Diplodocus now common scientific opinion that "Seismosaurus hallorum" is a species of "Diplodocus". This genus of dinosaurs lived in what is now mid-western North America at the end of the Jurassic period. "Diplodocus" is one of the more common dinosaur fossils found in the middle to upper Morrison Formation, between about 154 and 152 million years ago, during the late Kimmeridgian age. The Morrison Formation records an environment and time dominated by gigantic sauropod dinosaurs, such as "Apatosaurus", "Barosaurus", "Brachiosaurus", "Brontosaurus", and "Camarasaurus". "Diplodocus" is among the most easily identifiable dinosaurs, with its typical sauropod shape, long neck and tail, and
Apatosaurus on the anterior end. The pelvic girdle includes the robust ilia, and the fused (co-ossified) pubes and ischia. The femora of "Apatosaurus" are very stout and represent some of the most robust femora of any member of Sauropoda. The tibia and fibula bones are different from the slender bones of "Diplodocus" but are nearly indistinguishable from those of "Camarasaurus". The fibula is longer and slenderer than the tibia. The foot of "Apatosaurus" has three claws on the innermost digits; the digit formula is 3-4-5-3-2. The first metatarsal is the stoutest, a feature shared among diplodocids. The name "Apatosaurus ajax" was