Biometric study of Nephrolepidina praemarginata (Douvillé, 1908) from the Madonie Mountains (Sicily)

A wealth of data exists on the evolutionary trends of Oligo-Miocene Nephrolepidina from the Mediterranean area concerning both embryonic and nepionic acceleration (Drooger, 1993). New biometrical data on statistically significant populations of Nephrolepidina are here presented. A succession of the Caltavuturo Formation, near Portella Colla (Madonie Mts., Sicily), contains several breccia layers rich in larger foraminifers (Benedetti & Pignatti, in press). Resedimented Nephrolepidina populations from these levels have been investigated biometrically. A total of 24 parameters have been measured from oriented equatorial sections of megalospheric specimens and 8 factors have been derived from selected parameters. According to De Mulder (1975), the degree of enclosure of the deuteroconch on the protoconch (factor A) and the number of adauxiliary chambers (parameter C), are sufficient for the recognition of N. praemarginata (Douvillé, 1908). The investigated samples can be referred to chronobiozone SBZ22A of Cahuzac & Poignant (1997) (Upper Rupelian) basing on the first occurrence of N. praemarginata and Eulepidina formosoides. The recognition of the expected trends trough time of increase in embryo size, degree of enclosure of the protoconch by the deuteroconch and average number of adauxiliary chambers, is hampered by oscillation of mean values. Factor dII/dI versus parameter C evidences a more definite positive trend compared with other measures; on the contrary the mean values of protoconch size for each population show an inverse trend especially for the younger samples. This is interpreted as a consequence of resedimented tests from progressively deeper environments; protoconch size decrease is expected for populations far from the ecological optimum, according to the biometrical investigations on recent nummulitids (Pécheux, 1995).