The processes of fossilization (taphonomy) preserve evidence of organisms as mineralized or imprinted traces within sediments. Fossils may be altered remains of the actual organism or imprints such as body impressions, trails and footprints, or holes made by burrowing creatures.

(Left - fossilization by various mechanisms with trace fossils at center.)

Fossilization potential (FP) is the likelihood that an organism will leave some trace of its existence, and this is increased by:
● hard body-parts (exoskeletons, endoskeletons)
● marine rather than land habitat (longer history of marine life, greater likelihood of burial in sediments)
● possibility of rapid burial by sediment (slides, volcanic ash)
● burial of soft-bodied organisms in fine-grained sediment (imprints)
● anoxic environment at burial (swamps, tar sands, resin, rapid burial under sediment)
● 'mummification' by desiccation, ice
● perfusion of burial site by mineralizing solutions (petrification)

The chances that a particular fossil remain will be discovered by paleontologists and earth scientists depends upon:
● geologic stability of biostrata
● protection of soft body parts from scavengers and decomposition
● erosion of overlying strata
● freedom from permanently overlying snow or ice
● geographic location and accessibility (physical, political, educational)

Index fossils are particularly useful in biostratigraphic correlation of the age of rock formations [diagram, shale fossil index]:
● rapid evolution constricts the timeframe (range) during which a particular species lived, enabling more exact identification of the relative age of strata
● abundant and widespread occurrence relatively unaffected by environment increases the likelihood that the species will have fossilized across different rock strata formed contemporaneously
● easy identifiability aids cross-correlation

Index fossils, or guide fossils include:
graptolites (Paleozoic)
trilobites (extinct arthropods from Cambrian to Permian extinction)
ammonites (cephalopods from Late Silurian or Early Devonian to KT-boundary)
● some corals
brachipods (Paleozoic)
● some echinoids (e.g., Micraster in Cretaceous)

While a lagerstätte (pl. lagerstätten) is a fossil bed that displays exceptional quality of preservation, the vast majority of organisms that have lived have left no fossilized traces or their fossilized remains will never be uncovered or discovered. The fact that relatively few organisms leave any trace has been twisted by creationists in fallacious arguments against the fact of biological evolution.

biostratigraphyTimeline Earth LifeHadeanArchaeanProterozoicPhanerozoic

Labels: , , , ,


<< Home

. . . evolving since 12/24/06