As any school-age dinosaur fan can tell you, the classic dinosaur name “Brontosaurus” was overwritten long ago and replaced with “Apatosaurus.” However, a recent study by Emanuel Tschopp, Octavio Mateus, and Roger Benson resurrects Brontosaurus, concluding (among other things) that Brontosaurus and Apatosaurus were two different dinosaurs living side-by-side. But contrary to many media reports, paleontologists have never claimed that Brontosaurus didn’t exist – they just temporarily suppressed its name.

Confused? Nobody would blame you – this debate has much more to do with our cumbersome system of scientific naming that it does with the biology and diversity of prehistoric animals. Every living thing has one internationally recognized scientific name, which usually means something in Greek or Latin. A complete scientific name consists of two parts: a genus (plural is genera) and a species. A genus may be associated with only one species, or it may serve as a container for multiple closely-related species.

Unfortunately, there is no agreed-upon threshold of anatomical difference that separates two species in one genus from two species in different genera. This means that a genus is often not just a label, but a hypothesis. When a scientist places a species into a genus that already includes other species, he or she is predicting that all those species are more closely related to one another than they are to anything else. Sometimes that choice is correct, but other times new discoveries or new ways of looking at old specimens require a reshuffling of names.

In 1877, leading 19th century paleontologist O.C. Marsh gave a partial dinosaur skeleton from Colorado the name Apatosaurus ajax (Apatosaurus is the genus, ajax is the species). Two years later, Marsh named a similar set of fossils Brontosaurus excelsus. However, when paleontologist Elmer Riggs looked at both specimens in 1903, he decided that Brontosaurus excelsus was too similar to Apatosaurus to deserve its own genus name. According to the rules of scientific naming, older published names always trump newer ones, so Brontosaurus excelsus became Apatosaurus excelsus. You might think of it as a second “flavor” of Apatosaurus – still a unique kind of dinosaur, but not different enough to warrant a separate genus.

More than a century later, Tschopp and colleagues have reversed Riggs’ decision, upgrading “excelsus” from the species level to the genus level. That means that the name Brontosaurus excelsus is back, but the animal itself never went anywhere. This might seem rather arbitrary, and to some degree it is. Tschopp and colleagues used sophisticated computer programs to organize and analyze hundreds of anatomical measurements, but even with this technology some judgement calls are required.

The important takeaway from all this is that scientific names are not always hard-and-fast definitions. It’s not always obvious where the boundaries lie between different kinds of animals. This is especially true when dealing with dinosaur fossils, which are usually incomplete. Names are hypotheses, interpreted from the best available data at the time. Sometimes new information will change those interpretations, and that’s okay. Science is an ongoing, self-correcting process, and the changing status of “Brontosaurus” is an example of that process in action.

References

Brinkman, P. (2006). Bully for Apatosaurus. Endeavour 30:4:126-130.

Gould, S.J. (1991). Bully for Brontosaurus: Reflections in Natural History. New York, NY: W.W. Norton and Company.

Tschopp, E., Mateus, O., and Benson, R. (2015). A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda). PeerJ 3:857.