In 1877, Yale professor O.C. Marsh published a description of the first excavated bones of a large dinosaur sent to him from Morrison, Colorado. These bones were isolated vertebrae and a sacrum (the fused vertebrae between the pelvic bones). Marsh named the animal Apatosaurus (“deceptive lizard”) because of certain skeletal features of the tail that he found deceptively similar to those of a group of marine lizards called mosasaurs. Two years later, Marsh received 25 crates of bones of another large dinosaur discovered at Como Bluff, Wyoming. This specimen—the skeleton mounted here—turned out to be very complete. Described by Marsh in 1879, he named this second dinosaur Brontosaurus (“thunder lizard”).
Marsh recognized the close similarity between the two giant dinosaurs, but because Apatosaurus had three sacral vertebrae in its hip region and Brontosaurus had five, he gave them two different names. It was not known at the time that the number of sacral vertebrae is related to age: two additional vertebrae fused to the original three sacral vertebrae as the animal got older. In 1903 paleontologist Elmer Riggs proposed that the animal called Apatosaurus was really a young Brontosaurus (albeit a very large youngster). Because the name Apatosaurus had been published first, it had priority over “Brontosaurus.”
But in 2015, a new study revealed more than seven differences between Apatosaurus and Brontosaurus. Marsh’s idea that they were two separate animals was likely correct, after all. Brontosaurus was reborn. The skeleton on display here—YPM 1980—is the original specimen and namesake of the famous Brontosaurus.