Figure 11.29 Scleractinian morphology: (a) longitudinal and (b) transverse sections, and (c) mode of septal insertion.
algae and stromatoporoids were usually more important. Nevertheless, frameworks dominated by colonial tabulates, and to a lesser extent rugosans, do occur, particularly during the Mid Paleozoic. Growth bands on the latter have provided us with a Paleozoic calendar (Box 11.9).
Pioneer and climax communities have been described from a number of Silurian and
Devonian successions (Fig. 11.34). The scler-actinians gradually became the dominant reef builders during the Mesozoic and Cenozoic. Modern coral reef associations have been documented in detail from eastern Australia, the eastern Pacific and the Caribbean.
The Great Barrier Reef on the continental shelf of eastern Australia is the largest coral structure on Earth, approaching 3000 km
Box 11.7 Kilbuchophyllida and iterative skeletalization
Did the scleractinian corals have a long cryptic history through the Paleozoic? When the coral Kil-buchophyllum (Fig. 11.30a) was described from the Middle Ordovician rocks of southern Scotland, it caused a sensation, at least amongst coral workers. Kilbuchophyllum seemed to have patterns of septal insertion and a microstructure identical to those of modern scleractinians, and quite unlike the contemporary rugosans and tabulates. At first, some paleontologists said this was an aberrant local form, but specimens have been found in the Silurian too. It is unlikely that Kilbuchophyllum was the stem group for the scleractinians; however, clearly other groups of soft-bodied anemones with the potential of skeletalization were around early in the history of the group. Following the end-Permian mass extinction, when the rugose and tabulate corals finally disappeared, calcification of other scleractinian-type morphs during the Triassic marked a new start of another highly successful calcified coral group. Similarly calcified, scleractinian-type polyps are known from the Permian, implying that this skeletal type re-evolved iteratively, that is time and time again. But what did the naked scleractinian-type polyps look like? Hou Xian-guang (Yunnan University) and his colleagues (2005) have described the sea anemone-like Archisaccophyllia from the Early Cambrian Chengjiang fauna (Fig. 11.30b; see p. 386). This organism may well have been one of a group of naked polyps that generated various scleractiniomorph corals during the Paleozoic and probably were responsible for seeding the Mesozoic radiation of the most successful reef builder in the oceans today.
Box 11.8 Reef building through time
Reefs were not just corals! Throughout geological time, a whole range of mainly modular organisms have contributed to these calcareous structures (Wood 2001), providing, too, significant carbonate factories often spalling off the continental shelves into deeper waters. While Early Paleozoic shallow-marine environments were dominated by various microbes, bryozoans, corals and sponges (including archaeocyaths and stromatoporoids), the Mesozoic and Cenozoic were characterized by scleractinian corals (Fig. 11.32). Through time, the more restricted environments were home to stromatolites during the Paleozoic and early Mesozoic; these too popped up after some extinction events, in for example the Early Silurian and Early Triassic, as disaster species. The later Mesozoic and Cenozoic saw the arrival of serpulid and oyster reefs in these more stressed brackish or hypersaline habitats. Deeper-water environments were the domain of the spicular sponges together with occasional symbiotic (ahermatypic) scleractinian corals.
(brackish or hypersaline)
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