Patterns of Extinction
Several mass extinctions have occurred throughout earth history. Cycles
found to be associate with these events has resulted in paleontologists
proposing mechanisms to explain these events. It seems that mass
extinctions occur at regular time intervals with a periodicity of
approximately 26 million years. This concept was first suggested by
Alfred Fischer and Michael Arthur, but has been further expanded upon
by various researchers utilizing data collected by David Raup and John
Sepkoski. Raup and Sepkoski constructed graphs utilizing incidences of
extinction of marine families through time. The rate of extinction was
represented on a graph as a series of rising and falling peaks.
Surprisingly, these peaks coincided with most major events of
extinction throughout earth history. However, a significant number of
paleontologists believe Raup and Sepkoski's research to be flawed,
arguing that the dates used in the study are incorrect, and that the
spacing pattern is not always 26 million years. Until new evidence can
be brought forth, the question of periodicity will clearly remain a
source of debate within the paleontological community.
The
concept of periodicity has important implications for determining which
factors cause extinction. Hypotheses invoking catastrophism have
particularly been advanced utilizing this concept, which imply
extra-terrestrial forces as extinction-causing agents. This is because
only astronomical forces are known to operate on such a precise
periotic time schedule. Contrary to catastrophism are hypotheses which
focus on gradualism. These gradualistic hypotheses invoke various
terrestrial extinction mechanisms including volcanism, glaciation,
global climatic change, and changes in sea level. Most recently
hypotheses centered on the new non-linear science of complexity have
emerged. Under these hypotheses species-species interactions lead to
occasional instability resulting in cascades which may ripple through
entire ecosystems, with potentially devastating results.
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