synonyms or reclassified as diplodocines. Brontosaurus has long been considered a junior synonym of Apatosaurus; its type 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. Description 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, reaching in length and in body mass. A 2015 study that estimated the mass of volumetric models of Dreadnoughtus, Apatosaurus, and Giraffatitan estimates CM3018 at , similar in mass to Dreadnoughtus. Some specimens of A.ajax (such as OMNH1670) represent individuals 1130% longer, suggesting masses twice that of CM3018 or , potentially rivaling the largest titanosaurs. However, the upper size estimate of OMNH1670 is likely an exaggeration, with the size estimates revised in 2020 at in length and in body mass based on volumetric analysis. The skull is small in relation to the size of the animal. The jaws are lined with spatulate (chisel-like) teeth suited to an herbivorous diet. The snout of Apatosaurus and similar diplodocoids is squared, with only Nigersaurus having a squarer skull. The braincase of Apatosaurus is well preserved in specimen BYU17096, which also preserved much of the skeleton. A phylogenetic analysis found that the braincase had a morphology similar to those of other diplodocoids. Some skulls of Apatosaurus have been found still in articulation with their teeth. Those teeth that have the enamel surface exposed do not show any scratches on the surface; instead, they display a sugary texture and little wear. Like those of other sauropods, the neck vertebrae are deeply bifurcated; they carried neural spines with a large trough in the middle, resulting in a wide, deep neck. The vertebral formula for the holotype of A.louisae is 15cervicals, 10dorsals, 5sacrals, and 82caudals. The caudal vertebra number may vary, even within species. The cervical vertebrae of Apatosaurus and Brontosaurus are stouter and more robust than those of other diplodocids and were found to be most similar to Camarasaurus by Charles Whitney Gilmore. In addition, they support cervical ribs that extend farther towards the ground than in diplodocines, and have vertebrae and