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.
Reference
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.
Sauropod Vertebra Picture of the Week 
April 14, 2009 at 8:42 pm
On the use of the term “opisthoplatyan” in place of Tidwell et al.’s equivalent “distoplatyan”: here’s what I wrote in the review:
The authors use the term “procoelous-distoplatyan”, from Tidwell et al. (2001:162), to describe vertebrae whose centra are concave anteriorly and flat posteriorly, thereby distinguishing them from vertebrae that are simply “procoelous”, a term that is generally taken to indicate both an anterior concavity and a posterior convexity. The term “distoplatyan”, although introduced seven years ago, has not been widely used – in fact, I was able to find only two subsequent papers that have used it (Harris, 2006a, b) – so I think it is not too late to change this term. In its place I recommend “opisthoplatyan” for two reasons. Most importantly, “distoplatyan” is restricted to use in the tail, as only there can the direction “distal” be used to indicate posterior, whereas “opisthoplatyan” is applicable throughout the axial column. Secondly, “opisthoplatyan” and its complement “proplatyan” neatly parallel the existing terms opisthocoelous and procoelous. I have discussed this with Jerry Harris, the only author to have used “distoplatyan” since Tidwell et al., and he is in agreement with my suggestion.
Hopefully Virginia and Ken will also agree.
April 15, 2009 at 12:51 pm
Compare fig. 11 of González Riga et al 2008 with fig. 8 of Bonaparte et al 2006 again, Ligabuesaurus has “always” been in a trichotomy with Phuwiangosaurus and higher titanosaurs. What does change is the clade that gets named Titanosauria. Under any published stem-based defintion of Titanosauria, Malarguesaurus actually is a titanosaur.
April 15, 2009 at 3:11 pm
Hi Brad – you’re correct, González Riga et al. (2008) regarded Malarguesaurus and so on as outside of Titanosauria because they chose to follow Bonaparte & Coria (1993) in recognising a node-based Titanosauria (for the Andesaurus + ‘Titanosauridae’ clade). They opted not to adopt the more inclusive, branch-based Titanosauria used by Sereno (1998). I’m not sure which view is regarded as most current, but I agree with González Riga et al. (2008) that the node-based definition better matches the spirit of Titanosauria as originally used by Bonaparte & Coria (1993). I should probably change the article to account for this, thanks for pointing it out.
April 15, 2009 at 4:10 pm
To make matters worse, don’t forget that Upchurch et al. (2004:309), in their Dinosauria II chapter which is probably THE definitive overview of sauropods, made Titanosauria a homodefinitional synonym of Somphospondyli: “Titanosauria, a clade named by Bonaparte and Coria (1993), is a stem-based taxon defined as Titanosauriformes more closely related to Saltasaurus than to Brachiosaurus.”
Given how wildly the base of Titanosauria fluctuates between these definitions (and especially in light of the bumper crop of new basal titanosauriforms in recent years), I think Titanosauria is close to useless as a formal taxon right now, and I tend to use only the informal term “titanosaurs”. Stick with Somphospondyli and Lithostrotia is my advice: each of them has only a single published definition. Once the Phylocode is implemented — which might actually be some time not too far off — we can look again at Titanosauria, figure out whether the name is worth saving, and come up with consensus on which definition (if any) to enshrine.
April 15, 2009 at 5:01 pm
I agree with you Mike (on the problematical nature of the name Titanosauria). However, Upchurch et al.’s branch-based Titanosauria is arguably counter to the spirit of the name: Bonaparte & Coria (1993) coined it for the ‘Andesauridae’ + ‘Titanosauridae’ clade, not for all taxa closer to ‘Andesauridae’ + ‘Titanosauridae’ than to Brachiosaurus. Even a node-based Titanosauria is not necessarily useful though, given the fuzziness in this region of the tree.
Incidentally, having just spent a few weeks poring over the contents of Dinosauria II, let me say that Upchurch et al.’s sauropod chapter is one of the best contributions therein.
April 16, 2009 at 10:23 am
This is all just a bunch of malargue.
Someone had to say it…. I bet you’re just glad it was me!
April 16, 2009 at 10:31 am
How big would this sauropod have been? That vertebra looks smaller than some…
May 20, 2009 at 3:36 pm
[…] Well, yet again I’ve done my best to concentrate on CAUDAL vertebrae, given that we have an obvious (and understandable) bias towards cervicals and dorsals. Someone has to speak up for tails. For previous instalments in the Sauropods of 2008 series please see the articles on Eomamenchisaurus, Dongyangosaurus, and Malarguesaurus. […]
July 8, 2016 at 7:10 pm
[…] alternately called Procoelous/Distoplatyan or Procoelous-Opisthoplatyan (See the first comment here for a discussion of the […]