“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 ranging from one to ten centimeters). 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 most modern workers hold that they are trace fossils.
Brooksella alternata star cobbles (centimeter scale) (FMNH 16739 - Geology Department, Field Museum of Natural History, Chicago, Illinois, USA)
A recent 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!
The above info. is synthesized from Ciampaglio et al. (2005) and Ciampaglio & Babcock (2006), and from info. provided by Loren Babcock (pers. comm.).
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.