Astraeospongium meniscus sponge (Porifera) fossil found in the Brownsport Formation of Hardin County, Tennessee. This Silurian period fossil as the distinction of being included on the 2011 Science Olympiad fossil list. This Astraeospongium (class Calcarea) existed in the Silurian through Devonian periods.
See this nice write up on the web page of the The Virtual Petrified Wood Museum.
Thanks to Herb for the fossil and new insight on Silurian sponges. Next picture shows under side of the fossil.
As of this writing two of these sponges were for sale on eBay in an estate collection auction. The fossils are from Decatur County, Tennessee and the auction ends March 20, 2011.