Reconstructing early sponge relationships by using the Burgess Shale fossil Eiffelia globosa, Walcott

Abstract

The relationships of the sponge classes are controversial, particularly between the calcareous and siliceous sponges. Specimens of the putative calcarean Eiffelia globosa Walcott from the Burgess Shale show the presence of diagnostic hexactinellid spicules integrated into the skeletal mesh. The arrangement of these spicules in Eiffelia is shown to be precisely equivalent to that of early protospongioid hexactinellids, and sponge growth occurred through an identical pattern to produce identical skeletal body morphology. The difference in spicule composition of the classes is interpreted through the observation of taphonomic features of Eiffelia that suggest the presence of at least two mineralogically distinct layers within the spicules. These results support molecular analyses that identify the calcarean-silicisponge transition as the earliest major sponge branch and suggest that the heteractinids were paraphyletic with respect to the Hexactinellida.

Fig. 1.

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.)

Fig. 2.

Morphology of Eiffelia globosa (ROM 57022). (A) Bedding-plane view, reflective patches are fragments of HF-resistant carbonaceous film. Note the slight color/textural difference between the shale matrix and spicules where the film has peeled away (both aluminosilicate). The poorly defined material interspersed between the spicules is also HF-resistant. (B) Detail of spicule ray showing bilayered structure and fragments of carbonaceous film. (C) Perpendicular-to-bedding thin section through spicule ray showing divergence of the carbonaceous film in the cortical layer.

Fig. 3.

Camera lucida drawings from the type material of Eiffelia globosa showing tetraradiate spicules. (A) USNM 200648. (B) USNM 200653. (C and D) USNM 200638. Tetraradiate spicules are shaded in A-D.(E) USNM 200656. (F) Artificial construction obtained by tracing of E with all remaining hexaradiates replaced by tetraradiates, revealing an irregular quadruling habit (see Discussion). (G) Illustration of perfect quadruling as seen in Protospongia. (Scale bar: A, 1 mm; B-F, 2 mm.)

Fig. 4.

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.

Fig. 5.

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.