Amphicoelias (pronounced Am-fi-see-lias, meaning 'Doubly Hollow') is a genus of herbivorious sauropod dinosaur and is the largest animal to have ever live, bigger than the blue whale . Two species have been named, A.fragillimus and A.altus. A.fragillimus is known from a single bone which was lost more than 100 years ago and the size estimates are based of A.altus and other diplodocid sauropods.
A.fragillimus is based of a femur and a neural arch, both are now lost. A.fragillimus was described in 1884. The type species , A.altus is more complete consisting of two vertebrae, a pubis (hip bone), and a femur (upper leg bone). 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. A.altus was described in 1878.
Amphicoelias is a massive animal. A. altus measured around 25m long, a similar size to smaller species of its close relative Diplodocus. However, A.fragillimus is much larger. Its sizes are
based on drawings of the original fossil and related diplodocids. It was estimated by Gregory S. Paul in 1994 to measure 40-60m long. There was a re-evaluation of Amphicoelias' size in 2006 by Ken Carpenter. Carpenter presented an estimated total length of 58m (190ft), which he noted fell within the range presented by Paul in 1994 (40–60m, or 131–196ft). Carpenter pointed out that even the lowest length estimates for A. fragillimus were higher than those for other giant sauropods, such as the diplodocid Supersaurus (32.5m, 107ft), the brachiosaurid Sauroposeidon (34m, 111ft), and the titanosaur Argentinosaurus (30m, 98 ft). Carpenter presented more speculative, specific proportions for A. fragillimus (again, based on a scaled-up Diplodocus), including a neck length of 16.75m (55ft), a body length of 9.25m (30ft), and a tail length of 32m (105ft). He estimated the total forelimb height at 5.75m (19ft) and hind limb height at 7.5m (25ft), and the overall height (at the highest point on the back) at 9.25m (30ft). By comparison the blue whale, which is on average the longest living creature, reaches 30–33m (98–110ft) in length.
These figures from Carpenter assume the known neural arch came from the largest in the body, when it is perfectly possible it did not. This would suggest that despite the huge size of these estimates, they are a touch conservative.
While A. fragillimus was relatively thin, its enormous size still made it very massive. Weight is much more difficult to determine than length in sauropods, as the more complex equations needed are prone to greater margins of error based on smaller variations in the overall proportions of the animal. Carpenter used Paul's 1994 estimate of the mass of Diplodocus carnegii (11.5 tonnes) to speculate that A. fragillimus could have weighed up to 122.4 metric tonnes. The heaviest blue whale on record weighed about 195 tonnes, and the heaviest dinosaur known from reasonably good remains, the Argentinosaurus, weighed 80–100 tonnes, although if the size estimates can be validated, it would still be lighter than Bruhathkayosaurus, which is estimated to have weighed 139 tonnes.