Ceratosaurs of the Late Triassic and Early Jurassic

The clade Ceratosauria includes two groups, the Coelophysoidea and the Neoceratosauria. The first of these is made up of theropods from the early radiation of the dinosaurs discussed in Dawn of the Dinosaur Age. The group Neoceratosauria includes later theropods that span the Late Jurassic to Late Cretaceous Epoch and that retained some of the primitive features associated with ceratosaurs.

The Coelophysoidea

The Coelophysoidea ("hollow forms") were small to medium-sized carnivores that measured from 3 to 10 feet (1 to 3 m) long with the exception of Dilophosaurus, Gojirasaurus, Liliensternus, and Lophostropheus, which may have reached 20 feet (6 m). Their skulls were low, long, narrow, and tapered at the tip of the snout. Head ornamentation ranged from small ridges (Coelophysis) and crests (Syntarsus) to large, double crests (Dilophosaurus) that ran along the top of the skull between the nasal area and the brow. As with all dinosaurs, the maxilla—the main bone of the upper jaw—had a large opening in front of the orbit. There was a distinctive gap along the upper tooth row where the premaxilla, or front top jawbone, was connected to the maxilla. The body plan of coelophysoids included a long, flexible neck; a lightweight body; and a long, narrow tail. Two excellent examples of coelophysoids from the early radiation of theropods are Coelophysis and Dilophosaurus, described below.

Coelophysis ("hollow form"). This was a small theropod about the size of a large dog; it is known from Late Triassic formations of New Mexico and Arizona. Coelophysis was first described and named by Edward Drinker Cope in 1889, but his material was fragmentary. Not much was understood about this early dinosaur until the discovery of the Ghost Ranch quarry in New Mexico. The paleontologist most closely associated with Coelophysis was Edwin Colbert of the American Museum of Natural History.

In the summer of 1947, Colbert was at the end of his tossil-hunting visit to New Mexico. He and his colleagues, after successfully completing the excavation of a phytosaur—a crocodilelike archosaur—were preparing to close up camp and move on to another state to look for more fossils. As field paleontologists often do, they spent their last hours walking about, prospecting for new fossils and making plans for the next field season. Suddenly, one member of the team came running excitedly over a hill. In his hand was the claw of a small dinosaur. Further investigation revealed that more bones were just below the surface where the claw had been found. Colbert decided to stay a few more days and thoroughly evaluate the site. Early during the process he wrote in his field notebook,

Continued work in dinosaur quarry. This quarry keeps developing more and more, as we work it. Obviously it is not an isolated skeleton with associated odd bits of bone, but a comparatively extensive deposit containing a number of skeletons. (Edwin Colbert, The Little Dinosaurs of Ghost Ranch,)

Colbert himself later said that his entry was "the understatement of the year." His two-week field trip was extended for two months in 1947 and also occupied the summer of 1948. The Colbert team's work resulted in the collection of nearly a thousand specimens from a bone bed that measured roughly 30 by 30 feet (9 by 9 m). From the jumbled and tangled bones found on the Ghost Ranch site emerged

Coelophysis fossil

specimens that ranged in age and size from juveniles to adults and that included several complete, articulated skeletons. It was a once-in-a-lifetime discovery. Colbert's work on this little dinosaur made Coelophysis one of the best understood of all dinosaurs.

One of the mysteries of Ghost Ranch was how and why the little dinosaurs met their fate. Colbert reasoned that the animals were traveling together as a group or herd, and all died at the same time, presumably as victims of a mass drowning. They were buried quickly by sand and silt, which prevented scavengers from eating the carcasses. What the herd of Coelophysis was doing is still a mystery, and the discovery marks one of the rare cases of finding several individuals of one theropod species in one location. The Coelophysis group was clearly larger than a hunting pack. A reasonable explanation is that they may have been migrating or seeking refuge from some natural disaster.

Dilophosaurus ("two-crested lizard"). Known for its peculiar, double-crested head ornamentation, Dilophosaurus was a medium-sized theropod measuring 20 feet (6 m) long. It is known from Early

Jurassic deposits of Arizona on the basis of one partial skeleton, two subadult specimens, and several fragmentary skeletons. A crested skull found in China was described as Dilophosaurus in 1993, but additional investigation of that material suggests that it may belong to a different, as-yet-unnamed dinosaur. About twice as long as Coelo-physis, Dilophosaurus had a lightweight skeleton that was more similar to its smaller kin than to the bulkier forms normally associated with medium to large theropods. Dilophosaurus had the long, lightly built skull of all coelophysoids, but its skull appears taller because of the two parallel head crests that extend along the skull roof from just above the nostrils to just behind the eyes. Seen in profile, the skull of Dilophosaurus looks twice as tall as it is because of the crests. There is not enough fossil evidence to speculate as to whether the crest was exclusive to or larger in Dilophosaurus males, but if it was, one might surmise that the crest probably made for an impressive display of physical prowess when showing off to a potential mate. The sacrum, or fused vertebrae that connect with the pelvis, number only four in Dilophosaurus, a primitive feature. Most later and more derived the-ropods had five or more sacral vertebrae.

Dilophosaurus was relatively lightweight and agile for its length; it weighed perhaps half a ton. This compares with later, more heavily built predatory dinosaurs that may have weighed twice as much at the same length. The forelimbs of Dilophosaurus were moderately long, and the three main fingers of its hands were adorned with fairly long, curved claws. The teeth of Dilophosaurus varied in size. The longest, most slender, knifelike teeth were in the upper jaw and premaxilla. Among the prey of Dilophosaurus were small reptiles and amphibians as well as large, herbivorous "prosauropod" dinosaurs that Dilophosaurus probably attacked using a combination of its slashing hand claws and ripping teeth.

The Neoceratosauria

Neoceratosaurs ("new horned lizards") were medium to large predators that measured up to 33 feet (10 m) long. Their skulls were large, tall, and broad, with wide snouts, strong jaws, and large, bladelike teeth. The skull opening behind the orbit was twice or more the size of the orbit. The skulls of neoceratosaurs from the Southern Hemisphere exhibit extensive pitting and sculpting of the bone. Neoceratosaur skulls were adorned with a variety of bony ornaments, including a single stout horn on top of the skull and forward of the eyes (Ceratosaurus); a pair of outwardly pointing brow horns over the eyes (Carnotaurus); and a bony dome or horn on top of the skull between the eyes (Majungatholus). The body plan of a neoceratosaur included a short, stocky neck; long hind limbs; a stout tail; and strong but short forelimbs. The Neoceratosauria lived in the later half of the Age of Dinosaurs and are discussed in more detail in other volumes of The Prehistoric Earth.

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