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151:“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.
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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
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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
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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
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272:– You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.
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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.
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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):
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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.
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Brooksella alternata
Walcott, 1896 - fossil sponges from the Cambrian of Alabama, USA. (FMNH 16739 - Geology Department, Field Museum of Natural History, Chicago, Illinois, USA) (centimeter scale)
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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).
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The above info. is synthesized from
Ciampaglio et al. (2005, 2006), Ciampaglio & Babcock (2006), and from info. provided by Loren Babcock (pers. comm.).
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Subdivision
Working Group, Nanjing, August 18-24, 2005, abstracts and short papers. Acta Micropalaeontologica Sinica 22(Supplement): 21-23.
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Brooksella alternata fossil sponges ("star cobbles") (Conasauga
Formation, Middle Cambrian; Coosa River Valley, Alabama, USA).jpg
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siliceous concretions. So, Brooksella isn’t a trace fossil - it’s a sponge!
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