Apatosaurus fossils were first found in 1877 in the Morrison Formation, a location that boasts a wealth of dinosaur fossils from the Jurassic Period. Much of the skeleton was found, with the skull being the main element that was missing.
Museums were keen to mount skeletal reconstructions of these massive long-necked dinosaurs after their discovery, to attract visitors who wanted to view the bones of these ancient massive creatures. A mount of Apatosaurus was constructed for the American Museum of Natural History, but since no Apatosaurus skull was known at the time, the skull was reconstructed based on that of Camarasaurus. In 1909 an Apatosaurus skull discovery would show its head was actually long and narrow like Diplodocus, rather than deep and short like that of Camarasaurus.
In the early 1900s, it was also determined that Apatosaurus and another dinosaur, Brontosaurus, likely represented different ages of the same dinosaur. Since Apatosaurus was the earlier discovery, its name was kept and Brontosaurus was no more. However, the American Museum of Natural History labeled their mount Brontosaurus, and thus the name became popular among the public.
The skull would not be officially mentioned in scientific literature until the 1970s, and the first mount of an Apatosaurus with an accurate skull would appear in the Carnegie Museum in 1979.
In 2015, new studies showed significant differences between Apatosaurus and the remains originally named Brontosaurus, so Brontosaurus was once again given status as a distinct genus from Apatosaurus.