Box Rugosan life strategies

Despite the apparent simplicity of rugosan architecture, these corals may have pursued a number of different life strategies (Fig. 11.24). A number of corals, for example Dokophyllum, probably sat upright in the sediment rooted by fine holdfasts extending from the epitheca. Other taxa, such as Holophragma, were initially attached to a patch of hard substrate but subsequently toppled over to rest on the seabed. Grewingkia was cemented to areas of hard substrate. The small discoidal Pal-aeocyclus, however, may have been mobile, creeping over the substrate on its tentacles. A number of strongly curved rugosans, for example Aulophyllum, probably lay within the substrate, concave upwards. Successive increments of growth were directed more or less vertically giving the coral exterior a stepped appearance. Many other solitary corals exhibit a similar terraced theca, which may be due to changes in growth direction associated with adjustments following toppling of the corallum during slight turbulence or storms.

Dokophyllum

Rhizosessile

Figure 11.24 Rugose solitary life strategies displaying attached, fixosessile, rhizosessile and recumbent life modes. (Based on Neuman; B.E.E. 1988. Lethaia 21.)

Rhizosessile

Figure 11.24 Rugose solitary life strategies displaying attached, fixosessile, rhizosessile and recumbent life modes. (Based on Neuman; B.E.E. 1988. Lethaia 21.)

phaceloid Syringopora, with long, thin, cylindrical corallites, characterize the coral faunas of the period. By the Late Permian the group was very much in decline following a long period of deterioration after the Frasnian extinctions; only five families survived to the end of the period.

Scleractinian corals

The scleractinians are elegant zoantharian corals with relatively light, porous skeletons composed of aragonite (Fig. 11.29). Both solitary and colonial modes exist with even more varied architectures than those of the rugo-

sans. Secondary septa are inserted in all six spaces between the primary or cardinal septa. In further contrast to septal insertion in the rugosans, each cycle of six is fully completed before the next cycle of insertion commences. Tabulae are absent, although dissepiments and dissepimentaria are developed. Moreover the scleractinian skeleton, although relatively light and porous, has the stability of a basal plate which aids anchorage in the substrate. Additionally, the scleractinian polyp can secrete aragonite on the exterior of the coral-lite, often in the form of attachment structures. Both adaptations provided a much greater potential for reef building than the less

Figure 11.25 Some rugose corals: (a, b) cross and longitudinal sections of Acervularia (Silurian); (c, d) cross and longitudinal sections of Phillipsastrea (Devonian); (e) Amplexizaphrentis (Carboniferous); and (f, g) cross and longitudinal sections of Palaeosmilia (Carboniferous). Magnification approximately x2 (a-d), x3 (e), x1 (f, g). Note that here and elsewhere, age assignments refer to the specimen figured and not to the entire stratigraphic range of the taxon. (Courtesy of Colin Scrutton.)

Figure 11.25 Some rugose corals: (a, b) cross and longitudinal sections of Acervularia (Silurian); (c, d) cross and longitudinal sections of Phillipsastrea (Devonian); (e) Amplexizaphrentis (Carboniferous); and (f, g) cross and longitudinal sections of Palaeosmilia (Carboniferous). Magnification approximately x2 (a-d), x3 (e), x1 (f, g). Note that here and elsewhere, age assignments refer to the specimen figured and not to the entire stratigraphic range of the taxon. (Courtesy of Colin Scrutton.)

Figure 11.26 Some tabulate corals: (a, b) cross and longitudinal sections of Favosites (Silurian); (c, d) cross and longitudinal sections of Syringopora (Carboniferous); and (e) Aulopora (Silurian). Magnification approximately x2. (Courtesy of Colin Scrutton.)

Figure 11.27 Tabulate morphology: (a) transverse and (b) longitudinal sections of Favosites. The insets on (a) show the lateral and upper surfaces of the entire Favosites colony.

Figure 11.27 Tabulate morphology: (a) transverse and (b) longitudinal sections of Favosites. The insets on (a) show the lateral and upper surfaces of the entire Favosites colony.

stable rugose and tabulate corals of the Paleozoic. Finally, scleractinians have a distinctive ultrastructure composed of aragonite and a widespread development of coenosarc. Although scleractiniomorph corals are now known from both the Cambrian and Ordovician (Box 11.7), the scleractinians first appeared in the Mid Triassic with forms such as Thamnasteria becoming quickly widespread throughout Europe.

