File:Stromatoporoid (Ashlock Formation, Upper Ordovician; Paint Lick Elementary School outcrop, Garrard County, Kentucky, USA) 8.jpg

English: Fossil stromatoporoid sponge in the Ordovician of Kentucky, USA.

This fossil sponge is eroded from Upper Ordovician fossiliferous limestones of the Ashlock Formation, which is correlative with the famous Cincinnatian outcrop belt in the Cincinnati, Ohio area. I've seen two or three asserted members identified for this area, so I'm not sure about the sub-formation level stratigraphy at this site. The fossils are similar to what's seen in the Maysvillian and/or Richmondian Stages of the type Cincinnatian. Observed fossils here included silicified stromatoporoids, brachiopods (including Platystrophia ponderosa - also known as Vinlandostrophia ponderosa - and Hebertella), ramose bryozoans, encrusting sheet bryozoans, bivalves, gastropods, nautiloid cephalopods, and corals. The rocks range from micrites (lime mudstones) to wackestones to packstones.

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).

Shown here is a broken cross-section through a fossil stromatoporoid (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). 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: Ashlock Formation, Cincinnatian Series, Upper Ordovician

Locality: Paint Lick Elementary School Outcrop - roadcut along (new) Route 52, across the road from Paint Lick Elementary School, near the town of Point Leavell, Garrard County, Kentucky, USA. (37° 35’ 17.25” North latitude, 84° 28’ 15.49” West longitude)