In my blog-post announcing Haestasaurus as the new generic name for the misassigned species “Pelorosaurus” becklesii, I briefly surveyed the three phylogenetic analyses in the paper. Of the third — the one based on the Mannion et al. (2013) Lusotitan matrix using both discrete and continuous characters — I wrote that it …

… recovers Haestasaurus as a titanosaur — as sister to Diamantinasaurus and then Malawisaurus, making it a lithostrotian well down inside Titanosauria.

My mistake! I was working from the result of an earlier version of that analysis. In the final version included in the paper, things are rather different:

As you can see, Haestasaurus is indeed a titanosaur in this analysis — but not a derived one at all. In fact, it’s part of the most basal clade of titanosaurs, along with Janenschia and Dongbeititan. In this tree, we have a really nice, big Brachiosauridae, containing 19 OTUs split fairly evenly between two subclades.

[Side-note: Upchurch et al. (2015) uses phylogenetic definitions that I’m not crazy about. I prefer the arrangement that I followed in my brachiosaur paper (Taylor 2009), in which Titanosauriformes = Brachiosauridae + Titanosauria is a node-stem triplet. Hopefully, some time soon, the wretched PhyloCode will finally be implemented, and we’ll be in a position to nail down a single set of definitions for the whole community to use.]

Anyway, the upshot of all this is that all three phylogenetic analyses in the paper return Haestasaurus as a pretty basal macronarian, and on the balance of evidence it’s likely not a titanosaur after all. (That’s why the name “Haestatitan“, which was in some earlier drafts of the paper, was changed to Haestasaurus. Kind of a shame, given how mundane -saurus names are, but probably the wisest course of action.)

What is the takeaway lesson from this? It’s not just “Haestasaurus is not a derived titanosaur”. It’s that all our phylogenetic hypotheses are just that — hypotheses. Papers that publish only a single cladogram are always at risk of being misinterpreted as conveying much more certainty than they really do, and Paul and Phil are to be commended for including the whole messy story in this paper. The position of Haestasaurus shifts around far too easily for us to have a strong sense of what it is, and it’s good that the paper makes that clear.

(It also makes me glad that way back in Taylor and Naish (2007), I and Darren didn’t give a more precise position of Xenoposeidon than that it’s probably some kind of neosauropod. And even that is not something I would put money on.)

References

• Mannion, Philip D., Paul Upchurch, Rosie N. Barnes and Octávio Mateus. 2013. Osteology of the Late Jurassic Portuguese sauropod dinosaur Lusotitan atalaiensis (Macronaria) and the evolutionary history of basal titanosauriforms. Zoological Journal of the Linnean Society 168(1):98–206. doi:10.1111/zoj.12029
• 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.
• Taylor, Michael P. and Darren Naish. 2007. An unusual new neosauropod dinosaur from the Lower Cretaceous Hastings Beds Group of East Sussex, England. Palaeontology 50(6): 1547-1564. doi:10.1111/j.1475-4983.2007.00728.x
• Upchurch, Paul, Philip D. Mannion and Michael P Taylor. 2015. The Anatomy and Phylogenetic Relationships of “Pelorosaurus” becklesii (Neosauropoda, Macronaria) from the Early Cretaceous of England. PLoS ONE 10(6):e0125819. doi:10.1371/journal.pone.0125819