The percentage of scleractinian erect species decreased
until the late Cretaceous (Turonian), but increased markedly – particularly in multiserial forms with inferred rates
of rapid regeneration – after that time. This spectacular
rise of various morphologies of branching forms was coincident with the appearance of new groups of predatory
excavators.
All families of modern scleractinian corals that dominate reefs today spread throughout the Tethys Sea during
the Eocene. The poritids, their relatives the actinids, and
the favids (which had survived the Cretaceous extinction)
dominate most coral reef communities throughout much
of the Cenozoic (McCall et al., 1994). Although branching
acroporoids appeared in the Eocene, they did not dominate
reefs until early Pleistocene. The rise of this group – with
its particularly remarkable powers of regeneration from
fragmentation and rapid growth – would then seem to be
independent of any known changes in predation style.
Patterns of sediment removal and storage
Unlike modern reefs, anecdotal evidence suggests that
a greater proportion of reef framework is preserved in situ
before the Jurassic (Wood, 1999). Many Palaeozoic reefs
commonly preserve intact reef frameworks, even of fragile
biota such as frondose bryozoans (Figure 6b) or platy
stromatoporoid sponges. Such preservation was aided, in
part, by abundant and probably rapid synsedimentary lithification, particularly cementation.
We might predict that substantial aprons of sediment
may not have been present on pre-Eocene reefs before
the evolution of reef fish, and likewise in the absence of
the grain size reduction activities of clionid sponges, echinoids, fish, mean sediment grain size may have been more
coarse prior to the late Jurassic, perhaps resulting in
a reduced net loss of carbonate to the system through the
removal of fines. It is possible also that the modern style
of coral reef lagoon may also not have appeared until the
late Jurassic or later (Wood, 1999, 2010).
The role of mass extinctions
Reefs have long been thought to be highly susceptible to
mass extinction events, with major reef-building biota typically going largely or wholly extinct. Metazoan reefs may
take some 2–8 Myr to recover after such events, sometimes longer than other communities, and post-extinction
reefs are often composed of entirely new groups of skeletal metazoans.
In some cases, either calcimicrobes or microbialites can
dominate post-extinction reefs, but these elements were
often already part of the pre-extinction reef community.
Shallow-water Frasnian (late Devonian) reefs show abundant large skeletal metazoans (particularly stromatoporoid
sponges, and subsidiary corals) as well as extensive fenestral microbialite and calcimicrobes both as free-standing
mounds (Figure 5b). Succeeding Famennian reefs growing in the aftermath of a mass extinction were composed
of frame-building microbialite and calcimicrobes (e.g.,
Renalcis and Rothplezella) and a diverse community of
spiculate sponges. The first metazoan reefs to form after
the end-Permian extinction in the Anisian (Lower Triassic) bore large framework cavities, populated with the calcified microbe Cladogirvanella.
Tropical shallow marine carbonate production has been
noted to fall after some mass extinctions (e.g., the late
Devonian and end Permian), but not others (end Cretaceous). Changes in seawater saturation or chemistry may
also explain the response of reef and other carbonateproducing biota to mass extinctions. When skeletal groups
first appear, they tended to adopt the mineralogy favored
by ambient seawater chemistry, but subsequent extinction
events may have preferentially removed those groups and
selected for new skeletal biota whose skeletal mineralogy
reflected the new ocean chemistry (Kiessling et al., 2008).
Summary
Reefs have shown some unifying features over their
3.5-billion-year history. Reefs have shown zonation in
response to environmental gradients from their inception,
and metazoan reefs have been differentiated into open
surface and cryptic reef communities for their entire
$550-million-year history (Wood et al., 2002).
Tectonics drives many aspects that control reef growth
including sea level, circulation patterns, climate, and
evolving seawater chemistry change over long timescales.
Reefs have always been most numerous on passive continental margins at low latitudes where they typically form
prograding or aggrading barriers (with growth during
transgressive and highstand periods and erosion during
longer low stand periods), or fringing reefs. By contrast,
reef-forming toward the cratonward side of foreland
basins is relatively rapid and reef geometries more aggradational. Finally, reefs forming in strike-slip basins and on
thrust complexes are affected by the vagaries of local tectonic movements, which may be highly episodic but
involve substantial displacements and are thus highly
unpredictable (James and Wood, 2010).
Climate is a major controlling force on the evolution of
reefs on both short and long timescales, as the latitudinal
range of carbonate-producing species is largely governed
by temperature and carbonate supersaturation. Reef growth
therefore shows cyclicity at all scales in response to shortterm oscillations (e.g., Milankovitch and glacial–interglacial
cycles) as well as to longer-term climatic intervals, driven by
slower, tectonically driven processes.
Global climate has oscillated through greenhouse and
icehouse phases, in concert with aragonite and calcite seas,
respectively. During icehouse times of continental glaciation, high eustatic sea-level changes created aggrading
reef growth, common subaerial exposure and conspicuous
regional disconformities in reef complexes. These large
changes in sea level caused ramps to have steep gradients,
and platform tops to have significant depositional relief
characterized by pinnacle reefs and erosional topography.
Icehouse reefs are typically dominated by heterotrophs or
autotrophs with aragonitic or high-Mg mineralogies.
