Re: Fossils Are Seldom Perfect

[Cliff Green <dinonaut@emerytelcom.net>]

Dear Jaime and List,

    You aren't kidding about that one. I have done some fossil cast
reconstruction.Some bones will actually rot and become soft like cooked
spagetti noodles before fossilization takes place.
     I remember many years ago, working on a 3-D version of the juvenile
camarasaur fossil from Dinosaur National Monument. Dinolab wanted the bones
on both sides to match.One humerus was normal, but the one that the dinosaur
layed on was squished almost flat. Probably from the weight of the dinosaur,
as it was decomposing. The two bones hardly looked alike at all, and I
believe that Dinolab finally made a reverse copy of the perfect humerus on
the computer.

Cliff

----- Original Message -----
From: "Jaime A. Headden" <qilongia@yahoo.com>
To: <dinosaur@usc.edu>
Sent: Monday, June 07, 2004 11:53 PM
Subject: Fossils Are Seldom Perfect


In fact, with a very few rare exceptions, no fossil IS perfect. It's a
modified, permineralized structure that is in the shape of bone, or at
least that's how it started out. Then, depending on it's depositional
environment, it either cracks, shattered, becomes pulled apart, drifts or
is displaced or _torn_ apart, or has a few defects due to the
preservation, such as injection of

  Erosion or distortion post-burial can pull portions of the bone in
directions that would, at first glance, appear "natural," and therefore
obscure the original shape of the bone. In the example below, which orbit
is the natural size?
http://fossils.valdosta.edu/fossil_pages/fossils_per/t23.1.html

  Opalizations occurs to turn bone into a singular mineral structure that
is, by it's very nature, extremely fragile. This isn't Morrison bone in
ironstone or mudstone, and a major example is the holotype femur of
*Kakuru* (courtesy Dann Pigdon and the owner):
www.alphalink.com.au/ ~dannj/kakuru/

  This is a radius (which doesn't look like one due to it's deformation
and partially "blasted" appearance):
http://paleo.amnh.org/fossil/img_show.html?file=http://paleo.amnh.org/i/2106
-08.jpg

  The bone here shows water-borne damage and swelling, distorting the
shape, surface and diameter of the bone:
http://paleo.amnh.org/fossil/show.html?cat_num=FR%2024382

  The most prominent damager in fossils, and the bane of many, is that of
pyrite in coal-measures, as exemplified by the Bernissart coal-mines which
produced *Goniopholis* and *Iguanodon,* now encapsulated in air-tight
chambers and coated to stall the decay that pyrite causes on contact with
oxygen. Yet still, the bone is damaged and fractured down to microscopic
levels, meaning that when first recovered, this bone is imperfect at best,
and basically, only shapes can be made out adequately.

  Then there's dolomitized bone. This problem of fracture along planes in
a lagerstätten is nowhere better exemplified than in the holotype of
*Megalancosaurus,* in which the halves of bone, including maxillae, are
split such that bone slab and counterslab include halves of both sides of
bone, which had been previously squished flat. The division at the
fracture plane, the point in the bedding layers of greatest weakness
(usually but not always the fossil inclusion) causes the plane to usually
crack unevenly and this is even more problematic in dolomite. Ash is
decidedly a weak bonding agent, so in the Liaoning fossils, it becomes
much more of a problem when portions of the slab break off because of the
inclusions, through the inclusions, etc. Especially when so much weight is
borne on top, ANY layered material will cause the inclusions to distort.
Fossil bone becomes a casuality to geology. Cracks and erosion to wind,
water, or time, happen, and limit what we can tell.

  Cheers,

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making
leaps in the face of adversity, that a simple skip is so hard to do.  We
should all learn to walk soft, walk small, see the world around us rather
than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)




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