Allosaurus fragilis Marsh 1877
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Reconstruction of Allosaurus showing the location on the skeleton of the ungual bone that was cast for this reproduction.
The bones of the Allosaurus hand (after Gilmore 1920).
The bones of the Allosaurus hand (manus) showing the location of the ungual bone of digit 1 (after Madsen 1976).

Systematics: Saurischia, Theropoda, Carnosauria, Allosauridae, Allosaurus fragilis

Adult size: 33 to 42 feet (10 to 14 meters); approx. 1.5 tons

Catalog no.: YPM – PU 14554

Etymology: allos meaning “different” and sauros meaning “lizard” (Greek); fragilis meaning “fragile” (Latin)

Age: Morrison Formation, Late Jurassic Period (155 to 145 million years ago)

Locality: Cleveland–Lloyd Quarry, Emery County, Utah, USA

Collector: William L. Stokes, 1939

In the late Jurassic the arch predator of North America was Allosaurus fragilis. Allosaurus grew to lengths of over 30 feet, and weighed as much as 1.5 tons. It shared many similarities with its carnosaur relatives—Tyrannosaurus rex and Albertosaurus libratus—the biggest meat-eaters that ever walked the earth. Like them, it walked on two legs, using its tail as a counterbalance. Its massive head had sharp teeth serrated like steak knives for slicing flesh. While smaller than T. rex, Allosaurus had longer and more useful arms.

In 1877, Yale University’s Othniel Charles Marsh (1831–1899) described a few fragmented dinosaur bones from Garden Park in southern Colorado as a new genus and species of carnivorous dinosaur, Allosaurus fragilis (“different lizard”). In 1939, under the direction of William L. Stokes, a field party from Princeton University began work in what would become known as the Cleveland–Lloyd Quarry in central Utah. This site proved to be extremely rich, and eventually gave up the bones of more than 50 Allosaurus specimens of various sizes, from juvenile to adult. Other dinosaur species were also found, but it is highly unusual to find this many predatory dinosaurs in one place. Possibly this was a single group killed by a catastrophic event, or it could be that this site represents a repeated use of the same habitat. Regardless, no articulated skeletons have been found at the quarry, so most specimens from the site are composites made from parts of different individuals. All have the black color distinctive of this quarry.

Allosaurus fragilis had three fingers on each hand (manus) and would have used its hands to grasp prey. These three functional fingers, designated digits 1, 2 and 3 (the thumb, middle and index fingers), are still present in birds today. Although the hands of living theropods (modern birds) are highly modified, fossils like Allosaurus indicate that theropod hands were originally suited to seizing and securing prey. Like all living animals with powerful, gripping hands, the longest finger bones (phalanges) support the claws. The claws themselves—as indicated by the shape of the ungual phalanges—are reminiscent of those in predatory cats: quite large, sharply pointed and recurved, the claws were moved by heavy tendons attached to prominent knobs (the flexor tubercles) at their bases.

This cast from the Peabody Museum’s Allosaurus specimen is of the ungual (the last bone of a finger or toe) of the second (middle) finger of the left hand. The ungual was covered by the actual claw, which did not survive the fossilization process. It could have been twice as big as the ungual itself, and much sharper. The cast was made from a mold of the fossil.