are marine organisms of class Anthozoa (Kingdom
Animalia, Phylum Cnidaria,
Class Anthozoa) that typically live in large colonies of identical
individuals. They are important reef builders in tropical seas,
and are secretors of calcium carbonate (CaCO3) that forms their
hard skeleton. Class Anthozoa likely appeared in the Precambrian,
though the fossil record is sparse and inconclusive. Several soft-bodied
Vendian animals are now thought to be early Anthozoa. Sequence
analyses also supports the theory that Anthozoa are the first
branch of the Phylum Cnidaria (Bridge et al. 1992).
a few CaCO3-secreting corals or coral-like organisms are known
from the early Cambrian (e.g., Sorauf and Savarese 1995), and
the Cothoniida, are known from the middle Cambrian. It was not
until the Ordovician that mineralized corals became prodigiously
important to marine ecosystems. By the middle Ordovician, the
Tabulata, and Rugosa corals, and the smaller Heliolitida were
widely prevalent. Despite the appearance of corals during the
Ordovician, reef ecosystems continued to be dominated by algae
and sponges and in some cases by bryozoans. However, there apparently
were also periods of complete global reef collapse due to global
now extinct tabulate corals were colonial. Colonies resembled
honeycombs and comprised hexagonal cells known as corallites having
a calcite skeleton similar in appearance to a honeycomb. Adjacent
cells are joined by small pores. Their defining characteristic
is horizontal internal partitions (tabulae) within each cell,
but reduced or absent vertical internal partitions (septae). Tabulate
corals are normally smaller than rugose corals and exhibit significant
variability in shape, from flat to conical to spherical.
also extinct Rugosa or Rugose coral were ubiquitous from the middle
Ordovician to late Permian. Solitary forms (i.e., solitary polyps)
are commonly called horn corals owing to their horn/conical-shaped
chamber having a wrinkled,
or rugose, wall. Some of these solitary rugose corals grew to
almost a meter. Other Rugosa formed large colonies (e.g., Lithostrotion).
the middle Ordovician in the Paleozoic major coral groups were
contributors to massive reef systems, while stromatolites reefs
were in steady decline. The Heliolitids met
extincttion in the upper Devonian. The Rugosa and Tabulata recovered
from the late Devonian extinction only to completely disappear
in the PT extinction event at the end of the Permian some 245
million years ago.
scleractinians (Order Scleractinia) appeared in the Triassic.
In fact, all corals after the than the PT extinction as well as
all extant corals, belong to the Scleractinia. They are differentiated
from the Rugosa by their patterns of septal insertion where the
scleractinian septa are appear in sets of six, maintaining a hex
symmetry throughout life. The scleractinian corals are markedly
different than the Paleozoic rugose corals, which leads scientists
to believe they evolved separately from anemones instead of from
the earlier Rugosa. The scleractinians have formed great reefs
on Earth from the Mesozoic on.
do not have a medusa stage in their development as do other cnidarians.
Rather, they release sperm and eggs that form a planula that attaches
to a surface on which they grow. A coral head contains thousands
of genetically identical polyps, each typically a few millimeters
in diameter. Over thousands of generations, the polyps lay down
a skeleton that is characteristic of their species, and recognizable
are usually a few millimeters in diameter, and are formed by a
layer of outer epithelium and inner jellylike tissue known as
the mesoglea (see figure to right). They exhibit radial symmetry
with tentacles surrounding a central mouth, the only opening to
the stomach or coelenteron, where food is ingested and waste is
expelled. The polyp grows by extension of vertical calices that
are sometimes septated to form a new base plate. Over numerous
generations the extensions form the large calcareous structure
from which coral reefs are comprised. The calcareous exoskeleton
is built by the deposition of the mineral aragonite by the polyps
from calcium in seawater.
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from mitochondrial gene structure. Proceedings of the National
Academy of Sciences of the USA 89:8750-8753.
C. A., D. M. Odorico, M. ten Lohuis, J. E. N. Veron, and D.
J. Miller (June 1995). "Systematic relationships within
the Anthozoa (Cnidaria: Anthozoa) using the 5'-end of the 28S
rDNA". Molecular Phylogeny and Evolution 4 (2): 175–183
P. A. & Jell, J. S. 1976. Early Middle Cambrian corals from
western New South Wales. - Alcheringa, 1 (2): 181-195.
J., Debrenne, F., Gandin, A. & Gravestock, D. 1991. The
oldest tabulate coral and the associated archaeocyatha, Lower
Cambrian, Flinders Ranges, South Australia. - Géobios,
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J.E., and M. Savarese. 1995. A Lower Cambrian Coral from South
Australia. Palaeontology 38:757-770.
Tynan, M.C. 1983. Coral-Like Microfossils from the Lower Cambrian
of California, Journal of Paleontology 57(6):1188-1211.