Hexactinellida: Systematics

Not a great deal of research has been directed toward understanding the phylogenetic history of hexactinellids. Spicule morphology has been the primary criterion for determining hexactinellid systematic groupings. For instance, in the phylogenetic hypothesis shown below, the Hexactinellida is split into two major groups, the Hexasterophora and the Amphidiscophora, based on the presence or absence, respectively, of six-rayed microscleres (small spicules) called hexasters.

Similarly, the groups within the Amphidiscophora and Hexasterophora have been separated based upon spicule morpholgy. However, it is known that the silica concentration in sea water can have an effect on spicule morphology. This suggests that spicule morphology may be too variable to reveal evolutionary relationships. Of course, the phylogenetic hypothesis shown here may be correct. However, it should be tested with analyses based on other types of characters.