A death assemblage of marine snails. |
Gastropods
can be beautifully preserved, as shown by the two examples below.
These are from the Type collection, and are specimens from the Cretaceous
and the Tertiary
periods. |
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This Tertiary snail shell has
been preserved even to retaining its fine surface texture.
A modern shell on the beach will often show much more abrasion
than this specimen! |
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Even the color patterns are
preserved on this snail shell from the Cretaceous! |
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Most of the gastropods that we see
in the Cretaceous
rocks around Austin are much less well preserved. Gastropod
shells are composed of aragonite,
an unstable form of calcium carbonate; the older the fossil, the
more the shell will be altered. |
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In this example the shell has
been recrystallized and the inside filled with sediment. |
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In this similar specimen, the
recrystallized shell has begun to disintegrate. |
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This specimen has reached the
final stage, where nothing is left but the hardened mud that
originally filled the shell. This style of preservation
(called a steinkern),
is how gastropods are most often observed in Cretaceous
rocks around Austin. |
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