Corals secrete a skeleton that may turn into a fossil.
Corals are sometimes better known for the reefs that they create, but they are actually 500 million years old and, depending upon the particular species, may be abundantly represented in the fossil record. This makes corals unique compared to other organisms that are similar to them.
Coral Skeletons
Corals are invertebrates that inhabit the phylum cnidaria, which includes jellyfish and sea anemones. Each coral contains a colony of tiny polyps, and this colony, depending on the coral species, secretes calcium carbonate to form an outer skeleton that is eventually left behind as a remnant of the organism after it is gone, so we only get to see the organism indirectly.
Fossilization
Soft tissue like skin and brain very rarely fossilize unless preserved instantly. It is usually just hard parts like bones that fossilize because of the traumatic process that is required to reach fossilization. Soft tissue is usually weathered away, so quite a few invertebrates don't fossilize very well. Because the coral is mostly made up of soft digestive tissue, only its outer skeleton will fossilize.
History
Corals first evolved in the middle Cambrian geological period, around 510 million years ago, but fossils from this time are relatively rare. They did not begin to radiate or spread out until the Ordovician period about 490 million to 440 million years ago, when the corals began covering the oceans and eventually became ubiquitous.
Tabulate and Rugose
Tabulate and rugose corals were diverse orders that evolved in the Ordovician and left behind calcite skeletons, a form of calcium carbonate, in a number of shapes. Rugose fossils in particular look wrinkled and shaped like a horn. Both orders died out or began to dwindle during a major extinction event at the end of the Permian 250 million years ago.
Scleractinia
Scleractinian corals, which still exist to this day, filled the niche left by tabulate and rugose, but the scleractinia is quite different from either. They evolved during the middle Triassic period about 245 million to 230 million years ago and became common during the Jurassic. These species use a form of calcium carbonate called aragonite, which doesn't actually fossilize as well as calcite.
Tags: million years, calcium carbonate, about million, about million million, form calcium, form calcium carbonate, left behind
