Fig. 2

Hexactinellid framework and basic spicule types. Scanning Electron Micrographs (SEM). a hexactine megasclere (Dictyocalyx sp., Euplectellidae Corbitellinae), the eponymous character for the class. Other megasclere (structural spicule) types are derived from hexactins by reduction of rays. b dictyonal framework of the “regular” (euretoid) type (Conorete gordoni, Euretidae), as found in most sceptrulophorans. c dictyonal framework with “haphazard” connection of hexactins (Dactylocalyx pumiceus), as found in Dactylocalycidae. d dictyonal framework of the aulocalycoid type (Aulocalyx australis), as found in Aulocalycidae. Note that in addition to proper ray fusion (as in b, c, and e; see [2] and Additional file 4 for details), synapticular fusion – cementation of spicules by siliceous bridges – is also common in this type of framework construction. e dictyonal framework of the Lychniscosida (Neoaulocystis zitelli, Aulocystidae; facial view). Instead of regular hexactins, lantern-like spicules (lychniscs) are the building-blocks of the frameworks in this paleontologically important relict-group. f oblique view of surface lychnisc of N. zitteli. g lyssacine construction type of parenchymal skeleton (Atlantisella sp., Euplectellidae Corbitellinae), as found in Lyssacinosida and Amphidiscophora. In this type of body plan, spicules either do not fuse at all, or (often older) parts of the skeleton fuse by synapticular bridging (only in Lyssacinosida); proper ray fusion as in the dictyonal body plan never occurs. h detail of g, showing synapticular bridging (lower right). i amphidisc (Hyalonema populiferum, Hyalonematidae), the diagnostic microsclere and defining autapomorphy of subclass Amphidiscophora. j hexaster (Farrea omniclavata, Farreidae), the diagnostic microsclere and defining autapomorphy of subclass Hexasterophora. The pictured spicule is an oxyhexaster, meaning that the secondary ray tips are pointed (without ornamentation); for further examples of hexasters see Fig. 3. k septrule (C. gordoni), the diagnostic spicule type and defining autapomorphy of Sceptrulophora. The pictured spicule is a scopule; for further examples of sceptrules and discussion of their evolution, see [19]. l unicate (F. omniclavata), a spicule type found in most species of Sceptrulophora and Amphidiscophora. All scale bars without lettering = 100 μm