Archosauria: Rauisuchiformes

Taxa on This Page

1. Ornithosuchidae X
2. Rauisuchia X
3. Rauisuchiformes

Venaticosuchus rusconii, a typical ornithosuchid. This animal filled the role of intermediate- sized predator. Length 2 meters, late Carnian of south-west Pangea (Gondwana). Drawing from The Cambridge Encyclopedia of Life Sciences, ed. Adrian Friday & David S. Ingram, © Cambridge University Press 1985. Some parts of the original retouched.

The Ornithosuchids

Introduction

The ornithosuchids are yet another of the many archosaur lineages that constituting the confusing Triassic family tree of these animals. They have been variously considered dinosaur ancestors, crocodile relatives, and an separate branch of Crurotarsi. Much of this confusion is due to the fact that Triassic archosaur relationships rest to a large degree on ankle structure, with some, like dinosaurs, having bird-like ankles, and others, like "pseudosuchians", having croc-like ankles.

The ornithosuchids were also one of a number of groups of Triassic archosaurs which evolved a fully-erect (bird- and mammal-like) limb posture, as opposed to the sprawling and semi-erect postures of more primitive archosauromorphs, and even of crocodiles today. Ornithosuchus itself was originally considered a "pseudosuchian thecodont," then reclassified as an early theropod dinosaur, then later as a dinosaur-uncle, and is now considered to be a side-branch on the line to crocodiles. Thus far, ornithosuchid fossil remains are known only from Scotland and South America, although it can be assumed their distribution was much broader.

In the past, the ornithosuchids and close relatives were distinguished from most other advanced archosauriforms by a different sort of ankle structure: the so-called "Crocodile-Reversed" (CR) joint. Both the crocodylomorph "Crocodile-Normal" (CN) and the ornithosuchid "Crocodile-Reversed" (CR) ankle joints were ways of increasing the mobility of the animal; both had a definite advantage over the older protosuchian/erythosuchian ankle, which was a multi-part hinge. The trouble with this scheme is that ornithosuchid ankle structure is ambiguous, and it is unclear exactly how the ankle joint worked. So determining the phylogenetic position of these interesting and clearly highly specialized animals is not so simply accomplished.

Early books illustrate Ornithosuchus as a theropod-like biped, with long strong legs and diminutive fore-arms. And whilst these creatures could indeed move on their hind legs (and probably did for bursts of speed) they may have been equally comfortable on all fours, as indicated in the illustration of Venaticosuchus, right.

Ecology

The ornithosuchids were medium-sized or large (2 to 3.3 metres long) armoured carnivores, with mouths full of over-sized teeth. They were clearly active hunters, and the hindlimbs were much larger than the forelimbs. It used to be thought that this meant the creature was an obligate biped (like the large carnivorous dinosaurs for example), but they were more probably only facultative bipeds; that is although they were able to run on their back legs when they wanted to, they probably preferred quadrupedal locomotion.

Like those other characteristic late Triassic archosaurs, the phytosaurs, aetosaurs, and poposaurs, the ornithosuchids appear seemingly out of nowhere during the Carnian. These were active, fast-moving animals, with fully erect and upright posture, just like dinosaurs and mammals. It is possible that they evolved this posture from sprawling ancestors independently of other erect groups. Alternatively, if they can be derived from a prestosuchid (or rauisuchid?) ancestor (and that is not certain) they may have retained the stance that way. In any case, superior locomotive ability obviously conferred great adaptive advantages, and hence developed among a number of only distantly related archosaurs. (this posture developed in at least three or four distinct clades)

Although not very common, as would be expected from a predator near or at the top of the food chain, the ornithosuchids are widely distributed , being known from both Scotland (Laurasia) and Argentina (Gondwana). The name "bird crocodile" seems to nicely describe all the great archosaurs of Mesozoic, combining as they did features of both crocs and birds.

Phylogenetic Relationships

The ornithosuchids have had a rather uncertain phylogenetic history. Originally they were classified among the "Pseudosuchia," as one of a number of small bipedal running thecodonts, along the lines of Hesperosuchus, Saltoposuchus, and so on. It is now known that most of these latter forms are either basal crocodylomorphs Sphenosuchia) or very close to the crocodylomorph stem.

In the early 1960s Ornithosuchus was re-classified as an early and very primitive member of the theropod dinosaurs (Walker, 1964). However, Ornithosuchus, like other members of its family, have a number of primitive (plesiomorphic) features, such as a double row of bony plates down its back a short broad pelvis attached to the backbone by only 3 vertebrae, and five toes on each hind foot (as opposed to three in the case of theropod dinosaurs). These characteristics are shared by most basal archosaurs, and show that Ornithosuchus is only distantly related to the dinosaurs.

During the 1980s an influential paper by Gauthier (Gauthier, 1986) followed by Benton & Clark, 1988, placed the ornithosuchids as the most basal group of the Dinosaur-Pterosaur line Ornithodira). This was termed the "Ornithosuchia" in contrast to the Crurotarsan (or Crocodylotarsan) clade. The 1990s saw the pendulum swing again, with first Sereno (Sereno, 1991), and then Parrish and others publishing archosaur phylogenies that reject Gauthier's thesis and place the Ornithosuchidae on the Crurotarsan position. This has been the standard position since. So despite having a dino-bird-like ankle, ornithosuchids are still on the crocodylian side of the tree. Unless the phylogeny changes again of course!

