A group of invertebrate marine fossils of the phylum
Cnidaria, characterized by radial symmetry and a lack of cells
organized into organs. They are related to jellyfish, hydroids,
and sea anemones, all of which possess stinging cells. Corals
are the best preserved of this phylum because they secrete a
hard calcareous skeleton. The animal is basically a simple sac
with a central mouth, surrounded by tentacles, that leads to a
closed stomach. Cnidarians are passive predators, catching
food that wanders by in their tentacles. Corals and other
cnidarians produce alternating generations of two body
forms. Medusa are forms that reproduce sexually to form
polyps that are asexual forms from which the medusa may
bud off. The corals belong to a subclass of the anthozoan
cnidarians known as the Zooantharia. The jellyfish belongs
to the Scyphozoa class, and the Hydrozoa class includes both
fresh and saltwater cnidaria dominated by the polyp stage.
Corals can live in a range of conditions from shallow
tidal pools to 19,700 feet (6,000 m) depth. They have a cylindrical
or conical skeleton secreted by the polyp stage organism,
who lives in the upper exposed part of the structure. The
skeleton is characterized by radial ridges known as septa that
join the skeleton’s outer wall (known as the theca), and may
have flat floors that were periodically secreted by the polyp.
Corals range from the Early Ordovician Tabulata forms,
joined in the Middle Ordovician by the rugose corals. They
both experienced a major extinction in the Late Devonian,
from which the rugose forms recovered stronger. Both forms
became extinct in the Early Triassic and were replaced by
modern coral forms known as Scerlactina, which apparently
arose independently from different soft-bodied organisms.
Most corals grow in colonial communities and form
reefs that provide numerous advantages including shelter for
larvae and young stages. Modern corals can survive only in
shallow waters that range in temperature from 77°F to 84°F
(25°C to 29°C), at depths of less than 300 feet (90 m). Reefs
have various forms including fringing, barrier, and atoll reefs.
See also CARBONATE; PLATFORM; REEF.
Cordillera See ROCKY MOUNTAINS.
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