Fossil cnidarians have been found in rocks formed about 580 million years ago, and other fossils show that corals may have been present shortly before 490 million years ago and diversified a few million years later.

What was the first cnidarian?

Another theory is that the original cnidarian was a planula-like organism that preceded both polyp and medusa. In either case, Hydrozoa is considered to be the most ancient of cnidarian classes, and Trachylina is thought to be the most primitive extant order of that group.

What group did cnidarians evolve from?

Fortuitously, the first two cnidarian genomes to be sequenced come from Anthozoa (Nematostella) and Hydrozoa (Hydra); the last common ancestor of these two classes was the stem cnidarian (Fig. 1).