File:Stromatoporoid in limestone (Columbus Limestone, Middle Devonian; abandoned quarry, Kelleys Island, Lake Erie, Ohio, USA) 2 (48637371288).jpg

Fossil stromatoporoid sponge in limestone in the Devonian of Ohio, USA.

This fossil sponge is from the Columbus Limestone, a significant carbonate unit in the Devonian of central and northern Ohio. It's actually part of a much more widespread sheet of Devonian carbonates that extends from New York State to the Midwest. The Columbus Limestone represents deposition in a subtropical, shallow-water, carbonate platform environment. The rocks are principally micritic limestones, fossiliferous wackestones, and fossiliferous packstones. Some chert nodules are present in the unit. Fossils are typical Paleozoic shallow marine invertebrates - favositid corals, rugose corals, stromatoporoids, brachiopods, crinoids, blastoids, bryozoans, trilobites, bivalves, gastropods, cephalopods, rostroconchs, and tentaculites. Microfossils include conodonts and charophyte oogonia. Other fossils in the Columbus Limestone include vertebrates (fish), land plants (rare), and trace fossils. Some horizons in the Columbus Limestone have silicified fossils.

Sponges are sessile, benthic, filter-feeding organisms. They are not metazoan animals, as they lack organs or tissues - they are called parazoans. Sponges are essentially colonies of cells (the cells can live independently for short periods of time). Most sponges are marine, but some occupy freshwater environments.

Sponges construct organic or mineralized, multi-element skeletons. Individual pieces of a sponge skeleton are called spicules. The group first appears in the fossil record in the Neoproterozoic and extends to today, in the Holocene. Some sponges make skeletons composed of opal spicules (SiO2·nH2O - hydrous silica), while others are calcareous (calcite or aragonite) or make spicules of organic material (spongin - a tough, proteinaceous, organic compound).

The rounded mass seen here is a stromatoporoid fossil (these are often misidentified as stromatolites, which are bacterial constructs). They occur in Paleozoic and Mesozoic sedimentary rocks (Ordovician to Cretaceous). The Mesozoic-aged stromatoporoids may represent a separate group. Stromatoporoids have a layered, calcitic skeleton, usually with small vertical pillars between individual layers (laminations) (click on the photo to zoom in to see the layers). If preserved, the top living surface has small mounds (mamelons) with radiating canals (astrorhizae). This group of fossils is similar to a living group of sponges called the sclerosponges - some researchers place the stromatoporoids with the sclerosponges. Stromatoporoids were important components of some Paleozoic and Mesozoic shallow-water reefs.

Classification: Animalia, Porifera, Stromatoporoidea

Stratigraphy: Columbus Limestone, Eifelian Stage, lower Middle Devonian