29 June 2016

Day of the Diplodocids Redux

And he means business -- just check out that switchblade comb.
[by the author]
Remember Tschopp et al 2015? You know the paper... Brontosaurus is back! Huzzah and tally-ho and all that. I wrote about it here, nearly 15 months ago. I'm bringing it back up now because it will be relevant to a post that I've had in the works for a little while now. That should hopefully materialise here within the next little while but before it does, let's look back at good ol' Brontosaurus and see what was really important about its resurrection from the open grave of taxonomic obsolescence...

Ah, Brontosaurus. The name all of us in the palaeo community were too good to admit we never stopped using, when it was just us, alone where nobody would overhear, with our sketchpads and our toys and our old-timey books.... Brontosaurus. THE Thunder Lizard. THE dinosaur, quite probably. Brian Switek didn't write a book called My Beloved Apatosaurus, now did he? No, no, no, when we walked around museums with our families or read a children's book aloud, it was always strict business, 'Well, ACTUALLY, Brontosaurus is a junior synonym of Apatosaurus, which was named and described earlier, so that's the CORRECT name,' that line of talk. But not in our heads, and certainly not in our hearts. Brontosaurus, baby, it's great to have you back... but you never really left us. Not really.

Of course, that's just thing. Brontosaurus never went anywhere - not culturally, of course, but not scientifically either. The animal Brontosaurus excelsus was always its own unique taxonomic entity. Had Elmer Riggs, back in 1903, argued that Brontosaurus excelsus was not different enough to warrant species-level separation from Apatosaurus ajax, well, that might have been something else, but it was always clear that B. excelsus was its own unique species. Riggs argued instead that Brontosaurus excelsus wasn't different enough to warrant genus-level separation from Apatosaurus, even though it was clearly not the same as any species of Apatosaurus. The thing about arguing synonymy for taxonomic ranks, though, is that there aren't actually rules for how inclusive or exclusive things like species - or even worse, genera - have to be. When you're working on fossil animals, without the benefit of life appearances and breeding patterns, nailing down a species is tricky enough. Genera, even in the world of modern zoology, are pretty much totally arbitrary.

Bad photo of the Field Museum's FMNH 25112, a mystery diplodocid formerly
identified as a specimen of 
Apatosaurus louisae. Tschopp et al 2015 find that this
animal is not an
Apatosaurus specimen at all, and may not even be an
apatosaurine! A new, undescribed genus and species of dinosaur, standing
proud on display in one of the nation's great museums -- isn't that exciting?
[photo by the author]

The Rules of the (Name) Game

This is where Tschopp and colleagues come in. For - I believe - the first time in palaeontology, they provide a numerical guideline for breaking groups of related animals down into quantifiable units called 'genera' and 'species'. This is important because it allows for genera and species to be used consistently within a particular group. The authors warn that selecting degrees of difference between different species and different genera must be done on a group-by-group basis, but this paper shows that it is something that can indeed be done.

The number of differences used (13 or more differences to are needed to separate species into genera) in the revision of Diplodocidae was selected to keep useful concepts of Apatosaurus and Diplodocus alive. This may seem to be arbitrary -- and to a point, it is -- but it reflects the goal of modern taxonomy, which is to classify organisms based on their evolutionary relationships to one another, and to allow scientists to discuss and study these organisms. Splitting these genera too thin, or alternatively,  lumping even more wildly than the most ardent of Greg Paul fanboys would make any discussion of the evolutionary and morphological diversity of Diplodocidae too complicated to be useful. It just so happened that in the process of evaluating the diplodocids along these lines, which do reflect genuine evolutionary relationships, a group of animals long considered by scientists to be Apatosaurus fell outside the boundaries of Apatosaurus under this paper's definition. These animals formed a natural group, meaning they shared an evolutionary history with each other to the exclusion of other animals, and as such needed a name. The oldest, and thus scientifically rightful, valid name that was available happened to be Brontosaurus.

And lo, Brontosaurus riseth again, through that age-old miracle of low-level taxonomic reshuffling.

Two Brontosaurus excelsus prepare to slam down in the form of ancient combat
known only as BRONTOSMASH! This depiction is based on recent studies that
suggest apatosaurine necks were specially adapted for combat, and may have
supported keratinous knobs or spikes along their lower edges. Read more
over at SV-POW! -- where this very drawing was featured!
[by the author]
I think this is a very important takeaway from this paper. Perhaps it is a bit slash-and-burn of me, and seeing as I'm not actually a working palaeontologist (yet!) I don't know if this opinion might actually be more hurtful than helpful, but I think dinosaur workers need to work with and embrace an idea of numerical divisions. The tyrannosaur workers, the ankylosaur workers, all those myriad people who work on small fluffy birdy things, decide your numbers. Decide how much variety a genus shall encompass, how much variety a species shall encompass, what groupings must remain intact for your clades to have any sort of internal usefulness, and then take it from there. Surely try to maintain useful order and prevent total anarchy, but don't be afraid to let old names rise from the muddy backwaters of synonymy like a Dalek out of the Thames, or lump genera together, even if they have really cool names. Hopefully, by doing this, there will be some sort of standards in place for future work on dinosaur groups that can help establish some more solid clarity in the very human and artificial, but also very important, pursuit of classifying and communicating about nature.

-- M

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