REINTERPRETATION OF BROOKSELLA FROM THE CONASAUGA FORMATION (CAMBRIAN) OF GEORGIA AND ALABAMA, USA

The Cambrian fossil Brooksella was first described by Walcott from cherty “star cobbles” that occur abundantly in the Conasauga Formation of the Coosa River Valley of Alabama and adjacent areas of Georgia. The reddish-brown nodules, usually one to 10 cm in diameter, weather free from fine-grained gray shales. Walcott assigned three species to Brooksella: B. alternata, the type species of Brooksella; B. confusa, and B. cambria. Furthermore, Walcott assigned Brooksella to the cnidarian order Scyphomedusae. Subsequent interpretations of Brooksella indicate uncertainty about the origin of the fossil. Some fossils referred to Brooksella from the Coosa Valley and elsewhere have been interpreted as trace fossils, or as features of inorganic origin.

Restudy of Brooksella from the Conasauga Formation suggests that the “star cobbles” represent body fossils of simple ellipsoidal construction. Brooksella specimens show wide morphologic variation, including a variable number of radially disposed lobes divided by deep radial grooves, and often a central opening on one side. Lobes sometimes terminate in small openings. The “star cobbles” do not regularly have lobes numbering in multiples of four, nor do they show tentacles or gonads, as expected if they had a cnidarian affinity. Seilacher and Goldring interpreted Brooksella from Alabama as a trace fossil on the basis of inferred radial tunnels and teichichnoid backfill structures. Whereas it is evident that radial internal cavities occupy the lobes, we have been unable to find backfill in any specimen, and therefore reject the trace fossil interpretation.

Morphology of the "star cobbles" seems to be most consistent with a poriferan interpretation. Numerous newly collected “star cobbles” from Georgia have siliceous spicules preserved surficially and internally. Internally, the fossils have a distinctive spongy appearance, which is consistent with a poriferan interpretation. The three-dimensional nature of most “star cobbles” suggests rapid fossil diagenesis of siliceous (probably hexactinellid) sponges. Additionally, the “star cobbles” show great variability in external shape, and morphologic patterns are gradational, suggesting that a single species name (B. alternata) should be used to embrace all forms described from the Coosa Valley.