This is a story about Apatosaurus and Brontosaurus, both named by paleontologist Othniel Charles Marsh in 1877 and 1879 respectively. The two sauropods have had a long, complicated history of classification that everyone, even people outside of the paleontological community, are at least partially familiar with.
At first, Marsh named Apatosaurus ajax based on material he recovered from the Morrison Formation in 1877, noting a mosasaur-like anatomical feature on its tail. The dinosaur’s name, “Deceptive lizard,” is a reference to this enigma. He later also named Brontosaurus excelsus based on bones from the hip of a sauropod.
In 1903, however, paleontologist Elmer Riggs argued that Marsh’s two sauropods were not different enough anatomically to be recognized under two different genus names. He pointed out that the shape of the shoulder blades and the bones in the hips were more or less the same between the two. And so following Riggs’ 1903 paper, the two genera were synonymized and the newer name “Brontosaurus” was dropped. Although it should be noted, Riggs still regarded them as different species within the same genus.
This didn’t stop Brontosaurus’ loyal fans, however. The majority of dinosaur-themed media at the time such as books, toys, comics, and postcards continued to recognize Brontosaurus despite what Riggs had argued in 1903. In fact, it seems as though Brontosaurus simply skyrocketed in fame within dinosaur pop-culture after it had been considered a synonym of Apatosaurus by Riggs. Brontosaurus went on to become one of the names that the general public would consider synonymous with the very word “dinosaur.”
Fast forward to the middle to late 20th century, we see people becoming increasingly aware that Brontosaurus “didn’t exist” — and was just another name for Apatosaurus. Mention the name “Brontosaurus” in front of a large enough audience and you’d most likely get one or two people correcting you. This would be the case for over a century in paleontology. People even said that their “childhood was ruined” because their favorite dinosaur “didn’t exist” — which is a misconception since the animal in question has always existed simply as another species within the genus Apatosaurus.