Heterastridium (Hydrozoa) from the Norian of Iran and Turkey

The spherical hydrozoan genus Heterastridium Reuss is reported worldwide from Norian marine deposits in association with reef and other shallow-water organisms as well as with deep water organisms at some localities, the latter being result of transportation. Untill now 29 taxa of Heterastridium were described by previous authors, either as "species", "subspecies", "variety" or "forma". Three species (H. conglobatum Reuss, H. lobatum Reuss and H. disciforme Gerth) are considered to be valid species. All other former taxa are synonyms of Heterastridium conglobatum representing different ontogenetic growth stages. Heterastridium is an index fossil of the Norian. Based on almost 1000 fossils from central Iran, on about 80 specimens from south Turkey and on about 100 thin-sections from morphologically differently structured and sized specimens, the following results have been obtained: (a) Generally, the surface of the small specimens, about 20 mm in diameter, is covered by large elements, called humps or spines in this paper. The development of the surface elements starts from the centre as small columns, becoming thicker by splitting when the specimen reached a diameter of about 20 mm. This first ontogenetic stage is characterised by small humps and/or warts. (b) By repeated spliting of the humps the middle-sized (young adults) and large specimens (adults) developed. The adult stage of Heterastridium conglobatum is characterised by small and equally distributed elements (warts). (c) Morphologically differently sized and structured specimens, described as "species", "subspecies", "variety" or "forma" by previous authors, are different ontogenetic growth stages of the species Heterastridium conglobatum Reuss. (d) In addition to H. conglobatum, Heterastridium lobatum Reuss and H. disciforme Schafer & Grant-Mackie are valid species, which are differentiated by external morphological features (shape) and by an internally different architecture. Heterastridium has been classified as hydrozoan by previous authors and this view is followed here. This genus is a cosmopolitan index fossil of the Norian stage. Its occurrences in the Carnian and Rhaetian rocks, as described by some authors, is not proven and doubtful. Heterastridium is known from most continents, the main occurrence being the Tethyan realm. Some specimens of Heterastridium contain a biogenic object in the centre. It can be assumed that the colonisation of the core object by Heterastridium has most probably taken place at the sea floor followed by a rolling and may be later intermittently floating mode of life. The internal differences of Heterastridium and the Cretaceous genus Parkeria Carpenter are discussed.