A Truly Exceptional Allosaurus

The remains of dinosaurs are entombed in strata all over the world, but many notable specimens have also been buried in museum collections and obscure bits of technical literature. One such dinosaur, first reported in June of 1878, may be part of a truly exceptional Allosaurus that was given a different name.

In 1877, Allosaurus was new to science. The predatory dinosaur had been described only the previous year by Othniel Charles Marsh on the basis of a paltry number of fragments, including parts of the backbone and limbs. As such, little was known of this dinosaur when Marsh’s rival, Edward Drinker Cope, received several vertebrae from an unknown dinosaur excavated in the vicinity of Cañon City, Colorado. The central portions of the vertebrae were distinguished by having a concave, cup-shaped surface on the back end, which led Cope to attribute them to an “opisthocoelous” dinosaur similar to the sauropod Camarasaurus. Almost every unique scrap of bone received a name in those days, and Cope called this mysterious dinosaur Epanterias amplexus.

A significant part of his collection eventually wound up at the American Museum of Natural History, and in 1921 the paleontologists Henry Fairfield Osborn and Charles Craig Mook of that institution published a re-analysis of many of the sauropods Cope had described. Included in the lot was Epanterias. Rather than being a sauropod, though, Osborn and Mook found Epanterias to be a theropod “which at present cannot be separated from Allosaurus Marsh.” Still, the bones were of special significance because they appeared to be one-fifth larger than the corresponding bones from other theropod dinosaurs found in the Late Jurassic Morrison formation. Cope did not know it at the time, but he had described an especially large representative of a dinosaur his rival had named just a year before.

Just how big was Cope’s Allosaurus? That is difficult to say with certainty. So little of it was found that paleontologists can only estimate. In an email sent to the Dinosaur Mailing List in 2003, Mickey Mortimer estimated the “Epanterias” specimen to have been nearly 40 feet long. If this is accurate, then the biggest Allosaurus specimens would have grown as big as Tyrannosaurus, and as a consequence this indicates that most known Allosaurus specimens come from relatively young animals. Perhaps, in time, a more complete specimen of such a super-sized Allosaurus will be found.

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.