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) – Class

Hydrozoa

(Precambrian-Recent) – Class

Scyphozoa

(Precambrian-Recent)

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

fish) is comprised mostly of molds and casts. (jelly • 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. • 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

• 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.