Problems of Interpreting the Fossil Record
Poor preservation and exposure; dissolution
Bioturbation and reworking
Reduced sample size Signor-Lipps effect
Gradual extinction appears sudden.
Range shortened; extinction seems to have occurred earlier than it did.
Range shortened; species appear more rare than they were; extinction seems to have occurred earlier than it did.
Ranges extended upward and downward; sharp layering is smeared out. Reworked and survivor species confused. Sudden extinction appears gradual.
Species appear too early; those that did not live together may be found together.
Sudden extinction appears gradual. Sudden extinction appears gradual.
exactly at the K-T boundary. To perform these tests, geologists needed to be able to pinpoint the exact level in a bed of rock at which a species became extinct. This brief review has shown what a formidable challenge such a requirement presents (summarized in Table 3). The fossil record begins in imperfection and is then altered by sedimentary and biologic processes; what was gradual may appear falsely sudden, what was sudden may appear falsely gradual; the highest fossil is never found.
In 198", when the Alvarez theory broke upon the scientific world, geologists had already been aware of these problems (except for the Signor-Lipps effect) for a long time. Since they had not been focusing on extinction, however, they had not been attempting to determine exactly where a fossil species disappeared. Their initial response was to rely on the data already collected, and their prior interpretations of it, but those soon proved inadequate to provide a proper test. Paleontologists returned to the field to collect the much larger sample fossil databases that were required to probe the Alvarez theory. Gradually, enough data became available to allow researchers to test in detail its predictions for mass extinction.
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