Classification and phylogeny of Hexactinellida (Porifera)

Development of the present classification scheme of file class Hexactinellida was delayed because authors of the early species descriptions did not figure their specimens and few of them had access to compound microscopes necessary to visualize characters important in determining relationships. When microscopic information on spicule symmetry became available, Wyville Thomson in 1868 recognized the distinctive triaxial symmetry of the group and named it Vitrea. Schmidt's 1870 name, Hexactinellidae, was without the contamination associated with Thomson's Vitrea, and gained widespread Support. Through a series of proposals and refinements, the present scheme recognizes two main lineages, Amphidiscophora and Hexasterophora, differing in shape and type of their microscleres. Since these structures are generally not retained in fossil material, paleontologists have long had an independent classification for hexactinellids, but the trend to unify the systems is now nearly complete. Hypotheses on phylogeny of the group remain virtually dependent upon scarce paleontological information, pending data from nucleic acid and protein sequence analyses that are not yet available for unravelling relationships among hexactinellids.