Amphicoelias was a genus of sauropod dinosaur, which may have included the largest dinosaur ever discovered ("A. fragillimus"). If it truly existed, "A. fragillimus" would be the longest vertebrate by a considerable margin, and it would have a mass that rivals the heaviest animal known, the blue whale. However, because the only fossil evidence for its existence was lost shortly after being discovered in the 1870s, evidence survives only in drawings, and doubt has been expressed about their accuracy.