Ceratosauria Coelophysoidea and Neoceratosauria

Although relatively less known than Tetanurae, the stem-based clade Ceratosauria (= "horned lizard") includes abundant specimens of some interestingly varied theropods, whose remains have been found on all continents except Antarctica. Two stem-based clades are within Ceratosauria, Coelophysoidea and Neocer-atosauria. Coelophysoideans were the dominant theropods soon after the beginning of the geologic range for dinosaurs. Herrerasaurids in the Late Triassic were supplanted by coelophysoideans, which are represented abundantly by Coelophysis. Coelophysoideans in turn were succeeded by Early Jurassic species, such as the Syntarsus and Dilophosaurus, as well as Ceratosaurus of the Late Jurassic.

A clade within Neoceratosauria from the Early and Late Cretaceous, Abelisauridae, comprises the bulk of neoceratosaurs for the latter part of the Mesozoic. Abelisaurids include Abelisaurus, Carnotaurus, Majungasaurus, Masiakasaurus, and several other relatively large theropods that mostly lived in the Cretaceous southern continent of Gondwana (Chapter 4), as indicated by specimens from South America, India, and Africa. In contrast, coelophysoideans were apparently rare or extinct by the end of the Jurassic.

Important synapomorphies of ceratosaurs include:

■ Fusion of astragalus and calcaneum in the ankle.

■ Sacrum fused to pelvis.

■ At least seven sacral vertebrae, which were fused to form a synsacrum.

FIGURE 9.5 Early Cretaceous neoceratosaur and abelisaurid Carnotaurus of Argentina, showing cranial ornamentation typical of ceratosaurs. Museo de Ciencias Naturales, Madrid, Spain.

■ Two fenestrae on the pubis.

■ Four digits, but with digit IV reduced so that digits I through to III were the most functional.

■ Two pairs of cavities (pleurocoels) in the cervical vertebrae.

Many ceratosaurs are also well known for the development of prominent "headgear" in some species, evident as large bony horns or crests on the dorsal surfaces of their skulls. In particular, Dilophosaurus of the Early Jurassic had a pair of pronounced crests that stuck out of each side of its skull. Another example was Carnotaurus of the Early Cretaceous, a ceratosaur that displayed orbital horns (Fig. 9.5). One proposed purpose for these seemingly non-functional features is that they were used for sexual display, but whether they are indicative of males or females is unknown.

Other than skeletal evidence, soft-tissue impressions have been found for one ceratosaur, the aforementioned Early Cretaceous abelisaurid Carnotaurus. These impressions display rows of low-profile conical scales interpreted as external molds of the actual skin. Feathers are currently unknown from ceratosaur remains. Nevertheless, some artists have depicted such frills on Coelophysis in reconstructions, despite the current lack of scientific evidence for them. One example of an Early Jurassic "sitting theropod" trace fossil, attributed to a ceratosaur, purportedly had feather impressions. Nevertheless, more detailed scrutiny revealed that the "feathers" were wrinkle marks in the sediment caused by the animal's movement.

Aside from skeletal evidence, ceratosaurs are not well understood. Theropod tracks of the Late Triassic and Early Jurassic have been tentatively linked to ceratosaur tracemakers on the basis of correlation with known body fossils of the same age. Even so, no one has yet identified tracks made by specific genera, such as Coelophysis or Dilophosaurus. No toothmarks or coprolites of ceratosaurs have been interpreted, thus little is known about whether any of them were primarily predators or scavengers. One specimen of Coelophysis, which had bones of another, smaller Coelophysis in its rib cage, was regarded for decades as evidence of cannibalism in this species. However, a more careful examination of the specimen later revealed that the bones were underneath and not inside the rib cage, meaning that (as far as we know) Coelophysis did not eat its own species, let alone its young. Similar to herrerasaurids, no eggs, embryos, or nests of ceratosaurs have been identified, despite the numerous body fossils found of some adult ceratosaurs (that is, Coelophysis and Syntarsus). However, this circumstance may be partially a result of the preservation bias against dinosaur eggs in general, which are not abundantly represented in the geologic record until the Early Cretaceous (Chapter 8). Abundant juvenile remains of Coelophysis afford a rare view of the growth and development of one species of ceratosaur, but embryos are unknown, even for this well-studied dinosaur.

Despite Coelophysis being the most abundant dinosaur represented by skeletal remains and the common preservation of Syntarsus, which is represented by more than 30 specimens, ceratosaurs are not as well-studied as tetanurans. About 25 species have been placed in Ceratosauria (Table 9.1), but some of these species are named

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