Introduction

Back when the Xenoposeidon paper came out, we suggested that Xeno could be the first repesentative of a new sauropod “family”, and then discussed at some length: what is a “family” anyway? Now that the Brachiosaurus paper is out, and I’ve argued that the species “Brachiosaurus” brancai is generically distinct from Brachiosaurus altithorax, it’s time to talk about what a genus is (and so what “generically distinct” means).

In an unnecessarily snarky aside at the end of the last entry, I implied that Randy Irmis would be the one to say that, because my phylogeny recovered “Brachiosaurus” brancai as the sister taxon to Brachiosaurus altithorax, it could and should remain in the genus Brachiosaurus, irrespective of the morphological differences between the genera.  He didn’t quite do that — although Daniel Madzia very nearly did — but Jaime Headden certainly did over on the Dinosaur Mailing List, and even ended up asking: “So my question is this: Why do we need Giraffatitan, and cannot have a Brachiosaurus proteles etc.?”

There are plenty of possible responses to this, but before we plough into that, here is a pretty picture:

Ligament rugosities on the neural spines of Brachiosaurus dorsals

Brachiosaurus altithorax holotype FMNH P25107, presacral vertebrae 5-7, neural spines in right posterolateral view

The more retentive among you SV-POW! veterans might remember way back in the very first month of this blog when I showed you what I said were the last four presacral vertebrae of the Brachiosaurus altithorax holotype FMNH P25107.  Actually, I don’t know what I was thinking — they were presacrals 4-7, not 1-4, but that’s not the point.  The point is that Mike From Ottawa (whatever happened to him?) asked about the very rugose anterior surfaces of the neural spines, and I replied:

What the photo doesn’t show (but if you stay tuned long enough you’ll probably see one that does) is that the posterior faces of the neural spines have very similar rugosities. In life, these would have been the anchor points for epaxial muscles and ligaments. In Brachiosaurus altithorax (but not B. brancai) these have a distinctive inverted-triangle shape. In the most posterior pair of B. brancai dorsals, which are co-ossified, the ligament joining their neural spines is itself ossified. Picture to follow some time, I guess :-)

I am finally following up on the first half of that promise: the picture above shows the three most anterior of those same four presacral vertebrae from the Brachiosaurus altithorax holotype, but this time in right posterolateral view, so you can see the posterior faces of the neural spines.  And you’ll notice that on the back of each spine, as well as on the front, there’s a large and extremely rough inverted triangle.  I’ve yet to see anything at all like that in any other sauropod — Giraffatitan and the Archbishop included.  Very distinctive.

Right then — back to genera!

Rampant genera on the loose!

Here’s a practical reason to reject the idea that if two taxa are sisters, then they should be regarded as congeneric: Jaime wants to retain the species brancai within Brachiosaurus because it is (in the current analysis) the sister to the type species Brachiosaurus altithorax.  He then wants to put the species proteles into Brachiosaurus because the species we all know as Sauroposeidon proteles is (presumably) the sister to the Brachiosaurus–altithorax-and-brancai clade.  But by induction, if we accept Jaime’s policy, whatever is sister to that clade must also be subsumed into Brachiosaurus, so that we end up losing Titanosauria, Camarasauridae, Diplodocoidea, etc. — Diplodocus carnegii becomes a species of Brachiosaurus.  The good side of this scheme is that eventually, we’ll work our way up to the base of Amniota, at which point I become a member of the species Brachiosaurus sapiens.  That, I could get on board with.  But in other respects, this classification would not be so hot.

I’m assuming that no-one really wants this, and so that advocates of the Sister-Taxa-Are-Congeneric school (hereafter STAC) recognise that you have to draw a line somewhere.  But where?  And how do you choose where?  [Only time will tell whether I just coined an AHATWNUABPANTA.]

How to choose between specific and generic separation

At this point, I am reminded of when I used to be on a mailing list for wannabe writers. Lots of dogma on that list — people saying “don’t overdo adverbs” and “make sure you have enough incidental detail” and so.  People trying to nail down an algorithm for good writing.  But the best advice I saw on that list was from Jane MacDonald: “My personal advice is don’t overdo, or underdo, anything in your writing.  Do it exactly right.”(*)  That’s my attitude to drawing genus boundaries.  It is, frankly, an art; and there are no substitutes for taste, experience, judgement, familiarity with the group in question and all those other touchy-feely qualities that uber-cladists would love to find a way to abolish if they could.  But they can’t.  There is no algorithm for this.  I also think of an observation by computer scientist Bjarne Stroustrup, the inventor of the C++ programming language: “Design and programming are human activities; forget that and all is lost.”  The same is true of palaeontology.  (And of, well, everything.)

