Also see:
Cnidaria Fossils
Coral Fossils

Corals are marine organisms of class Anthozoa (Kingdom Animalia, Phylum Cnidaria, Class Anthozoa) that typically live in large colonies of identical individuals. They are important reef builders in tropical seas, and are secretors of calcium carbonate (CaCO3) that forms their hard skeleton. Class Anthozoa likely appeared in the Precambrian, though the fossil record is sparse and inconclusive. Several soft-bodied Vendian animals are now thought to be early Anthozoa. Sequence analyses also supports the theory that Anthozoa are the first branch of the Phylum Cnidaria (Bridge et al. 1992).

Only a few CaCO3-secreting corals or coral-like organisms are known from the early Cambrian (e.g., Sorauf and Savarese 1995), and the Cothoniida, are known from the middle Cambrian. It was not until the Ordovician that mineralized corals became prodigiously important to marine ecosystems. By the middle Ordovician, the Tabulata, and Rugosa corals, and the smaller Heliolitida were widely prevalent. Despite the appearance of corals during the Ordovician, reef ecosystems continued to be dominated by algae and sponges and in some cases by bryozoans. However, there apparently were also periods of complete global reef collapse due to global disturbances.

The now extinct tabulate corals were colonial. Colonies resembled honeycombs and comprised hexagonal cells known as corallites having a calcite skeleton similar in appearance to a honeycomb. Adjacent cells are joined by small pores. Their defining characteristic is horizontal internal partitions (tabulae) within each cell, but reduced or absent vertical internal partitions (septae). Tabulate corals are normally smaller than rugose corals and exhibit significant variability in shape, from flat to conical to spherical.

The also extinct Rugosa or Rugose coral were ubiquitous from the middle Ordovician to late Permian. Solitary forms (i.e., solitary polyps) are commonly called horn corals owing to their horn/conical-shaped chamber having a wrinkled, or rugose, wall. Some of these solitary rugose corals grew to almost a meter. Other Rugosa formed large colonies (e.g., Lithostrotion).

From the middle Ordovician in the Paleozoic major coral groups were contributors to massive reef systems, while stromatolites reefs were in steady decline. The Heliolitids met extincttion in the upper Devonian. The Rugosa and Tabulata recovered from the late Devonian extinction only to completely disappear in the PT extinction event at the end of the Permian some 245 million years ago.

The scleractinians (Order Scleractinia) appeared in the Triassic. In fact, all corals after the than the PT extinction as well as all extant corals, belong to the Scleractinia. They are differentiated from the Rugosa by their patterns of septal insertion where the scleractinian septa are appear in sets of six, maintaining a hex symmetry throughout life. The scleractinian corals are markedly different than the Paleozoic rugose corals, which leads scientists to believe they evolved separately from anemones instead of from the earlier Rugosa. The scleractinians have formed great reefs on Earth from the Mesozoic on.

Anthozoans do not have a medusa stage in their development as do other cnidarians. Rather, they release sperm and eggs that form a planula that attaches to a surface on which they grow. A coral head contains thousands of genetically identical polyps, each typically a few millimeters in diameter. Over thousands of generations, the polyps lay down a skeleton that is characteristic of their species, and recognizable as fossils.

Polyps are usually a few millimeters in diameter, and are formed by a layer of outer epithelium and inner jellylike tissue known as the mesoglea (see figure to right). They exhibit radial symmetry with tentacles surrounding a central mouth, the only opening to the stomach or coelenteron, where food is ingested and waste is expelled. The polyp grows by extension of vertical calices that are sometimes septated to form a new base plate. Over numerous generations the extensions form the large calcareous structure from which coral reefs are comprised. The calcareous exoskeleton is built by the deposition of the mineral aragonite by the polyps from calcium in seawater.

References:

• Bridge, D., C.W. Cunningham, B. Schierwater, R. DeSalle, and L.W. Buss. 1992. Class-level relationships in the Phylum Cnidaria: evidence from mitochondrial gene structure. Proceedings of the National Academy of Sciences of the USA 89:8750-8753.
• Chen, C. A., D. M. Odorico, M. ten Lohuis, J. E. N. Veron, and D. J. Miller (June 1995). "Systematic relationships within the Anthozoa (Cnidaria: Anthozoa) using the 5'-end of the 28S rDNA". Molecular Phylogeny and Evolution 4 (2): 175–183
• Jell, P. A. & Jell, J. S. 1976. Early Middle Cambrian corals from western New South Wales. - Alcheringa, 1 (2): 181-195.
• Lafuste, J., Debrenne, F., Gandin, A. & Gravestock, D. 1991. The oldest tabulate coral and the associated archaeocyatha, Lower Cambrian, Flinders Ranges, South Australia. - Géobios, 24 (6): 697-718
• Sorauf, J.E., and M. Savarese. 1995. A Lower Cambrian Coral from South Australia. Palaeontology 38:757-770.
  Tynan, M.C. 1983. Coral-Like Microfossils from the Lower Cambrian of California, Journal of Paleontology 57(6):1188-1211.