Why do we have so few complete, undistorted sauropod necks?
October 9, 2015
Since I posted my preprint “Almost all known sauropod necks are incomplete and distorted” and asked in the comments for people to let me know if I missed any good necks, the candidates have been absolutely rolling in:
- The Kaatedocus siberi holotype SMA 0004 (thanks to Oliver Demuth for pointing this out)
- The Futalognkosaurus dukei holotype MUCPv-323 (thanks to Matt Lamanna)
- The referred Rapetosaurus specimen FMNH PR 2209 (Matt Lamanna again)
- The as-yet unnnamed DGM ‘Series A’ titanosaur from Peirópolis (Matt Lamanna again)
- The Camarasaurus lewisi holotype BYU 9047 (thanks to John D’Angelo)
- Dicraeosaurus hansemanni — the mounted “m” specimen, probably MB.R.4886 (thanks to Emanuel Tschopp)
- Maybe some Omeisaurus specimens (Emanuel Tschopp again)
I will be investigating the completeness of all of these and mentioning them as appropriate when I submit the revision of this paper. (In retrospect, I should have waited a week after posting the preprint before submitting for formal review; but I was so scared of letting it brew for years, as we’re still doing with the Barosaurus preprint to our shame, that I submitted it immediately.)
So we probably have a larger number of complete or near-complete sauropod necks than the current draft of this paper suggests. But still very few in the scheme of things, and essentially none that aren’t distorted.
So I want to consider why we have such a poor fossil record of sauropod necks. All of the problems with sauropod neck preservation arise from the nature of the animals.
First, sauropods are big. This is a recipe for incompleteness of preservation. (It’s no accident that the most completely preserved specimens are of small individuals such as CM 11338, the cow-sized juvenile Camarasaurus lentus described by Gilmore, 1925). For an organism to be fossilised, the carcass has to be swiftly buried in mud, ash or some other substrate. This can happen relatively easily to small animals, such as the many finely preserved stinkin’ theropods from the Yixian Formation in China, but it’s virtually impossible with a large animal. Except in truly exceptional circumstances, sediments simply don’t get deposited quickly enough to cover a 25 meter, 20 tonne animal before it is broken apart by scavenging, decay and water transport.
Secondly, even when complete sauropods are preserved, or at least complete necks, distortion of the preserved cervical vertebrae is almost inevitable because of their uniquely fragile construction. As in modern birds, the cervical vertebrae were lightened by extensive pneumatisation, so that they were more air than bone, with the air-space proportion typically in the region of 60–70% and sometimes reaching as high as 89%. While this construction enabled the vertebrae to withstand great stresses for a given mass of bone, it nevertheless left them prone to crushing, shearing and torsion when removed from their protective layer of soft tissue. For large cervicals in particular, the chance of the shape surviving through taphonomy, fossilisation and subsequent deformation would be tiny.
So I think we’re basically doomed never to have a really good sauropod neck skeleton.