Monday, April 6, 2009

What's In A Name? Brontosaurus Returns?

To elaborate on the previous story:
As you all probably know, the reason the name Brontosaurus was abandoned in the first place is that it turned out to be a subjective junior synonym of Apatosaurus. (NB: It was NOT because it was a chimera. The whole "wrong head" thing is a separate issue and overstated anyway. That head was a decent educated guess at the time!) Basically, Apato was named first, people decided they belonged to the same genus (note: not species), and so the name Brontosaurus was sunk since it was coined second.

[Image: Apatosaurus mount at the AMNH. According to he SV-POW goss mentioned previously, this will remain Apatosaurus even after Brontosaurus is resurrected. Photo by Erika & Shannon, from Wikipedia, some rights reserved.]

If you're really observant and know something about taxonomy, you can see why this was not an open and shut case or a definitely permanent change. First of all the synonymy was subjective, not objective. Apato and Bronto appeared to belong to the same genus, but were not named for the same exact specimen, so the synonymy was more of an opinion. Especially given that it was generic, not specific synonymy. When species get sunk, it tends to stick. Whether two species belong to the same genus, however, is entirely dependent on whether you're a lumper or a splitter. Tom Holtz coined the term "genericometer" for the mental device people use to demarcate genera, and to illustrate just how subjective it is. With a finely-tuned genericometer, every genus only has one species (often over-splitting taxa). Loosen the settings on yours, and suddenly Daspletosaurus torosus is looking a lot like it should get renamed Tyrannosaurus torosus (probably a case of over-lumping).

This whole business has been simplified somewhat by phylogenetic taxonomy. Broadly, when all taxa should also be clades, it's easy enough to split and lump based on relationships. Daspletosaurus is less closely related to Tyrannosaurus than a few other species, so if you lump one you must lump all (or else the taxon Tyrannosaurus would become polyphyletic, an unnatural grouping), which doesn't make much sense and is not very useful in communicating about dinosaurs.

[Image: Drawing of the Brontosaurus skeleton by O.C. Marsh, 1896. Public domain.]

On the other hand, Apatosaurus and Brontosaurus are each others closest relatives, so you're not leaving anything out by lumping them. It forms a natural group, so it makes sense.

Or do they form a natural group? When a new paper was published on Supersaurus in 2007, the authors pointed out that it shared many features with apatosaurs, and belonged to the clade Apatosaurinae. In an offhand way with little elaboration, they mentioned Supersaurus has some characters that link it to Apatosaurus louisae in particular. This is significant, since that's the non-Brontosaurus species. If Supersaurus is closer to A. louisae than to A. excelsus (aka Brontosaurus), that means Apatosaurus would not form a natural group unless Supersaurus was also sunk into Apatosaurus, which, like in the tyrannosaur example, makes little sense.

Word on the tubes is that this is exactly the case. Supersaurus is likely closer to Apatosaurus than either are to Brontosaurus, and that means Brontosaurus needs to be brought back as a valid name. Sources say exactly this is in the works now, and a new paper will show that Supersaurus does indeed split up the Apatosaurus band. Look for news media sources trumpeting the return of one of the most famous dino names ever in the next year or so.

Stay tuned for more Bronto- related news, Brontosaurus, and otherwise... (hint hint?).