Mesozoic and Cenozoic corals
Extant Scleractinia have similar numbers of zooxanthellate and azooxanthellate genera and species. These two groups are not taxonomically separate although they are almost all sharply eco-physiologically distinct. Zooxanthellate Scleractinia (sometimes called ‘reef-building corals’)are primarily colonial and are responsible for the construction of most Mesozoic to present coral reefs. Azooxanthellate Scleractinia are primarily solitary and most occupy deep water.
Structure: The simplest skeleton of an individual polyp, the corallite, is a tube-like structure, the corallite wall. Radially arranged vertical plates, the septa, are the dominant structures within the wall. Corallites may be solitary or joined together by horizontal plates and other structures, collectively called the coenosteum. Some polyps or colonies have an epitheca, a thin film of skeleton surrounding the lower wall or colony perimeter. Complex colonies of many types are derived from this basic structure.
The sac-like body cavity of the coral polyp is the coelenteron, which serves many functions including digestion and the circulation of fluids for respiration and nutrition. It is partitioned by vertical mesenteries that have coiled filaments along their inner margins.
Most Scleractinia have tentacles which are smooth in corals that feed on detritus but most taxa have stinging cells (nematocysts) for defense and food capture. Nematocysts are commonly grouped into wart-like batteries.
Cnidaria are the simplest organisms to have discrete nervous, muscular and reproductive systems and in Scleractinia all these are well developed. Some corals have separate male and female sexes but most are hermaphrodites.
This order of hydrozoans is represented by several extinct genera and one common extant genus, Millepora with approximately 50 species. All are zooxanthellate.
Milleopora have growth forms ranging from branching to submassive and encrusting. Tiny polyps are mostly imbedded in the skeleton where they are linked by a network of minute canals, the cyclosystem. All that can be seen on the smooth surface are the pores of two types of polyps, gastropores and dactylopores. Reproductive ampullae, which produce medusae, can also be seen on the colony surface. Generations of sexual medusae alternate with generations of asexual polyps. Almost all records are Cenozoic, but as polyps are very small they are seldom distinct in fossils.
The second group of hydrozoans are mostly found only in deep water. All are azooxanthellate. Two genera, Distichopora and Stylaster are commonly found in caves and under overhands in shallow reef environments.
Distichopora are ornate corals that branch in one plane. They have no cyclosystem; instead, gastropores are aligned along the edge of branches and these are flanked on each side by a row of dactylopores. Reproductive ampullae are clustered towards the ends of side branches.
Stylaster also branches in one plane but branches are fine, tapered and delicate. Gastropores are linked by individual cyclosystems and surrounded by dactylopores. These alternate on the sides of branches, giving the latter a zig-zag pattern. Wart-like reproductive ampullae occur on the sides of older branches. Almost all records are Cenozoic.
Heliopora coerulea or ‘blue coral’ is the sole member of Order Helioporacea. Heliopora is zooxanthellate, blue or greenish underwater, but the skeleton, composed of fibrocrystalline aragonite, Throughout this time it appears to remain a single species, in which case it would have by far is always permanently blue. Polyps are small and superficial and are interconnected by minute solenial tubes. Heliopora is easily recognised in fossil outcrops by its color and therefore can be traced back to the Cretaceous. the greatest geological longevity of any coral.
The second group of octocorals (corals which have eight tentacles) to form skeletons is part of the very large Order Alcyonacea. One genus, Tubipora the ‘organ-pipe’ coral, has a skeleton. Only one species, T. musica, is currently named. It has an isolated taxonomic position.
Tubipora are zooxanthellate. The skeleton is permanently coloured dark red. Polyps are long and tubular and are interconnected by horizontal tabulae or stolons which form transverse platforms. Corallites grow from the platforms, not from the branching of corallites. Living colonies have brown pinnate tentacles.