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. 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 polyps are restricted to a thin surface layer. The size of these colonial polyps are enormous and create entire coral islands such as 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 artifacts.