Tutorial 7: the sauropod family tree
August 19, 2009
We really should have covered this ages ago … Here we are, blithering on about brachiosaurids and diplodocoids and all, and we’ve never really spelled out what these terms mean. Sorry!
The family tree of a group of animals (or plants, or fungi, or what have you) is called its phylogeny. The science of figuring out a phylogeny is called systematics. And once you’ve got a phylogeny, the business of naming the parts of it (and of course choosing which parts to name) is taxonomy.
For a long time, sauropod systematics was completely up in the air, so that McIntosh’s (1990) review article on sauropods in The Dinosauria (first edition) said, rather despairingly, that “although recent discoveries are beginning to clarify the problems of sauropod phylogeny, were are still very far from being able to construct a cladogram” (p. 399). Happily, this changed rapidly thereafter, with the first published numerical phylogenetic analysis appearing in Russell and Zheng’s (1993) description of the new Mamenchisaurus species M. sinocanadorum. More importantly, in the same year Paul Upchurch submitted his (1993, duh) dissertation on sauropods, and this contained a much larger analysis which was subsequently published as Upchurch (1995). This paper raised the bar significantly, with an analysis of 27 taxa using 174 characters. Three years later, Upchurch (1998) published a major revision of his own work; in the same year, the other major school of sauropod phylogeny launched with a JVP memoir (Wilson and Sereno 1998), which featured only 10 taxa and 109 characters, but discussed and illustrated them in more detail. Wilson (2002) followed this up with a much larger analysis of 27 taxa and 234 characters, and Upchurch et al. (2004), in the second editi0n of The Dinosauria, saw his 27×234 and raised him to 41×309. The good news is that, by this time, the two schools’ phylogenies, having started out rather different, were converging on a consensus topology with only two significant disagreements, which we’ll come to in a minute.
Since then, Jerry Harris (2006) created a union matrix from the character scores in the Wilson (2002) and Upchurch et al. (2004) matrices, and also threw in a few additional characters from other less ambitious phylogenetic analyses. This analysis came up with a tree that was very similar to Wilson’s, and subsequent work by Wilson and Upchurch (2009) indicates that Upchurch is now also substantially in agreement with this arrangement.
So here it is!
I plucked this from Jerry’s paper, and coloured it in to show two of the more important groups. Evolution begins at bottom left, so let’s quickly tour the group.
- First of all, note the outgroups. Sauropods’ nearest relatives are the other saurischian dinosaurs, theropods and prosauropods. (They’re shown the wrong way round here, because in an unrooted tree it makes no difference. Ignore that.)
- The most basal sauropods include things like Vulcanodon and, it turns out mostly from the work of Adam Yates (e.g. Yates 2007), a whole bunch of things that, if you looked at them you’d probably guess were prosauropods.
- Sauropods as we know them really begin at the boundary of the group Eusauropoda (“true sauropods”), which is roughly speaking everything more derived than Vulcanodon. (I won’t discuss the naming of nodes and branches in detail in this post, as it would quickly get too long. Maybe in a later tutorial.) This group I have coloured pink in the diagram above.
- Basal eusauropods include quite a few genera, and the order in which they branched off the “main line” leading to the neosauropods is not clear — as the unresolved polytomy above shows. Cetiosaurus (which for some reason is not shown in this figure) is generally considered quite derived; some of the Chinese sauropods (Mamenchisaurus, Omeisaurus, etc.) may form a group of their own, but that’s not clear.
- Most of the best-known sauropods fall within the great group Neosauropoda (“new sauropods”), which is coloured purple above. A few genera float around the root of this group, including Haplocanthosaurus and Jobaria, both of which are sometimes considered neosauropods, and sometimes non-neosauropod sauropods (or what I informally call “eosauropods”, or “dawn sauropods”).
- Otherwise the great split within Neosauropoda is between the diplodocoids (on the left) and the macronarians (on the right) — the groups including Diplodocus on one hand, and Saltasaurus on the other.
- The most basal diplodocoids are the rebbachisaurids over on the left.
- Most other diplodocoids fall into the group Flagellicaudata (“whip-tails”), which is itself composed of dicraeosaurids and diplocodids. (It’s not clear where in that dichotomy, or maybe just outside it, Suuwassea falls.)
