The coral animal essentially becomes a living veneer over its own dead body. And within these enduring dead bodies are contained significant clues to the ancient history of both coral and our world. The calcium skeleton provides an excellent fossil record and core samples yield fascinating geological insights. Corals have survived, in one form or another, for over 500 million years, so they must be doing something right. Changes in the elevation of the sea due to passing ice ages and tectonic land movements are accurately reflected by ancestral reefs, now stranded high above and below our modern-day sea level.
Corals leave daily growth rings in their skeleton, similar to trees.
In 1963, John Wells made a remarkable discovery, since confirmed by subsequent researchers. Because corals grow so much faster during the day when the algae are active, careful study of these rings in fossilized corals indicate the earth's rotation may be slowing down due to atmospheric friction. Other inferences are equally astounding. While these prototypical coral communities lie sequestered in the annals of extinction, many of today's reefs originated in relatively ancient times. Some atolls of the Central Pacific are built on coral foundations borne of the Eocene era some 50 million years back.