corals and related fossils

corals and related fossils The corals, jellyfish, sea anemones, and hydroids are all members of the phylum Cnidaria, a group of fairly simple aquatic organisms of both solitary and colonial habit. They differ from higher invertebrate groups in lacking cells organized into organs, and in their radial symmetry and the possession of stinging cells or cnidoblasts, which give the phylum its name. The basic structure of the animal is a simple sac in which a central mouth surrounded by tentacles leads to a closed stomach or enteron (Fig. 1a); sea anemones exhibit this form. They are passive predators, catching in their tentacles small organisms that blunder into them. Their fossil record is uneven, because many groups do not have a skeleton; corals, however, secrete a massive calcareous skeleton and because of this are the most important fossil group.

Cnidarians show an alternation of generations in which two body forms, the polyp and the medusa alternate (Fig. 1a). The medusae reproduce sexually, giving rise to the polyps, which reproduce asexually by budding off medusae, and so on. All cnidarian groups are modified versions of one or other of these, and three major classes are recognized. The Hydrozoa are marine and freshwater cnidaria in which the polyp stage dominates; however, they only rarely produce a massive calcareous skeleton and are therefore of only marginal importance in the fossil record. The Scyphozoa, or jellyfish, are an exclusively marine class in which the medusoid stage dominates. Although they do not secrete a skeleton they do occur in the fossil record and are known from as far back as the Proterozoic, where they occur in the Ediacara Fauna. Included within this class by some authorities are the extinct conulariids whose conical, four-sided, chitinous skeletons occur in marine rocks from the Cambrian to Triassic. The major class is the Anthozoa. This class includes the corals and encompasses a group in which there is no trace of the medusoid form. This is the largest group today, with about 6000 living species, and is also the largest fossil group with about 2300 genera known from the Proterozoic Ediacara Fauna to the Present.

The main anthozoan subclass, which includes the corals, is the Zoantharia. These organisms live in most marine environments, from tide pools to depths of 6000m or more. They are characterized by the presence of a calcareous skeleton that consists of a cylinder or cone, secreted by the polyp, which inhabits the upper part (Fig. 1b). The base of the polyp forms radial folds that secrete ridges, the septa, that are extended upwards as the polyp grows and are joined at the outer ends by the theca or skeletal wall. Horizontal floors, or tabulae, may be laid across the theca periodically as the animal grows.

The earliest corals are the Tabulata, colonial forms with conspicuous tabulae but poorly developed septa, which appear in the Early Ordovician. By the Middle Ordovician they are common and diverse elements of the marine fauna and are joined by the Rugosa, a group both solitary and colonial in which the septa are grouped in four quadrants. The Tabulata maintained numerical superiority until the Devonian, when a burst of activity brought the Rugosa level. The Late Devonian saw a wholesale extinction of coral faunas from which the Rugosa recovered with a further proliferation of families in the Mississippian (Early Carboniferous). Both became extinct by the Early Triassic, at which time they were replaced by the modern corals, the Scleractinia, which appear to have risen independently from soft-bodied ancestors. During the early Mesozoic the Scleractinia steadily increased in numbers, diversity, and distribution culminating in extensive reef development in the Late Jurassic. Additional periods of widespread reef development occurred in the Cretaceous and the Late Tertiary.

The development of reefs is caused in part by the colonial form of many corals. This is a successful strategy allowing rapid growth without the dangers of larval and immature stages; and with the ability to grow laterally allowing them to take advantage of whatever space is available in areas of high competition. Modern reef corals also take advantage of a symbiotic relationship with zooxanthellae, algae that live within their tissues. These hermatypic corals are restricted to shallow tropical waters, where they exist to 90 m depth in waters at 25–29 °C. This beneficial arrangement allows them to build massive fringing reefs, barrier reefs, and atolls, representing successive stages in the growth of coral around a sinking volcanic island, coral growth keeping pace with subsidence. The ahermatypic forms are less restricted environmentally and are found down to 6000 m and in temperatures at low as 1 °C.

Corals are generally too long-ranged to be useful zonally, although they have been used in Europe in the Early Carboniferous, where A. Vaughan established a zonal scheme in 1905 based on corals and brachiopods. They have, however, been useful in geochronometry. Many rugose and scleractinian corals show fine daily growth increments, which are often grouped in monthly and yearly annulations by growth constrictions. Study of Palaeozoic corals by Colin Scrutton of the University of Durham (England) has indicated that there were an average of 400 days in the Devonian year, a figure that corresponds well with astronomical estimates of the slowing of the Earth's rotation due to tidal friction.

David K. Elliott

Bibliography

Clarkson, E. N. K. (1993). Invertebrate palaeontology and evolution. Chapman and Hall, London.
Muscatine, L. and and Lenhoff, H. M. (1974). Coelenterate biology. Academic Press, New York.