The infinite variety: the beginning of life
Cnidarians and the Fossil Record
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- Coral Reefs under threat
Cnidarians and the Fossil Record Although Cnidarians are relatively simple organisms and appeared fairly early in the history of life, fossil evidence for them was only recently found (1940's) in the Flinders Range, southern Australia in rock strata that has been dated at about 650 million years. 
Photograph of the fossil coral Heliophyllum taken by Dlloyd. Heliophyllum halli from the Devonian. Locality - Arkona, Ontario, Canada. Complete matrix free specimen which measures 4.3 cm across. Coral Reefs under threat Not all Cnidarians are soft-bodied, and some produce skeletons of limestone in a similar way to the sea sponges and are better known as corals. These animals secrete their skeletons from their base. Each polyp is connected with its neighbours by strands that extend laterally. As the colony develops new polyps form, leaving a limestone skeleton that is riddled with tiny cells were polyps once existed. Live polyp are restricted to a thin surface layer. The size of these colonial polyps are enormous and create entire coral islands called atolls and created the Great Barrier Reef running parallel to the east coast of Australia. This coral reef extends for over a sixteen hundred kilometres and is the greatest animal construction prior to man's artefacts. 
Portion of a Pacific atoll showing two islets on the ribbon or barrier reef separated by a deep pass betwen the ocean and the lagoon. coral reef coral reef 
A coral reef is a type of biotic reef developing in tropical waters. Although corals are major contributors to the overall framework and bulk material comprising a coral reef, the organisms most responsible for reef growth against the constant assault by ocean waves are calcareous algae, especially, although not entirely, species of red algae. Water temperature of 20–28 °C (68–82 °F) is an optimal range for proper growth and health of coral reefs. Coral reefs are found in all oceans of the world, except the Arctic Ocean, generally between the Tropic of Cancer and the Tropic of Capricorn, because reef-building corals live in these waters. Reef-building corals are found mainly in the photic zone (less than 50m), where the sunlight reaches the ground and offers the corals enough energy. The corals themselves do not photosynthesise, but they live in a symbiotic relationship with types of microscopic algae that photosynthesise for them. Because of this, coral reefs also grow much faster in clear water, which absorbs less light. Such reefs take a variety of forms, defined as the following; Download 2.01 Mb. Share with your friends:
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