A common group of fossil organisms that lived during the Ordovician, Silurian, and Devonian periods. They are preserved in rocks of these systems as large (cabbage-sized) calcium carbonate fossils in the shape of crusts, plates, domes, fingers, bulbs, cylinders, and bushes consisting internally of a three-dimensional network of regularly repeating structural elements such as pillars, laminae, cyst plates, and walls (illus. a,b,c). Although recognized as a class, or subclass, of sponges, these fossils, unlike most sponges, are lacking in siliceous or calcareous spicules (spikelike supporting structures). The term stromatoporoid has been used for a very similar group of Mesozoic fossils that are discussed here as sphaeractinids. Unlike the Paleozoic stromatoporoids, some of the sphaeractinids show the remains of siliceous spicules and can be assigned to several subclasses of the sponge class Demospongiae. Stromatoporoid has also been used as a descriptive term to apply to a grade of evolution of a wide range of different groups of sponges in which a calcareous basal skeleton of stromatoporoid-like architecture was secreted. The implication of such usage is that the Paleozoic fossils are not a natural group but a collection of disparate lineages of sponges that are at a similar stage of evolution. The term stromatoporoid is best restricted to the fossils of Ordovician, Silurian, and Devonian age defined by their lack of spicules and characteristic internal structures that can be grouped as the class Stromatoporoidea of the Porifera.