Quality of the Fossil Record -- Incomplete or Complete? Inadequate or Adequate?
From death to burial to fossilization to collection there are several factors (filters) that an organism must survive to enter the fossil record. David Raup1 explained the nine factors below that may prevent an organism from entering the fossil record and could lead to incompleteness of the fossil record:
1. Anatomic filters: Organisms with hard parts, e.g., skeletons or shells, are much more likely to be preserved than soft-bodied organisms.
2. Biological filters: Population sizes can affect fossilization rates. Prolific organisms are more likely to enter the fossil records than rare species.
3. Ecological filters: Organisms that live in high sedimentary habitats, e.g., shallow seas, lakes, swamps, and rivers are more common in the fossil record than those that inhabit terrestrial habitats.
4. Sedimentary filters: High depositional paleoenvironments are more likely to have left behind increased concentrations of fossiliferous strata whereas past environments with high erosion typically leave no trace of organisms living there.
5. Preservation filters: Conditions in the sediment must be optimum after an organism's burial for fossilation to occur. For example, if groundwater within the sediment is too acidic, all traces of the organism may be destroyed; or, if sediment is further eroded by a submarine landslide or other physical erosion, bones, shells, or traces may be broken or destroyed.
6. Diagentic filters: Over geologic time, mineralizing waters are likely to transform fossils in buried strata. This can either enhance fossils or destroy them altogether.
7. Metamorphic filters: Metamorphic processes can destroy or severely deform fossiliferous rocks and delete specimens from the record.
8. Vertical movement filters: The vast majority of fossils are preserved in sedimentary rock that has been buried by younger strata and will remain buried forever unless tectonic movements raise the strata containing fossils to the surface.
9. Human filters: Finally, a fossil must be found to enter the fossil record. And after collection, the specimen must be registered in order to enter the collective paleontological knowledge base.
Do these filters mean that the fossil record is incomplete? Well, as long as there is one fossil undiscovered that would add to our knowledge, then one could say it is incomplete. So all sciences are incomplete in that no one ever knows when all knowledge is ascertained2. Rather than asking whether the fossil record is incomplete, Michael Benton writes, "A more sensible question might be: how adequate is the fossil record? Is it good enough to show us the broad outlines of the evolution of life or are the fossils so sporadic that we can learn very little from it?"3
1. Raup, D.M., 1972, Taxonomic diversity during the Phanerozoic: Science, v. 177, p. 1065-1071.
2. Paul, C.R.C., 1998, Adequacy, Completeness and the Fossil Record, in Donovan, S.K. and Paul, C.R.C., eds., The Adequacy of the Fossil Record, p.1-22.
3. Benton, Michael, 2009, The completeness of the fossil record: Significance, September, p. 117-121