fossil formation process, called fossilization,
is a complex and a rarity for given organism. Hard parts of
an organism, such as a mineralized shell of an ammonite, or
exeskeleton of an arthropod are much more likely to become fossilized
than soft parts. One key to fossilization is burial, particular
by sediment, in a marine environment. The crinoid is an example
of an animal that left a poorly fossilized due to its delicate
parts. This explains why most crinoids, for example, are found
only as stem pieces. Since crinoids were not usually buried
quickly, their hard stem parts are far more frequently found
as fossils. Observations of rare living crinoids have shown
that they will rapidly disarticulate within a few days of death.
Rapid burial, in contrast, prevents this disintegration, and
thus explains a few localities where beds of delicate crinoids,
starfish and brittle stars are preserved in their entirety.