Cnidaria Cnidaria are named for stinging cells called cnidoblasts. The name Coelenterata means "hollow" (coel) + "gut" (enteron). Radial symmetry. The cnidarian classes Anthozoa (corals) and Hydrozoa have calcified skeletons of aragonite and calcite and a good fossil record The long fossil record of the class Scyphozoa (jelly fish) is comprised mostly of molds and casts. Class Octocorallia is not well represented in the fossil record because of its poorly calcified skeletons.
CORALS Corals have a hard calcareous skeleton, and may be solitary or colonial. Colonies are composed of many polyps living together. The skeletal parts formed by polyps are called corallites. Each corallite is a small (several millimeters to several centimeters in diameter), roughly circular or hexagonal opening, with internal radial partitions called septae in the Rugose and Scleractinian corals. Tabulate corals lack septae. Geologic range: Late Precambrian (Proterozoic) to Recent Corals live attached to the sea floor, primarily in warm, shallow marine environments.
Class ANTHOZOA Geologically the anthozoans are the most important of the cnidarians because their polyps often produce calcitized skeletons that are readily preserved as fossils. They can be either solitary or colonial. Common forms of anthozoans include corals, sea-anemones, and sea-pens. Anthozoans differ from other Cnidaria in that they have no medusoid stage. They are exclusively marine and occur at various depths from shallow to deep water.
Order Rugose Most rugose corals are solitary and conical Septae are visible in the circular opening of the cone. Some rugose corals are colonial, having hexagonal corallites with septae (such as Hexagonaria from the Devonian of Michigan). Geologic range: Ordovician to Permian -
Tabulate Tabulate corals are colonial and resemble honeycombs or wasp nests. They lack septae. Halysites is called the chain coral because its coral tubes are attached in wavy lines resembling a chain. Geologic range: Ordovician to Permian - all extinct.
SUBCLASS ZOANTHARIA ORDER SCLERACTINIA Scleractinian corals are the modern corals. Most are colonial, but some are solitary. Many are reef-builders. Skeletal material is deposited between corallites Geologic range: Triassic to Recent.
Scleractinian Scleractinian corals can be either colonial or solitary. Their originally aragonitic skeletons have dissepiments, tabulae, and septa just as in the Paleozoic rugosans. Although there are superficial similarities, scleractinian corals differ from rugosa corals by their skeletal mineralogy and by their method of septal insertion during growth. Scleractinian corals also have six primary septa, but in contrast to rugosa corals, subsequent septa are added in all six of the resulting spaces. An important distinction between the two orders is that for the Scleractinia the septa are inserted between every two pre-existing septa in later growth stages.
Scleractinian Scleractinian ("hard-rayed") corals first appeared in the Middle Triassic and refilled the ecological niche once held by tabulate and rugose corals.tabulaterugose They are probably not closely related to the extinct tabulate or rugose corals, and probably arose independently from a sea anemone-like ancestor. Their pattern of septa differs markedly from that of the Rugosa, being basically six-rayed.pattern of septa For this reason, scleractinians are sometimes referred to as hexacorals.
Coral Morphology The morphology of coral colonies can be grouped into three broad categories: –(i) encrusting forms which are often sheet-like such as this specimen. –(ii) massive forms which are domal or hemispherical –(iii) erect forms which are branching or palmate
encrusting forms massive forms erect forms
Paleoenvironments Corals occur as framework organisms in reef environments and as important constituents in level-bottom communities. As a group they are very sensitive to physical and chemical conditions such as fluctuating sea level, turbidity, and salinity. Of all of these factors which may result in differing growth morphology, the overall shape of coral colonies is most responsive to water (= wave + current) energy. However, it should be noted that the morphologic response is quite different when a coral is in a reef setting or in a level bottom setting.