Tehuelchesaurus

Carballido, José L., Rauhut, Oliver W. M., Pol, Diego & Salgado, Leonardo, 2011, Osteology and phylogenetic relationships of Tehuelchesaurus benitezii (Dinosauria, Sauropoda) from the Upper Jurassic of Patagonia, Zoological Journal of the Linnean Society 163 (2), pp. 605-662 : 634-635

TEHUELCHESAURUS

Alternative positions for the phylogenetic position of Tehuelchesaurus were tested by moving the taxon within the reduced consensus tree in MacClace 4.06 (Maddison & Maddison, 2003). First, a position of Tehuelchesaurus as the sister taxon of Omeisaurus , as proposed by Rich et al. (1999), and retrieved in the phylogenetic analysis of Alifanov & Averianov (2003) and Upchurch et al. (2004), results in a tree that is nine steps longer than the MPTs, representing a considerably suboptimal topology. When this grouping ( Tehuelchesaurus + Omeisaurus ) is forced to be monophyletic, a single synapomorphy is retrieved for this clade. Character 224 (equally developed distal femoral condyles) is shared between these two taxa and represents a unique reversal of a eusauropod synapomorphy. Other characters noted by Rich et al. (1999) are interpreted either as plesiomorphies or synapomorphies within a larger clade under the present tree topology, such as the presence of opisthocoelous posterior dorsal vertebrae. The latter character is very conservative among camarasauromorphs and is recovered as a synapomorphy of the clade containing Omeisaurus , Mamenchisaurus and neosauropods, as it is present in the latter two taxa, and also in the basal macronarian Haplocanthosaurus, which has a weakly developed convexity in the anterior face of posterior dorsal centra (Hatcher, 1903: pl. II).

Moving Tehuelchesaurus into more or less derived positions within camarasauromorphs requires fewer steps. Only one additional step is needed to place this taxon in a position more derived than Galvesaurus , thus being more closely related to the Janenschia / Tastavinsaurus clade and more derived camarasauromorphs. Such a placement would be supported by the shared derived character of posterior dorsal vertebrae that are longer than wide, and, possibly, by the lateral bulge of the femoral shaft, as the latter character is unknown in Galvesaurus .

Placing Tehuelchesaurus as less derived than Galvesaurus results in a tree that is two steps longer than the MPTs. However, such a placement would only be supported by ambiguous synapomorphies of Galvesaurus with more derived camarasauromorphs, of which the character state is unknown in Tehuelchesaurus , such as ventrally transversely concave and extremely elongate cervical centra that are at least four times as long as they are high posteriorly and distoplatyan anterior caudal vertebrae. A further possible synapomorphy of Galvesaurus with more derived camarasauromorphs might be the presence of pneumatopores in the dorsal ribs. This character is also present in titanosauriforms, Euhelopus and Chubutisaurus , but absent in Tastavinsaurus .

Moving Tehuelchesaurus into a position more derived than the Janenschia / Tastavinsaurus clade also requires two additional steps. This placement would be supported by the tapering posterior margins of the anterior dorsal pleurocoels (which would here be optimized as synapomorphies at this node, being convergently developed in Haplocanthosaurus and Europasaurus ).

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.

To further test the robustness of the phylogenetic position of Tehuelchesaurus recovered here, a Templeton test for several of the alternative topologies was conducted. The positions tested were for Tehuelchesaurus as sister taxon to Omeisaurus , as originally proposed by Rich et al. (1999) and found by Alifanov & Averianov (2003) and Upchurch et al. (2004), for Tehuelchesaurus as immediate outgroup to neosauropods, as a non-camarasauromorph macronarian, and as a basal titanosauriform. The test was performed following the protocol recently summarized in Wilson (2002).

Slightly different values were obtained depending on the position of the problematic Tendaguru sauropod Tendaguria (see above). For all the positions tested the smaller P -values were always recovered when Tendaguria was placed within neosauropod dinosaurs. With Tendaguria in this position, the placement of Tehuelchesaurus as sister group to Omeisaurus can be rejected by the data with 95% confidence (P = 0.029, and thus <0.05). Nevertheless, when Tehuelchesaurus is forced to be the sister group to Noesauropoda (P = 0.057) or a basal titanosauriform (P = 0.052) the P -values are close to the significance cut-off level (i.e. 0.05), so these positions seem very unlikely as well. In contrast, the only slightly suboptimal topologies retrieved when Tehuelchesaurus is placed as a non-camarasauromorph macronarian result in P -values of the Templeton test that are far from being significant (P> 0.1). If Tendaguria is placed as a non-neosauropod, only the P -value for a sister-group relationship with Omeisaurus is close to the cut-off level (P = 0.059).

In summary, the analysis presented here provides strong evidence against a placement of Tehuelchesaurus amongst basal eusauropods (‘cetiosaurids’ in the traditional sense), as was originally suggested by Rich et al. (1999), and later recovered by Alifanov & Averianov (2003) and Upchurch et al. (2004) in their analyses. In contrast, the results indicate that Tehuelchesaurus is a basal, non-titanosauriform macronarian. However, placement of taxa within basal macronarians is often poorly supported, indicating that there is still ample scope for research in this part of the phylogeny.