File:Brooksella alternata (fossil sponge) (Conasauga Formation, Middle Cambrian; near Rome, Georgia, USA) 5.jpg

English: Brooksella alternata Walcott, 1896 - fossil sponge from the Cambrian of Georgia, USA.

“Star cobbles” are hard, three-dimensional, multilobate siliceous concretions weathered out from shales of the Conasauga Formation (Middle Cambrian) in the Coosa River Valley of Alabama & Georgia. Most are three to six centimeters in diameter, but they range from one to ten centimeters in size. Star cobbles were given scientific names by Charles Walcott in 1896: Brooksella alternata, Brooksella confusa, and Laotira cambria. What star cobbles represented has been debated in the paleontological literature ever since. Walcott called them jellyfish (Phylum Cnidaria), but many researchers considered them to be trace fossils (burrows).

A restudy of Conasauga star cobbles has concluded that these structures are body fossils, not burrows, and that Walcott’s three species belong to one highly variable species, called Brooksella alternata.

Brooksella specimens, as seen in cross sections, have no backfills, so they are not burrow fillings. They are always preserved in silica, and have a simple ellipsoidal construction with radial lobes. They vary in shape and lobe number, but they often have a central depression, and the lobes often terminate in visible openings. Most telling, sponge spicules are present on the surfaces of some specimens (even Walcott saw these, but concluded they weren’t part of Brooksella itself).

Brooksella alternata is now considered to represent a hexactinellid sponge. The central depression is inferred to be the osculum. Hexactinellids make their skeletons with siliceous spicules, and rapid diagenesis of a siliceous sponge skeleton is a great way to make three-dimensionally preserved siliceous concretions. So, Brooksella isn’t a trace fossil - it’s a sponge!

Classification: Animalia, Porifera, Hexactinellida, Reticulosa, Protospongiidae

Stratigraphy: Conasauga Formation, Middle Cambrian

Locality: undisclosed site near the town of Rome, Floyd County, northwestern Georgia, USA

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Synthesized from info. provided by:

Loren Babcock (pers. comm.)

Ciampaglio, C.N., L.E. Babcock, C.L. Wellman, A.R. York, & H.K. Brunswick. 2006. Phylogenetic affinities and taphonomy of Brooksella from the Cambrian of Georgia and Alabama, USA. Palaeoworld 15: 256-265.

Ciampaglio, C.N., C. Wellman, H. Brunswick, A. York & L.E. Babcock. 2005. Reinterpretation of Brooksella from the Conasauga Formation (Cambrian) of Georgia and Alabama, USA. in The Fourth International Symposium on the Cambrian System and the Tenth Field Conference of the Cambrian Stage Subdivision Working Group, Nanjing, August 18-24, 2005, abstracts and short papers. Acta Micropalaeontologica Sinica 22(Supplement): 21-23.

Ciampaglio, C.N. & L.E. Babcock. 2006. Reinterpretation of Brooksella from the Conasauga Formation (Cambrian) of Georgia and Alabama, USA. Geological Society of America Abstracts with Programs 38(3): 4-5.