Pseudoplocephalus: A Brontobyte of Sauropods

Palaeontology emergency alert! This is not a drill! Brontosaurus is back!

YES I FINALLY GOT TO USE THIS ON THE BLOG. Success!

I mean, Brontosaurus never really left. That's the nice thing about taxonomy – once a name is out there, it's there forever, even if we decide later on that it might represent the same kind of animal that another name does. And so every now and then, we get to bring an old name back from the dead. Today, Tschopp and colleagues have published some very good support to indicate that Brontosaurus really is distinct from Apatosaurus after all, and we can all use that name and stop telling people that Brontosaurus isn't real. OMG, WHAT A RELIEF.

To recap: Brontosaurus has not been an accepted name in the palaeontological community for more than 100 years, but because of its use in some museum exhibits, and things like the 1964 World's Fair and the "Rite of Spring" passage in Fantasia, for example, the name has become entrenched in the popular consciousness in a way few other dinosaur names have. It is very disappointing to learn that palaeontologists don't call that big dinosaur Brontosaurus, but the decidedly less evocative name Apatosaurus instead.

Click for sauropod-size. With many thanks to the authors and PeerJ for creating such a useful diagram, which I'm sure will be reproduced often and with much gratitude by palaeontologists, teachers, and other science communicators.

The new paper is staggering in its length (almost 300 pages!) and the amount of work it represents, and I'm not a sauropod specialist, so I'll summarize it here without delving into sauropod anatomy very much:

Two of the Big 3 diplodocids: Apatosaurus (in the back) and Diplodocus (foreground) face-off at the Carnegie Museum.

Tschopp et al. did a specimen-level phylogeny of diplodocids, the sauropods like Apatosaurus and Diplodocus, but not Brachiosaurus or Camarasaurus. This means that individual specimens were coded, rather than species. Often, phylogenetic studies have just looked at the 'classic' diplodocids Apatosaurus, Barosaurus, and Diplodocus (the 'Big 3', shall we say?). And most of those studies elide the many species represented by these three genera. So a specimen-level phylogeny is a much-needed approach to resolve some questions about diplodocid diversity.
They then used some techniques to quantify differences among specimens – pairwise dissimilarity, and apomorphy counts – that would help justify dividing clusters of individuals into different genera. There isn't a rule in palaeontology that individuals need to be a certain amount 'different' from each other in order to be a new genus or species, so the authors looked at how many unique characters separate some sauropods that everyone seems pretty comfortable calling different species and genera. Apatosaurus ajax and Apatosaurus louisae had 12 different features, and Diplodocus carnegii and Diplodocus hallorum had 11 different features. So 13 different characters was set as the baseline for separating out genera in the specimen phylogeny. Using the same approach, they also set 6 differences as the baseline for separating species within a given genus. These numbers only apply to this particular analysis, but it's an interesting approach that I think would be worth considering for other dinosaur phylogenies.
Using this, they wind up doing some taxonomic reshuffling:

a. Diplodocus longus lacks any diagnostic features at the species level and is a nomen dubium, which is bad because it's also the type species for Diplodocus. A petition to the ICZN to switch the type species to D. carnegii is in the works. Diplodocus includes the species D. carnegii and D. hallorum (née Seismosaurus)

b. Dinheirosaurus (from Portugal) is a junior synonym of Supersaurus, and so Supersaurus is a cross-continental genus represented by two species.

c. Diplodocus hayi passes the threshold for generic distinctiveness from Diplodocus and gets a new name, Galeamopus hayi. Specimens of Galeamopus are actually more complete than Diplodocus, which means that Diplodocidae is best represented by Galeamopus at present if you need a diplodocid for whatever you're working on.

d. And finally, and arguably most significantly, Brontosaurus passes the threshold for generic distinctiveness from Apatosaurus. There are three species within Brontosaurus: B. exelsus ('classic' Brontosaurus), B. parvus (née Elosaurus), and B. yahnahpin (née 'Eobrontosaurus').

The third of the Big 3 diplodocids, the iconic rearing Barosaurus at the American Museum of Natural History.

I really hope this taxonomic shuffling gains wide acceptance, because 1) I think their approach and reasoning are pretty sound, and 2) it's going to be SO MUCH EASIER not to have to constantly 'debunk' Brontosaurus with non-palaeontologists.The oft-repeated story that "Brontosaurus" wasn't real because it had the head of one animal and the body of another is wrong, but the real story, about the rules of taxonomy and how we define species, is much more difficult to explain. (It's interesting, but it's not as easily parsed to a lay audience.) And let's face it, Brontosaurus was a really good name and it was sad that it had to be synonymized. The story of Brontosaurus now has a new and interesting chapter – our ideas about the biology of Brontosaurus have changed, but now we can talk about changes to how we think Brontosaurus looked and lived, rather than just focusing on a quirk of taxonomy. So let your Brontosaurus flag fly high, dinosaur fans, because Brontosaurus is back and that's awesome.

Old-timey sauropod in the little diorama at the Smithsonian, back in 2011.

Big taxonomic revisions are hard and important but often don't feel as 'sexy' as some of the other research that gets publicized. I like thinking about alpha taxonomy (uh, perhaps obviously) and I like doing this kind of research, and I think it's really important that we recognize how important this kind of work is – alpha taxonomy is really foundational to a lot of other studies. If you don't know how many species you have, or where they lived, or what anatomy belongs with each species, how can you do projects that look at the evolution of certain features through time, or understand changing ecosystems?

For example, given that there's at least 14 species of diplodocid in only 11 million years of Morrison Formation, it's unlikely that there's a slice of time in there in which there's only one diplodocid species. (And remember, diplodocids weren't the only sauropods in the Morrison – this is also the home of Brachiosaurus and Camarasaurus and Suuwassea and who knows what else.) This is a pretty good reason to reject what I like to call the "Highlander hypothesis", i.e. There Can Only Be One ___(ankylosaur, tyrannosaur, whatever)___ in a given formation, something that I've encountered in conversations on occasion. It's understandable that we would feel unease at the idea of high species/generic diversity in such massive dinosaurs, because how are they dividing up ecosystem space? But over and over again it seems like lots of similarly-shaped dinosaurs were occupying similar times and spaces in terms of what we see in the rock record, which I find very interesting indeed. (Now what we need is a really good stratigraphic framework for putting all of these diplodocids into chronological and geographical context.) We can only do a good job of addressing these kinds of questions by having good data to put into those studies, and that data comes from taxonomic revisions like this one.

And revising taxonomy is probably a never-ending job, because we need to keep reassessing our definitions of genera and species as we get more information through new specimens. Let's make sure we all support this kind of research as palaeontology continues to evolve with new techniques, questions, and approaches. Bully for Brontosaurus, and bully for alpha taxonomy.

Stray observations:

The concept of a 'relatively small' animal that is 12-15 metres long amuses me. (re: Kaatedocus, page 2)
The 'brontobyte' image at the top of this post is an old joke from my Currie lab days; a brontobyte is actually 10^27 bytes. But I think it would be a good collective noun for sauropods, and it also feels appropriate given the large number of sauropod species recovered by Tschopp et al. In fact, we need more collective nouns for dinosaurs, and so I'd like to propose brontobyte for sauropods and armada for ankylosaurs, to join terror of tyrannosaurs.