Still not agreed on is the position of ornithosuchids within the Crurotarsi. Parrish (1993) placed Ornithosuchidae basal to all other crurotarsans (the Crocodylotarsi), Juul (1994) has them as the sister taxon to the "Paracrodylomorpha" (= Crocodylomorpha + Gracilisuchus + Postosuchus), and Benton (1999) and Benton and Walker 2002 tentatively find that Ornithosuchidae were sister group to the "Rauisuchia" (except for Fasolasuchus all Prestosuchidae) + Postosuchus. Since the Prestosuchidae and the Poposauridae / Rauisuchidae (Postosuchus' exact position there is uncertain) together may form only a paraphyletic, if not polyphyletic, assemblage, this is not as helpful as it may seem.

Our own analysis finds the Ornithosuchidae as the most basal group of the Rauisuchia, a large clade of vaguely theropod-like Crurotarsi which includes both the Prestosuchidae and the Rauisuchidae. Then again, it may also include Doswellia, which not really like much of anything else at all. At the present time, none of the possible arrangements of Triassic Crurotarsi can really be excluded, but it is relatively certain that the Ornithosuchidae will occupy a comparatively basal position in whatever phylogeny ultimately prevails.

MAK 000123 & 030730. ATW040124.

Image: Left: Ornithosuchus courtesy of the  Elgin Museum.

Descriptions

Rauisuchiformes: Rauisuchus + Aetosaurus

Range: from the Early Triassic

Phylogeny: Crurotarsi : Phytosauridae + * : Rauisuchia + Suchia.

Characters: skull <50% length of presacral vertebral column reversal) [$π03]; foramina for entrance of cerebral branch of internal carotid on anterolateral surface of basisphenoid [$π03]; medial wall of vestibule almost completely ossified [$π03]; lagenar recess present, elongate & tubular [$π03]; external foramina for abduscens nerve not entirely within prootic [$π03]; auricular recess extends onto internal surface of supraoccipital and/or epiotic [$π03]; antitrochanters on ischium & ilium [$π03]; pubis acetabular margin recessed [$S91] [2] [$π03]; pubis longer than ischium & > 3x width of acetabulum [$S91] [2];.

Notes: [1] "[π03]" refers to our own analysis of published data. [2] Sereno has a very different phylogenetic framework. However, his clade Suchia + Ornithosuchidae is generally equivalent to Rauisuchiformes as used here. ATW031226

References: Sereno (1991) [S91].

Rauisuchia: Rauisuchus > Aetosaurus.

Range: from the Early Triassic

Characters: subnarial fenestra formed as part of joint between premaxilla & maxilla [B84] [$P93] [1]; maxilla anterior to antorbital fenestra shorter than exposure posterior to anterior margin of fenestra [$π03]; antorbital fenestra tends to be keyhole-shaped [B84]; basisphenoid with elongate open trough on ventral surface formed by elongated basipterygoid processes [$P93]; ribs with anterolaterally projecting flange on proximal portion [$P93]; ilium low [B84]; acetabulum faces ventrally [B84] [2]; supraacetabular crest present [$π03]; digits V with 2 Phalanges [$P93] [$π03]; osteoderms not sculptured (reversal) [$π03].

Notes: [1] see diagram at Batrachotomus. In the current phylogeny, this is probably not a valid synapomorphy. The character is present outside Rauisuchia in prestosuchids, and absent inside Rauisuchia in Gracilisuchus, most rauisuchids and crocodylomorphs). [2] Benton's influential note [B84] proposed that rauisuchians developed an erect stance convergently and differently from the dinosaurs, by angling the acetabulum ventrally rather than putting an angled neck on the femur.  [3] "[π03]" refers to our own analysis of published data.

References: Benton (1984) [B84], Parrish (1993) [P93]. ATW031226

Ornithosuchidae: Ornithosuchus; Riojasuchus.

Phylogeny: Rauisuchia : (Tarjadia + (Prestosuchidae + (Doswellia + (Poposauridae + Rauisuchidae)))) + * : Ornithosuchus + Venaticosuchus + Riojasuchus.

Discussion: Ornithosuchids are so dinosaur-like that the were originally believed to be carnosaurs. In fact when the major divergence of the archosaurs into the crocodile and dinosaur lineages was originally recognized, the dinosaur line was named after its earliest known representatives: "Ornithosuchia." It now appears that the Ornithosuchidae are in fact on the other side of the archosaur split. That is, the Ornithosuchids are not ornithosuchians. As a result, the dinosaur  line is now referred to as the Ornithodira, although many sources retain the older name. The other branch has also suffered its own nomenclature problems, since this lineage was known as the Pseudosuchia or "false crocodiles." Unfortunately, as an unintended result of applying cladistic principles to Nineteenth Century nomenclature, the "false crocodiles" now include the true crocodiles (the less confusing term "Crurotarsi" is used at this site). Just to compound the difficulties, there are still those who believe that the Ornithosuchidae are more directly related to dinosaurs than crocodiles.

Notes: "[π03]" refers to our own analysis of published data.

Image: Left: Riojasuchus skull courtesy of Steve Harvey (Wiccart). Ornithosuchus image courtesy of Prof. Paul E. Olsen.

Links: Lecture 10 - Triassic: Newark, Chinle; ornithosuchus.jpg.

References: Sereno (1991) [S91], Sues 1992) [S92]. MAK030730, ATW040118.