Antarctosaurus - a glorious sauropodomorph

So ya want sauropodomorphs do ya? Here is the first of a series of three in a mini 'sauropodofest'



The titanosaurs were quite rightly described as “the last great frontier in dinosaur phylogenetics” by Jeff Wilson. Indeed at this stage we have little idea of how the various titanosaurs are related to each other beyond a vague notion that some forms such as Andesaurus and Malawisaurus are basal to other more derived forms, of which Saltasaurus is the classic example. A big part of the problem is that although the group is diverse and many genera have been named, most are known from very incomplete remains. Skulls in particular have proved quite elusive. For a long time the best-known skull was that Antarctosaurus whichmannianus. The specimen was collected by Dr R Whichmann from the right bank of the Rio Negro, 15 km south west of General Roca, Argentina in 1912. The site was probably in the Campanian (Late Cretaceuous) Anacleto Formation. The specimen includes cranial remains and some postcranial pieces that are unquestionably that of a derived titanosaur (eg. there is a biconvex first caudal,amongst other derived titanosaurian characteristics). The skull is severely fragmented and most of the pieces are missing. Von Huene (1929) reconstructed the skull as similar to Diplodocus but with a steeper snout. This iconic reconstruction was in no small part responsible for the widespread view that titanosaurs were the derived descendants of diplodocoids. As our understanding of sauropod anatomy and evolution improved it became clear that titanosaurs were actually closer to deep-skulled sauropods (now named Macronaria) such as Camarasaurus and Brachiosaurus. Thus the Antarctosaurus skull became an anomaly. Some speculated that the skull really did belong to a diplodocoid and didn’t go with the postcranium(Jacobs et al. 1993). However there is precious little evidence of diplodocoids surviving so late in the Cretaceous. Salgado and Calvo (1997) suggested the skull had been restored incorrectly and presented a new reconstruction that presented a brachiosaurid aspect. As our knowledge of titanosaur skulls increased (thanks to skull pieces from Saltasaurus and the recognition that the so-called diplodocoids Nemegtosaurus and Quaesitosaurus were really titanosaurs) it became clear that the posterior portion of the Antarctosaurus skull was indeed that of a titanosaur. It showed several titanosaur synapomorphies such as pendant distal tips of the paroccipital processes (the braincase ‘wings’) and apparent exclusion of the squamosal from the margin of the upper temporal fenestra. However doubt about the association of the skull pieces continued. Wilson (2002) listed three derived characters of the dentary that were shared with the bizarre diplodocoid Nigersaurus (supercroc’s side-kick). It is true that jaw is remarkably diplodocoid-like, particularly with a sharp right-angled bend between the main ramus of the jaw and the toothbearing symphyseal ramus but this feature has been shown to have evolved convergently in at least one other titanosaur, Bonitasaura (Apesteguia 2004). The three characters Wilson suggested were shared specifically with Nigersaurus were 1, extension of the tooth row lateral to the main ramus of the jaw, 2, a marked increase in the number of dentary teeth and 3, restriction of the teeth to the transverse section of the jaw (= symphyseal ramus). Of these character 1 is indeed present, though only just, as you can see in the figure below.



Dentary pair of Antarctosaurus in dorsal view, created by mirroring the right dentary in photoshop. Original drawing from Huene (1929).

Character 2 is not present with three or so alveoli extending onto the main ramus, behing the symphyseal ramus, similar to the condition seen in Bonitasaura. Character 3 is not present either. There are 15 alveoli (Powell 2003), which is typical for basal macronarians, and only slightly more than in other titanosaurs (13 in Bonitasaura and Nemegtosaurus, 11 in Rapetasaurus) and a far cry from the 34 tooth columns in each dentary of Nigersaurus. Further to this the dentary lacks some diplodocoid synapomorphies that would have to have to be regarded as reversals if a special relationship with Nigersaurus is accepted. These are an increase to more than four replacement teeth per alveolus and the loss of mesial and distal carina of the tooth crowns. As in Bonitasaura there are three replacement teeth per alveolus (Apesteguia 2004) and the tooth crowns also retain mesial and distal carinae, despite their elongate diplodocoid-like shape. Elongate but carinate teeth are also present in other titanosaurs, e.g. Rinconsaurus and Ampelosaurus. Lastly the dentary displays a vertical symphyseal axis, a derived characterstic of titanosaurs. In summary there is one weak similarity with the diplodocoid, Nigersaurus, one derived characteristic of the Diplodocoidea that is convergent in some titanosaurs, the absence of two derived characters of Diplodocoidea and the presence of one derived character of titanosaurs. Following from this I think there is little reason to believe that the dentary is not that of a titanosaur. Given the scarcity of titanosaur skulls in general it seems very likely that all the cranial pieces of Antarctosaurus, which were found together at one site, belong to a single individual. Given this, I present above my own reconstruction of Antarctosaurus, using our improved knowledge of titanosaur anatomy to fill in the missing parts.



References

Apesteguia S (2004) Bonitasaura salgadoi gen. et sp. nov.: a beaked sauropod from the Late Cretaceous of Patagonia. Naturwissenschaften 91: 493-497.

Huene VF (1929) Los saurisquios y ornithisquios del Cretaceo Argentino. Annales del Museo de La Plata 3: 1-196.

Jacobs LL, Winkler DA, Downs WR, Gomani EM (1993) New material of an Early Cretaceous titanosaurid sauropod dinosaur from Malawi. Palaeontology 36: 523-534.

Powell J (2003) Revision of South American titanosaurid dinosaurs: palaeobiological, palaeobiogeographical and phylogenetic aspects. Records of the Queen Victoria Museum. 111: 1-173.

Salgado L, Calvo JO (1997) Evolution of titanosaurid sauropods. II: the cranial evidence. Ameghiniana 34: 33-48.

Wilson JA (2002) Sauropod dinosaur phylogeny: critique and cladistic analysis. Zoological Journal of the Linnean Society 136: 217-276