Middle Silurian to Late Devonian interspersed with
periods of apparent collapse (Copper, 1994).
The world’s Devonian reefs may have been every bit as
well formed and biologically diverse as today’s reefs and,
with an estimated area of 5 Â 10
6 km
2 (including interreef areas), may have been as much as twice as extensive
(Copper, 1994). The Late Devonian mass extinction, like
that at the end of the Ordovician, has been linked to multiple causes. Many bolides may have struck the Earth
around this time (McLaren and Goodfellow, 1990 and
others), although this is unlikely to be linked to the extinction (reviewed by Racki, 2005). A precipitous drop in
atmospheric carbon dioxide owing to uptake by vascular
plants, low global temperatures, and wildly fluctuating
sea levels have all been blamed (Copper, 2001). The
extinctions appear to have varied geographically, although
they primarily affected marine life: reef development
ceased almost completely. Some corals and stromatoporoid
sponges lived on, but there was no recovery for the vast
coral-sponge reef-building communities anywhere in the
world. Instead, the survivors were microbes. These continued to build carbonate structures, but these have none of the
characteristics of “reef ” or “coral reefs” built by skeletonsecreting organisms.
Reefs – although still not structures comparable to
today’s coral reefs – existed sporadically during much of
the Carboniferous and Permian – through times of oscillating temperatures and sea levels, including the periodic
development of polar ice caps. Some of these reefs, especially those of the Late Permian, were very extensive.
Their fossil content indicates that they were exceptionally
diverse relative to their forebears, for they contained extensive arrays of all of the major marine groups, especially
molluscs, echinoderms, and crustaceans, as well as rugose
and tabulate corals.
The end Permian mass extinction
The fossil record of the end Permian mass extinction
reveals a staggering loss of life: 82% of all genera and
perhaps 80–95% of all marine species went extinct
(Sepkowski, 1995, 2002; Erwin, 2006). All rugose and
tabulate corals and most other calcifying organisms ceased
to exist. The cause of this catastrophe was one of the foremost questions in palaeontology a century ago, and it
remains so today. At one point or another, wildly fluctuating temperatures in response to sharp increases in atmospheric carbon dioxide, acid rain, and lack of oxygen in
shallow waters after protracted times of plenty have all
been held responsible. The oceans are widely believed to
have turned anoxic and to have contained free hydrogen
sulphide (Wood, 1999). Recent studies using biomarkers
and isotopic data (Grice et al., 2005) are particularly
revealing, one study suggesting that hydrogen sulphide
generated by bacteria in deoxygenated oceans was so
abundant that they released enough hydrogen sulphide to
drive terrestrial extinctions (Ward, 2008). Other studies
suggest that a sudden release of methane occurred at this
time (Ryskin, 2003; Erwin, 2006). Recently, a comprehensive palaeophysiological study clearly implicates carbon
dioxide (Knoll et al., 2007), perhaps associated with the
Siberian Traps, the biggest volcanic outpouring of all
time. Whatever the cause (reviewed by Knoll et al.,
2007), the impact on reefs was sudden by geological standards and devastating. Some members of most marine
phyla survived, yet reefs did not reappear for about 10 million years, the greatest hiatus in reef building in all of
Earth history. What emerged in the Middle Triassic was
the Scleractinia in place of their Palaeozoic predecessors.
There is much in common between the ancient reefs of
the mid-Palaeozoic and the first scleractinian reefs that
took their place, not so much in the animals that built
them, but in the stop-start nature of their intervals of
growth, their biodiversity, and the sheer size of the structures they formed.
The end Triassic mass extinction
For 20 million years Triassic scleractinian corals, in concert with calcifying sponges and algae, went on to build
some of the most spectacular reefs of all time. There was
another substantial extinction of corals early in the Late
Triassic, followed by a brief interval of dramatic recovery.
Then, once again, the Earth plunged into a full-scale mass
extinction. The end Triassic mass extinction (Stanley,
1988, 2001) – the fourth in the history of reefs – is estimated to have claimed about half of all marine invertebrates. The main taxa affected were conodonts (all),
ammonites (almost all), and bivalves and gastropods (over
half of all species). One-third of all scleractinian families
went extinct and fewer than 25% of all genera are known
to have survived (Beauvais, 1984). Nevertheless, this
mass extinction, like its predecessors, was not only
marine; perhaps 80% of all land quadrupeds also went
extinct. The cause, again, has been much debated without
a clear outcome (Wood, 1999; Flügel and SenowbariDaryan, 2001). Atmospheric carbon dioxide reached
extreme levels for the Mesozoic (perhaps eight times
today’s level) (Hautmann, 2004; Hautmann et al., 2008),
and high greenhouse temperatures certainly resulted.
Short-term sea-level fluctuations and various changes in
ocean chemistry have been implicated by several
palaeontologists, although none gives convincing evidence. The extinction was relatively abrupt (in geological
time), making the reason for it particularly obscure. Whatever the cause, reefs again remained rare or absent
throughout the world for a vast amount of time, perhaps
6–8 million years.
The end Cretaceous mass extinction (K/T)
K/T has been extensively studied because of its indelible
association with the demise of the dinosaurs. However,
many well-known animal groups besides the dinosaurs
went extinct at the K/T boundary. Virtually no large land
animals survived. Plants were also greatly affected,
although this is less clearly documented. Once again
672
MASS EXTINCTIONS, ANOXIC EVENTS AND OCEAN ACIDIFICATION
periods of apparent collapse (Copper, 1994).
