Rauisuchia Translation and Pronunciation Guide

Introduction

Ben Creisler

Rauisuchians ("Rau's crocodiles") have been aptly described as animals "trying to become dinosaurs." But recent research (Benton, 1986) confirms that they were NOT dinosaurs, and represent instead a distinct group of Triassic archosaurs that anticipated carnivorous dinosaurs in surprising ways. This insight has cleared up years of confusion. Most older books on dinosaurs listed Teratosaurus, Cladeiodon, Poposaurus, etc., as primitive theropods--the formidable blade-like teeth found in most forms closely resemble those of large meat-eating dinosaurs. The taxonomic error stemmed in part from scientific descriptions that mixed limb material and other postcranial remains of true Triassic dinosaurs such as prosauropods with teeth and parts of skulls belonging to rauisuchians or ornithosuchians. The resulting paleontological chimeras fit once widely accepted theories about dinosaur evolution, but in reality muddled the facts.

Rauisuchians ranked as the top terrestrial predators of the Middle and Late Triassic, thanks to huge skulls armed with powerful biting jaws that would be a match for many later theropods. However, rauisuchians were typically low-to-the-ground quadrupeds, archosaurian versions of lions (sometimes termed "bear-crocs") with fairly short erect limbs--a sharp contrast to the meat-eating dinosaurs that succeeded them, tall bipeds with long ostrich-like hind legs. Once widely accepted reconstructions of Teratosaurus, and more recently Postosuchus, depicted the animals as bipeds with rather dinosaur-like hind limbs, a notion now rejected. While it appears that some rauisuchians such as the small chatterjeeids could walk or run on two legs, rauisuchids and poposaurs were typically built to move on all fours, achieving a nimble upright non-sprawling stance thanks to a vertical femur in a strong hip joint and an ankle and foot design more advanced than in modern crocodiles. Some forms had small armor plates in two rows along the trunk, with a single row above and below the tail.

At the moment, considerable debate surrounds the phylogenetic status of the proposed group Rauisuchia, which may represent a number of independent lines of archosaurs with advanced forms of a crocodile-like ankle--archosaurs that adopted a similar body plan, but may not all be descendants of a single evolutionary stock. Two defined groups are generally recognized, though diagnoses differ among reseachers. Following Long and Murry, poposaurs are more lightly built with vertically oriented ilia, while rauisuchids have long, well-developed sacral ribs and horizontally oriented ilia. A range of other Middle and Late Triassic carnivorous archosaurs found in Russia, India and China show similarities to established European and North and South American taxa, suggesting they belong in the Rauisuchia, but some of the material is fragmentary or currently lacks thorough descriptions. A number of small gracile Middle Triassic forms from Russia (Tsylmosuchus, Vytchegdosuchus, Dongusuchus) have elongated sigmoidally curved necks, in contrast to the shorter robust necks found in some later forms. However, the distinction between gracile and robust species may not be a simple family level distinction, and the two body types may have developed independently within different lineages.

The peculiar sail-backed archosaurs Lotosaurus, Ctenosauriscus and the poorly known Spinosuchus appear to have some rauisuchian affinities, and are included here as well, though more detailed research is needed to clear up their exact status. The recent discovery of thin, greatly elongated dorsal spines in Middle Triassic deposits at Devon, England, suggests the existence of sail-backed archosaurs that must have closely resembled the famous sail-backed pelycosaurs of the Permian, but to date fossil material remains frustratingly incomplete.

Remains of rauisuchians are currently known from the Middle and Late Triassic of North America, South America, Europe, India, China and Africa. The largest known species reached 7 meters (23 ft.) (Saurosuchus), though most were from 2 to 5 meters (7 to 15 ft.) long. Rauisuchians apparently did not survive beyond the end of the Triassic.

(Parrish (1993) embedded the Crocodylomorpha in the Rauisuchia, but crocodylomorphs are treated as distinct from rauisuchians for purposes of these lists (See Crocodylia).)

1. Benton, M. J. 1986. The late Triassic reptile Teratosaurus--a rauisuchian, not a dinosaur. Palaeontology 29:293-301.
2. Long, R. A. & P. A. Murry, 1995. Late Triassic (Carnian and Norian) Tetrapods from the Southwestern United States. New Mexico Museum of Natural History & Science. Bull. 4. 254pp.
3. Parrish, J. M. 1993. Phylogeny of the Crocodylotarsi, with reference to archosaurian and crurotarsan monophyly. Journal of Vertebrate Paleontology. 13(3): 287-308.