In 1877, O.C. Marsh published a description of the first excavated bones of a
large dinosaur — isolated vertebrae and a sacrum (the fused vertebrae between
the pelvic bones) — sent to him from Morrison, Colorado. Marsh gave the animal
the genus name 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.
Though not quite as large as Apatosaurus, this specimen turned out to be
very complete. He described and named this second dinosaur “Brontosaurus”
(“thunder lizard”) in 1879. This specimen is the mounted skeleton on display in
the Peabody’s Great Hall.
Marsh recognized the familial relationship 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. What was not known at the time was that the number of sacral vertebrae is related to age: as the animal got older two additional vertebrae fused to the original three sacral vertebrae. It was paleontologist Elmer Riggs who in 1903 realized that the animal called Apatosaurus was really a young “Brontosaurus” (albeit a very large youngster).
According to rules of scientific nomenclature (how new species are named), because the name Apatosaurus had been published first it had priority over “Brontosaurus.” To show that “Brontosaurus” is no longer a valid scientific name it is written unitalicized within quotation marks.
However, because Marsh first described the genus “Brontosaurus” based on this mounted skeleton — which you can see in the Museum’s Great Hall — this individual specimen keeps its name as part of its permanent official record. So the Yale Peabody Museum is the only museum in the world that can accurately say, “This is Brontosaurus.”