Sauropods of 2008: Malarguesaurus
April 12, 2009
Here’s another article in my ‘sauropods of 2008’ series. Previous entries have looked at Eomamenchisaurus and Dongyangosaurus, both of which are Asian. This time round we look at a new South American taxon: Malarguesaurus florenciae González Riga et al., 2008. In marked contrast to the majority of recent SV-POW! articles, this article really is going to be short!
While the majority of new South American sauropods are titanosaurs, Malarguesaurus is a basal titanosauriform. More specifically, González Riga et al. (2008) found it to be a non-titanosaurian somphospondylian, closer to titanosaurs that to Ligabuesaurus, Chubutisaurus or Euhelopus, and the sister-taxon to Phuwiangosaurus from Thailand. Some of these results might seem surprising, as Ligabuesaurus (itself only named in 2006) and Phuwiangosaurus have both previously been regarded as titanosaurs. However, note that González Riga et al. (2008) only found these taxa to be outside of Titanosauria because they employed a restricted, node-based version of Titanosauria that is less inclusive than the branch-based version used by some other authors. The node-based version is closer to the spirit of the name as originally employed by Bonaparte & Coria (1993). Anyway… Malarguesaurus is from an outcrop of the Turonian-Coniacian Portezuelo Formation that crops out in Mendoza Province, Argentina: it’s known from caudal vertebrae, limb bone fragments, ribs and chevrons [image below, from González Riga et al. (2008), shows proximal caudal vertebra in (A) anterior, (B) lateral and (C) posterior views. Scale bar = 50 mm].
Once upon a time it was thought that sauropod caudals were either platycoelous or amphiplatyan (as they are in brachiosaurs and camarasaurs), slightly procoelous (as they are in diplodocoids), or strongly procoelous (as they are in titanosaurs). Discoveries made over the past few decades have shown that things can be much more complicated than this, with some titanosaurs being opisthocoelous (Opisthocoelicaudia skarzynskii for one), and some possessing a combination of different articular types: in Rinconsaurus for example, the caudals are variously procoelous, amphicoelous, opisthocoelous and biconvex. Malarguesaurus also exhibits a combination of different articular types: its distal caudals are procoelous while those from elsewhere in the tail are procoelous-opisthoplatyan*. In fact, the authors regard this combination of caudal morphologies as diagnostic for the taxon: the vertically oriented neural spines on the proximal caudals, with their concave posterior borders, are also diagnostic (González Riga et al. 2008).
* These terms are all familiar, I’m sure. However, ‘procoelous-opisthoplatyan’ hasn’t been used much before, and might in fact be unique to this paper. In fact… the authors cite pers. comm. with a certain Mike P. Taylor for the invention of this term (Mike was a reviewer). It refers to a vertebra in which the anterior face is slightly concave while the posterior face is flat. Tidwell et al. (2001) referred to the same sort of morphology in the titanosauriforms Cedarosaurus and Venenosaurus, but termed it ‘procoelous/distoplatyan’.
Incidentally, the Malarguesaurus paper is another of those annoying pieces of literature that will prove problematical when it comes to citing the date of publication: the paper is dated 2009, but was actually published in 2008. I know it was definitely published in 2008 as I had a final, published version of the relevant journal issue in that year. So Malarguesaurus is a 2008 sauropod, not a 2009 one.
Bonaparte, J. F. & Coria, R. A. 1993. Un nuevo y gigantesco sauropodo titanosaurio de La Formacion Rio Limay (Albiano-Cenomaniano) de le provincia del Neuquen, Argentina. Ameghiniana 30, 271-282.
González Riga, B. J., Previtera, E. & Pirrone, C. A. 2008. Malarguesaurus florenciae gen. et sp. nov., a new titanosauriform (Dinosauria, Sauropoda) from the Upper Cretaceous of Mendoza, Argentina. Cretaceous Research 30, 135-149.
Tidwell, V., Carpenter, K. & Meyer, S. 2001. New titanosauriform (Sauropoda) from the Poison Strip Member of the Cedar Mountain Formation (Lower Cretaceous), Utah. In Tanke, D. H. & Carpenter, K. (eds) Mesozoic Vertebrate Life. Indiana University Press (Bloomington & Indianapolis), pp. 139-165.