Amphicoelias is a genus of herbivorous sauropod dinosaur. It includes what has sometimes been estimated to be the largest dinosaur specimen ever discovered, originally named “A. fragillimus“. Based on surviving descriptions of a single fossil bone, scientists had over the years estimated A. fragillimus to have been the longest known animal at 58 metres (190 ft) in length, with potentially a mass of up to 122.4 tonnes (134.9 short tons). However, because the only fossil remains were lost at some point after being studied and described in the 1870s, evidence survived only in drawings and field notes. More recent analysis of the surviving evidence, and the biological plausibility of such a large land animal, has suggested that the enormous size of this animal were over-estimates due partly to typographical errors in the original 1878 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. fragillimus, in the same paper. However, all subsequent researchers have considered A. fragillimus to be a synonym of A. altus. Even by 1881 however, it was recognized that A. altus could not be distinguished from other genera, as the features described by Cope were misinterpreted and are widespread. 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 dentition of Amphicoelias is homodont. Its teeth are shaped like long slender cylindrical rods, are spaced apart and project forward towards the front of the mouth. 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 has suggested that Amphicoelias is probably the senior synonym of Diplodocus.
Producing an estimate of the complete size of A. fragillimus requires scaling the bones of better-known species of diplodocid (a family of extremely long and slender sauropods) in the assumption that their relative proportions were similar. In his original paper, Cope did this by speculating on the size of a hypothetical A. fragillimus femur (upper leg bone). Cope noticed that in other sauropod dinosaurs, specifically A. altus and Camarasaurus supremus, the femora were always twice as tall as the tallest dorsal vertebra, and estimated the size of an A. fragillimus femur to be 12 ft (3.6 m) tall.