Ornithosuchidae is anextinctfamily ofpseudosuchianarchosaurs (distant relatives of moderncrocodilians) from theTriassic period. Ornithosuchids werequadrupedal and facultativelybipedal (e.g. likechimpanzees), meaning that they had the ability to walk on two legs for short periods of time. They had distinctive, downturned snouts, unique, "crocodile-reversed" ankle bones, and several other features that distinguish them from other archosaurs. Ornithosuchids were geographically widespread during theCarnian andNorian stages of theLate Triassic with members known fromArgentina,Brazil, and theUnited Kingdom. Four genera, comprisingOrnithosuchus,Venaticosuchus,Dynamosuchus,^[1] andRiojasuchus are presently known.^[2] The family was first erected by German paleontologistFriedrich von Huene in 1908.^[3]

Ornithosuchids Temporal range:Late Triassic,228–203.6 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Life restoration ofOrnithosuchus scavenging on therhynchosaurHyperodapedon
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Archosauria
Clade:	Pseudosuchia
Family:	†Ornithosuchidae von Huene,1908
Type species
†Ornithosuchus woodwardi Newton, 1894
Genera
†Aenigmaspina? †Dynamosuchus[1] †Ornithosuchus †Riojasuchus †Venaticosuchus

Ornithosuchids can be identified by the presence of an archeddiastema, a gap between the teeth at the front of the snout. When the jaw is closed, two large, curved,dentary (lower jaw) teeth fit into the diastema, which is positioned between thepremaxilla andmaxilla. Two shallow depressions are on the wall of the diastema to accommodate these teeth. The large dentary teeth ofOrnithosuchus andRiojasuchus are placed behind a smaller, procumbent dentary tooth that sticks out from the jaw. This type of tooth position is not seen in any other basal archosaurs. Another characteristic feature of ornithosuchids is their unusual downturned, overhanging snout, seen inRiojasuchus andVenaticosuchus, but notOrnithosuchus.^[4]

Several other features distinguish ornithosuchids from all other early archosaurs.Ornithosuchus andRiojasuchus both possess a small fenestra, or hole, between thepalatine andpterygoid bones of thepalate, i.e. the roof of the mouth. The contact between thenasal andprefrontal bones of the skull is small or absent, excluded by a large contact between thefrontals andlacrimals. In other archosaurs, includingrauisuchians,aetosaurs,pterosaurs, anddinosauromorphs, the nasal-prefrontal contact separates the frontal from the lacrimal.^[4]

The postcranial skeleton is nearly completely known inRiojasuchus, incomplete inOrnithosuchus, and entirely unknown inVenaticosuchus. As a result, whether all of the postcranial traits seemingly unique to ornithosuchids actually occurred in all members of the family is uncertain. Ornithosuchids known from decent postcranial remains typically had about 9cervical (neck), 14-15 dorsal (back), threesacral (hip), and over 20 caudal (tail)vertebrae. Above each vertebra was a pair of bony scutes known asosteoderms.^[2]

Thefemur (thigh bone) has a pronounced anterior trochanter. The anterior trochanter, sometimes known as the "lesser trochanter" (but unrelated to thelesser trochanter of the femur in humans), is a ridge on the outer surface of the femur, near thefemoral head. It was probably an insertion point for m. iliofemoralis cranialis, which helps to raise the leg. Most archosaurs and archosaur relatives lack a distinct anterior trochanter, but ornithosuchids are an exception, along with mostdinosauromorphs (dinosaurs and their close relatives).^[5]

Much like the femur, thefibula (outer shin bone) also has a distinctive point for muscle insertion. The muscle in question is the iliofibularis, which helps to straighten the limbs. In most archosaurs, the iliofibularis is inserted onto the fibula by means of a tiny ridge on the proximal part of the fibula, near the knee. However, ornithosuchids have a much larger, knob-shaped iliofibularis insertion point located about midway down the shaft of the fibula. Phytosaurs and aetosaurs also share a knob-like attachment point midway down the fibula, so whether the case in ornithosuchids is a unique case of convergent evolution, or alternatively the retention of a trait independently lost by several archosaur lineages is unclear.^[6]

Unlike most other early archosaurs, the pedalunguals (the distalmost bones of the feet that form claws) are laterally compressed. They are sharp and recurved. The unguals are very deep, being taller than they are long, especially on the inner digits. This type of claw is not seen in any other Triassic archosaur except for pterosaurs.^[4]

Major archosaur groups have often been distinguished from each other based on the structure of their ankles. In most crurotarsans, theastragalus has a convex projection that fits into a concave space in thecalcaneum. This condition is often referred to as a "crocodile-normal" ankle, as it is the most common ankle type in crurotarsans. Ornithosuchids are unique among crurotarsans, and all other archosaurs, in their possession of a "crocodile-reversed" ankle, in which the placement of the concavity is reversed; instead of being on the calcaneum, it is on the astragalus. In ornithosuchids, the calcaneum bears a convex projection that is analogous to the convex projection on the "crocodile-normal" astragalus.^[4]

Ornithosuchidae is generally considered to be within the largercladeSuchia, which includes aetosaurs, rauisuchians, and crocodylomorphs.^[7][8][9] It was given a phylogenetic definition by Paul Sereno in 1991 as the last common ancestor and all descendants ofOrnithosuchus,Riojasuchus, andVenaticosuchus.^[4] Sterling Nesbitt in 2011 gave an alternative definition consisting ofOrnithosuchus woodwardi and all archosaurs closer to it than toRutiodon carolinensis,Aetosaurus ferratus,Rauisuchus tiradentes,Prestosuchus chiniquensis,Crocodylus niloticus (the Nile crocodile), orPasser domesticus (the house sparrow).^[6] Below is a cladogram based on Nesbitt (2011), showing the placement of Ornithosuchidae in Archosauriformes.^[6]

• Palaeos Vertebrates