Palaeobiological perspectives on variability and taxonomy of scleractinian corals

Scleractinians are well known for their exceptional variability and difficult taxonomy. Until the end of the nineteenth century, these corals were studied outside their natural habitat. In situ investigation brought to attention their variability and led to description of formae and attempts to depart from the typological taxonomy. Studies commenced in 1950s of extant scleractinians of Jamaica, the South Pacific, Madagascar, the Red Sea, Cuba, and Australia demonstrated instances of coralla unclassifiable in the described species because they possess intermediate characters, and sometimes one corallum exhibits the characteristics of more than one species. Since 1984, discoveries about scleractinian life history and molecular data further challenged the conventional taxonomy. The coral holobiont is now being studied as a totality of the coral animal, its endosymbiotic zooxanthellae, and its associated community of microorganisms. Molecular genetics and studies of life history combined with morphological variability and variability in geological time are needed for scleractinian taxonomy. The input of palaeontologists with temporal aspect as well as an enormous amount of morphological data is invaluable, as demonstrated by several examples based on detailed morphological observations later supported by molecular and life history information. Efforts to resolve the variability vs. taxonomy dilemma for fossil scleractinians would benefit from further actuopalaeontological work: studies at all levels of biological organization, including ancient DNA and evolutionary genetics, the rich fossil record, fractals and RLQ analysis, palaeopathological research, sclerochronology, the ecology and skeletogenesis of extant deep sea corals.