LAZARUS-LIKE REAPPEARANCE AND GEOGRAPHIC RANGE SHIFT OF SOME HOMALOZOAN CLASSES AND PLEUROCYSTITIDS (ECHINODERMATA) FROM THE HIRNANTIAN (UPPER ORDOVICIAN) TO THE LOWER DEVONIAN

Bilaterally symmetrical echinoderms including the homalozoan classes Homoiostelea (Cam.–Dev.) and Stylophora (Cam.--Penn.); and the blastozoan family Pleurocystitidae (Ord.--Dev.) have a flattened theca and at least one elongated appendage. They are well adapted to living “snowshoe-like” on soft siliciclastic and limey muds in off-shore, mid -- to outer shelf environments. During the Hirnantian Extinction event (U.Ord.) many early echinoderm classes went extinct and the three mentioned homalozoan and blastozoan groups apparently went extinct, but did not. Post glaciation sea level rise was slowed, modified mostly by tectonic events (e. g. closing of the Iapetus Ocean), but through out most of the Silurian these forms are missing in similar off-shore facies. In the upper Silurian rare stylophorans are found in England (Placocystites) and in Australia (Victoriacystis). There are no regional middle or upper Ordovician forms. In the lower Devonian representatives of all three recumbent groups reappear in the fossil record and in some cases they are similar to their regional Ordovician representatives and in many cases not. Special attention is paid to the Cravatt Member of the Bois d’Arc Formation near Clarita, Oklahoma. Pleurocystitids that reappear are in Germany ( Regulaecystis) and Laurentia (Turgidacystis graffhami). Stylophorans that reappear are in Germany (Rhenocystis); Gondwana -- South Africa (Placocystella); South America (Australocystis); Australia (Victoriacystis); New Zealand (Allanicytidium); Laurentia -- New York (Anomalocystites) and Oklahoma (Victoriacystis aff. holmesorum). Homoiosteles that reappear are in Germany (Dehmicystis); Australia (Rutroclypeus) and in Laurentia -- (a Gondwana-like genus Claritacarpos smithi). This Lazarus-like reappearance suggests that Hirnantian genera migrated into deeper water with lowering of sea level, and with slow rise of Silurian sea level these groups which had adapted to deeper water conditions did not reenter shallower waters with rising sea level until late Silurian- lower Devonian where, in most cases, their tenure was brief. There is clear evidence that in the Silurian some of these organisms migrated between existing continental plates and reappeared far from their origin.