The scleractinians developed a wide range of morphologies (Fig. 11.31). For example, Montlivaltia is a small, cup-shaped coral common from the Early Jurassic to the Cretaceous. Thecosmilia is a small, dendroid to phaceloid colonial form with similar coral-lites that ranges from the Middle Jurassic to the Cretaceous; the massive cerioid Isastraea has a similar range. Scleractinians are now the dominant reef-building animals in modern seas and oceans where they form reef structures in a variety of settings, usually in the tropics.

Synecology: corals and reefs_

Virtually all fossil corals were benthic. Two ecological groups have been recognized among Recent scleractinians. Hermatypic corals are associated with zooxanthellae (dinoflagel-lates) and are restricted to the photic zone to maintain this symbiosis. Symbiosis between the dinoflagellates and cnidarians is widespread across the living representatives of the phylum, with algae associating not only with corals but also anemones and gorgonians.

The zooxanthellae are endosymbionts living in the tentacles and mouth of the cnidarian where they recycle nutrients, accelerate the rate of skeletal deposition and convey organic carbon and nitrogen to the cnidarian in return for support and protection from grazers. Hermatypic corals are commonly multiserial forms, with small corallites displaying a high degree of integration. Ahermatypic corals, lacking algal symbionts, are commonly solitary or uniserial compounds with large, poorly integrated corallites.

Some have suggested that coral morphology may help predict the presence of symbi-onts in fossil coral communities. It is probable that many tabulates were zooxanthellate whereas the rugosans were not. In broad terms, there may be parallels between the platform and basin associations of rugose and tabulate corals of the Paleozoic and reef-building and non-reef-building scleractinian corals of the Mesozoic and Cenozoic.

Reefs are biological frameworks with significant topography (Box 11.8). Three main types of structure occur in tropical shallow water: (i) fringing reefs develop directly adjacent to land areas; (ii) barrier reefs have an intervening lagoon; and (iii) atolls completely surround lagoons and are usually of volcanic origin. The last will continue to grow as the volcanic island subsides until eventually only a barrier reef, enclosing a lagoon, remains.

Paleozoic corals were not particularly successful reef builders; many preferred firm substrates and lacked structures that allowed anchorage and aided stability; calcareous

Box 11.6 Computer reconstruction of colonies

The colonial tabulate Aulopora had a long geological history and mainly occupied an encrusting niche, coating brachiopods, stromatoporoids and other, larger corals. Aulopora grew by dichoto-mous branching, pursuing a creeping or reptant life mode, efficiently siting its corallites adjacent to potential sources of food at, for example, the inhalant currents through brachiopod commissures. Colin Scrutton (University of Durham) has reconstructed colonies of the free-living animals in three dimensions using a computer-based technique (Fig. 11.28). Serial sections of the colony were digitized and assembled on a micro-VAX mainframe with software routinely used for building up three-dimensional views of diseased kidneys. Both the ontogeny of the procorallites and the astogeny of the colony as a whole were established in considerable detail by these techniques. With the development of desk and laptop microcomputers such modeling is now, more or less, routine.

Halysitids were tabulate corals that dominated some Ordovician and Silurian assemblages. As each colony grew, budding chains were able to find their way back to the colony, instead of heading off in random directions. Perhaps they could sense the gradient of a diffusive field of "pheromones", their waste products or the depletion of nutrients set up by the colony. In a simulation by Hammer (1998), new protocorallites are introduced into random positions, simulating "polyplanulate" asto-genesis and the diffusive zones are established by numerically solving the differential equation for diffusion and decay.

Other fossil simulations are available at http://www.blackwellpublishing.com/paleobiology/.

Dendroid Coral Colony
Figure 11.28 Aulopora morphology: computer-generated reconstructions of (a) the plan, (b) the lower side, and (c) the direction of the procorallite; (d) reconstruction of the colony. (Courtesy of Colin Scrutton.)

mesentery septum epithecal wall

mesentery septum

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