TAXONOMY AND TAPHONOMY OF THE FIRST REPORTED SPONGE FROM THE FORT PAYNE CHERT OF NORTHWESTERN GEORGIA, USA

Sponges are major components of modern marine communities, but are noticeably absent in most Paleozoic fossil assemblages because of the delicate structure of their skeletons. This is especially true within the Mississippian of the Valley and Ridge in the southeastern region of the United States which has yeilded few taxa. Even so, the lack of described sponge taxa is somewhat enigmatic within the Fort Payne Chert given that many of the silica-rich beds are composed of dense accumulations of spicules along with the suggestion that sponges are the source of the silica that replaced the original carbonates in the formation.

A new sponge-dominated assemblage has been recently discovered within the Lavender Member of the Fort Payne Chert, near Rome, Georgia. The exposure of rock is extremely limited, yet has yielded hundreds of articulated siliceous sponges along with rare vertebrate material and molds of bivalves, crinoids, and bryozoans. The most common sponge morphotype is a flattened conical form with radiating spicules. The spicules lack any processes and occur in loose bundles similar to those in Belemnospongia parmula, from the St. Genevieve Limestone of Alabama, though they lack the well-defined outline. In addition, there are elongated undulating cylindrical forms and structures with oriented spicules that may represent root tufts.

The Lavender Member has a distinctive lithology (thin and irregularly bedded siltstones and shales) compared with the rest of the formation which likely facilitated the articulated preservation of the sponges. Silicification is limited to small mottled areas at the top of the exposure and is likely constrained by the impermeable shales. The diagenetic alteration of the spicules is distinctly non-uniform with moldic preservation in thicker siltstone layers and likely original material retained in the protected thin silt layers. Even within these thin layers the calcareous fossils are moldic, which is likely the result of Recent weathering. Nevertheless, this deposit has a complex diagenetic history that needs more detailed study.