Stromatoporoidea is a class of aquatic invertebrates common in the fossil record from the Ordovician through the Devonian. They were especially abundant in the Silurian and Devonian. These invertebrates were important reef-formers throughout the Paleozoic and the Late Mesozoic. The group was previously thought to be related to the corals and placed in the Phylum Cnidaria. They are now classified in the sponges (Phylum Porifera), specifically the sclerosponges. There are numerous fossil forms with spherical, branching or encrusting skeletons of laminated calcite with vertical pillars between the laminae.Stromatoporoids are useful markers whose form and occurrence can diagnose the depositional environment of sedimentary strata. Paleozoic stromatoporoids hosted various symbiotic endobionts such as rugosans, syringoporids, worms and cornulitids.