Ceratosauria

	Timescale	Evolution	Paleontology	Paleogeography	New/Updated
Mesozoic: Triassic \| Jurassic		Terrestrial Evolutionary Biotas: Mesozoic Tetrapods Kingdoms of Life > Animalia > Vertebrata > Tetrapoda > Reptilia/Sauropsida > Archosauria > Dinosauria > Theropoda

Suborder Ceratosauria

The Coelophysoidea, Ceratosauroidea/Neoceratosauria, and, usually, the the Abelisaurs, are together considered as the Ceratosauria, the most primitive taxon of "neotheropods" (conventional or non-herrasaur theropods). They are united by a number of features, including a hand with four fingers (the fourth digit is present, although strongly reduced, in ceratosaurs, but absent in all more advanced theropods), the lightly-built jaw support bones and roof of the mouth, and the robust central metatarsal, and a number of more specialised features. However, the "neoceratosaurs" lack many of the skull specializations of the coelophysids, so it is possible that they may not be that closely related after all.

The evidence for a monophyletic Ceratosauria is still debatable. All recent published studies which include multiple "ceratosaurs" (Gauthier, 1986; Novas, 1992; Perez-Moreno, 1993; Holtz, 1994; Sereno et al. 1994, 1996; etc.) find that Ceratosaurus and Abelisaurs are closer to coelophysoids than to tetanurines. (implying a monophyletic clade). But it has been unofficially suggested that the coelophysoids and neoceratosaurs may not form a clade apart from other known theropods. In addition, a recent paper by Rauhut strongly supports Ceratosaur paraphyly. There are also some derived (specialised) features found in neoceratosaurs (Ceratosaurus and Abelisaurs) which are not found in coelophysoids. This could be potential evidence that neoceratosaurs are closer to tetanurines than to coelophysoids. In this case, the neoceratosaur/coelophysoid characters would have to be convergences and/or primitive neotheropod features lost in Tetanurae, and the Ceratosauria are paraphyletic or even polyphyletic.

My own opinionated comment on all this: It seems unreasonable to retain the Ceratosauria as a monophyletic taxon. If they are, then they were not the ancestors of the other theropods, and that it is necessary to postulate a "ghost lineage" of unknown advanced theropods going all the way back to the early late Triassic (Carnian age), living alongside the well-known primitive ceratosaurs of the time. One may ask why a primitive common ancestor should give rise to both primitive and advanced descendents, but only the advanced descendents remained invisible in the fossil record for some millions of years. Worse, there must be a mysterious and similarily invisible taxon transitional between the Herrerasauria and the Tetanurae. No such form has ever been found. Surely it is much more logical to have the Ceratosauria evolve from Herrerasauria ancestors, and then, several million years later, in turn give rise to Megalosaurian proto-tetanurae.

The following is a suggested and tentative Linnean classification of the Ceratosauria

Superfamily Coelophysoidea - Primitive lightly-built forms
family Procompsognathidae
family Coelophysidae

Superfamily Ceratosauroidea
family Ceratosauridae
family Elaphrosauridae

Superfamily Abelisauroidea
family Abelisauridae
family Noasauridae
family Velocisauridae
The status of the Superfamily Abelisauroidea is still uncertain, they may be Ceratosaurs or Megalosaurs.

Coelophysis bauri - skull The Coelophysoidea represent the earliest theropod radiation, flourishing during the late Triassic and early Jurassic. These graceful animals were characterised by a long, low body, a long neck, and short three-fingered hands for clutching prey. The long fairly lightly built skull (left) bears a distinctive kink between the premaxilla (upper jaw tip) and maxilla (in main upper jaw). It was thought that this kink meant that coelophysids could not attack live prey due to skull weakness, but it appears to have been reinforced. Gregory Paul argued that Spinosaurid Megalosaurs like Baryonyx, which also have such a kink, were be late coelophysoids, but this is now no longer considered the case.

Coelophysiods probably died out during the Toarcian turnover, perhaps due to the extinction of the prosauropod dinosaurs (their main food source perhaps) at the same time. It can be assumed that the Coelophysiodea were the ancestors of all later theropod dinosaurs.

drawing of Coelophysis skull courtesy of Professor Paul Olsen

My Linnean/Evolutionary Systematic sympathies are showing - I have used the term "Ceratosauroidea" rather than "neoceratosauria" to distinguish these medium to large, persistantly primitive theropods from their Coelophysoid ancestors. The most characteristic genus, the large late Jurassic Ceratosaurus, is distinguished by a small nasal horn, most certainly used for sexual display. Although Ceratosaurs are usually grouped with Abelisaurs, I feel the latter are sufficiently distinct to warrant their own superfamily status (in any case their exact phylogentic position is still a little dubious). Like the Megalosaurs, these were typically Jurassic forms. Elaphrosaurus, variously considered an ancestral ornithomimosaur and a late surviving Coelophysid, probably belongs here. It is possible that some stragglers (e.g., Kelmayisaurus) continued on to the late Jurassic.

The abelisaurs are an important Cretaceous group known mainly from Gondwana. Most were large and a few had strange horns and crests. There was a tendency in one lineage for the hands and arms to be reduced so they became even smaller than the tiny forelimbs of the Tyrannosaurs. Another lineage curiously resembled the dromaeosaurs ("velocoraptors") of Asiamerica. The abelisaur phylogenetic placement is still very questionable. They have some tyrannosaurid characters, as well as ceratosaur and carcharodontosaur characters. Their position is still far from being resolved. However, the general, and increasingly likely, consensus is that they are late surviving ceratosaurs. Another interesting possibility may be an abelisaur-spinosaur relationship. Abelisaurs, spinosaurs and carcharodontosaurs are all Gondwanan theropods for which relationships have been proposed in various ways, and which are all difficult to place in a phylogeny.

Books and Web Links

Book and Journal References

Rowe, T., & Gauthier, J., "Ceratosauria", pp. 151-168, in Weishampel, D. B., Dodson, P., & Osm�lska, H. (eds.), 1990: The Dinosauria. University of California Press, Berkley, Los Angeles, Oxford, 1990

Rauhut, O. W. M. 2000. The interrelationships and evolution of basal theropods (Dinosauria, Saurischia). Unpublished PhD-thesis, University of Bristol, Bristol, 583 pp. (see also Rauhut's Thesis)