This species possessed the longest cervical rib of any described sauropod dinosaur, measuring . The oldest known unequivocal sauropod dinosaurs are known from the Early Jurassic. "Brontosaurus" is a member of the family Diplodocidae, a clade of gigantic sauropod dinosaurs. "Apatosaurus" is a member of the family Diplodocidae, a clade of gigantic sauropod dinosaurs. The reasons mammals cannot reach the much larger size of sauropod dinosaurs are unknown. The Morrison Formation records an environment and time dominated by gigantic sauropod dinosaurs. In 1999, a huge Cretaceous sauropod about 95 million years old was unearthed near Winton. Most known instances of preserved sauropod gastroliths are from Jurassic animals. The exact relationships with other basal sauropod genera remain unclear. However, none were found after they examined a large number of sauropod skeletons. "Apatosaurus" was the second most common sauropod in the Morrison Formation ecosystem, after "Camarasaurus". Paleontologist Richard Owen named the first sauropod, "Cetiosaurus", in 1841. All sauropod remains from this bonebed appear to come from the same genus of sauropod. This expedition also recovered the peculiar spiny sauropod "Amargasaurus". Finds of sauropod footprints in the 1930s eventually put Hay's theory to rest.