An extinct group of fossil organisms generally referred to as the Mesozoic stromatoporoids. As originally proposed by Othmar Kühn in 1927, the order included the Mesozoic stromatoporoids (Sphaeractiniidae), the heterastridiids (a group of obscure spherical Mesozoic fossils commonly regarded as hydrozoans), and the spongiomorphs. Kühn placed all of these in the class Hydrozoa. The discovery of spicules (spikelike supporting structures) in some genera of the sphaeractinids has allowed some genera formerly considered Mesozoic stromatoporoids to be classified in the Porifera as representatives of the class Demospongiae and the subclasses Ceractinomorpha and Tetractinomorpha. This discovery has opened the probability that all the sphaeractinids will eventually be referable to living sponge families and orders, if traces of spicules can be found in them. Both Mesozoic and Paleozoic stromatoporoids are now considered to be poriferans; the Paleozoic representatives are best considered a class of the phylum separated from the Mesozoic forms by time, lack of spicules, and microstructure. Those genera of the Mesozoic group that, at present, cannot be assigned to a living poriferan taxon owing to the lack of (or our failure as yet to discover) spicules are a disparate orphan group that can best be informally referred to as sphaeractinids or Mesozoic stromatoporoids.