Stromatoporoid sponge from the Devonian of Michigan, USA.

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).

Seen here is a stromatoporoid fragment. They occur in Paleozoic and Mesozoic sedimentary rocks (Ordovician to Cretaceous), but the Mesozoic-aged stromatoporoids may represent a separate group. Stromatoporoids have a layered, calcitic skeleton, usually with small vertical pillars between individual layers (laminations). The top living surface has small mounds (mamelons) with radiating canals (astrorhizae) - this specimen has preserved mamelons. Stromatoporoids are 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: top beds of the Charlevoix Limestone or the basal beds of the Petoskey Formation, upper Traverse Group, upper Middle Devonian