A new reef classification model with insights into Phanerozoic evolution of reef ecosystems

Organic reefs are largely calcareous build-ups of sessile organisms, commonly in shallow marine ecosystems, with a long Precambrian to Modern history. Despite a long history of study, the reef classification is still open to development due to reefs encompassing diverse organic and inorganic components with various imprecise and synonymous terms. In this study, a new classification model for organic reefs is expressed in two hierarchical levels. Level 1: Reef outline geometry has two major forms: bioherms (prominent relief and laterally constrained) and biostromes (low relief and large lateral extension interbedded with non-reef sediments), within which constructional components highly vary. Level 2: Four reef constructor groups form end-members in a pyramid diagram: macroskeleton (S), microbia (Mi), matrix (Ma) and cement (C). Thus five reef types are recognized depending on the composition and structure of these constructors: (i) S-supported reef; (ii) Mi-supported reef (calcimicrobial and agglutinated microbial); (iii) Ma-supported reef (cluster and carbonate mud mound); (iv) C-supported reef; and (v) hybrid component-supported reef (comprising two or more major types of reef components). This arrangement makes a more streamlined and accessible reef classification than prior schemes. Two associated non-reef habitats that may grade into reefs are relevant: debris biostromes/banks and level-bottom communities containing taxa found in adjacent reefs. Based on this ‘SMiMaC’ model, the structural evolution of reef ecosystems is demonstrated for the Phanerozoic, characterized by S-supported and hybrid-supported, and Mi-supported and Ma-supported structures under cooling and intense warming climates, respectively.