Brontosaurus
"Brontosaurus" was a large, long-necked quadrupedal animal with a long, whip-like tail, and forelimbs that were slightly shorter than its hindlimbs. The largest species, "B. excelsus", weighed up to and measured up to long from head to tail. For comparison, the current largest land animal, the average male African bush elephant is 3.20 m (10.5 ft) tall at the shoulder and has a body mass of 6,000 kg (13,228 lb) or 6.6 short tons.
Apatosaurus
"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 be , by measuring the vertebral column. Current estimates are similar, finding that the individual was long and had a mass of . A 2015 study that estimated the mass of volumetric models of "Dreadnoughtus", "Apatosaurus", and "Giraffatitan" estimates CM3018 at , similar in mass to "Dreadnoughtus". Past estimates have put the creature's mass as high as . Some specimens of "A.ajax" (such as OMNH1670) represent individuals 1130% longer, suggesting masses twice that of CM3018 or , potentially rivalling the largest titanosaurs.
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.