Page 181 - Introduction to Paleobiology and The Fossil Record
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168 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
could find no evidence for selectivity during
the KT event for ecological characters of biv-
hiatus Tertiary alves and gastropods, such as mode of life,
Cretaoeous body size or habitat preference. He did fi nd
that the probability of extinction for bivalve
genera declined predictably depending upon
the number of major biogeographic realms
Tertiary
hiatus they occupied, and the positive survival benefi t
Cretaoeous
of a wide geographic range has been found
for many other groups during other mass
(a) extinctions. Also, genera containing many
species survived better than those with few.
Ecological characters that may be impor-
Tertiary tant in normal, or background, times often
Cretaceous have little influence on survivorship during
times of mass extinction. Jablonski (2005),
for example, showed that epifaunal bivalves
search zone
have shorter generic durations than infaunal
bivalves in the Jurassic and Cretaceous, sug-
gesting that in evolutionary terms it is better
to burrow. However, during the KT event,
there was no difference in the pattern of sur-
vival and extinction of epifaunal and infaunal
bivalves.
last known fossil This confirms a general principle of mass
extinctions, which is that normal evolution-
ary processes break down. So, if during normal
times, it is advantageous to be large, to be
secretive, to burrow, to move fast, or to have
a particular diet or breeding mode, these posi-
(b)
tive characters may make no difference at all
Figure 7.5 Gaps and missing data can make when the crisis hits. Natural selection hones
gradual extinction events seem sudden (a) or and shapes the adaptations of species on the
sudden events seem gradual (b). In both scale of generations and normal levels of envi-
diagrams the vertical lines represent different ronmental change; mass extinctions seem to
species. (a) The real pattern of fossil species represent a different scale of challenge, much
distribution is shown on the left, and if there is a too great for the normal rules to apply. Mass
large or small hiatus, or gap, at the KT extinctions probably occur too far apart, and
boundary (middle diagram), a gradual loss of too unpredictably, for the normal rules of evo-
species might seem artifi cially sudden (right-hand lution to apply. As Steve Gould said, mass
diagram). (b) It is likely that the very last fossils extinctions re-set the evolutionary clock.
of a species will not be found, and a sudden
extinction might look gradual; this can only be
detected by intense additional collecting in the Periodicity of mass extinctions
rocks that include the supposed last fossils There are many viewpoints on the causes of
(shaded gray). mass extinctions, but a fundamental debate
has been whether each event had its own
unique causes, or whether a unifying principle
linking all mass extinctions might be found.
larger number of microscopic planktonic If there was a single cause, it might be spo-
species also died out. radic changes in temperature (usually cooling)
The best evidence of selectivity during mass or in sea level, or periodic impacts on the
extinctions has been against genera with Earth by asteroids (giant rocks) or comets
limited geographic ranges. Jablonski (2005) (balls of ice).