The world’s Devonian reefs may have been every bit as
well formed and biologically diverse as today’s reefs and,
with an estimated area of 5 Â 10
6 km
2 (including interreef areas), may have been as much as twice as extensive
(Copper, 1994). The Late Devonian mass extinction, like
that at the end of the Ordovician, has been linked to multiple causes. Many bolides may have struck the Earth
around this time (McLaren and Goodfellow, 1990 and
others), although this is unlikely to be linked to the extinction (reviewed by Racki, 2005). A precipitous drop in
atmospheric carbon dioxide owing to uptake by vascular
plants, low global temperatures, and wildly fluctuating
sea levels have all been blamed (Copper, 2001). The
extinctions appear to have varied geographically, although
they primarily affected marine life: reef development
ceased almost completely. Some corals and stromatoporoid
sponges lived on, but there was no recovery for the vast
coral-sponge reef-building communities anywhere in the
world. Instead, the survivors were microbes. These continued to build carbonate structures, but these have none of the
characteristics of “reef ” or “coral reefs” built by skeletonsecreting organisms.
Reefs – although still not structures comparable to
today’s coral reefs – existed sporadically during much of
the Carboniferous and Permian – through times of oscillating temperatures and sea levels, including the periodic
development of polar ice caps. Some of these reefs, especially those of the Late Permian, were very extensive.
Their fossil content indicates that they were exceptionally
diverse relative to their forebears, for they contained extensive arrays of all of the major marine groups, especially
molluscs, echinoderms, and crustaceans, as well as rugose
and tabulate corals.
The end Permian mass extinction
The fossil record of the end Permian mass extinction
reveals a staggering loss of life: 82% of all genera and
perhaps 80–95% of all marine species went extinct
(Sepkowski, 1995, 2002; Erwin, 2006). All rugose and
tabulate corals and most other calcifying organisms ceased
to exist. The cause of this catastrophe was one of the foremost questions in palaeontology a century ago, and it
remains so today. At one point or another, wildly fluctuating temperatures in response to sharp increases in atmospheric carbon dioxide, acid rain, and lack of oxygen in
shallow waters after protracted times of plenty have all
been held responsible. The oceans are widely believed to
have turned anoxic and to have contained free hydrogen
sulphide (Wood, 1999). Recent studies using biomarkers
and isotopic data (Grice et al., 2005) are particularly
revealing, one study suggesting that hydrogen sulphide
generated by bacteria in deoxygenated oceans was so
abundant that they released enough hydrogen sulphide to
drive terrestrial extinctions (Ward, 2008). Other studies
suggest that a sudden release of methane occurred at this
time (Ryskin, 2003; Erwin, 2006). Recently, a comprehensive palaeophysiological study clearly implicates carbon
dioxide (Knoll et al., 2007), perhaps associated with the
Siberian Traps, the biggest volcanic outpouring of all
time. Whatever the cause (reviewed by Knoll et al.,
2007), the impact on reefs was sudden by geological standards and devastating. Some members of most marine
phyla survived, yet reefs did not reappear for about 10 million years, the greatest hiatus in reef building in all of
Earth history. What emerged in the Middle Triassic was
the Scleractinia in place of their Palaeozoic predecessors.
There is much in common between the ancient reefs of
the mid-Palaeozoic and the first scleractinian reefs that
took their place, not so much in the animals that built
them, but in the stop-start nature of their intervals of
growth, their biodiversity, and the sheer size of the structures they formed.
The end Triassic mass extinction
For 20 million years Triassic scleractinian corals, in concert with calcifying sponges and algae, went on to build
some of the most spectacular reefs of all time. There was
another substantial extinction of corals early in the Late
Triassic, followed by a brief interval of dramatic recovery.
Then, once again, the Earth plunged into a full-scale mass
extinction. The end Triassic mass extinction (Stanley,
1988, 2001) – the fourth in the history of reefs – is estimated to have claimed about half of all marine invertebrates. The main taxa affected were conodonts (all),
ammonites (almost all), and bivalves and gastropods (over
half of all species). One-third of all scleractinian families
went extinct and fewer than 25% of all genera are known
to have survived (Beauvais, 1984). Nevertheless, this
mass extinction, like its predecessors, was not only
marine; perhaps 80% of all land quadrupeds also went
extinct. The cause, again, has been much debated without
a clear outcome (Wood, 1999; Flügel and SenowbariDaryan, 2001). Atmospheric carbon dioxide reached
extreme levels for the Mesozoic (perhaps eight times
today’s level) (Hautmann, 2004; Hautmann et al., 2008),
and high greenhouse temperatures certainly resulted.
Short-term sea-level fluctuations and various changes in
ocean chemistry have been implicated by several
palaeontologists, although none gives convincing evidence. The extinction was relatively abrupt (in geological
time), making the reason for it particularly obscure. Whatever the cause, reefs again remained rare or absent
throughout the world for a vast amount of time, perhaps
6–8 million years.
The end Cretaceous mass extinction (K/T)
K/T has been extensively studied because of its indelible
association with the demise of the dinosaurs. However,
many well-known animal groups besides the dinosaurs
went extinct at the K/T boundary. Virtually no large land
animals survived. Plants were also greatly affected,
although this is less clearly documented. Once again
672
MASS EXTINCTIONS, ANOXIC EVENTS AND OCEAN ACIDIFICATION