The Ceratosauria ("horned lizards") is a group that includes many of the earliest theropods but that also includes several later theropods that retain several primitive features. The earliest ceratosaurs lived during the Late Triassic and Early Jurassic Epochs. Several other prominent lines of ceratosaurs persisted, particularly in regions of the Southern Hemisphere (now South America, India, and Madagascar), well into the Late Cretaceous. The name Ceratosauria is a nod to Ceratosaurus (Late Jurassic, Colorado, Utah), a predatory dinosaur with a small horn on its snout, named in 1884 by American paleontologist Othniel Charles Marsh. The clade Ceratosauria was originally created by Marsh to include only Ceratosaurus. More than 100 years later, in 1986, Jacques Gauthier retained the name as that of a redefined clade of a much larger number of theropods based on his extensive cladistic analysis.
Ceratosaurs range widely in length, from the swan-sized Segis-aurus (Early Jurassic, Arizona) to the hulking, 36-foot (11 m) long monster, Carnotaurus (Late Cretaceous, Argentina). Although they varied widely in size, ceratosaurs were united by several common anatomical traits that distinguished them from the other major clade of theropods, the Tetanurae. Among their unique skeletal traits were the fusion of several bones of the hind limb (toes, upper
Ceratosaurus confronting Brachiosaurus, with Rhamphorynchus in the foreground ankle bones); a modified knee joint; sacral vertebrae that were fused to each other and to the ilia; and the particular shape and flaring of individual pelvic bones. The ceratosaur hand had four fingers, although the fourth was greatly reduced in size—a primitive feature that was lost entirely in later theropods.
Adult skeletons of ceratosaurs also possess visually distinctive structures that may have served as a means of sexual display to attract a mate, or that perhaps came into play as males engaged in head-butting or bumping contests to win a position of dominance in their pack. Several ceratosaurs, including Dilophosaurus, Ceratosau-rus, and Carnotaurus, had distinctive head crests, ridges, or horns; however, because so few individual specimens are known of these taxa, paleontologists at present have no basis on which to determine whether these features represent a sexually dimorphic trait.
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