First report of Stromatocystites (Echinodermata) from the Middle Cambrian of Turkey: Palaeobiogeographic implications

Stromatocystites is one of the oldest and most primitive known echinoderms. This genus is relatively common in early to middle Cambrian deposits of Baltica (Sweden) and Gondwana (Australia, Bohemia, Newfoundland, Spain). It possibly also occurs in younger (Late Cambrian) strata of Montagne Noire (S. France). Stromatocystites is characterised by rounded to slightly pentagonal outlines, a flattened, biscuit-shaped body consisting in 1) a domed, polyplated, oral (upper) surface bearing five ambulacra and numerous respiratory openings (sutural pores); and 2) a slightly concave aboral (lower) surface. Such a morphology is extremely plesiomorphic within echinoderms. Stromatocystites differs from basal blastozoans (e.g., Lepidocystis) by the absence of free ambulacra (brachioles), from basal crinoids (e.g., Titanocrinus) by the absence of body wall expansions (arms), and from basal edrioasteroids (e.g., Cambraster) by the absence of a well-differentiated marginal ring. We report here on the recent discovery of two well-preserved specimens of Stromatocystites, collected within a shale interval in the upper part of the Middle Cambrian (Drumian) Koruk Formation of Hakkari-Çukurca area (southeastern-most Anatolia, close to the border with Iraq). One specimen shows an almost complete oral surface, whereas the other one exhibits a typical lower surface. These two specimens are the oldest fossils of echinoderms ever reported so far from Turkey, and the first record of Stromatocystites in this part of the world. From a palaeobiogeographic point of view, the new Turkish fossils are particularly interesting, as they occur in a peri-Gondwanan region intermediate in latitude between western (Bohemia, Newfoundland, Spain) and eastern (Australia) occurrences of Stromatocystites. The morphology of the two Anatolian specimens is apparently closer to S. pentagularis (Bohemia, Newfoundland, Sweden), than to S. flexibilis (Bohemia), S. reduncus (Australia), or S. walcotti (Newfoundland). The occurrence of Stromatocystites in southeastern Turkey is in good accordance with the Mediterranean-Acado-Baltic affinities observed for other faunal elements reported from the same area (e.g., trilobites). It also confirms the existence of regular faunal exchanges, and thus the palaeogeographic closeness, of Baltica and various peri-Gondwanan regions in middle Cambrian times. As a consequence, future field work in the middle Cambrian of Hakkari-Çukurca area will possibly yield additional echinoderm taxa typical of Mediterranean-Acado-Baltic regions, as for example the eocrinoid Cigara and/or the stylophoran Ceratocystis, which both occur along with Stromatocystites in both Bohemia and Sweden.