Amphicoelias ("biconcave") is a genus of herbivorous sauropod dinosaurs that lived in late Jurassic North America. Its type species is Amphicoelias altus, though the genus also 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 (135 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.

In The Land Before Time

Juvenile Amphicoelias
A sauropod that can potentially be pinpointed as an Amphicoelias fragillimus appears in The Land Before Time TV series episode, The Hidden Canyon. The identification comes from the fact that, while the size of a normal Land Before Time adult "Longneck", it uses a model usually only used for young characters, such as Littlefoot and Ali. This could potentially mean that the individual represents a juvenile of an even larger dinosaur; likely the behemoth A. fragillimus described by Cope in 1870.