The rudists of Southern Istria - An example of environmentally induced succession within Santonian limestones

Rudist bivalves thrived commonly in the shallow carbonate platforms of the Cretaceous Tethyan realm. The presence of two vertically separated Rudist a ssemblages along two well-preserved Upper Cretaceous sections of cape Mrler a (southern parr of the Istrian peninsula) is interpreted as environmentall y induced faunal replacement. The first succession begins with pelagic limestones, followed by a mixed ra diolitid-hippuritid assemblage, and by monogeneric floatstones-biostromes o f radiolitids. The lower pelagic unit represents relatively open marine con ditions while the uppermost radiolitid unit originated under more restricte d marine conditions. Radiolitid floatstones with rare Gorjanovicia bouquets represent the beginn ing of the second succession. A mixed radiolitid-hippuritid assemblage, cha racterised by radiolitid floatstones where rare large Vaccinites individual s occur, replaces the Radiolitid assemblages in vertical succession. In both sections, sea level changes were the most important factor causing the replacement of rudist assemblages. Slight differences between the two s uccessions could be attributed to differences in submarine topography. Radi olitids and hippuritids might have occupied different biotopes within subti dal environments of the Adriatic carbonate platform. Hippuritids preferred more stable and deeper subtidal environments. By contrast, radiolitids pref erred the shallowest parts of subtidal areas. Therefore, these two major rudist groups may be used for determination of d ifferent paleoecological conditions.