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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 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