LATE APTIAN RADIOLITIDAE (RUDIST BIVALVES) FROM THE MEDITERRANEAN ANDSOUTHWEST ASIATIC REGIONS - TAXONOMIC, BIOSTRATIGRAPHIC AND PALEOBIOGEOGRAPHIC ASPECTS

The onset of the Radiolitidae in the mid-Aptian was subsequently follo wed by a diversification and a geographic spreading of the group durin g the late Aptian. From the Mediterranean and Southwest Asian regions are recorded four species of Eoradiolites and one species of Praeradio lites, all well defined. Moreover at least two species of Eoradiolites and two species of Praeradiolites, with a poorly defined taxonomic st atus, are also present. The geographic distribution of the Mediterrane an faunas runs from Southern France, Spain, Italy, Bosnia, East Serbia , Lebanon to Algeria. The southwest Asian faunas are documented from W estern China, North India, Pakistan, Afghanistan and Iran. The age of the radiolitid-bearing localities is discussed and/or documented after micropalaeontological assemblages. Mediterranean and Southwest Asian faunas are remarkably different: the former group is dominated by typi cal cellular shelled forms (Eoradiolites katzeri-E. plicatus being the cardinal taxa) whereas the latter is dominated by acellular forms (Eo radiolites gilgitensis being the cardinal taxon). Palaeogeographic rec onstruction of the late Aptian palaeoenvironments place the Mediterran ean fauna on the northern and the southern margin of the ''Mediterrane an Seuil'' without any significant dissimilarities in composition, whi le the southwest Asian faunas are restricted to the northern margin of Tethys. These two bioprovinces are both located in the tropical domai n of the northern hemisphere. The existence in southern Russia of a wi de marine embayment connected to high latitude cool waters suggests a possible thermal barrier between the Southwest Asian and Mediterranean faunas, close to the Caucasus. The corresponding faunal groups, with common origin, could have derived from isolation of founder population s coupled with some adaptive divergence during the rapid dispersion of the family at an early stage of evolutionary maturity.