GENERAL EVOLUTION OF CARBONATE REEFS
467
until the late Cretaceous (Turonian), but increased markedly – particularly in multiserial forms with inferred rates
of rapid regeneration – after that time. This spectacular
rise of various morphologies of branching forms was coincident with the appearance of new groups of predatory
excavators.
All families of modern scleractinian corals that dominate reefs today spread throughout the Tethys Sea during
the Eocene. The poritids, their relatives the actinids, and
the favids (which had survived the Cretaceous extinction)
dominate most coral reef communities throughout much
of the Cenozoic (McCall et al., 1994). Although branching
acroporoids appeared in the Eocene, they did not dominate
reefs until early Pleistocene. The rise of this group – with
its particularly remarkable powers of regeneration from
fragmentation and rapid growth – would then seem to be
independent of any known changes in predation style.
Patterns of sediment removal and storage
Unlike modern reefs, anecdotal evidence suggests that
a greater proportion of reef framework is preserved in situ
before the Jurassic (Wood, 1999). Many Palaeozoic reefs
commonly preserve intact reef frameworks, even of fragile
biota such as frondose bryozoans (Figure 6b) or platy
stromatoporoid sponges. Such preservation was aided, in
part, by abundant and probably rapid synsedimentary lithification, particularly cementation.
We might predict that substantial aprons of sediment
may not have been present on pre-Eocene reefs before
the evolution of reef fish, and likewise in the absence of
the grain size reduction activities of clionid sponges, echinoids, fish, mean sediment grain size may have been more
coarse prior to the late Jurassic, perhaps resulting in
a reduced net loss of carbonate to the system through the
removal of fines. It is possible also that the modern style
of coral reef lagoon may also not have appeared until the
late Jurassic or later (Wood, 1999, 2010).
The role of mass extinctions
Reefs have long been thought to be highly susceptible to
mass extinction events, with major reef-building biota typically going largely or wholly extinct. Metazoan reefs may
take some 2–8 Myr to recover after such events, sometimes longer than other communities, and post-extinction
reefs are often composed of entirely new groups of skeletal metazoans.
In some cases, either calcimicrobes or microbialites can
dominate post-extinction reefs, but these elements were
often already part of the pre-extinction reef community.
Shallow-water Frasnian (late Devonian) reefs show abundant large skeletal metazoans (particularly stromatoporoid
sponges, and subsidiary corals) as well as extensive fenestral microbialite and calcimicrobes both as free-standing
mounds (Figure 5b). Succeeding Famennian reefs growing in the aftermath of a mass extinction were composed
of frame-building microbialite and calcimicrobes (e.g.,
Renalcis and Rothplezella) and a diverse community of
spiculate sponges. The first metazoan reefs to form after
the end-Permian extinction in the Anisian (Lower Triassic) bore large framework cavities, populated with the calcified microbe Cladogirvanella.
Tropical shallow marine carbonate production has been
noted to fall after some mass extinctions (e.g., the late
Devonian and end Permian), but not others (end Cretaceous). Changes in seawater saturation or chemistry may
also explain the response of reef and other carbonateproducing biota to mass extinctions. When skeletal groups
first appear, they tended to adopt the mineralogy favored
by ambient seawater chemistry, but subsequent extinction
events may have preferentially removed those groups and
selected for new skeletal biota whose skeletal mineralogy
reflected the new ocean chemistry (Kiessling et al., 2008).
Summary
Reefs have shown some unifying features over their
3.5-billion-year history. Reefs have shown zonation in
response to environmental gradients from their inception,
and metazoan reefs have been differentiated into open
surface and cryptic reef communities for their entire
$550-million-year history (Wood et al., 2002).
Tectonics drives many aspects that control reef growth
including sea level, circulation patterns, climate, and
evolving seawater chemistry change over long timescales.
Reefs have always been most numerous on passive continental margins at low latitudes where they typically form
prograding or aggrading barriers (with growth during
transgressive and highstand periods and erosion during
longer low stand periods), or fringing reefs. By contrast,
reef-forming toward the cratonward side of foreland
basins is relatively rapid and reef geometries more aggradational. Finally, reefs forming in strike-slip basins and on
thrust complexes are affected by the vagaries of local tectonic movements, which may be highly episodic but
involve substantial displacements and are thus highly
unpredictable (James and Wood, 2010).
Climate is a major controlling force on the evolution of
reefs on both short and long timescales, as the latitudinal
range of carbonate-producing species is largely governed
by temperature and carbonate supersaturation. Reef growth
therefore shows cyclicity at all scales in response to shortterm oscillations (e.g., Milankovitch and glacial–interglacial
cycles) as well as to longer-term climatic intervals, driven by
slower, tectonically driven processes.
Global climate has oscillated through greenhouse and
icehouse phases, in concert with aragonite and calcite seas,
respectively. During icehouse times of continental glaciation, high eustatic sea-level changes created aggrading
reef growth, common subaerial exposure and conspicuous
regional disconformities in reef complexes. These large
changes in sea level caused ramps to have steep gradients,
and platform tops to have significant depositional relief
characterized by pinnacle reefs and erosional topography.
Icehouse reefs are typically dominated by heterotrophs or
autotrophs with aragonitic or high-Mg mineralogies.
GENERAL EVOLUTION OF CARBONATE REEFS
467