Freitag, 21. November 2008

Tectonics & Paleo (III):
The shearing of fossils and how to reverse it

Let's start in 2D: You have a flat fossil and regard only the deformation in the two dimensions of the fossil plane:

Easy, you may say. Fossils specimens like that are good strain indicators and it's not difficult to deduce the amounts of simple shear and flattening/lengthening necessary to transform the undeformed into the deformed specimens or vice versa:

There was a story, my tectonics prof told me from his study time when he was working for a famous German paleontologist: He was doing the retrodeformation of fossils using some kind of algorithm/ computing procedure - but only, until his sponsor found that he could reach the same effect by holding the fossil oblique over a photocopying machine (you can imagine what a disillusionment that was...).

In some cases, however, the problem is not as simple as in the example displayed above. Insect wings from Madygen and other localities often display a considerable amount of deformation but occur isolated and as palaeontological samples they have not been taken oriented (i.e. referenced to a system of external coordinate axes).

What was the original shape?

This question is crucial if you want to define and distinguish taxa (how many unnecessary species have been erected because the similarity of fossil specimens got lost in deformation?) but also searching for intraspecific variation, e.g. branching points that are highly variable in individuals of the same species.

My "bureau-mate" Olivier Bethoux, paleoentomologist, is currently doing his postdoc research working on that problem. I won't say much about his solution which involves morphometrics/ landmark analysis but keep you informed about results when they are published.


220mya hat gesagt…

I've really enjoyed all of your posts - keep them coming! I do vertebrate paleontology, but I really really enjoy reading posts about other aspects of the Triassic. And, Madygen is such a famous but also unknown locality that its really valuable and exciting to learn about it.

Insect wings are certainly easy to retro-deform compared to most vertebrates! A textbook example of plastic deformation wreaking havoc with systematics is the archosaur group Parasuchia (phytosaurs). A completely undeformed skull specimen may not exist.

MBW hat gesagt…

Ah, thanks for the feedback! (And that it is positive.)

I didn't realize that plastic deformation is a large problem for old vertebrate skulls, imagined that they are rather brittly deformed, i.e. gone to pieces ... so you better find the left and right halves of the same skull in order to have some indication of the sort of deformation.

(I'm also working on vertebrates but there is still much work to be done before I have further news on that.)

220mya hat gesagt…

Here are a couple of papers on retrodeforming vertebrates (I'm sure you've already seen them):

Angielczyk, K.D., and H.D. Sheets. 2007. Investigation of simulated tectonic deformation in fossils using geometric morphometrics. Paleobiology 33(1):125–148. DOI: 10.1666/06007.1

Motani, R. 1997. New technique for retrodeforming tectonically deformed fossils, with an example for ichthyosaurian specimens. Lethaia 30(3):221-228. Link
(the DOI doesn't seem to work)

Boyd, A.A., and R. Motani. 2008. Three-dimensional re-evaluation of the deformation removal technique based on "jigsaw puzzling". Palaeontologia Electronica 11(2):7A, 7 p. Link

MBW hat gesagt…

Thanks for the refs - as I wasn't really bothered with tectonic deformation in vertebrates till now (and not much with insects either), the papers were in fact new to me.

Scanned them through - apparently the authors are not using much of a statistical approach as the number of individuals is rather not sufficient for that.