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CORALS
Corals, sea anemonies and jellyfish are fully marine organisms that are all members of the major group
(Phylums) called the Cnidaria. Soft-bodied sea anemones and jellyfish decay quickly after death which
prevents fossilisation except under unusual circumstance. Corals, however, have a long geological history
due to their hard skeleton which remains after the soft polyp body has decayed. Today, such stony corals
are mosty common and diverse in the tropics, for example, the Great Barrier Reef in Australia.
Corals are important for understanding past environments (palaeoenvironments) because they have a
narrow range of tolerance.
The tentacles gather food suspended in the water column. Food is passed to the gut (enteron) through an
opening which acts as both the mouth and the anus. Food is broken down and taken into the internal wall
the endoderm. A series of folds call mesenteries increase the surface area of the gut wall.
CORAL SHAPE
Most modern stony corals have aragonitic skeletons which recrystallise to calcite when fossilised. Each
coral skeleton (corallum) has few characters. All have an outer wall or epitheca which may be rough
rugose) with growth rings.
Cnidarians are with clearly recognisable forms are known from the Cambrian. Tabulate corals (colonial)
became highly successful from the Ordovician and were a major part of Palaeozoic reefs along with
bryozoans and stromatoporoids. Rugose corals (colonial and solitary) were also present and the colonial
species were also involved in building reefs.
PALAEOZOIC CORALS
1. Tabulate corals are the simplest of three main skeletal coral groups and belong to the order
Tabulata. Tabulates are colonial, have tabulae and have variable shapes such as polygonal,
chain-like or simple branching. They lack internal supports for the mesantaries called septa.
2. Rugose corals are relatively complex, may be compound or solitary. They possess taluae, septa,
dissepiments and an axial canal.
MESOZOIC – RECENT CORALS
3. Scleractinian corals are colonial or solitary – compound corals may be massive and ‘brain-like’,
arranged in linear series: branching; or composed of subparallel, cylindrical corallites. They have
tabulae and septa, but no dissepiments or axial canal complex.
TABULATE CORAL
Halysites
(chain coral)
RUGOSE CORAL
Streptelasma
(Solitary coral)
A colony of tabulate chain coral from
the Silurian of Gotland, Sweden.
A solitary horn coral (Ordovician
of the USA),
Scale in mm
Scale in mm
SCLERACTINIAN CORAL
Acropora
(colonial coral)
Acropora from the Pleistocene
of Yemen.
Scale in mm
CORAL EVOLUTION
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Cnidarians are known from the Cambrian.
Tabulate corals became highly successful from the Ordovician and were a major part of
Palaeozoic reefs along with bryozoans and stromatoporoids.
Rugose corals have a similar range to tabulates and maybe found in close association.
Corals were strongly affected by the late-Devonian mass extinction.
Rugose corals became more common during the Carboniferous.
A major extinction event during the Permian devastated coral reefs and completely
extinguished tabulate and rugose corals.
Scleractinian corals during the mid-Triassic, probably evolved from an ancestral sea anemone.
Useful websites:
http://www.nhm.ac.uk/research-curation/earth-sciences/fossil-invertebrates/fossil-invertebrateresearch/corals/index.html
Dive into the Natural History Museum’s Coral Reefs exhibition 2015