Anthozoa: Morphology, Geological History and Evolutionary Trends

The class Anthozoa includes both solitary and colonial forms and may include both living and extinct forms.
The class includes exclusively marine polyps of lower Ordovician to present day with an apparent Radial symmetry.
They are found in warm and shallow waters upto a depth of 200 metres at a temperature range of 18-28 degree celcius.
Class Anthozoa (both Solitary and reef building corals) include more than 6000 species. They are marine, warm and shallow (200 mts) organisms. Though the corals are sedentary, their wide geographical distribution is due to free swimming larval stage.

Morphology

In a simple coral body may be conical or cylindrical. The majority of Anthozoa has an exoskeleton which is made up of Calcite of horny matter.
The exoskeleton is known as Corallite , and a colony of corallite is known as Corallum. The outer wall of corallite is called Theca and a calcite coat is known as Epitheca .
The terminal end of corallite is the epical end and triangular open depressed end is the Calyx .In the cetre of the Calyx there is the axis of the corallite which is known as Columella . The interior of Corallite is divided into septa, tabulae and dissepiments. The corallite possess Biradial Symmetry.

Alcyonaria – Also known as Octacorallia because of the presence of eight mesentries and tentacles. Skeleton has the form of an axial rod of horny material as in Gorgonia or carbonate of lime in Corallium , the red coral .
Schizocorallia – Ordovician to Jurassic , branching , massive corals with the corallite having complete horizontal or slightly curved tabulae . e.g- Tetradium
Tabulata – forms with strong platy and porous dissepiments tabulae are included in this subclass . The corallite may be prismatic or it may resemble honey-comb .e.g – Favosites ,Ccleistopors , Aulopora
Zooantharia or Hexacoralia – number of septa is six . also known as stony coral . endoskeleton composed of minute fibrous aragonite crystals . The corallites are held together by calcareous plates. The dissepiments may be rod like or it may be in the form of tabulae .
Rugosa – The fossils of this subclass possess rough appearance. Since the number of septa is four , it is known as TetraCorallia.e.g – Zaprentis , L

First record of tabulate corals is known from Early Ordovician period. They attained their climax in Silurian-Devonian.
They were declining in number since late Devonian and become extinct towards the end of paleozoic.
Tetracorals also appeared in Ordovician and attained their climax by Silurian-permian.
They also failed to cross Permian boundary. Fossils of hexacorals first appeared in middle Triassic rocks.
They gradually in number since then and become dominant after tertiary. They are abundant in the present day reef structure .

Two hypotheses have been proposed to explain the origin of corals or zoantharian.
The first postulates that living hexacorals, extinct Palaeozoic tabulates and tetracorals had a common ancestor in some as yet unknown early Palaeozoic anthozoan stem.
Absence of fossil hexacorals in Palaeozoic may be due to their lack of skeletal parts.
By middle Triassic most of them developed skeleton forming habit and they soon occupied the niches left by extinct tetracorals and tabulates.
The second hypothesis holds that hexacorals descended directly from tetracorals. The transition is supposed to have taken place in late Permian – early Triassic times.