These dinosaurs were herbivorous, but grass didn't exist until the late Cretaceous period. Therefore, Jurassic ferns were probably most of what the Amphicoelias ate.
Amphicoelias was possibly either the or one of the biggest dinosaurs ever. It reached a maximum size of 40-60 meters long.
Amphicoelias was described by Edward Drinker Cope in 1878, who believed that the remains (a single vertebra) were from the largest dinosaur ever. This bone is now lost, so we cannot confirm its measurements.