Problems posed by derived fossils
Dear Editor, Derived or reworked fossils are those eroded out of an earlier formation and redeposited in younger strata. These fossils survived the process of derivation that destroyed the rock in which they were originally enclosed. Derived fossils present palaeontologists and other geoscientists with numerous problems.
Biostratigraphy is the method by which strata are dated via the use of Zone Fossils—those that characterise a particular time period. In order for a fossil to qualify, it must be relatively abundant, easy to recognise, geographically widespread, and have a limited stratigraphic range. If a Zone Fossil is reworked, its presence as a derived fossil would lead to an erroneous biostratigraphic age—it would make the stratum appear older than it is.
Derived fossils can also cause extinct species to appear as fossils in strata deposited after their extinction point, a phenomenon termed “dead clade walking”. The term was first coined by David Jablouski (2002), referring to short-lived survivors of mass-extinctions. One commonly cited example is that of Palaeocene dinosaurs that, according to Sullivan (2003), are presently believed to be Cretaceous fossils reworked into the Cenozoic Ojo Alamo Sandstone Formation of the Midwestern United States, rather than post-Mesozoic survivors.
Some species have narrow environmental tolerances so are used in palaeoenvironmental reconstructions, and derived fossils can screw environmental interpretations. An example of this comes from the Eocene Barton Beds of Southern England. Here contemporaneous and reworked dinoflagellates are present. Unless reworked fossils are identified and excluded, this will lead to incorrect assumptions about the depositional setting or contradictory results.
Jack Wilkin
Jablonski, D. (2002) Survival with recovery after mass extinctions. PNAS 99 (12), 8139-8144.
Sullivan, R. M. 2003. No Paleocene dinosaurs in the San Juan Basin, New Mexico. Geological Society of America Abstracts with Programs 35 (5), 15.