Members of the Ceratosauria represent the earliest substantial radiation of carnivorous dinosaurs. The earliest ceratosaurs lived during the Late Triassic and Early Jurassic. Several other prominent lines of ceratosaurs persisted, particularly in regions of the Southern Hemisphere (now Argentina, India, and Madagascar) well into the Late Cretaceous Epoch. The name Ceratosauria is a nod to Ceratosaurus (Late Jurassic, Colorado and Utah), a predatory dinosaur with a small horn on its snout, named in 1884 by American paleontologist Othniel Charles Marsh. With the exception of the Gondwanan Cretaceous taxa, ceratosaurs were largely extinct by the Middle Jurassic. Among the ceratosaurs that lived among the giants of the Late Jurassic were two curious holdovers from the early evolution of theropods, Ceratosaurus and Elaphrosaurus.
Ceratosaurus was a medium- to large-sized predator, the original specimen of which measured about 20 feet (6.1 m) long. Very fragmentary remains of other Ceratosaurus-like dinosaurs have been found in such places as Portugal and Tanzania; the Tanzania specimen suggests a dinosaur that may have been more than 40 feet (12 m) long.
The skull of Ceratosaurus was tall and broad, with a wide snout, strong jaws, and large, bladelike teeth. Its head was adorned with a single stout horn on top of the skull in front of the eyes, as well as smaller hornlets over the eyes. It has been suggested that Ceratosaurus, with a tail shaped somewhat like that of a crocodile, may have preferred hunting in the water for aquatic prey. The Ceratosaurus body plan included a short, stocky neck, long hind limbs, a stout tail and strong but short forelimbs.
Marsh's original specimens, discovered in the 1880s, accounted for most of what was known of this theropod until the recent discovery, in the years since 1999, of new specimens, including a juvenile. One study of the braincase of a new specimen included work done by radiologist R. Kent Sanders and paleontologist David K. Smith using computed tomography, or CT scans. Although the brains of fossil animals are never preserved, the endocranial cavity in which the brain was located can sometimes reveal much about the sensory abilities of a given dinosaur. A scientist familiar with the physiology of the vertebrate brain can trace channels and spaces within a braincase that once hosted connections to nerves and sensory mechanisms of the brain and body. In the case of Ceratosaurus, the position of the brain and associated nerve canals confirms a posture for the head and neck that was more horizontal than erect, eyesight that was probably about average, and a well-formed sense of smell comparable to that of birds.
Elaphrosaurus is a less well understood ceratosaur from the Late Jurassic of Tanzania. The original specimen, described in 1920, lacked a head and most of the forelimb. Measuring about 17 feet (5 m) long, Elaphrosaurus had a slender body and a long neck and was undoubtedly a more agile runner than the bulkier Ceratosaurus.
The lightweight body of Elaphrosaurus once led paleontologists to classify it as an ostrichlike dinosaur; but more recent cladistic analysis has demonstrated that its feet, vertebrae, pelvis, and leg bones were more closely aligned with those of ceratosaurs.
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