Amphicoelias fragillimus was a sauropod herbivore dinosaur, possibly the largest dinosaur (or animal of any kind) that ever existed. Its veracity is contested because the only bones of the dinosaur, a partial vertebra and a gigantic femur, have since been lost. The vertebra fragment, located by an employee of the American paleontologist Edward Cope in 1877, measured 1.5 m (5 ft) in length. The vertebra it was a part of during the life of the dinosaur would have been 2.7 m (8.8 ft) in length. Extrapolating the length of the vertebra to the total size of the dinosaur, based on similar species, has given an estimated length of 40-60 m (131-196 ft), with a mass of up to 122 tonnes (135 tons), longer than a blue whale but approximately two-thirds its weight.

Amphicoelias的意思是 "双空"，指的是这种动物的椎体壁很薄，这对于这种体型的动物携带自己的骨架是必要的。 物种名称的第二部分，fragillimus，是指化石的脆弱性。 它是在科罗拉多州的泥岩中发现的，这种岩石很脆弱，容易破碎成不规则的小方块。 化石的草图只从一个角度完成，这很不寻常，因为爱德华-科普以详细和从多个角度绘制每个重要化石而闻名。 历史学家认为，这可能是因为化石在第一次草图完成后破碎成碎片。

Amphicoelias was a diplodocid dinosaur, a family of sauropods known for their extremely long and slender bodies. Like other sauropods, an adult Amphicoelias probably would have had little need to worry about predators, spending all its time grazing foliage at the same height as its head, which would have been about 9 m (30 ft) off the ground. It is uncertain how old the dinosaur would have been at death or how fast it grew: it depends on its metabolism. If it were warm-blooded like mammals, it would have grown to full size in about 10 years, growing an average of roughly 20 ft/year, 1.7 ft per month, or a few inches per day. If it were cold-blooded like reptiles, it would have required a full century to reach the size at which it died. Scientists continue to debate whether dinosaurs in general were cold or warm-blooded, and the consensus seems to be that they were somewhere in between.

不幸的是，19世纪70年代后所有关于Amphicoelias的报告都表明，有关的骨头已经失踪。 因此，每当恐龙被介绍为可能是有史以来最大的生物之一时，就会有相当大的争议。