Until about 25 years ago, Brontosaurus would have been on anyone's short list of the world's most famous dinosaurs, along with Tyrannosaurus Rex, Triceratops and Stegosaurus. But today, under the scientifically correct (and much less impressive) name Apatosaurus, this late Jurassic sauropod has slipped down into B-list territory, along with such reliable but unexciting dinosaurs as Compsognathus and Deinonychus.
What went wrong? Well, the story begins in 1877, at the height of the Bone Wars (the sometimes-underhanded competition between Edward Drinker Cope and Othniel C. Marsh about which paleontologist could discover and name the most dinosaurs). That year, Marsh examined the incomplete fossil of a juvenile sauropod, a type of plant-eating dinosaur that paleontologists were only just beginning to understand. He assigned this specimen, which had been discovered in the western U.S., to a new genus, Apatosaurus, Greek for "deceptive lizard"--not a foreshadowing of the confusion to come, but a reference to the fact that the bones he examined had initially been mistaken for those of a mosasaur, or marine reptile.
Enter (and Exit) Brontosaurus
So far, so good. Unusually, the next chapter in the story of Apatosaurus didn't involve Edward Drinker Cope, who ordinarily would have jumped with both feet onto an error committed by his arch-rival. Rather, Marsh inflicted the damage on himself: two years later, he examined the fossil of a much larger sauropod that had been discovered in Wyoming, for which he erected the genus name Brontosaurus ("thunder lizard") and the species name excelsus ("highest" or "sublime"--"excellent," if you will).
As luck would have it, Brontosaurus, not Apatosaurus, was the name used when the first-ever reconstructed sauropod went on display at the Yale Peabody Museum of Natural History in 1905, instantly propelling this dinosaur to the top tiers of the public's imagination. Given the lack of knowledge available at the time, this "Brontosaurus" was a bit of a chimera, incorporating parts (especially its feet and its thick, heavy skull) from the better-attested sauropod Camarasaurus. In fact, it wasn't until the mid-1970's that the correct skull--relatively small and tapered compared to that of Camarasaurus--was finally attached to the long, slender neck of Apatosaurus.
So why is Brontosarus now Apatosaurus? Well, after Marsh had done his work, a paleontologist named Elmer Riggs examined both fossils and concluded that what Marsh called Brontosaurus was in fact an adult specimen of Apatosaurus. Under the rules of scientific nomenclature, Brontosaurus was jettisoned, and Apatosaurus was deemed the "correct" name. It may surprise you to learn that Riggs published this conclusion way back in 1903, yet the name Brontosaurus managed to stick around for decades; some scientific errors take a long time to correct themselves!
Will Brontosaurus Have its Revenge?
After the Brontosaurus/Apatosaurus debacle, a list of the various species assigned to this dinosaur may seem anticlimactic, but they're still important to know. When Elmer Riggs reverted Brontosaurus back to Apatosaurus, he made a bit of a compromise, retaining the species name excelsus. (Marsh had originally erected the Apatosaurus species name ajax, after the famous warrior of Greek myth.) Since then, two new species have taken their place alongside Apatosaurus excelsus: Apatosaurus louisae in 1915 (after Louise Carnegie, the wife of the famous plutocrat and dinosaur enthusiast Andrew Carnegie) and Apatosaurus parvus in 1994 (this specimen had originally been assigned to its own genus, the now discarded Elosaurus).
There is a fourth named species of Apatosaurus, but it's the subject of some debate. Apatosaurus yahnahpin was identified in 1994; shortly thereafter, the maverick paleontologist Robert Bakker--who has never tried to hide his disappointment at the disappearance of the name Brontosaurus--assigned this species to a newly erected genus, Eobrontosaurus ("dawn Brontosaurus"). However, most other paleontologists believe that Eobrontosaurus yahnahpin was really a species of Camarasaurus, and Bakker's genus name is not widely accepted in the scientific community.