Also see:
Cnidaria Fossils

Phylum Cnidaria comprises incredibly diverse creatures, including colonial siphonophores, massive medusae jellyfish, box jelly fish, corals. Cnidarian kinship through a common ancestor is based on the common characteristic of stinging cells called nematocysts that they all possess. In fact, the name Cnidaria (silent C) comes from the Greek word "cnidos" that means stinging nettle.

The Cnidarian fossil record reaches back further than any other animals to the Vendian. Those Cnidarians lacking mineralized skeletons are, of course, exceedingly rare, while cnidarians that possessed hard skeletons, in particular the corals, have left a prodigious fossil record. Coral fossils are sparse in the Cambrian Period, since a major radiation did not occur until the Lower Ordovician. Paleozoic corals included the tabulate corals, rugose corals, and heliolitid corals, all of which met extinction at the end of the Permian Period, along with 95% of all marine invertebrates. Scleractinian corals first appeared in the Middle Triassic, and rapidly expanded into ecological niches once dominated by tabulate and rugose corals, becoming the dominant reef-building organisms in shallow tropical marine environments.

Scyphozoans are extremely rare as fossils, being comprised of soft bodies that can only be preserved under extremely rare conditions. A few possible but poorly known scyphozoans have been described from the Vendian (Late Precambrian), but ichnofossils are known as far back as the Middle Cambrian.

1. The Rugosa, also called the Tetracoralla, are an extinct order of coral that were prodigious in the Middle Ordovician through the Late Permian and left an extensive fossil record. See this plate of beautiful rugosa coral form from Ernst Haeckel's Kunstformen der Natur, circa 1904.