One thing to watch out for in fossil specimens is the spectre of distortion. While most people are probably comfortable with the idea that obviously not all fossils are complete and bits are often missing, they can suffer other kinds of damage. Fossilisation does, after all, largely require that bones be buried under millions of tons of sediment for millions of years and so it should not be a surprise that tough though bones (or fossilised bones) are, the Earth has the capacity to affect them.

In other words, a bone might be buried in pristine condition, but that doesn’t mean it will come out in pristine condition. First off it might simply be crushed – something of course particularly seen in lagerstaat specimens like those from Liaoning or the Solnhofen. These are certainly crushed, but the crushing need not change the bones, or rather, that shapes of those bones. Pretty much all of this Archaeopteryx is effectively flat, whereas the bones were not 2D in life, so rather by definition the specimen is crushed. However, it’s pretty clear that the arm bones have retained their original shape – as if a photo was taken of them, but the skull is rather crushed with parts broken and moved.

Think of a drinks can. Take a look at the end. Now stamp on the can to flatten it and look at the end again. If you did it right, it will still look more or less the same, despite the obviously catastrophic crushing the can as a whole has suffered. From the point of view of a palaeontologist, it’s not so bad. Sure you can only see one end of the can, but what you can see is actually a pretty accurate representation of what it looked like before your foot got involved.

So much so straightforward, but often unmentioned is that things can be distorted too. Not broken or crushed necessarily, but simply shifted. To continue our can metaphor you can flatten it on it’s side but what is like to happen is for it to flatten out and look much fatter than it was originally. And that what were the sides (OK, it’s a cylinder, but bear with me) are now visible to the left and right of the front. Or if it went sideways a bit, the front could lie to one side. These are generally easy to spot as the bones end up looking super fat with small articular ends. You can also crush the can upwards such that the base of your now 2D representation of a can had the base covering most of the face.

Finally the whole thing might remain in 3D but still shifted slightly from its true form. One face could be folder over, or crushed in. It might have moved like a parallelogram as it were with the top and bottom staying parallel to each other, but no longer one directly above the other.

All of this can and does happen to fossils. Sometimes, even most of the time, it’s very obvious. But some can be quite insidious and hard to spot, or it may not be clear which side has been distorted and which is the original. The surrounding matrix can be very informative in this respect, but of course what is usually the most helpful in these situations it to get multiple specimens. Even if they are all crushed, these will be in different ways with different levels of distortion and working out what bits when wrong where can become much clearer.

Still it’s another issue to be wary of and of course another potential source of both error and disagreements between researchers. Ah the joys of geology. This gig would be much easier if we had proper dead dinosaurs to work from and not these scraps of modified bone and rock. Still, there’s always the possibility a couple of rexes will stumble through a time portal at some point. I can live in hope, though in the meantime, we’re not short of fossils.

5 Responses to “Distortion”

  1. 1 Andy 31/01/2011 at 3:02 pm

    One of our fossil preparators uses the term “Hell Creek Tweak” to describe this condition in some fossils from that formation. Very memorable, and accurate.

  2. 2 Mark Robinson 31/01/2011 at 3:43 pm

    “This gig would be much easier if we had proper dead dinosaurs to work from and not these scraps of modified bone and rock.”

    Hah. Time to switch to studying animals from the Holocene of Siberia!

  3. 4 Scott Elyard 31/01/2011 at 9:18 pm

    Raven and I both liked Jerry Harris’ term for mooshed specimens: meltybone and smashybird (as heard at SVP in Bristol, 2009).

  1. 1 A few more words on distortion « Dave Hone's Archosaur Musings Trackback on 17/02/2011 at 10:13 am
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