RE-EVALUATION OF THE MIDDLE CAMBRIAN SPONGE, BROOKSELLA ALTERNATA, FROM THE CONASAUGA FORMATION, GEORGIA AND ALABAMA, U.S.A

The identity of Brooksella alternata has fascinated paleontologists since Charles Walcott described the alleged fossils along the Conasauga River in Georgia and Alabama. Walcott thought Brooksella was a medusoid jellyfish; later workers suggested a feeding or gas bubble trace, and more recently, a hexactinellid sponge. We re-examined the evidence that Brooksella was a sponge that possessed radial canals, oscula, ostia, and hexactinellid spicules. These features were previously reported as external and internal radial lobes, a central depression, surface pores, and an external meshwork of spicules, respectively. Surface morphologic features of Brooksella (n = 72) were observed and measured (body diameter, lobe length and width, osculum diameter). Internal morphology was examined with computed tomography and XRD analysis was used to determine Brooksella’s composition. Thin sections were microprobed to identify mineral associations and to look for spicules. We also collected in situ Brooksella to determine whether its orientation in the shales was consistent with a living sponge.

Results indicate that Brooksella is highly variable in morphology, especially in the number of lobes, ranging from 3 to 15. Some lobes were well defined, others not; none had a canal opening or internal canals as previously reported, suggesting they might not be radial canals. Some specimens have a central protuberance rather than a depression, or lacked a central depression all together, calling into question whether they had oscula. Small surface pits were made by lichen and small rootlets, and were likely not ostia. Hexactinellid spicules or traces were not present on the surface or in thin sections. Brooksella’s composition and internal structure are similar to concretions from the Conasauga: quartz grains with minor amounts of calcite and small, oxidized, root-like holes partly filled with iron oxide and barite crystals. In situ Brooksella were rare and were oriented with their “oscula” and lobes downward, rather than upward if this was a once-living sponge. Furthermore, shale laminations were displaced by the growth of the putative sponge. We therefore think that the sponge designation is insufficiently supported, and we favor a concretional mode of formation for Brooksella.

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