Evolution and Systematics

Given animals in the phylum Cnidaria have fewer cell types than a single organ contains in most other metazoans, they are often seen as the simplest eumetazoans¹. The ancestry of extant cnidarians have been thought to lie with medusoid and pennatulacean-like animals of the Ediacaran fauna from the late Precambrian², as they have adopted a body morphology with an epithelial arrangement that ensures external sources of oxygen and nutrients are accessible by internal tissues. 

Modern molecular and morphological analyses suggest that all classes in the phylum Cnidaria are evolutionarily derived from a sessile polyp^3,4. This means that the adult was a sexual polyp with a planula larvae stage, and there was no medusa³.

There are a number of views on the phylogenetic position of the class Anthozoa. Werner (1973) argued that it diverged from other classes early in the evolution of the Cnidaria. On the other hand, Grasshoff (1984) has argued that the class Anthozoa is the stem of Cnidaria, with other classes having become secondarily simplified with the evolution of medusae.

The oldest Anthozoan has been thought to be the subclass Octocorallia; however that would only hold true if Ediacaran sea pens are definitely octocorals. If they are not octocorals, the earliest representative of Octocorallia would be a gorgonian of middle Cambrian age⁵, and octocorals would have been younger than the first possible actinian body fossil from the lower Cambrian⁶. 

Whether Octocorallia and Hexacorallia shared a common ancestor is unknown⁷. If this stand were to be taken, the ancestor must have been very early and prior to the development of coloniality. This is because both Ediacaran colonial 'octocorals' and solitary anemones, and even the lower to middle Cambrian representatives of these groups are known to be very ancient. 

Schmidt (1974) has also suggested that Hexacorallia are derived from a common ancestor with advanced endomyarian actiniarians, based on analysis of nematocysts and other microanatomical features. This analysis has also suggested that the order Actiniaria and the order Madreporaria (which includes scleractinian corals) share a common ancestor.