Description
The type species of Amphicoelias, A. altus, was named by paleontologist Edward Drinker Cope in December 1877 (though not published until 1878) for an incomplete skeleton consisting of two vertebrae, a pubis (hip bone), and a femur (upper leg bone). Cope also named a second species, A. latus, in the same paper. However, all subsequent researchers have considered A. latus to be a synonym of A. altus. In 1921, Osborn and Mook assigned additional bones to A. altus—a scapula (shoulder blade), a coracoid (shoulder bone), an ulna (lower arm bone), and a tooth. Henry Fairfield Osborn and Charles Craig Mook noted the overall close similarity between Amphicoelias and Diplodocus, as well as a few key differences, such as proportionally longer forelimbs in Amphicoelias than in Diplodocus. The femur of Amphicoelias is unusually long, slender, and round in cross section; while this roundness was once thought to be another distinguishing characteristic of Amphicoelias, it has since been found in some specimens of Diplodocus as well. A. altus was also similar in size to Diplodocus, estimated to be about 25 m (82 ft) long. While most scientists have used these details to distinguish Amphicoelias and Diplodocus as separate genera, at least one (John Foster, in his 2007 book Jurassic West) has suggested that Amphicoelias is probably the senior synonym of Diplodocus.
The third named Amphicoelias species, A. fragillimus, is known only from a single, incomplete 1.5 m tall neural arch (the part of a vertebra with spines and processes), either last or second to last in the series of back vertebrae, D (dorsal) 10 or D9, that would have measured 2.7 m (8.8 ft) long in life. In addition to this single vertebra, Cope's field notes contain an entry for an "mmense distal end of femur” located only a few tens of meters away from the giant vertebra, and it is likely that this undescribed leg bone belonged to the same individual animal as the neural spine.
In 2010, a monograph was made available, but not formally published, by Henry Galiano and Raimund Albersdorfer in which they referred a fourth species to Amphicoelias, as "A. brontodiplodocus" based on several complete specimens found in the Dana Quarry of Big Horn Basin, Wyoming and held in a private collection. The specific name referred to their hypothesis based on these specimens that nearly all Morrison diplodocid species are either growth stages or represent sexual dimorphism among members of the genus Amphicoelias, but this analysis has been met with skepticism and the publication itself has been disclaimed by its lead author, explaining that it is "obviously a drafted manuscript complete with typos, etc., and not a final paper. In fact, no printing or distribution has been attempted" (see comment on previous reference).
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