Geology Reference
In-Depth Information
10.2.3.3 Corals
members of the exclusively marine, usually sedentary
Anthozoa. The number and symmetry of vertically ar-
ranged infoldings of the gut wall in the body cavity of
the polyps and the type and composition of the skel-
eton serve to divide the class into various groups, such
as the soft corals, sea fans (Octocorallia), sea anemo-
nes, and the stony or true corals.
Octocorals
occur from
tropical to polar regions and from tidal zones to deep
sea. The skeleton of octocorals consists of small cal-
citic sclerites (0.10 to 1 mm) that are common in re-
cent sediments but very rare in the fossil record.
True
corals
, commonly collected in the subclass Zoantharia,
Corals are common constituents of limestones. Paleo-
zoic rugose corals and tabulate corals are widely dis-
tributed in shelf and ramp environments and contrib-
ute to the formation of reefs. Mesozoic and Cenozoic
scleractinian corals are important reef builders in tropi-
cal and subtropical warm-water zones but also occur
in temperate and cold-water environments.
Major groups:
Corals are Cnidaria comprising the
hydrozoans, scyphozoans and anthozoans. Corals are
Box 10.6.
Major coral groups.
Technical terms used are:
axial structure
- structural element occurring in the center of
corallites;
corallum
- the skeleton of a coral colony;
corallite
- the skeleton of an individual coral polyp;
coenosteum
-
colonial skeletal deposits formed between corallites;
dissepiments
- blister-like calcareous elements between septa;
edge
zone
- a fold in the body wall extending over the edge of the corallum;
epitheca
- outer skeletal sheet laid down by the
edge zone;
microarchitecture
- the spatial arrangement of microstructural elements;
microstructure
- basic elements of
septa and walls;
protosepta
- initial septa formed at first;
septa
- radial and vertical element extending from the wall
toward the center of corallites;
tabulae
- horizontal elements within the whole or a part of the corallite;
trabecula
-
fibrous microstructural element corresponding to vertically stacked cones and related to skeleton formation;
wall
-
peripheral calcareous element bounding the corallites.
Rugosa
- Pl. 83/3, 8, 10
Criteria:
Calcitic skeleton. Solitary or colonial. Cup-
shaped, conical or cylindrical solitary corals ranging in di-
ameter from a few millimeters to several centimeters. Cen-
timeter- to meter-sized massive or dendroid colonies with
variously integrated corallites. Major and minor septa. Long
major septa inserted serially in four positions; shorter mi-
nor septa inserted between major septa. Septa usually with
trabecular microstructure. Dissepiments and tabulae always
present. Axial structures present or absent, often of septal
origin. Walls formed by laterally fused septa dissepiments
of additional thickened stereoplasm.
Significance:
Many rugose corals serve as good indi-
cators of paleoenvironment and age.
Distribution:
Ordovician to Late Permian.
Heliolitida
- Pl. 83/5, 6
Criteria:
Calcitic skeleton. Exclusively colonial. Mas-
sive, dome-shaped or laminar forms. Slender corallites are
separated by extensive coenosteum consisting of smaller
tubes. Septa generally present, commonly 12 in number,
either spinose or laminar. Tabulae common. No evidence
of communication between corallites. Similar to tabulate
corals in having corallites with tabulae but different in hav-
ing corallites embedded apparently within a colonial tissue
developed between the polyps.
Significance:
Sometimes reef builders.
Distribution:
Late Ordovician to Middle Devonian.
Heterocorallia
-
Pl. 83/2
Criteria:
Calcitic skeleton. Elongate solitary corallites
with four axially connected protosepta. Subsequent septa
in each quadrant remain attached to earlier septa, dividing
successively to form characteristic Y-shaped patterns. Tabu-
lae present.
Significance:
Predominantly restricted to the Viséan, use-
ful as index fossils.
Distribution
: Uppermost Devonian and Early Carbonifer-
ous.
Tabulata
-
Pl. 41/1; Pl. 83/1, 4, 7, 9, 11; Pl. 135/1, 2, 4;
Pl. 143/7, 8
Criteria:
Calcitic skeleton. Exclusively colonial. Sheet-
like to dome-shaped, or erect-branched, or chain-like forms.
Colonies generally small, but colonies with a diameter of
several meters are also known. Slender tubelike corallites
ranging in size between 0.5 to about 5 mm. Pores or tubes
between corallites. Septa absent or short, commonly ex-
pressed as rows of spines or low ridges. Tabulae common
and numerous. Dissepiments present or absent.
Significance:
Important reef-building organisms in the
Silurian and Devonian, often together with stromatopor-
oid sponges.
Distribution:
Early Ordovician to Late Permian. Com-
mon in Middle Ordovician, Silurian, and Devonian. Less
common in the Carboniferous and Permian.
Scleractinia
-
Pl. 84/1-6, Pl. 115/1, Pl. 116/3, Pl. 117/1;
Pl. 134/2; Pl. 147/1, 2, 4
Criteria:
Aragonitic skeleton. Solitary or colonial. Vary-
ing strongly in shape and colony form. Sixfold symmetry
of septal insertion (hence the name hexacorals). Complex
microarchitecture and microstructure of septa and walls.
Significance:
Prominent reef builders since the Late Tri-
assic and even more since the Late Jurassic. High carbon-
ate production in shallow-marine environments and peri-
platform areas by accumulation of coral debris.
Distribution:
Middle Triassic to Holocene.
