CNIDARIANS

From Eldredge (1991) 


INTRODUCTION

The phylum Cnidaria (Coelenterata in some texts) includes both solitary and colonial organisms that have radial and/or bilateral symmetry. Typical cnidarians alternate each generation between a fixed polyp stage and a free living medusoid stage. Most cnidarians are considered carnivores because of their ability to actually catch food with their stinging cells called nematocysts. Some groups, particularly the reef-corals employ photosynthetic algae (zooxanthellae) within their tissues in a symbiotic relationship to aid in supplying food needed for their rapid growth.

The cnidarian classes Anthozoa (corals) and Hydrozoa have calcified skeletons of aragonite and calcite and a good fossil record, whereas 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. The general form of coral colonies may be quite similar in unrelated anthozoans (e.g., some colonial Tabulates and Scleractinians) because form represents a basic response to long-term environmental conditions (i.e., limiting factors such as light, turbidity, and especially wave and current energy).

The first part of the lab introduces you to the taxonomy of the Cnidarians and their geologic ranges. The second part concentrates on aspects of coral morphology, coloniality, and integration that are used to deduce ancient environments


CLASSIFICATION & GEOLOGIC RANGES

Phylum Cnidaria

Class Anthozoa (Precambrian-Recent)

Order Tabulata (Ordovician-Permian)

Order Rugosa (Ordovician-Permian)

Order Scleractinia (Triassic-Recent)

Subclass Octocorallia (Precambrian-Recent)

Class Hydrozoa (Precambrian-Recent)

Class Scyphozoa (Precambrian-Recent)

 


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.


Morphologic Terms (see accompanying figures)

Calyx: the bowl-shaped depression or "seat" in which the living polyp resides.

Corallite: the skeleton produced by one polyp, which may or may not be part of a colony

Epitheca: the outermost skeletal layer of a corallite which sometimes shows growth lines.

Tabula (plural tabulae): a horizontal partition (or floor) dividing the corallite skeleton.

Septum (plural septa): vertical blade or partition within the calyx of a corallite that are normally radially arranged.

Dissepiment: small curved plate in a corallite near the tabulae that is convex inward and upwards.

Mural pores: the small holes in the epitheca of some tabulate corals.

Columella: an axial rod in a corallite usually formed by the fusion of two or more septa that typically forms a topographic prominence in the central part of the calyx.


Order TABULATA

The exclusively colonial Tabulate corals occur only in the Paleozoic. Their calcite skeletons typically have a lateral wall (epitheca) that separates each rather small corallite. Each of the corallites typically have a tabula that serve as the floor for the polyp. Septa in tabulate corals are either absent or inconspicuous. Although their growth forms vary, they often occur in "honeycomb" or chain-like morphologies.


Figure 1 - Tabulate Morphology

From McRoberts (1998)


See examples: Favosites which has well developed tabula, and which has well developed mural pores; and Haylisites.


Order RUGOSA


Figure 2 - Rugosan Morphology

From McRoberts (1998)


The Paleozoic rugosan corals can be either solitary or colonial. Although they may have originally had aragonitic skeletons, all are now calcite. Rugosa corals are distinguished from the other Paleozoic group, the Tabulata, by having dissepiments and well developed septa in addition to the tabulae. As shown in the accompanying figure (Figure 3.1), rugosan corals have six primary septa and new septa are added in only four of the resulting six spaces with none added in the remaining two spaces. This septal arrangement is well illustrated in the external mold where the septa are preserved as gaps.

For examples of solitary forms which typically exhibit a cone or horn morphology (hence the informal name "horn corals") examine these specimens, - and This specimen has excellent dissepiments visible where the epitheca is worn-away. For examples of colonial rugosans view this specimen.


Figure 3 - Septal Growth Patterns

Modified from McRoberts (1998)


Order SCLERACTINIA

Scleractinian corals (including all modern coral species) 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. Good examples showing corallites and septal arrangement in a colonial form can be seen in this specimen. See also the colonial specimens, - - - noting the different growth morphologies. This specimen is a solitary coral collected from deep water; note its similarity to some of the solitary rugosans seen earlier.


Subclass OCTOCORALLIA

Although octocorals are very abundant in modern oceans, they do not have a good fossil record at all because of their lightly calcitized skeletons. Among the more common octocorals are sea whips and sea fans such as shown by these specimens. - - Other groups of octocorals have even a poorer fossil record because they have only calcitic spicules or non-calcified skeletons. One of the latter group are the sea-pens which have soft, feather-like skeletons.


Class HYDROZOA

Hydrozoans are a diverse group of cnidarians that inhabit a variety of marine and fresh-water environments. The more important groups (in terms of paleontology) construct their skeletons of calcite. These critters can sometimes superficially resemble corals in skeletal morphology and growth habits, or they can also occur as encrusting sheets or erect blades. Some hydrozoa such as the fire coral Millipora have thick calcareous lamellar skeletons with vertical tubes and cross partitions.


Class SCYPHOZOA

Scyphozoa (jelly fish) only occur in marine environments. The are typified by a reduced polyp stage and an extended free-swimming medusae stage. As one might imagine, fossil scyphozoans are rarely preserved as fossils; yet surprisingly they are probably represented in the famous Ediacara fauna of the Precambrian. Almost all fossil remains of scyphozoans occur as molds and less commonly casts. Some workers would place the Conularia as a Subclass of the Scyphozoa.


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