Bacteria. They were the first life forms on Earth and have managed to exploit virtually every environment on the planet. We find them deep underground in oil pools and even within solid rock. They occur in the deepest parts of the ocean and high up in the atmosphere. They are incredibly important as decomposers and as mutualistic "partners" in many symbiotic relationships. The cyanobacteria are important photosynthesizers that contribute free oxygen to the atmosphere. In this essential process, they are part of a trinity of O2-producers which includes the various species of marine algae and the land plants. They were also the very first reef-builders on Earth.
Evidence of ancient cyanobacterial growth has been found in rocks as old as 3.5 billion years. Known as stromatolites, these organosedimentary structures comprised the bulk of the fossil record during the Archean Eon and most of the Proterozoic Eon. Vast reefs were produced by the actions of cyanobacteria who were joined by the red algae (Rhodophyta), brown algae (Phaeophyta), and green algae (Chlorophyta) during the Proterozoic. These stupendous reef-building projects were curtailed during the Cambrian Period by the emergence of a host of "grazing" invertebrates which took advantage of the extensive bacterial and algal mats. Consequently, although stromatolites are still being produced today, they are nowhere near as abundant as they once were.
The Cambrian Period also witnessed the rise of the first skeletal reef-building organisms on Earth, the Archeocyathids. Unfortunately, their time on Earth would be brief. The entire phylum, consisting of 16 families and some 80 genera, disappeared from the fossil record just prior to the end of the Cambrian Period. But during that relatively short period of geologic time, these primitive "cup animals" constructed some large and impressive reefs, usually in association with stromatolites and sponges.
For the sponges also make their appearance during the Cambrian Period. Significantly for our story, this period of Earth's history also witnessed the rise of the massive, colonial stromatoporoid sponges. This group of formidable reef-builders would prove to be far more successful than the Archeocyathids, persisting and surviving in the shallow seas of our planet well into the Tertiary Period. Thriving in the warm waters of the early Paleozoic seas, the stromatoporoids will join a new group of reef-builders during the Ordovician Period.
During the middle part of the Ordovician Period, a major marine transgression resulted in widespread inundation of the continental cratons. As a result of this increase in the shallow marine environment, benthic organisms proliferated. Among these were three great reef-building groups and a significant contributor to the reefs being constructed at this time. The stromatoporoid sponges comprise one of the reef-building groups, but the other two are corals. Known as the tabulate corals and the rugose corals, these novel and revolutionary forms of life would become the premier reef-builders of the Paleozoic. It is this trinity of stromatoporoid sponges, tabulate corals, and rugose corals that would dominate reef-building during the Ordovician, Silurian, and Devonian Periods. They would be assisted in this effort by the trepostome bryozoans. These "stony bryozoans" constructed dense carbonate structures which contributed significantly to the coral reefs, helping to "glue" them together into a compact mass. The trepostomes flourished during the Ordovician but then steadily declined during the Silurian and Devonian Periods.
The tabulate corals were the principal reef-building corals during the Silurian Period. Their place was gradually taken by the rugose corals during the following Devonian Period. But both groups coexisted during this period and would continue to do so to the very end. Both groups are important in that the tabulate corals were the world's first true coral reef builders and the rugose corals literally set the stage for modern-day coral reef systems.
Near the end of the Devonian Period, a mass extinction event occurred in the marine ecosystem which resulted in a global collapse of the world's reef communities. The massive reef structures of the Silurian and Devonian Periods virtually disappeared, being replaced by smaller "patch" reefs dominated by sea lilies, bryozoans, brachiopods, and calcareous algae. Of course, "life will find a way" and this unoccupied niche in the shallow marine environment could not remain vacant for long. By the middle of the Mississippian Period, another group of important reef-building tabulate corals had emerged to fill the gap. The chaetetids, as they are called, became the dominant constructors of reefs during the Pennsylvanian Period. Their populations then declined somewhat, but they somehow managed to survive the great Permian extinction event.
This catastrophic die-off wiped out over 90% of the life forms on Earth. The slate was nearly wiped clean and the world would never be the same again. A new and vibrant group of reef-building corals would appear and come to dominate the reef systems of the world. Known as the scleractinians, this group of highly successful corals made their appearance during the Triassic Period, shortly after the demise of the tabulate and rugose corals. The scleractinians are still with us today. The huge coral reefs that flourish in the tropical waters of present-day Earth are a testament to their success.
Their history was one of success, but only when water conditions were nearly perfect. During the Jurassic Period, a group of unusual reef-building clams known as the rudistids actually came to dominate some portions of the shallow epeiric seas surrounding the land masses. In highly saline waters, rudistids frequently out-competed the scleractinians. But the corals would turn out to be the ultimate winners. The rudistid clams vanished from the fossil record at the end of the Cretaceous Period.
The Cretaceous Period witnessed the rise of another Cnidarian reef-building group that survived the Cretaceous extinction event and went on to dominate the warm Tertiary seas that followed. The colonial reef-building hydrozoans known as the milleporids joined the scleractinian corals in constructing massive reefs during this period of time.
Yet another group of Cnidarians appeared during the Cretaceous Period, survived the die-off, and then joined the milleporids and scleractinians in constructing reefs during the following Tertiary Period. The coenothecalians secreted massive CaCO3 skeletons which augmented and strengthened the structures produced by the other reef-building groups.
The Modern Era
The coral reefs of today are dominated by scleractinian corals, although some coenothecalians like the Blue Coral (Heliopora coerulea) are locally quite abundant. But again, only in waters where temperature, chemistry, and turbidity levels are nearly perfect do we find healthy, thriving coral. In less than perfect conditions, coral is easily stressed and can be out-competed by other benthic sessile, encrusting organisms. Red coralline algae in particular is notorious for colonizing and "taking over" coral reef substrate when water conditions are compromised.