Sponges have been in the news lately, so how could I resist writing about a recent paper on sponge relationships? Sponges (phylum Porifera) are found in three classes: the Demospongia, the Calcarea, and the Hexactinellida, all of which are quite ancient with forms identified from the Cambrian. Fossil sponges can be identified by the arrangement of their skeletons, which consist of collections of spicules with characteristic shapes and chemical constitutions. Spicules of various sizes are organized into an interlocking meshwork that generates the supporting framework of the animal. Two characteristics of the spicules that are used to classify them are 1) shape, in particular the angle that the rays diverge from one another, and 2) the chemical structure, whether they are based on calcium or on silicon.

• Demospongia. Spicules are siliceous, with a triradiate symmetry—the spicule rays typically diverge at 60° or 120°. Individual spicules may look like a caltrop, with points to the corners of an imaginary tetrahedron.
• Calcarea. Spicules are made of calcite, with a triradiate or tuning fork shape.
• Hexactinellida. Spicules are siliceous, with a hexactine or tetraradiate symmetry—the spicule rays diverge at 90° angles. Picture a child's set of jacks clustered together.

Our understanding of the relationships between these three, however, has been getting juggled about. My copy of Clarkson's Invertebrate Palaeontology and Evolution, for instance, groups the Demospongia with the Calcarea in the subphylum Gelatinosa on the basis of the organization of the soft tissues and spicule shape, and sets the Hexactinellida apart in the subphylum Nuda, while admitting that there are complications that make the groupings prone to radical revision. One alternative is to group them by whether their spicules are made of calcareous or silicaceous, which would mean that the Hexactinellida and Demospongia are sister lineages, with the Calcarea the odd man out.

Bitting and Butterfield are attempting to resolve these relationships by examining a Cambrian sponge, Eiffelia globosa. Eiffelia is a member of a somewhat problematic group of sponges called the heteractinids which have been classified in the Calcarea because they have spicules made of calcium carbonate, and hexaradiate spicules that are at least close in shape to those of calcareans. What the authors suggest, though, is that Eiffelia is actually a good transitional form that also has tetraradiate spicules and two mineralogically distinct layers to their spicules that may represent both a calcareous core and a silicaceous outer rind.

Here's what a fossil of a Cambrian sponge looks like: what is preserved is an array of spiny spicules.

Morphology of Eiffelia globosa (ROM 57023). (A) Whole specimen. (B-G) Details of A, showing tetraradiate spicules. Note the quadruled arrangement of tetraradiates in E and F, and evidence of a central vertical ray in C and G.(H) Spicule showing evidence of bilayered construction. (Scale bar: A, 3 mm; F, 0.50 mm; D, 0.45 mm; B, E, and G, 0.35 mm; C and H, 0.20 mm.)

The key feature here is that many of the spicules are tetraradiate in an animal that also has hexaradiate spicules. So which group should it belong in, the tetraradiate Hectinellida, or the triradiate Calcarea? Another datum that would help settle the question is to find out what mineral made up the spicules—but of course this is a fossil, which means there has been replacement in the half-billion years it has been imbedded in rock. One suggestive observation can be seen in (H) above, though: the spicules in Eiffelia are bilayered.

Botting and Butterfield propose that Eiffelia is transitional in regards to their mineral composition, too. What we're seeing is the primitive state, a calcareous spicule, with the deposition of a layer of SiO₂ in the matrix surrounding it. The diagram below illustrates the change, from a calcareous spike (A), to a silicaceous spike (C), by way of an intermediate form (B, Eiffelia).

Proposed transition from magnesium calcite (Calcarea) to opal (Silicospongea) spicules based on the bilayered structure exhibited by Eiffelia. (A) Calcarean spicule with outer sheath of collagen fibrils. (B) Secondary precipitation of opal onto the collagenous sheath (as represented by Eiffelia).(C) Further increase in opal precipitation, accompanied by reduction of the calcaean sheath to an axial filament and loss of hexaradial symmetry. Mg-Ca, magnesium calcite; ACC, amorphous calcium carbonate; CFS, collagen fibril sheath; Op, opal; Sl/Um, silicalemma/unit membrane or second collagen fibril sheath.

What it all means is that this seemingly problematic organism actually has a suite of features that bridge a couple of poriferan classes, and also tells us something about how the differences between those classes evolved.

As a stem-group hexactinellid, Eiffelia globosa joins a growing list of problematic Cambrian fossils recognized as key intermediate stages linking higher-order taxa. In this instance, the mosaic of calcarean and hexactinellid characters documents the morphological transition between two poriferan "classes", with the peculiarly bilayered spicules suggesting a heuristic, possibly even correct, model for understanding the mineralogical transition between the Calcarea and Silicispongea.

Here's a new family tree for the sponges based on this work:

Proposed phylogenetic relationships of extant sponge classes, Eumetazoa, and Eiffelia globosa. Eiffelia is shown as a stem hexactinellid (Silicispongea) and a derived member of the "Heteractinida" (stem Calcarea). Eiffelia and early hexactinellids share tetraradial hexactines and a quadruled spicule geometry, but the spicules of the latter are entirely siliceous.

The Hexactinellida and Demospongia, the two silicaceous classes, are now grouped together. The Calcarea are a separate branch, with Eiffelia on a branch in between the two.

Also note where the Eumetazoa (that's us!) have been placed, using molecular affinities referenced in the paper…as a group more closely related to the Calcarea. That makes the Porifera paraphyletic, which suggests that there might be some more shake-ups to come in the taxonomy of these animals.

(Oh, just in case you thought this was about some other kind of sponge relationships, I'll just mention that sponges are hermaphrodites capable of asexual reproduction, and Spongebob would find Spongedob's weird obsession with minor details of vertebrate behavior to be peculiar and trivial.)

Botting JP, Butterfield NH (2005) Reconstructing early sponge relationships by using the Burgess Shale fossil Eiffelia globosa, Walcott. PNAS 102(5):1554-1559.