Brontosaurus skeleton by Othniel Charles Marsh, circa 1880s.
The news that ‘Brontosaurus is back!’ made international headlines recently. Since 1903, paleontologists have been explaining that ‘Brontosaurus’ didn’t exist. It was considered just a subspecies of Apatosaurus… until now.
When a new species is named, a researcher must demonstrate that the species is unique, and different from any closely related species. If another researcher can prove that it is not notably different, the new species designation is rejected. So it was with Brontosaurus. In 1879, paleontologist Othniel Charles Marsh named a fossil skeleton he had found ‘Brontosaurus.' However, a second paleontologist named Elmer Riggs later demonstrated that the fossils were not sufficiently different from Apatosaurus (a species Marsh had named in 1877) to warrant their own designation. Because Apatosaurus had been named before Brontosaurus, Apatosaurus was the name that stayed, and ‘Brontosaurus’ became a nomen dubium (Latin for ‘doubtful name’).
In a 2015 study by Emanuel Tschopp and colleagues, the relationships between diplodocids (the group of long-necked dinosaurs that Apatosaurus and Brontosaurus belong to) were re-examined. The paleontologists used new, sophisticated techniques and different statistical analyses to resolve the diplodocid phylogeny (‘family tree’). When they ran their analysis, three Apatosaurus species that had originally been described as Brontosaurus grouped together. This meant that Apatosaurus and Brontosaurus were different enough to be considered different groups; hence Brontosaurus is a legitimate species again.
This study comes down, in part, to ‘what is a fossil species?’ Obviously we can't go back in time and see if Apatosaurus and Brontosaurs could mate to produce fertile offspring, which is the modern definition of an animal species. We have to rely on fossil evidence and on continuously evolving techniques to better understand the ancient world.