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Paleontology Lab II. CNIDARIANS. Phylum Cnidaria. Phylum Cnidaria Class Anthozoa (Precambrian-Recent) Order Tabulata (Ordovician-Permian) Order Rugosa (Ordovician-Permian) Order Scleractinia (Triassic-Recent) Subclass Octocorallia (Ordovician-Recent)

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Phylum cnidaria
Phylum Cnidaria

  • Phylum Cnidaria

    • Class Anthozoa (Precambrian-Recent)

      • Order Tabulata (Ordovician-Permian)

      • Order Rugosa (Ordovician-Permian)

      • Order Scleractinia (Triassic-Recent)

    • Subclass Octocorallia (Ordovician-Recent)

  • Class Hydrozoa (Precambrian-Recent)

  • Class Scyphozoa (Precambrian-Recent)


  • 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 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
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 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 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 ("hard-rayed") corals first appeared in the Middle Triassic and refilled the ecological niche once held by tabulate and rugose corals.

  • 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.

  • For this reason, scleractinians are sometimes referred to as hexacorals.

Coral morphology
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

Paleontology lab ii

encrusting forms

massive forms

erect forms


  • 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.