Reefs through geological time - a conservation paradox
Reefs, in some shape or form, have been around for a very long time. Approximately 3.5 billion (3,500,000,000) years ago microbialites (calcareous organo-sedimentary deposits) begin to appear in the fossil record. These benthic microbial communities produce their own hard substrate by sequestering raw inorganic materials from the surrounding seawater. For the next 2.5 billion years microbialites are represented by photosynthesizing cyanobacteria (blue-green algae) called stromatolites. These organisms biogenically precipitated ions over time to produce boulder-like structures that are recognized today as the oldest examples of reef-building organisms. Stromatolites are still found, more or less unchanged, in some parts of the world today. They appear to be found where high sedimentation rates or low nutrient levels exclude macroalgal competition, or highly supersaturated carbonate conditions favor microbial deposition. The Bahama banks (e.g., Lee Stocking Island), and Western Australia (e.g., Shark Bay) are perhaps the best known examples of stromatolitic reefs.
Stromatolites remained the principal reef-building organisms until the pre-Cambrian era (>600 million years ago, or Mya). From this point on, a greater diversity of organisms became involved in reef-building, and true reef communities began to take shape. Reef evolution over the last 600 million years (the Phanerozoic era - gr. "visible life") can be divided into three cycles, each separated by a significant extinction event. All reef extinctions appear to coinicide with mass marine extinctions.
The first cycle, from the pre-Cambrian (>600 Mya) to the mid Cambrian (540 Mya), was dominated by Archaeocyathids (sponge-like animals), stromatolites, and calcareous cyanobacteria and algae.
The second cycle, from the mid-Cambrian to the late Devonian (350 Mya) was dominated by algae-sponge-coral tripartite associations. The corals involved in these associations were ancient (non-scleractinian) tetracorals called rugose corals (e.g., Favistellata tabulata). Cyanobacteria, stromatoporoids (sponge-like animals) were also present, as well as unusual bivalve molluscs called rudists. These communities were complex and diverse.
The third cycle, from the late Devonian to the the late Permian/Triassic (220 Mya), was dominated by algae-bryozoan-coral assemblages. Also present were cyanobacteria, phylloid algae, tubiphytes, foraminifera (still around today), sponges, stromatoporoids and rudist bivalves.
Since the Triassic (i.e., over the last 220 million years), scleractinian corals have become increasingly dominant as reef-builders. Diverse (molecular, stable isotopic, ecological) evidence suggests scleractinian corals formed symbioses with algae soon after their appearance in the fossil record.
Holocene (Recent) reefs probably represent the most developed scleractinian reefs in geological history.