My neglected paddle-shaped dorsal neural spines, by Amargasaurus
July 3, 2008
If I say Amargasaurus cazaui, you say ‘long spines on cervical vertebrae’. Yes yes, it’s true that Amargasaurus had weird neck spines (and spines that, judging from their shape, really were spines rather than parts of continuous sail-like structures), but have you ever looked at its dorsals? That’s what we’re doing here: this is a posterior dorsal (from Salgado & Bonaparte (1991)) and, in contrast to the cervicals and anterior dorsals, its neural spine is not bifurcated.
Two things are really obvious. For starters, the neural spine is incredibly tall, being at least four times taller than the length of the centrum. It is in fact among the tallest of neural spines within Sauropoda. Secondly, seen in anterior or posterior view, the spine flares outwards towards its tip, forming a broad, rounded apex. Wilson (2002) and Rauhut et al. (2005) described this as ‘petal-shaped’, but it’s also been referred to as ‘paddle-shaped’ (Upchurch et al. 2004). Amargasaurus isn’t unique in these features: it’s a dicraeosaurid, and the other dicraeosaurids we know of (Dicraeosaurus hansemanni and D. sattleri, both from the Tendaguru Formation, and Brachytrachelopan mesai from the Upper Jurassic Cañadón Cálcareo Formation of Patagonia) have extremely similar vertebrae. Some rebbachisaurids also have paddle-shaped neural spines, but this presumably arose convergently because dicraeosaurids and rebbachisaurids are not close relatives within Diplodocoidea.
Finally, note the laminae. A prominent prespinal lamina (PRSL) is obvious. Given that the anterior dorsals have bifurcate neural spines (and given that both halves of the spine sport spinoprezygapophyseal laminae [SPRLs] on their anterior faces), the PRSL looks like a composite formed from the fusing of the two SPRLs (Wilson 1999). We can also see a prominent lamina running from the diapophysis to the neural spine apex, and it looks like a spinodiapophyseal lamina (SPDL). What’s unclear from the diagram is how extensive the spinopostzygapophyseal lamina (SPOL) was: did it contact the SPDL at its base? In Brachytrachelopan, Rauhut et al. (2005) described how the SPDL was short, and how it’s the SPOL that extends up the side of the neural spine. What appears more common however, is for the SPOL to merge with the more extensive SPDL, and for both to then form the so-called lateral lamina complex. It’s pretty clear that this is what happened in D. hansemanni (Janensch 1929), and it looks to me like it’s the case in Amargasaurus too.
All in all, these animals had pretty funky dorsal neural spines. As usual, the big question is: why?
References
- Janensch, W. 1929. Die Wirbelsäule der Gattung Dicraeosaurus. Palaeontographica Suppl. 7 (1), 3 (2), 37-133.
- Rauhut, O. W. M., Remes, K., Fechner, R., Cladera, G. & Puerta, P. 2005. Discovery of a short-necked sauropod dinosaur from the Late Jurassic period of Patagonia. Nature 435, 670-672.
- Salgado, L. & Bonaparte, J. F. 1991. Un nuevo sauropodo Dicraeosauridae, Amargasaurus cazaui gen. et sp. nov., de la Formacion La Amarga, Neocomiano de la Provincia del Neuquen, Argentina. Ameghiniana 28, 333-346.
- Upchurch, P., Barrett, P. M. & Dodson, P. 2004. Sauropoda. In Weishampel, D. B., Dodson, P. & Osmólska, H. (eds) The Dinosauria, Second Edition. University of California Press (Berkeley), pp. 259-322.
- Wilson, J. A. 1999. A nomenclature for vertebral laminae in sauropods and other saurischian dinosaurs. Journal of Vertebrate Paleontology 19, 639-653.
- Wilson, J. A. 2002. Sauropod dinosaur phylogeny: critique and cladistic analysis. Zoological Journal of the Linnean Society 136, 217-276.
Sauropod Vertebra Picture of the Week 
July 3, 2008 at 3:32 pm
“All in all, these animals had pretty funky dorsal neural spines. As usual, the big question is: why?”
Badminton?
July 3, 2008 at 6:21 pm
Ping-pong?
July 3, 2008 at 7:11 pm
The basal titanosaur Ligabuesaurus also has “paddles” on its anterior dorsals, though they’re shorter than seen in diplodocoids.
July 3, 2008 at 7:33 pm
“Two things are really obvious. For starters, the neural spine is incredibly tall, being at least four times taller than the length of the centrum. It is in fact among the tallest of neural spines within Sauropoda.”
