I acknowledge Dr. F. Gorrieri for useful suggestions and advice, Prof. G. Facchinetti who checked the mathematical part of the study, Carlo Corradini who supplied the biostratigraphy, and the efforts of two anonymous reviewers whose comments improve the manuscript. I also acknowledge Dr. Norman MacLeod for improvement of my English. MURST and CNR grants (resp. Prof. E. Serpagli) provided funding for this research.