…and that, ladies and gentlemen, is why you code Brachiosaurus and Giraffatitan separately
January 23, 2013
Now this is super-freakin’ cool, and I’ve been meaning to blog about it for a while. In Mike D’Emic’s recent titanosauriform phylogeny (D’Emic 2012), he (correctly) included Brachiosaurus and Giraffatitan as separate OTUs, and, hey, whaddayaknow, they’re not sister taxa anymore: Brachiosaurus is more closely related to a trio of Early Cretaceous North American brachiosaurids than it is to Giraffatitan.
The potential for someone to find this result was there ever since Mike broke Brachiosaurus and Giraffatitan apart, as a previously composite OTU, in his 2009 paper. It just hadn’t materialized. In fact, some authors have gone out of their way to not find this out, by keeping the old composite coding. That seems…unwise, in retrospect. Whether one agreed with Mike on the nomenclatural point of generic separation or not, not coding the two taxa as separate OTUs (especially after Mike had done that work for them) was a poor phylogenetic decision–in essence, it constrained Brachiosaurus and Giraffatitan to be sister taxa in the analysis, and outlawed any more interesting results–like the one obtained by D’Emic (2012)–before the software even started crunching trees.
So anyway, back to the coolness inherent in D’Emic’s tree. Of course, like all phylogenetic results this is just a hypothesis and it is subject to revision based on new information blah blah blah…but it is really interesting that there is now some phylogenetic support for an endemic radiation of brachiosaurids in North America (bonus goofy observation–you can’t spell ‘endemic’ without D’Emic). Or perhaps Lauriasia–I would kill to know where the British brachiosaurids (or basal titanosauriforms) fit into this story, and Lusotitan, and the apparently tiny Croatian carbonate platform brachiosaurs.
Also super-interesting that, if this tree is accurate, these endemic Early Cretaceous brachiosaurids were living alongside a giant basal somphospondyl in the form of Sauroposeidon, which came from heaven knows where. Look who it’s surrounded by–Ligabuesaurus is from Argentina, Tastavinsaurus is from Spain, and the euhelopodids are from eastern Asia. Evidently there was also a global radiation of basal somphospondyls. And why are all the Early Cretaceous North American brachiosaurids small–smaller than Brachiosaurus and Giraffatitan, anyway (at least until we find bigger individuals of the former)–while Sauroposeidon is so big? Or is that just an effect of tiny sample sizes, and one lucky strike in the form of the Sauroposeidon holotype?
So much cool stuff to think about. I don’t usually get this much enjoyment out of a tree unless it has lights and ornaments.
- D’Emic, M.D. 2012. The early evolution of titanosauriform sauropod dinosaurs. Zoological Journal of the Linnean Society 166: 624–671.
- Taylor, M.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.