The best preserved fossil sponges tend to be reef formers, and the group has played an important role in building or colonizing reefs through the Phanerozoic. All reef-building sponges have a predominantly calcareous skeleton. Archaeocyathids (Fig. 3.2) evolved into some of the first reef formers, during a brief period in the early Cambrian. They were small forms, generally around 10 cm in height, with a cup-like shape. Modular variations on this basic plan allowed them to increase in size and form frameworks for reefs. They were mainly tropical and inhabited water less than 30 m deep. Stromatoporoid reefs were common during the Silurian and Devonian, and brief but important intervals of sponge-reef formation occurred in the Permian, Triassic, and Jurassic. In general their importance declined during the Mesozoic and Cenozoic, possibly linked to the rise of colonial corals with symbiotic algae. However, they were still important members of reef communities throughout this time, particularly inhabiting caves and overhanging areas of reefs. These cryptic environments are well preserved in fossil reefs and yield a wealth of calcareous sponge species.
Most modern reefs thrive in nutrient-poor regions, such as around mid-ocean islands. This is because the symbiotic algae of modern corals form a plant base to the food chain. However, the bulk of geologically recognized reefs probably
grew in areas where nutrients were less limited because the framework organisms were filter feeders, and most sponge reefs fall into this category. The exception to this general rule are some Lower Palaeozoic stromatoporoid reefs, leading to the suggestion that this group may also have had symbiotic algae within their cells.
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