Brontosaurus was a distinct long-necked dinosaur species, one of the largest land animals ever.

In August 1877, fossil hunter Arthur Lakes sent Othniel Charles Marsh sketches of enormous vertebrae embedded in stone at Como Bluff, Wyoming. Marsh was already locked in fierce competition with his rival Edward Drinker Cope for naming rights over the creatures emerging from the American West, the conflict paleontologists later called the Bone Wars rewarded speed over precision. When Marsh formally named a new sauropod, Brontosaurus excelsus, from Como Bluff material in 1879, he was working fast. He had already described another sauropod, Apatosaurus ajax, two years earlier from the same formation. Both were large, long-necked herbivores. Marsh believed they were different enough to deserve separate names.

In 1903, Elmer Riggs of the Field Columbian Museum in Chicago compared the available specimens carefully and concluded that Marsh had been wrong. In a paper published in the museum's Geological Series, Riggs argued that the anatomical differences between Apatosaurus and Brontosaurus were too minor to support two distinct genera. Under the rules that govern biological nomenclature, when two names apply to the same animal, the older name takes priority. Apatosaurus had been named in 1877; Brontosaurus came two years later. Brontosaurus became a junior synonym — valid as a description of real fossils, but no longer a legitimate independent genus.

The correction was impeccably documented and largely ignored by the culture it was trying to reach.

The American Museum of Natural History had mounted a large sauropod skeleton under the Brontosaurus label, and in 1905 capped it with a skull drawn from a different quarry and, as later analysis established, from a different species — creating an iconic image of a large, small-headed beast that entered popular consciousness. The creature appeared in Disney's Fantasia in 1940 as a lumbering swamp-dweller. Sinclair Oil adopted it as their company logo, and its image graced gas stations across mid-century America. Children's books, encyclopedias, and toy manufacturers perpetuated the name for seven decades after Riggs had technically retired it.

Natural history museums began replacing their Brontosaurus labels with Apatosaurus in the late 1970s, and the Carnegie Museum of Natural History attached the anatomically correct skull to its sauropod skeleton in 1979. The effect on public awareness was minimal. Adults who had grown up with Brontosaurus experienced the correction not as scientific clarification but as something being taken from them. A minor cultural grievance accumulated: scientists had killed a beloved dinosaur.

In 2015, Emanuel Tschopp and colleagues at the Universidade Nova de Lisboa published the most comprehensive specimen-level analysis of diplodocid sauropods ever attempted — 81 specimens coded for 477 morphological characters. The analysis found that specimens assigned to Brontosaurus clustered consistently apart from those of Apatosaurus, with differences in the neck vertebrae, humerus, and pelvis proportions. Published in PeerJ, the study formally proposed reinstating Brontosaurus as a valid genus. Whether that proposal ultimately changes the scientific consensus, it does not revise the original error: for seventy years, children were taught Brontosaurus was a distinct animal when the paleontological record had established since Riggs's 1903 paper that the skeleton on display was Apatosaurus. The beloved Brontosaurus of mid-century classrooms and encyclopedias was the same animal as Apatosaurus — a name applied twice to the same fossil material during the competitive scramble of the Bone Wars, and the duplicate had no priority.