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Corals consist of marine creatures
called polyps, either on their own or, more typically,
in colonies. They belong to the phylum Cnidaria and the
class Anthozoa, which also includes sea anemones. The
earliest fossils of corals can found in Cambrian rocks
but they are much more abundant in Ordovician and
Silurian limestones and shales. These primitive corals
can be divided into two Orders: the rugose corals (Rugosa)
and the tabulate corals (Tabulata). Both became extinct
in the Permian. Modern corals fall into the order
Scleractinia, which appeared in the Triassic.
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Rugose corals occur in a wide range
of forms, both solitary and compound. They did not
generally fix themselves to the sea floor but lived near
reefs built by other creatures. Solitary forms are
usually horn-shaped, as shown left, whereas the compound
forms may be fasciculate (the corallites do not
touch) or massive (the corallites do touch, forming
a honeycomb-like structure. |
The calcareous skeleton, or corallum,
has a concave depression at its tip, in which the
soft-bodied polyp once rested. This is called a calice.
Within the corallum are thin, radial partitions called
septa, often surrounding a central axis or columella.
There are also horizontal partitions called tabulae
running up the length of each corallite. As the coral
grows older these septa multiply in a way that preserves
bilateral symmetry. Between the septa are small plates
called dissepiments. All this is enclosed by the outer
wall, or epitheca.
Tabulate corals may also be
fasciculate or massive, but the corallites are generally
smaller than their rugose counterparts. They all have
tabulae but lack some of the other structures found in
rugose corals. Some of the fasciculate varieties have
corallites connected by horizontal bars. Tabulate corals
were most abundant in the Silurian and Devonian periods.
Scleractinian corals first became
common in the Jurassic, and have survived to the present
day in both solitarily and colonial form. They never
have tabulae and only sometimes have dissepiments or
axial structures. The septa are in multiples of six and
the skeleton is light and porous. They are the most
successful reef-builders, many of them living in
symbiosis with algae, which feed the polyp by
photosynthesis.
See also: sponges
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