Friday, September 9, 2011

Tehuelchesaurus and how to describe the affinities of a taxon

I've often complained about the tendency for authors to view their most parsimonious cladogram as "the right" tree, such as here and here.  I note that it's usually more helpful to describe how parsimonious different hypotheses are, since while new analyses usually change the topology somewhat, they rarely support relationships that were strongly rejected before.  Carballido et al. (2011) recently redescribed the sauropod Tehuelchesaurus, and in addition to a detailed osteology and several other important discussions, their paper contains a phylogenetic analysis that did things just the way they should.  The analysis (249 characters, 45 taxa) is based on Wilson's (2002) analysis with added data, including numerous newly added macronarians.  Some characters were ordered, taxa which cause polytomies were deleted a posteriori, and Tehuelchesaurus emerged as a basal camarasauromorph sister to Galveosaurus, not a relative of Omeisaurus as originally thought by Rich et al. (1999).  But instead of simply letting the matter rest there, Carballido et al. included the section "Testing Alternative Positions for Tehuelchesaurus."  Music to my ears.  They tested not only the Omeisaurus alternative (9 steps longer), but also positions slightly more (1 step longer) and less (2 steps longer) than Galveosaurus, and noted the character support for all of these.  In addition, the authors wrote the following which basically covers any plausible position-

"Other positions within basal camarasauromorphs (in any position within the Janenschia/Tastavinsaurus clade, as sister taxon to Europasaurus, more basal than Europasaurus, and as sister taxon to Camarasaurus) and as a macronarian outside Camarasauromorpha, but more derived than Haplocanthosaurus, require three additional steps. Placing Tehuelchesaurus as sister taxon of Haplocanthosaurus results in a suboptimal tree four steps longer than the MPTs, and as the most basal macronarian needs even five additional steps. Even more steps are required to place this taxon in the Titanosauriformes (seven additional steps as a basal somphospondyl and eight additional steps as a basal brachiosaurid).

Any position outside Macronaria also results in considerably suboptimal tree lengths. Five additional steps are needed to make Tehuelchesaurus the most basal diplodocoid, but any position within higher diplodocoids results in trees that are at least ten steps longer than the MPTs. Likewise, placing Tehuelchesaurus outside Neosauropoda requires six additional steps, and any placement among basal, nonneosauropodan taxa results in trees at least nine steps longer than the MPTs. Thus, the possibility of a Jurassic Patagonian clade of sauropods, including Patagosaurus and Tehuelchesaurus, can also be rejected, as it requires 12 additional steps."

The paper succeeds in giving you a much clearer idea of Tehuelchesaurus' relationships than any one cladogram could.  Anybody describing a new taxon should follow their example.

Carballido, Rauhut, Pol and Salgado, 2011. Osteology and phylogenetic relationships of Tehuelchesaurus benitezii (Dinosauria, Sauropoda) from the Upper Jurassic of Patagonia. Zoological Journal of the Linnean Society. DOI: 10.1111/j.1096-3642.2011.00723.x


  1. Interestingly, Galveosaurus is sometimes thought to be a non-neosauropod too. Does constraining for a non-neosauropod Tehuelchesaurus cause a non-neosauropod Galveosaurus?

  2. No clue. I'd have to run the matrix myself, and there are more pressing matters to attend to.