Brontosaurus has long been considered a junior synonym of Apatosaurus. Its type species was reclassified as A. excelsus way back in 1903. However, a 2015 study concluded that Brontosaurus is a valid genus of sauropod distinct from Apatosaurus, and concluded that Brontosaurus should be "resurrected" as a valid name. Not all paleontologists agree with this division.

All mammals, including giraffes, have seven cervical (neck) vertebrae, but Apatosaurus needed 15 cervical vertebrae to support its long neck. As a result, it had a great feeding range and could probably bend its neck into a U-shape laterally.

Assuming Apatosaurus had an avian respiratory system and a reptilian resting-metabolism, a 1997 study estimates the animal would have needed to consume only about 262 liters (69 U.S. gal) of water per day.

Apatosaurus had a single claw on each forelimb and three on each hindlimb--for a total of eight.

The estimated lifespan of an Apatosaurus is believed to be around 100 years.