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.
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