Amphicoelias ( //, meaning 'biconcave', from the Greek αμφι, amphi: "on both sides", and κοιλος, koilos: "hollow, concave") is a genus of herbivorous sauropod dinosaur that includes what may be the largest dinosaur ever discovered, A. fragillimus. Based on surviving descriptions of a single fossil bone, A. fragillimus may have been the longest known vertebrate at 40 to 60 metres (130 to 200 ft) in length, and may have had a mass of up to 122 metric tons (135 short tons). However, because the only fossil remains were lost at some point after being studied and described in the 1870s, evidence survives only in drawings and field notes. Amphicoelias is present in stratigraphic zone 6 of the Morrison Formation.
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 (3.7 m) tall.
In 1994, using the related Diplodocus as a reference, Gregory S. Paul estimated a femur length of 3.1–4 m (10–13 ft) for A. fragillimus. The 2006 re-evaluation of A. fragillimus by Ken Carpenter also used Diplodocus as a scale guide, finding a femur height of 4.3–4.6 m (14–15 ft). Carpenter went on to estimate the complete size of A. fragillimus, though he cautioned that relative proportions in diplodocids could vary from species to species. Assuming the same proportions as the well-known Diplodocus, Carpenter presented an estimated total length of 58 m (190 ft), which he noted fell within the range presented by Paul in 1994 (40–60 m, or 131–196 ft). 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 (32.5 m, 107 ft), the brachiosaurid Sauroposeidon (34 m, 111 ft), and the titanosaur Argentinosaurus (30 m, 98 ft). Carpenter presented more speculative, specific proportions for A. fragillimus (again, based on a scaled-up Diplodocus), including a neck length of 16.75 m (55 ft), a body length of 9.25 m (30 ft), and a tail length of 32 m (105 ft). He estimated the total forelimb height at 5.75 m (19 ft) and hind limb height at 7.5 m (25 ft), and the overall height (at the highest point on the back) at 9.25 m (30 ft). By comparison the blue whale, which is on average the longest living creature, reaches 30 m (98 ft) in length.
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 tons) to speculate that A. fragillimus could have weighed up to 122.4 metric tons. The heaviest blue whale on record weighed about 195 tons, and the heaviest dinosaur known from reasonably good remains, the Argentinosaurus, weighed 80–100 tons, although if the size estimates can be validated, it would still be lighter than Bruhathkayosaurus, which is estimated to have weighed 139 tons.
The Morrison Formation environment in which Amphicoelias lived would have resembled a modern savanna, though since grass did not appear until the Late Cretaceous, ferns were probably the dominant plant and main food source for Amphicoelias. Though Engelmann et al. (2004) dismissed ferns as a sauropod food source due to their relatively low caloric content, Carpenter argued that the sauropod digestive system, well adapted to handle low-quality food, allows for the consumption of ferns as a large part of the sauropod diet. Carpenter also noted that the occasional presence of large petrified logs indicate the presence of 20–30 m (65–100 ft) tall trees, which would seem to conflict with the savanna comparison. However, the trees are rare, and since tall trees require more water than the savanna environment could generally provide, they probably existed in narrow tracts or 'gallery forests' along rivers and gulleys where water could accumulate. Carpenter speculated that giant herbivores like Amphicoelias may have used the shade of the gallery forests to stay cool during the day, and done most of their feeding on the open savanna at night.
Credit to Wikipedia for the article.