Although corals first appeared in the Cambrian period, some 542 million years ago, fossils are extremely rare until the Ordovician period, 100 million years later, when rugose and tabulate corals became widespread. Tabulate corals occur in limestone and calcareous shale of the Ordovician and Silurian periods, and often form low cushions or branching masses of calcite alongside rugose corals. Their numbers began to decline during the middle of the Silurian period, and they became extinct at the end of the Permian period, 250 million years ago. Rugose or horn 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 were also composed of calcite.
Fossils of about 15,000 bryozoan species have been found. Bryozoans are among the three dominant groups of Paleozoic fossils. The oldest species with a mineralized skeleton occurs in the Lower Ordovician. It is likely that the first bryozoans appeared much earlier and were entirely soft-bodied, and the Ordovician fossils record the appearance of mineralized skeletons in this phylum.
The Cambrian explosion took place around 540 to 520 million years ago (Mya). In this geologically brief period, all the major animal phyla diverged and these included the first creatures with mineralized skeletons. Brachiopods and bivalves made their appearance at this time, and left their fossilized remains behind in the rocks.
Coral's are still around today, but are facing challenging times due to rising sea levels, human pollution, and damage from irreparable harvesting of protected areas.
Arachnophyllum pentagonam polished colonial corals Beautifully polished specimens of Arachnophyllum pentagonam, a colonial coral. These are found in Assa Zag, Western Sahara Desert, Morocco. Devonian age, 350 MYO.