Origin of Biomes and Succession 263 



action usually moves these fragments back and forth until they 

 reach a protected area or are swept over the edge of the reef 

 platform to be deposited on its flanks. In this way, a vertical and 

 commonly also a lateral growth of the reef gradually comes about. 

 The association of algae in a reef deposit indicates deposition 

 within the photic zone. Reefs are most abundant in shallow waters 

 of the temperate and tropical regions, but they are also found 

 in water as deep as 100 fathoms (Teichert, 1958) and within a 

 few hundred yards of the Antarctic ice shelf in peculiar areas of 

 elevated temperature (Bullivant, 1959). The reefs provide an 

 ecological setting of food and protection in which lives a remark- 

 able assemblage of worms, fish, and many other diverse organisms 

 (Fig. 117) (Moore, 1958). 



Reef communities have persisted since the beginning of the 

 Cambrian to the present, although their faunal and floral com- 

 ponents and their geographic locations have changed continuously. 

 Presumed "algal heads" (Eozoon), formed of encrusting layers of 

 calcareous material in late pre-Cambrian strata in Glacier National 

 Park, Montana, are believed to represent the oldest "reefs" ( Fenton 

 and Fenton, 1933). In early and middle Cambrian times, the Ar- 

 chaeocyathinae, of unknown biologic affinities but similar in many 

 respects to calcareous sponges, grew in large dense colonies forming 

 extensive reefs in many areas. 



At the end of the early Ordovician time many new kinds of 

 reef-forming organisms evolved. The middle and later Paleozoic 

 reefs (bioherms) have faunas and floras with increasingly complex 

 colonial corals (tabulates and tetracorals ) , calcareous algae, bryo- 

 zoans, brachiopods, trilobites, echinodermal fragments, and gas- 

 tropods. In the Silurian and Devonian, stromatoporoids (encrust- 

 ing hydrozoans ) became another important reef-building organism. 

 Crinoids flourished in Mississippian and Pennsylvanian shelf areas 

 and together with colonial tetracorals and algae formed many of 

 the reefs of that time. In Permian times, aberrant productid bra- 

 chiopods {Prorichthofenia, Leptodus, and Scachinella), cemented 

 their shells to the substratum and to one another. They occurred 

 in many of the large reefs along with corals, sponges, crinoids, 

 and bryozoans (Newell et al., 1953). 



Many major Paleozoic reef builders became extinct during the 

 late Permian, and new faunal and floral components built reefs 

 during the Mesozoic and Cenozoic. In the Tethyan seas of south- 

 ern Asia, Europe, and North Africa, stromatoporoids, algae, sponges, 

 and hexacorals came to be the dominant faunas of many of the 



