus and its fossils were initially described in papers by José Bonaparte in 1967[3] and 1972. The skull was redescribed by M. Belén von Baczko, Julia B. Desojo, and Denis Ponce in 2016.[4] The postcranial skeleton was redescribed by von Baczko and Desojo in 2019.[5]

Restoration

The skull of the type specimen is 25 centimetres (9.8 in) long, and has a large, curved snout and short mandibles.

Distinguishing characteristics

Life reconstruction of Riojasuchus tenuisceps with speculative open nostril

Many characteristics were identified by Bonaparte in 1969. They are listed below:[3]

• a preorbital vacuity well bordered by protruding edges of the smaller preorbital opening, found in Ornithosuchus;
• 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.

Skull of referred specimen PVL 3828

Riojasuchus is a member of Ornithosuchidae, a family of facultatively bipedal carnivores that were geographically widespread during the Late Triassic.[3] Two other genera, Ornithosuchus and Venaticosuchus, are currently known. The group was originally considered to be related to dinosaurs, before many phylogenetical analysises.[1]

Below is a phylogenetic cladogram by Butler et al. in 2011 showing the cladistics of Archosauriformes, focusing mostly on Pseudosuchia:[7] Clade names follow Nesbitt 2011.[8]

Geological map of the Los Colorados Formation, Argentina, with star indicating the location where the specimens were collected

Fossils of Riojasuchas are found in the Los Colorados Formation, a Late Triassic unit in Argentina,[3] and date to approximately 217 to 215 million years ago.[1] At that age, it is the youngest ornithosuchid known.[1] It lived alongside the protosuchid Hemiprotosuchus; the sphenosuchid Pseudohesperosuchus; the stagonolepidid Neoaetosauroides; the melanorosaurid Strenusaurus; and the riojasaurid Riojasaurus.[3]