When you said “tallest of neural spines within Sauropoda”, did you mean in relation to the centrum, or that it was literally one of the tallest among the sauropoda? If the latter, I have some questions, how tall were the neural spines of Rebbachisaurus garasbae, the Apatosaurus species, and other diplodocids, in comparison? As they seem to also have a low “sail” or what-have-you along their dorsal column. So my main question is how unusual is the height of the neural spines in Amargasaurus (compared to its centrum) compared to other diplodicids, etc.?
Anyways, a thought-provoking article as always, Darren.
Thanks,
:) Zach
July 5, 2008 at 7:49 pm
Hmm… a few comments.
I thought the cervical spines were circular in cross section as in Dimetrodon and thus more likely to take part in a sail (Bailey, 1997).
Couldn’t paddle-shaped neural spines just as easily be a diplodocoid symplesiomorphy lost in diplodocids? Or do basal rebbachisaurids lack them?
Finally, since single neural spines are the primitive condition for diplodocoids, wouldn’t that suggest the laminae on the dual-spined vertebrae are split versions of the ancestral lamina rather than the other way around?
July 5, 2008 at 10:14 pm
Hi Mickey. The spines are indeed circular in cross-section and taper to points. Believe it or don’t, I didn’t have Dimetrodon and kin in mind when thinking of this, and the fact that round-in-cross-section neural spines are consistent with a sail would mean, yes, that paired sails are possible. However, the fact that these sails would be on the neck still seems to be a problem – to justify that argument I’d need to discuss neck flexibility however and I’d rather do that at another time.
Polarity of paddle-shaped neural spines: what you propose (that paddle-shaped neural spines were primitive for diplodocoids but lost in diplodocids) is plausible. However, MACN PV N35, a mid or posterior rebbachisaur dorsal described by Apesteguía (2007), doesn’t have a paddle-shaped spine, and is apparently a basal member of the group. Nigersaurus, which is a basal rebbachisaurid in some analyses (Gallina & Apesteguía 2005) also seems to lack this condition. We need more information to better evaluate the distribution of this character.
On the PRSL being a combined pair of SPRLs vs the SPRLs being a split PRSL, note that I said that ‘the PRSL looks like a composite formed from the fusing of the two SPRLs’, not that it IS a fusion of the two SPRLs. I agree with your reasoning and think it more likely that paired SPRLs represent the derived condition relative to the single PRSL.
Refs – –
Apesteguía, S. 2007. The sauropod diversity of the La Amarga Formation (Barremian), Neuquén (Argentina). Gondwana Research 12, 533-546.
Gallina, P. A. & Apesteguía, S. 2005. Cathartesaura anaerobica gen. et sp. nov., a new rebbachisaurid (Dinosauria, Sauropoda) from the Huincul Formation (Upper Cretaceous), Río Negro, Argentina. Rev. Mus. Argentino Cienc. Nat., n.s. 7, 153-166.
July 6, 2008 at 11:43 am
I would argue the extensive neural spines with prominent and distinct laminae are a feature related to elongated proximally in the column and retained posteriorly. The shape may be to support robust and broad tissues associated with the bifurcation. Have you compared the presence of a single spine with “paddle” shaped morphology to their elongation relative to centrum height or size in the animal in gross size? I would actually posit that the shape is particular to vascularization, and the higher and broader the spinal apex the greater the surface area and therefore a greater capacity for cooling at gross animal size (most dicraeosaurs appear to be fairly small, if I recall correctly, as is true of most rebbachisaurs?).
July 8, 2008 at 7:54 am
OT (I’m a few weeks behind on browsing SV-POW) but you really must link to this announcement, not only does it have a (chunk of reputed) sauropod vertebra, but an actual female hominid included for scale:
http://www.gns.cri.nz/news/release/20080624_large_dino.html
July 10, 2008 at 4:12 am
I was recently holding my toy Amargasaurus from the Boston Museum of Science line, and I noticed that its back was sculpted incorrectly, with bifurcated spines all the way back to the sacral region. I don’t have the Amargasaurus from the Carnegie Collection series, is it any more accurate? Come on Darren, you probably know this one. :p
July 25, 2008 at 5:35 am
That is indeed a big question. I offer a little answer: thermoregulation. (No idea, but it is a little answer.)
October 8, 2009 at 10:52 am
[…] somehow forgotten how frikkin’ weird it was. We’ve looked at Amargasaurus on SV-POW! before: in this article we drew attention to the fact that its dorsals are pretty interesting too, and as usual with […]