- Over in the other half of the Neosauropods, the first macronarians to diverge are the camarasaurids (which currently means, uh, Camarasaurus).
- Most of the other macronarians fall into Titanosauriformes, the group uniting brachiosaurids (yay!) with titanosaurs and their buddies. Everything closer to titanosaurs falls within Somphospondyli, and that includes Euhelopus — as it turns out. (Upchurch had found Euhelopus to fall outside Neosauropoda).
- Once you get past Euhelopus, you’re into Titanosauria (though there are various definitions which place the entry point differently).
- And once inside Titanosauria … well, all bets are off at this stage. There is a rough consensus that things like Malawisaurus and Andesaurus are pretty basal and Saltasaurus is, sort of by definition, derived. But apart from that, different studies have come up with wildly different phylogenies, with that of Curry Rogers (2005) being particularly left-field.
Without a doubt, Titanosauria is where the action is right now. As alluded to in the comments of Matt’s Isisaurus post, it’s a big, big group, encompassing many genera and huge morphological range. It’s also a long-lived group, spanning the whole of the Cretaceous; and it’s where most new genera are being named, as Argentina seems to be packed full of ’em.
Well, that’s all for now. Sorry it’s been wordier than usual — probably not much fun to read, but hopefully useful to refer back to in future.
Here’s the famous 8th cervical vertebra of the “Brachiosaurus” brancai lectotype HMN SII, this time in a left-lateral close-up of its left prezygpapophyseal ramus, showing the many pneumatic excavations. Enjoy!
- Curry Rogers, Kristina. 2005. The Evolutionary History of the Titanosauria. pp. 50-103 in: K. Curry Rogers and J. A. Wilson (eds.), The Sauropods: Evolution and Paleobiology. University of California Press, Berkeley.
- Harris, Jerald D. 2006. The significance of Suuwassea emiliae (Dinosauria: Sauropoda) for flagellicaudatan intrarelationships and evolution. Journal of Systematic Palaeontology 4: 185-198.
- McIntosh, John S. 1990. Sauropoda. pp. 345-401 in: D. B. Weishampel, P. Dodson and H. Osmólska (eds.), The Dinosauria, 1st edition. University of California Press, Berkeley and Los Angeles.
- Russell, Dale A., and Zheng, Zhong. 1993. A large mamenchisaurid from the Junggar Basin, Xinjiang, China. Canadian Journal of Earth Science 30(10/11): 2082-2095.
- Upchurch, Paul. 1993. The Anatomy, Phylogeny and Systematics of the Sauropod Dinosaurs. University of Cambridge, unpublished Ph.D. dissertation. 489 pp.
- Upchurch, Paul. 1995. The evolutionary history of sauropod dinosaurs. Philosophical Transactions of the Royal Society of London Series B, 349: 365-390.
- Upchurch, Paul. 1998. The phylogenetic relationships of sauropod dinosaurs. Zoological Journal of the Linnean Society 124: 43-103.
- Upchurch, Paul, Paul M. Barrett and Peter Dodson. 2004. Sauropoda. pp. 259-322 in D. B. Weishampel, P. Dodson and H. Osmólska (eds.), The Dinosauria, 2nd edition. University of California Press, Berkeley and Los Angeles. 861 pp.
- Wilson, Jeffrey A. 2002. Sauropod dinosaur phylogeny: critique and cladistic analysis. Zoological Journal of the Linnean Society 136: 217-276.
- Wilson, J. A. and Paul C. Sereno. 1998. Early evolution and Higher-level phylogeny of sauropod dinosaurs. Society of Vertebrate Paleontology, Memoir 5: 1-68.
- Wilson, Jeffrey A. and Paul Upchurch. 2009. Redescription and reassessment of the phylogenetic affinities of Euhelopus zdanskyi (Dinosauria – Sauropoda) from the Early Cretaceous of China. Journal of Systematic Palaeontology 7: 199-239. doi:10.1017/S1477201908002691
- Yates, Adam M. 2007. The first complete skull of the Triassic dinosaur Melanorosaurus Haughton (Sauropodomorpha: Anchisauria). pp. 9-55 in: Paul M. Barrett and David J. Batten (eds.), Special Papers in Palaeontology 77: Evolution and Palaeobiology of Early Sauropodomorph Dinosaurs. The Palaeontological Association, U.K.