A study of the morphological variability of Radiolites marinii Caffau & Plenicar (Radiolitidae), Late Cenomanian, Karst of Gorizia, Italy

The species Radiolites marinii Caffau & Plenicar is very abundant in the rudist assemblages of the limestone sequence of the area of "Archi", in the southeastern part of Karst of Gorizia, from the Upper Cenomanian age. The good preservation-state of several specimens found separately in the dolonstone allowed to analyse and describe in detail the external morphology of the shell of this species. The main feature of the external morphology is the difference between the upper and the lower part of the shell. Morphometrical values regarding the shell area (Sa), the inner area of the shell (Ia), the external perimeter (Ep) and the internal perimeter (Ip) of the shell were obtained from each transverse section by image analysis. The analysis of these data indicated that the change in morphology could be due to a strong biological stress that markedly influenced the physiological trend of calcitic secretion for the building up of the shell in individuals of this species.