Old Earth Ministries Online Earth History Curriculum

Presented by Old Earth Ministries (We Believe in an Old Earth...and God!)

This curriculum is presented free of charge for use by homeschooling families.

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Chapter 4 - The Silurian Period

Lesson 30: Corals


     Corals are marine organisms from the class Anthozoa and exist as small sea anemone-like polyps, typically in colonies of many identical individuals. The group includes the important reef builders that are found in tropical oceans, which secrete calcium carbonate to form a hard skeleton.

     A coral "head", commonly perceived to be a single organism, is formed from many individual but genetically identical polyps, each polyp being only a few millimeters in diameter. Over thousands of generations, the polyps lay down a skeleton that is characteristic of their species. An individual head of coral grows by asexual reproduction of the individual polyps. Corals also breed sexually by spawning, with corals of the same species releasing gametes simultaneously over a period of one to several nights around a full moon.

     Although corals can catch small fish and animals such as plankton using stinging cells on their tentacles, these animals obtain most of their nutrients from photosynthetic unicellular algae called zooxanthellae. Consequently, most corals depend on sunlight and grow in clear and shallow water, typically at depths shallower than 60 metres (200 ft). These corals can be major contributors to the physical structure of the coral reefs that develop in tropical and subtropical waters, such as the enormous Great Barrier Reef off the coast of Queensland, Australia.

Chapter 5: The Silurian Period


Lesson 26: Silurian Overview

Lesson 27: Graptolites

Lesson 28: Brachiopods

Lesson 29: Bivalves

Lesson 30: Corals


Pillar Coral

Pillar coral, Dendrogyra cylindricus

Other corals do not have associated algae and can live in much deeper water, with the cold-water genus Lophelia surviving as deep as 3,000 metres (9,800 ft).

     Although coral fossils have been found dating back to the Cambrian, coral reefs first appear in Silurian rocks.  Fossils are extremely rare until the Ordovician period, 100 million years later, when Rugose and Tabulate corals became widespread.


Rugose Corals

Rugosa Coral

Solitary rugose coral (Grewingkia) in three views; Ordovician, southeastern Indiana

     The Rugosa, also called the Tetracoralla, are an extinct order of coral that were abundant inMiddle Ordovician to Late Permian seas.

     Solitary rugosans (e.g., Caninia, Lophophyllidium, Neozaphrentis, Streptelasma) are often referred to as horn corals because of a unique horn-shaped chamber with a wrinkled, or rugose, wall. Some solitary rugosans reached nearly a meter in length. However, some species of rugose corals could form large colonies (e.g., Lithostrotion). When radiating septa were present, they were usually in multiples of four.

     Rugose corals have a skeleton made of calcite that is often fossilized. Like modern corals (Scleractinia), rugose corals were invariably benthic, living on the sea floor or in a reef-framework. Although there is no direct proof, it is inferred that these Palaeozoic corals possessed stinging cells to capture prey. They also had tentacles to help them catch prey. Technically they were carnivores, but prey-size was so small they are often referred to as microcarnivores.

     Rugose corals became dominant by the middle of the Silurian period, and became extinct early in the Triassic period. The Rugose corals existed in solitary and colonial forms, and are also composed of calcite.


Tabulate Corals

Tabulate Coral

Etched section of an Ordovician tabulate coral

     The tabulate corals, forming the order Tabulata, are an extinct form of coral. They are almost always colonial, forming colonies of individual hexagonal cells known as corallites defined by a skeleton of calcite, similar in appearance to a honeycomb. Adjacent cells are joined by small pores. Their distinguishing feature is their well-developed horizontal internal partitions (tabulae) within each cell, but reduced or absent vertical internal partitions (septae). They are usually smaller than rugose corals, but vary considerably in shape, from flat to conical to spherical.

     Around 300 species have been described. Among the most common tabulate corals in the fossil record are Chaetetes, Favosites, Halysites, Heliolites, Pleurodictyum, Sarcinula and Syringopora.

     Like rugose corals, they lived entirely during the Paleozoic, being found from the Ordovician to the Permian. With Stromatoporoidea and rugose corals, the tabulate corals are characteristic of the shallow waters of the Silurian to the Devonian. Sea levels rose in the Devonian, and tabulate corals became much less common. They finally became extinct in the Permian-Triassic extinction event.

     Tabulate corals occur in the limestones and calcareous shales of the Ordovician and Silurian periods, and often form low cushions or branching masses alongside Rugose corals. Their numbers began to decline during the middle of the Silurian period and they finally became extinct at the end of the Permian period, 250 million years ago. The skeletons of Tabulate corals are composed of a form of calcium carbonate known as calcite.


Other Corals


     The Scleractinian corals filled the niche vacated by the extinct Rugose and Tabulate species. Their fossils may be found in small numbers in rocks from the Triassic period, and become common in the Jurassic and later periods. Scleractinian skeletons are composed of a form of calcium carbonate known as aragonite. Although they are geologically younger than the Tabulate and Rugose corals, their aragonitic skeleton is less readily preserved, and their fossil record is less complete.

     At certain times in the geological past corals were very abundant. Like modern corals, these ancestors built reefs, some of which now lie as great structures in sedimentary rocks.

Fossils of fellow reef-dwellers algae, sponges, and the remains of many echinoids, brachiopods, bivalves, gastropods, and trilobites appear along with coral fossils. This makes some corals useful index fossils, enabling geologists to date the age the rocks in which they are found.

     Coral fossils are not restricted to reef remnants, and many solitary corals may be found elsewhere, such as Cyclocyathus, which occurs in England's Gault clay formation.


For more information on reefs, check out the excellent web resource known as "The Virtual Silurian Reef."


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Source: Corals, Coral Reef