Carnosaur theropods are divided into two principal families, the Allosauridae and Tyrannosauridae. The allosaurids include all carnosaurs of the Early Jurassic to Early Cretaceous age that have been studied thoroughly enough to be assigned a family lineage. All mature allosaurids were five meters or more in length, had large skulls, short, powerful necks, very short forelimbs, massive hind limbs, and long tails. Allosaurus is the best-known carnosaur genus and the archetypal allosaurid.
Remains of Allosaurus ("different reptile") were first recovered in Grand County, Colorado, in 1869, but a full skeleton was not discovered until 1883, in the Garden Park Quarry, Fremont County, Colorado. Since these initial discoveries, numerous allosaur remains have been recovered in North America, most notably from the Morrison Formation in the western United States. The most spectacular discovery occurred in Utah in 1927 at the Cleveland-Lloyd Quarry, where some ten thousand bones, representing at least sixty individual allosaurs, were identified. The quarry has provided examples of individual allosaurs ranging in size from twelve-meter-long adults to three-meter-long juveniles. It is speculated that the quarry may have been a predator trap, similar to the Pleistocene-age Rancho La Brea Tar Pits in Los Angeles, California. One unfortunate feature of the Cleveland-Lloyd Quarry's collection of bones is that no articulated skeleton has been recovered there. The stratigraphic range and large numbers of allosaur fossils recovered in North America suggest that packs of allosaurs pursued prey animals all across the Jurassic topography, and that allosaurs were the dominant predator for nearly twenty-five million years.
A unique feature of Allosaurus was its lightly constructed skull, which had distinctive shamrimmed ridges above and along the nasals and a tall, triangular hornlet in front of and above the eye orbits. The skull in an adult was about ninety centimeters long, relatively long in comparison to both its body size and to the skulls of other carnosaurs. Allosaur skulls have loose joints between some of the lower skull bones and also between several bones of the lower jaw. These flexible jaw joints probably allowed for expansion of the gullet so that large chunks of food could be swallowed. Above the jaws, the skull was lightly constructed, with several large, lateral spaces in front of and behind the eye. These spaces may have helped to lighten the skull, making it easier for Allosaurus to move its head. Allosaur eyes were twice as large as those of any other carnosaur, suggesting the animal hunted in both dim light and bright sunshine. Allosaurs had back and front serrated, recurved teeth, positioned far forward in the jaws and in front of the eye sockets, that could be used for stabbing, killing, and dismembering prey. Like tyrannosaurs, allosaurs had an enlarged transverse crest at the back of the skull, atop the braincase, supporting powerful cervical muscles. These muscles allowed the head to be held almost above the shoulders rather than extended forward. Allosaur forelimbs were large and powerfully muscled, having three large, sharply pointed, curving claws, the inner being the largest. These claw-tipped arms were probably used for mating, slashing, holding prey, and dismembering carcasses. Allosaur hind limbs were massive, with five-toed feet, of which only three toes supported weight. The allosaur body was rather squat and was balanced over the hip structure by a massive, muscular tail. Allosaurs were well-adapted predators, but their physiology suggests to many paleontologists that, individually, they may not have had the power or speed to overtake and kill large Late Jurassic sauropods. It has been suggested that allosaurs may have hunted in packs, ambushed prey, or been simply advantageous predators and carrion scavengers. The Cleveland-Lloyd Quarry discoveries suggest that, in that locale, allosaurs and other predatory theropods engaged in feeding frenzies when sauropods and other theropods wandered into a muddy bog and were trapped in slimy ooze. The immobilized prey were then swarmed by predators, who in turn became trapped in the bog and died.
carnosaurs: the largest meat-eating animals
to have lived on land
Mesozoic era: the middle era of the Phanerozoic eon, 250 million to 65 million years ago
sauropoda: large, quadrupedal herbivorous dinosaurs that lived from the Early Jurassic to the end of the Cretaceous
theropoda: "beast foot"; dinosaurs that lived from the Late Triassic to the terminal Cretaceous extinction event; most predatory and carnivorous dinosaurs belong to this group
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