Brooksella
alternata
“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.).
References Cited
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.