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.
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.
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.
Fossils, Sea Anemones, Sea Pens (Order Rugosa - see note
wasps or box jellyfish
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.