Present-day coral reefs exist within narrow limits of temperature, light, and seawater
aragonite saturation states. In the broad perspectives of geologic time, we are in an
icehouse supercycle with an ocean favoring the precipitation of aragonite. Current
climate change may be prematurely moving us into enhanced greenhouse conditions
and subjecting coral reefs to added stresses (Hoegh-Guldberg 2005). Thermal stresses
caused by rising temperatures have triggered numerous mass bleaching events among
living photosymbiotic corals (Chap. 3). Bleaching events signal a breakdown of symbiosis
but the long-term repercussions for reefs and their capacity for adaptation and
quick recovery have not been resolved. The fossil record, therefore, may assist.
Reef patterns observed throughout much of the Phanerozoic, following major
mass extinctions, include episodes of relatively sudden collapse followed by extended
reef eclipses, usually accompanied by decreased metazoan carbonate production.
This has been succeeded by slow reef recoveries, leading eventually to new reef
ecosystems. The geologically “sudden” response in many reef ecosystems to mass
extinction and the extended post-extinction reef eclipse following reef collapse
have direct relevance to current problems of coral bleaching. While difficult to
detect bleaching in the fossil record, crises recorded in many reef ecosystems of the
past certainly would have adversely affected photosymbiosis and ancient bleaching
is a logical consequence. This appears to have been the case for corals during the end-
Triassic when more than 95% of coral species, most judged zooxanthellate, died
out (Stanley and Swart 1995) and during the Cretaceous–Paleogene event when an
estimated 45% of coral species died out (Kiessling and Baron-Szabo 2004). During
warm intervals of the Cenozoic (e.g. Eocene, Miocene) zooxanthellate corals were
able to expand their ranges into higher paleolatitudes than those of the present day.
Although time-scales of paleoecological change within ancient reefs cannot be
resolved with the resolution available for their Holocene counterparts, stresses
associated with global mass extinction on ancient reefs most certainly involved
bleaching and disruption of photosymbiosis. In some cases global warming is
implicated but for others cooling is more likely. Several workers were quick to
equate the sudden collapse of ancient reef ecosystems following mass extinctions
to the breakdown of symbiosis (Talent 1988; Copper 1989; Rosen and Turnšek
1989; Stanley 1992; Perrin 2002), although this idea has not gone without challenge
(Rosen 2000).
The fossil record chronicles the rise, fall, and recovery of reefs. It is a sobering
record because of the longevity of post-extinction global reef gaps and the length
of time before reef recovery. Intervals when reefs are either entirely absent or
greatly reduced range from 106 years to as much as 10×106 years in duration. The
length of time for recovery has implications for the current environmental crisis.
Put into perspectives of the current biotic marine crisis, in which humans are both
directly and indirectly involved, the implications are bleak for the future evolution
of reefs (Myers and Knoll 2001). Although evolution is not predictable, meaningful
estimates on diversity trends and rates of recovery following mass extinctions, areemerging from the fossil record. A study of the role of zooxanthellate photosymbiosis
in the geologic past may provide new insights into both successes and failures
on living coral reefs. The integration of biology and the fossil record, especially
ecology, molecular biology, and life history of both corals and symbionts, offers
potentials to better understand the current coral reef problems, including the bleaching
phenomenon.