Page 81 - Introduction to Paleobiology and The Fossil Record

68 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 3.2 Retrodeformation of deformed fossils

Some fossils may be heavily deformed or distorted, so that they do not retain their original shapes.
These distortions may be the result of collapse or diagenesis, but they may indicate metamorphism
– that is, processes connected with tectonic activity, faulting, folding and mountain building. If a
mudstone is folded and, under high pressure, is changed into a slate, any contained fossils are likely
to be stretched and distorted. The deformation is very clear in symmetric fossils (e.g. Fig. 3.7), where
the form is stretched in such a way that the original symmetry has been lost. In a slab where numer-
ous fossils lie at different orientations, they will clearly be deformed in different ways, all subject to
the same forces in the rocks.
It is possible to restore the original shape of the fossil, a process called retrodeformation, meaning
“back deformation”. The outlines of one, or preferably several, deformed fossils are drawn, usually
in two dimensions, and these can be most easily restored to original symmetry in a standard computer
drawing software program by manipulating the shape dimensions. This method also allows the
analyst to calculate the amount by which the fossil was retrodeformed, and in which direction. This
can tell us much about the nature of the tectonic forces that were in operation.
Deformed fossils become commoner the farther back in time one goes, simply because of the
greater likelihood than any particular fossiliferous sediment has undergone metamorphism and
tectonism.
Find web references about retrodeformation of fossils at http://www.blackwellpublishing.
com/paleobiology/.

(b)

(a)

(c)
Figure 3.7 (a) Numerous examples of deformation of the brachiopod Eoplectodonta: in a
tectonized mudstone from the Silurian of Ireland. (b) A single deformed example (c. 20 mm wide)
of a Cambrian Billingsella fossil from the Himalayas (Bhutan) and (c) the same example
retrodeformed to its original shape.