FOSSIL INVERTEBRATES/Corals and Other Cnidaria 321 Corals and Other Cnidaria C T Scrutton, Formerly University of Durham, Durham, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction The phylum Cnidaria encompasses a varied group of primitive, dominantly marine, diploblastic metazoans of which the corals are by far the most important in the fossil record Other cnidarians generally have poor preservation potential but nevertheless exist as scattered fossils extending back probably to the Late Precambrian Cnidarian Organization and Classification All cnidarians consist fundamentally of a sac-like body with a mouth surrounded by tentacles (Figure 1) There are two variants: when the sac rests on its aboral surface with the mouth uppermost, it is a polyp; when it floats in the water with the mouth facing downwards, it is a medusa When both forms exist in the same species, medusae are formed on the polyp, from which they separate (bud off) and produce male and female gametes These fuse to form a free-floating larval stage, a planula, which settles and metamorphoses into a new polyp The phylum is also characterized by the presence of stinging cells for the capture of prey (nematocysts), which are particularly abundant in the tentacles and which give the phylum its name There are three main subdivisions (classes) of the Cnidaria, which differ principally in the interior structure of the sac, the enteron or gut, and in the prominence of the two body forms (Figure 2) The Hydrozoa have no internal partitions in the gut, and in most species both polyps and medusae are well developed The Scyphozoa have four radial flanges extending towards the centre of the gut, and the medusoid, or jellyfish, stage is dominant The polyp is reduced to a specialized factory for producing medusae (scyphistoma) In the Anthozoa, the gut is subdivided in a more complex and varied way by radial partitions called mesenteries, whilst the medusoid stage is entirely absent, and gametes are produced directly by the polyp The polyp stage may become modular by budding in both hydrozoans and anthozoans Anthozoan colonies are simple, usually with no differentiation of the modules However, in the Hydrozoa, modules may become specialized for different functions such as feeding and reproduction The ultimate expression of this is the formation of complex colonies of specialized modules, not only of polypoid but also of medusoid origin, which function as superindividuals These are the chondrophores and siphonophores, represented respectively in modern oceans by Vellela (by-the-wind-sailor) and Physalia (Portuguese Man-o’-War) The corals are anthozoans that have secreted a calcium carbonate skeleton beneath the polyp or polyp colony A few hydrozoan polyp colonies also secrete a carbonate skeleton Some other anthozoans secrete spicules or rarely coral-like skeletons, but most are soft-bodied – for example, the sea anemones Some hydrozoan polyps and scyphozoan scyphistomas may have an external organic periderm, but the medusae are essentially soft-bodied The bias in the cnidarian fossil record is directly related to the relative skeletonization of the different groups Rare solitary coral-like fossils from the Lower Cambrian of Siberia have been assigned to a fourth, extinct, class of cnidarians, the Hydroconozoa Geological History Precambrian Origins Figure Basic cnidarian anatomy (A) Polyp The polyp is shown without partitions in the gut, as in the Hydrozoa (B) Medusa The jelly like appearance of jellyfish is due to the thickening of the mesogloea, gelatinous connective tissue separ ating the ectoderm and endoderm in the roof of the medusoid bell All three of the principal classes have been claimed to be present in the Late Precambrian Ediacaran fauna (Precambrian: Vendian and Ediacaran) Here, internal and external moulds of nonmineralized organisms are preserved in siltstones and fine sandstones They are controversial, and alternative interpretations have been advanced for these fossils However, some are convincingly cnidarian As well as unassigned jellyfish, there are others of likely scyphozoan affinity (Kimberella; Figure 3B) and structures reasonably interpreted as the floats of hydrozoan chondrophores (Ovatoscutum and Chondroplon) The most striking anthozoan is the