Here’s the thing, folks: a genus, just like any other taxon, is there to be useful.  Its purpose is not to conform to a dogma, but to inform and enlighten.  In the new paper, I wrote that “generic separation is warranted since the two species are more different from each other than, for example, Diplodocus and Barosaurus Marsh, 1890″ (Taylor 2009:798).  I stand by that as a great way to figure out when the morphological differences  between two species merit generic separation: it’s all about conveying degrees of difference.  And, yes, of course I know that the morphological extent of a genus in sauropods is completely different from its extent in, say, botany, where the genus Quercus (oaks) has 700 species or something stupid.  Yes, I fully accept that there is no rigorous and absolute standard by which we can determine The Right Place to drop a genus boundary.  Sure.  But that doesn’t let us out from the responsibility of making the best judgements that we can, based on relevant prior art, recognised conventions, congruence with similar decisions and — there it is again — good taste.

(*) Actually, that is only the second best advice I saw on the wannabe writers’ mailing list.  The best advice of all came second-hand, and was passed on by Greg Gunther: “I was on an [email] list with Tom Clancy once.  Mr. Clancy’s contribution to the list was, ‘Write the damn book’.”  Top advice.

Nomenclatural stability

And so finally I come to Randy’s comment.  In response to his question, I guessed that when I put them all in a matrix together, the Archbishop will form a clade with Brachiosaurus, and Sauroposeidon with Giraffatitan.  Like this: ((Brachiosaurus altithorax, “The Archbishop”), (Giraffatitan brancai, Sauroposeidon proteles)).

And Randy said:

For the sake of discussion, if the topology is as you say, then I do support the generic separation of altithorax and brancai. Now, of course, as you might surmise, if the two sub-clades are well-supported, I would also advocate putting altithorax and the NHM Tendaguru taxon in the same genus, and brancai and Sauroposeidon in the same genus.

Now I yield to no man in my respect for Randy, whose work exceeds my own humble output by a truly humiliating factor, and who makes it even worse by being such a nice guy.  But I hope he will not take it the wrong way if I say that here, he is talking the purest arsegravy.  Suppose the topology came out the way I guessed, and we adopted his suggested nomenclature.  Then five minutes later Paul Upchurch comes along with a new analysis that finds the Archbishop closer to Giraffatitan after all: and suddenly Brachiosaurus archbishopus becomes Giraffatitan archbishopus.  Five more minutes pass and Jeff Wilson publishes his new phylogeny, in which “Sauroposeidon” proteles is sister to Brachiosaurus altithorax, and so what was briefly Giraffatitan proteles becomes Brachiosaurus proteles.  Later that afternoon Jerry Harris shows that Cedarosaurus is more closely related to Brachiosaurus altithorax than the species proteles is: at this point, presumably, either Cedarosaurus gets sunk into Brachiosaurus, as B. weiskopfae, or My Big Fat Brachiosaurus Genus gets smashed up and suddenly, woah, proteles needs its own genus after all and Sauroposeidon is back!

Hands up who wants to deal with tracking all that nomenclatural shifting back and forth?  Hmm, thought not.  Folks, when we name a new species of an existing genus we are betting the nomenclature on the phylogenetic hypothesis.  This is just a dumb thing to do in this day and age — especially if you work on dinosaurs which (A) are big and usually very incomplete and so their positions can’t  be known with certainty; (B) are trendy enough to be subject to a stream of new phylogenetic analyses; and (C) are in a field where pretty much everyone seems to be hot for mandatory monophyly of genera.

So I end with a plea: unless you know for certain that your new taxon is super-closely related to the type species of an existing genus, and unless you are sure that this isn’t going to change with subsequent discoveries, please put your new species in its own monospecific genus.  That way, nomenclature is independent from phylogeny, which is surely how we all want it.  A new monospecific genus is essentially a uninomial that happens to be spelled with a space in the middle.  And uninomials are nice: they rescue us from Linnaeus’s dumb mistake in lumbering nomenclature with binomials.

This has been an Unwelcome Education Product.

Acknowledgements

Many thanks to Jim Farlow for suggesting the title of this post, which I have cheerfully stolen.  I have no idea whether he agrees with the arguments presented in this article.

References

• Taylor, Michael P.  2009.  A re-evaluation of Brachiosaurus altithorax Riggs 1903 (Dinosauria, Sauropoda) and its generic separation from Giraffatitan brancai (Janensch 1914).  Journal of Vertebrate Paleontology 29(3):787-806.