A Critical Review of the Octocoralian Fossil Record (Cnidaria: Anthozoa)  

Octocorals have a rather disjunct representation throughout the geological record.  The earliest possibly date back to Ediacaran faunas and today they are as diverse and widely represented as soft and horny corals (Alcyonacea and Gorgonacea), sea pens (Pennatulacea), as well as blue corals and stoloniferans (Helioporacea and Stolonifera) and some other small groups.  They are generally characterized by a lightly chitinized exoskeleton (e.g. Gorgonacea) or endoskeletons with microscopic calcareous sclerites or axial rods (e.g. Alcyonacea, Pennatulacea). All have poor preservation potential, which is clearly the principal reason for their rarity and occasionally absence in the fossil record. 

Despite this, over the decades a number of Palaeozoic octocorallian fossils have been described.  In many of these, understanding higher-level taxa has not been possible, largely as a result of poor preservation of the microstructure.  After detection of skeletal carbonate hydroxylapatite in recent gorgonaceous octocorals, previously unknown in modern coelenterates, several Early Cambrian phosphatic ‘problematica’ have occurred regarding relationships to octocorals appear now in another light.  Similarly, fossils of the Ediacaran leaf-like presumed pennatulids have been described from Cambrian rocks – the earliest of these is Priscapennamarina from the Early Cambrian of eastern Yunnan, China.  The Middle Cambrian Burgess Shale taxon Thaumaptilon is also presumed to be a pennatulid, while another Burgess Shale fossil – Echmatocrinus – originally described as a crinoid, was later interpreted as a probable octocoral.  Some Neoproterozoic phosphatized embryos have been attributed to cnidarians, but interpretations of the Ediacarian and Cambrian pennatulid-like fossils and probable octocorals are not always accepted with regard to the further Phanerozoic fossil record.

The early fossil record, evolution and phylogeny of Octocorallia and Anthozoa, including recent molecular analyses, is still in a state of confusion, and more work is needed before either hypothesis can be accepted or rejected.