3.22.2013

Origins of Coral Reefs

A visitor to a coral reef millions of years ago would have witnessed a seascape that is quite different from the one that exists today. Over time, both the appearance and composition of reefs have changed dramatically. Reefs have been subjected to countless alterations over their history. Ice ages, mass extinctions, shifting of landmasses on continental plates, and fluctuating sea levels are just a few of the global events that reefs have endured.

Geologic records document the existence of reefs 2 billion years ago, in a period of time referred to as the Precambrian era. The architects of the ancient reefs were not coral but simple microbes called cyanobacteria. Then, as now, cyanobacteria were algaelike organisms that formed long, mucus-producing filaments. Their sticky filaments trapped and held debris and grains of sand. Individual algae, with their ensnared soil particles, stuck to one another, forming tall, gray towers, or stromatolites, that rose several meters upward from the seafloor. From 2 billion years ago to 500 million years ago, a period of 1.5 billion years, stromatolites flourished near coastlines.
Origins of Coral Reefs

About 600 million years ago, cyanobacteria were joined in their reef-building activities by archaeocyathids, spongelike animals with stony textures. The word archaeocyathid means “ancient cup” and aptly describes the appearance of these simple animals. The union of blue-green algae and these primitive animals yielded reefs of great durability. The partnership between the two lasted for the next 60 million years until communities were severely damaged by the first of many mass extinctions that have occurred in Earth’s history. Eventually, cyanobacteria and their stromatolite structures alone made a comeback, and the more primitive-style reefs returned.

Around 480 million years ago cyanobacteria teamed up with an animal more complex than the simple archaeocyathids. The new partners were bryozoans, animals with a mosslike appearance. Soon afterward, stony sponges, red algae, and the first of the true corals developed. All four types of organisms were capable of building a limestone covering over their bodies, a feature that protected them from the destructive action of the ocean waves. When these creatures died, their lacy, branching shapes added new dimensions to the reef structure. The association of cyanobacteria’s stromatolites with this new team of skeleton-making organisms lasted for 130 million years.

Around 350 million years ago, thousands of living things, including many species of corals, bryozoans, red algae, and sponges, were wiped out by a second mass extinction. Again, only the hardy cyanobacteria and their stromatolites survived. For the next 13 million years the cyanobacteria existed alone, once again building their drab, gray towers. Eventually several species of calcium carbonate–secreting green algae, stony sponges, and bryozoans joined them.

This latest wave of reef building continued for 115 million years, until another mass extinction struck 225 million years ago, claiming over half of the planet’s plants and animals. During this time several coral species suffered destruction and the stromatolites were reduced vastly in number, causing the reef population to once again disappear.

Reef builders were not able to start over for another 10 million years. When they did appear again, new families of coral, the ancestors of today’s coral populations, developed. For 130 million years, reefs expanded their locations, spreading from a few scattered sites to areas all around the world. At the same time, reef inhabitants changed. New varieties of sponges and mollusks moved in, and the role of cyanobacteria and their stromatolites as primary reef builders declined. Red algae, which had teamed with several new species of coral, acted as the major architects of the reefs of this time period.

About 65 million years ago a final mass extinction annihilated life-forms around the Earth. During this period, onethird of animal species, including the dinosaurs and many species of coral and other reef-building organisms, were lost. Ten million years passed before the reefs reappeared. The coral reefs that made a comeback grew vigorously and have endured till this day.