Apatosaurus considered a junior synonym of "Apatosaurus"; its only species was reclassified as "A.excelsus" in 1903. A 2015 study concluded that "Brontosaurus" is a valid genus of sauropod distinct from "Apatosaurus", but not all paleontologists agree with this division. As it existed in North America during the late Jurassic, "Apatosaurus" would have lived alongside dinosaurs such as "Allosaurus", "Camarasaurus", "Diplodocus", and "Stegosaurus". "Apatosaurus" was a large, long-necked, quadrupedal animal with a long, whip-like tail. Its forelimbs were slightly shorter than its hindlimbs. Most size estimates are based on specimen CM3018, the type specimen of "A.louisae". In 1936 this was measured to
Apatosaurus the time of the creature's maturity. This was first noted in 1903 and again in 1936. Long-bone histology enables researchers to estimate the age that a specific individual reached. A study by Eva Griebeler etal. (2013) examined long-bone histological data and concluded the "Apatosaurus" sp.SMA0014 weighed , reached sexual maturity at 21years, and died aged 28. The same growth model indicated "Apatosaurus" sp.BYU 601–17328 weighed , reached sexual maturity at 19years, and died aged 31. Compared with most sauropods, a relatively large amount of juvenile material is known from "Apatosaurus". Multiple specimens in the OMNH are from juveniles of an