Species richness and abundance patterns of Tethyan Cretaceous rudist bivalves (Mollusca : Hippuritacea) in the central-eastern Mediterranean and Middle East, analysed from a palaeontological database

Data about the stratigraphical and regional distribution of Cretaceous rudi st bivalves in the central-eastern Mediterranean and Middle East from the B arremian to the Maastrichtian have been compiled in a taxonomic databank sy stem. The investigated region comprises northern, central and southern regi ons of the Cretaceous Tethys. The regionally different quality of taxonomic data is critically discussed. Both species richness and abundance (number of species records) peak in the Early Aptian, Late Cenomanian and Early Cam panian. Temporal patterns in species richness are examined in relation to e volutionary innovations of the group. Crises in species richness and abunda nce during the Early and mid-Cretaceous were coeval with oceanic anoxia ass ociated with platform drowning. Also, regional emergence affected large par ts of the Apulian Plate and other central Tethyan carbonate platforms durin g the mid-Cretaceous period of globally high sea level, so that these crise s can be attributed to regional environmental perturbations, induced by eit her oceanic anoxia or tectonic movements. The Cenomanian-Turonian transitio n marks an important faunal turnover, as indicated by the demise of taxonom ic groups which are characterised by recumbent ecological morphotypes and p redominantly aragonitic shells. During the Turonian; they were replaced by associations of largely calcitic Radiolitidae and Hippuritidae, among which elevator morphotypes dominated. The stratigraphy of many post-Turonian dep osits in the studied region is too imprecise for a detailed analysis of Lat e Cretaceous distributional patterns, and the demise of rudists during the Maastrichtian in particular. The regional pattern of species richness and a bundance is similar throughout most of the investigated region. A notable e xception is the broad north-eastern African shelf from Libya to Lebanon, wh ere rudists were almost completely absent after the Coniacian, This coincid ed with shallow-water phosphorite and black-shale formation on the extended shelf of this region, indicating the upwelling of nutrient-rich waters and stagnant conditions. During this time (Campanian-Maastrichtian), species-r ich associations thrived along the north-eastern Afro-Arabian margin, now e xposed in south-eastern Turkey, Iran, the United Arab Emirates and Oman. Th is region forms the single distinct faunal province during the Cretaceous i n the studied region, as indicated by an endemism of 40% at the genus level . The low species richness and abundance of Hippuritidae on Turonian-Maastr ichtian central Tethyan carbonate platforms, when compared with regions hav ing mixed calcareous-siliciclastic sedimentation, is interpreted to argue a gainst a widespread photosymbiosis in the group. Several aspects of this st udy highlight the importance of regional tectonic and oceanographic control s of distributional, and perhaps also evolutionary, patterns of rudist biva lves. Their delineation still suffers from the regionally different quality of stratigraphical and taxonomic data. (C) 2000 Elsevier Science B.V. All rights reserved.