Evolutionary trends in the Mediterranean Nephrolepidina: new chronosubspecies and biostratigraphic constraints

Abstract

ABSTRACT The mean values of the grade of enclosure of the protoconch by the deuteroconch (Ai), the number of adauxiliary chamberlets (C), the number of annuli within 0.5 mm from the embryonic apparatus (N05) and the degree of stolonic distalization (FD5) were evaluated for several Nephrolepidina populations from the western Mediterranean. Data from two new populations were added to the various populations biometrically investigated in the last decades. The routinely employed Ai and C indexes alone seem to be not sufficient to provide biostratigraphic constraints for each Nephrolepidina chronospecies, whereas N05 appears to be dependent to the embryonic size. By including the index FD5, and thus the concept of neanic acceleration, it becomes possible to clearly separate the different chronosubspecies of Nephrolepidina from the Mediterranean bioprovince. Nephrolepidina praemarginata praemarginata is restricted to the late Rupelian SBZ22A. N. pramarginata elenamoroi n. ssp. is erected for those early Chattian (SBZ22B) populations with 32%<FD5mean <42%. N. morgani, characterised by 42%<FD5mean <50%, stands as a long-ranged species, spanning from the whole Chattian to the lowermost Aquitanian. N. tournoueri antonioi n. ssp. marks the Aquitanian (SBZ24) and is characterised by Aimean >45% and 50%<FD5mean <55%. N. tournoueri tournoueri characterises the Burdigalian (SBZ25), with Aimean >45% and FD5mean >55%.