Relative age inference in paleontology



    1. School of Geology and Geophysics, The University of Oklahoma, 830 Van Vleet Oval, Room 109, Norman, Oklahoma 73019; 10 December, 1979.
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Fossil taxa often occur in a regular vertical order in strata, yet, this regularity does not necessarily imply that the taxa succeeded one another in the same order in time. An argument for the time significance of such a regular pattern is strengthened if (1) the taxa involved were preserved in a wide range of depositional environments, and (2) the Eldredge-Gould model of evolution is correct: by contrast, (3) the areal extent of the pattern and (4) the inferred phylogeny of the taxa involved are not relevant. Paleontologists commonly do conclude that a homotaxial pattern, i.e. a definite orderly succession of fossil taxa, results from the taxa having occurred in essentially the same order in time. The conclusion may be justified as follows: If it is false, there should have been some local areas studied where the taxa (or first and last occurrences of taxa) occur in reverse order, or where two or more allegedly sequential taxa occur together in the same strata. But this is not the case: so the taxa probably succeeded one another in essentially the same order in time. The argument is broadly applicable, yet compelling only if (1) certain constraints involving the prior probability of the conclusion apply, and (2) transgression/regression can be ruled out as a cause of the pattern observed. Subsidiary defenses of the time significance of a homotaxial pattern involve other-group, radiometric, marker-bed, magnetic, seismic, or stable-isotope data. The use of fossils for time correlation of strata does not involve circular reasoning.