This Apatosaurus, collected in the late 1890s, was the first sauropod dinosaur ever mounted. Museum preparators labored over the specimen for several years before it went on view in 1905, and it has been a focal point of the collection ever since. Like the Tyrannosaurus rex, Apatosaurus (formerly called Brontosaurus) was disassembled in 1992 and remounted to reflect current ideas about how it looked and carried itself. At the Museum, a small model near the front left foot shows how it used to be mounted, with its tail dragging, looking as if it could barely hold up its own height. Scientists have found no evidence that these animals, or any other sauropods, dragged their tails. None of the numerous trackways of sauropod footprints that have been discovered have tail marks in them.

For many years, Apatosaurus stood not only with dragging tail, but with the wrong skull. As no skull was found with the original fossil, Henry Fairfield Osborn, then Chairman of the Department of Vertebrate Paleontology, had a plaster one sculpted. Because he believed that this animal was related to more primitive sauropods called camarasaurs, he had the skull made to look like that of a camarasaur. To this day, no Apatosaurus has ever been found with its skull directly attached, but one skull has been found lying close to an Apatosaurus skeleton. A cast of that skull now sits on top of this gigantic fossil's neck.

In addition to its new position and skull, the specimen's neck and tail have been lengthened; the former now contains fifteen vertebrae, and the latter is forty-nine feet long.