The controversial hypantra of Argentinosaurus
January 5, 2008
Newsflash: some sauropods were really, really big. But perhaps it’s not always obvious just how big some of them were… maybe this photo should help. It depicts one of the holotype dorsal vertebrae (MCF-PVPH-1) of the South American titanosaur Argentinosaurus huinculensis, with a person for scale (the entire holotype series consists of three anterior and three posterior dorsal vertebrae, part of a rib and a left fibula). For shame, I can’t remember where the photo originated, nor who the person is.
We’re looking here at the anterior surface of the vertebrae – the condyle is reconstructed and the label is obscuring the prespinal lamina that extends up the anterior face of the neural arch. Note that the neural spine is broad and anteroposteriorly flattened. Flanking the bottom of the label, on both the left and right, are the prezygapophyses, and the two flanges projecting directly below them supposedly (Bonaparte & Coria 1993) form the hypantrum (but read on). The hypantrum (plural: hypantra) is a sort of recess on the neural arch, always located dorsal to the neural canal and formed from two flanges arranged either side of the midline. A projecting structure on the posterior surface of the neural arch, termed the hyposphene, fits into the hypantrum (I remember which way round they go by simply remembering that ‘o’ [as in hyposphene] comes after ‘a’ [as in hypantrum]).
Hypantrum-hyposphene complexes are widespread in archosaurs and are thought to help add rigidity to the vertebral column. In dinosaurs they’re not present in ornithischians and, among sauropods, titanosaurs are well known for lacking them: a very detailed discussion of the hypantrum-hyposphene system in titanosaurs was provided by Apesteguía (2005) and is essential reading if you want to know more on this, err, specialised area. Argentinosaurus is thus odd in possessing them, and when this titanosaur was first described in 1993 (Bonaparte & Coria 1993), it was proposed that the presence of a hypantrum-hyposphene system in Argentinosaurus, Andesaurus and Epachthosaurus should be used to unite them in a new group, the Andesauridae. However, the distribution of other characters in these dinosaurs does not support this proposal and subsequent studies have not grouped the ‘andesaurids’ together. Furthermore, does Argentinosaurus really possess a hypantrum-hyposphene system? Salgado & Martínez (1993) and Salgado & Bonaparte (2007) argued that it didn’t, and explained that what Bonaparte & Coria (1993) had interpreted as such were actually modified laminae (the wording in both Salgado & Martínez (1993) and Salgado & Bonaparte (2007) is unclear, but they seem to mean the centroprezygapophyseal and centropostzygapophyseal laminae). Having said all that, particularly big hyposphenes do genuinely seem to be present in the mid and posterior dorsals of Argentinosaurus... and what’s the point of having hyposphenes if you don’t have hypantra? At the risk of getting too deeply involved in all of this, I’ll stop there.
Whatever, Argentinosaurus was big.
References
- Apesteguía, S. 2005. Evolution of the hyposphene-hypantrum complex within Sauropoda. In Tidwell, V. & Carpenter, K. (eds) Thunder-Lizards: The Sauropodomorph Dinosaurs. Indiana University Press (Bloomington & Indianapolis), pp. 248-267.
- Bonaparte, J. F. & Coria, R. A. 1993. Un nuevo y gigantesco sauropodo titanosaurio de la Formacion Rio Limay (Albiano-Cenomaniano) de le Provincia del Neuquén, Argentina. Ameghiniana 30, 271-282.
- Salgado, L. & Bonaparte, J. F. 2007. Sauropodomorpha. In Gasparini, Z., Salgado, L. & Coria, R. A. (eds) Patagonian Mesozoic Reptiles. Indiana University Press (Bloomington & Indianapolis), pp. 188-228.
- – . & Martínez, R. 1993. Phylogenetic relationships of the basal titanosaurids Andesaurus delgadoi and Epachthosaurus sp. Ameghiniana 30, 339.
Sauropod Vertebra Picture of the Week 
January 5, 2008 at 11:53 pm
An easier way to remember what’s what: the hypANTrum is on the ANTerior face of the neural arch, and the hyPOSphene is on the POSterior face. (Riggs, by the way, gets this consistently wrong throughout his 1904 monograph on Brachiosauridae.)
January 6, 2008 at 4:10 am
And exactly which part of the picture is the supposed hypantrum-hyposphene complex, and what is its function/purpose? That was a tiny bit perplexing…
By the way, while on the topic of Argentinosaurus, I know their are no known cervical vertebrae known for Argentinosaurus, but how reasonable is it to conclude that it had a short saltosaurid-like neck ( as per Ken Carpenter’s paper on Amphicoelias frgillimus), in other words, what group of titanosaurs is it most closely related to?
January 6, 2008 at 5:51 pm
The alleged hypantrum, m’lud, is the ill-define triangular hollow just above the centrum. As Darren said, the supposed function was to stabilise the vertebral column, making it more rigid. Among titanosaurs, Argentinosaurus is one of the most basal and Saltasaurus one of the most derived. Necks are not known for many titanosaurs, though, so modelling A. on S. is not as unreasonable as you might think. Rapetosaurus is probably a better bet, though.
January 7, 2008 at 4:41 pm
It’s funny, but that pic conveys better than many of the ‘silhouette of Apatosaurus together with silhouette of a human’ pics just how big those things were.
I continue to be surprised at how interesting sauropods are on the inside. On the outside, they pretty much all follow Ann Elk’s theory of the Brontosaurus, but on the inside, they are, well, in the current vernacular, eXtreme!
June 12, 2015 at 1:41 pm
[…] were definitely bigger sauropods than that — just not known from such complete specimens. Argentinosaurus was certainly bigger, for example. But there’s no way to put a meaningful whole-body mass […]