Palorbitolina lenticularis from the northern Adriatic region:: Paleogeographical and evolutionary implications

Samples bearing Palorbitolina were obtained from three localities on the islands of Cres and Losinj in the northern Adriatic. Although a relatively, small number (< 60) of specimens were studied, the size of the embryonic chamber and test diameters of Palorbitolina lenticularis indicate a Lower Aptian age. This is confirmed for the Adriatic carbonate platform by the presence of Lower Aptian index taxa. Both embryonic chamber and test diameter variation are pronounced. No change was observed concerning the relationship between stratigraphic horizon and the embryonic chamber diameter. A proportional relationship between the size, of the embryonic chamber and the test was determined. These data show that before it appeared on the Adriatic carbonate platform, Palorbitolina already colonized the predominantly mixed clastic-carbonate environments of the Mediterranean, as has been suggested by Velic & Sokac (1978). Although an essentially westwards water circulation during the late Barremian and early Aptian probably aided Palorbitolina in colonization of the Tethyan realm, it can neither explain the simultaneous existence of Palorbitolina in different and remote parts of the Tethys nor its dispersal, which must have been very rapid.