Signor and Lipps 1982
Tue Nov 25 2008
Sampling bias, gradual extinction patterns ad catastrophes in the fossil record.
Catastrophic hypotheses for mass extinctions are commonly criticized because
many taxa gradually disappear from the fossil record prior to the extinction. Pre-
sumably, a geologically instantaneous catastrophe would not cause a reduction in
diversity or a series of minor extinctions before the actual mass extinction. Two types
of sampling effects, however, could cause taxa to appear to decline before their actual
biotic extinction. The first of these is reduced sample size provided in the sedimentary
record and the second, which we examine in greater detail, is artificial range trunca-
tion. The fossil record is discontinuous in time and the recorded ranges of species or
of higher taxa can only extend to their last known occurrence in the fossil record. If
the distribution of last occurrences is random with respect to actual biotic extinction,
then apparent extinctions will begin well before a mass extinction and will gradually
increase in frequency until the mass extinction event, thus giving the appearance of a
gradual extinction. Otlœr factors, such as regressions, can exacerbate the bias toward
gradual disappearance of taxa from the fossil record. Hence, gradual extinction
patterns prior to a mass extinction do not necessarily eliminate catastrophic extinc-
tion hypotheses. The recorded ranges of fossils, especially of uncommon taxa or taxa
in habitats not represented by a continuous record, may be inadequate to test either
gradual or catastrophic hypotheses.
Catastrophic hypotheses for mass extinctions are commonly criticized because
many taxa gradually disappear from the fossil record prior to the extinction. Pre-
sumably, a geologically instantaneous catastrophe would not cause a reduction in
diversity or a series of minor extinctions before the actual mass extinction. Two types
of sampling effects, however, could cause taxa to appear to decline before their actual
biotic extinction. The first of these is reduced sample size provided in the sedimentary
record and the second, which we examine in greater detail, is artificial range trunca-
tion. The fossil record is discontinuous in time and the recorded ranges of species or
of higher taxa can only extend to their last known occurrence in the fossil record. If
the distribution of last occurrences is random with respect to actual biotic extinction,
then apparent extinctions will begin well before a mass extinction and will gradually
increase in frequency until the mass extinction event, thus giving the appearance of a
gradual extinction. Otlœr factors, such as regressions, can exacerbate the bias toward
gradual disappearance of taxa from the fossil record. Hence, gradual extinction
patterns prior to a mass extinction do not necessarily eliminate catastrophic extinc-
tion hypotheses. The recorded ranges of fossils, especially of uncommon taxa or taxa
in habitats not represented by a continuous record, may be inadequate to test either
gradual or catastrophic hypotheses.