Nephrolepidina and unispiralled Miogypsinidae from the Oligo-Miocene toe-of-slope succession of Gran Sasso (L'Aquila, Central Apennines - Italy): biometric and evolutionary remarks

Abstract

A biometric analysis was performed on twelve Nephrolepidina and ten Miogypsina and Miogypsinoides populations, coming from the Tre Valloni section, an Oligo-Miocene calcarenitic toe-of-slope succession outcropping in the southern sector of Gran Sasso (L'Aquila, Central Apennines - Italy). Miogypsina populations are investigated applying two essential parameters, i.e., the number of the nepionic chambers and the protoconch diameter. Previously detected evidences of reworking in the same material suggested to use some cautions to avoid misleading results. A mixed typological-statistical methodology is used, leading from starting "disordered" assemblages to final "virtual but reliable" populations, cleaned up from reworked specimens, in order to implement the accuracy of the biostratigraphic results. This goal is reached for the lower part of the succession only. According to the biometric limits proposed for the various chronospecies in the recent literature, the resulting populations are referred to Miogypsinoides ex. interc. complanatus-formosensis up to Md. bantamensis, and to Miogypsina basraensis up to M. ex. interc. gunteri-tani. This sequence of taxa, in general, is in agreement with the principle of nepionic acceleration, with a progressive reduction in the average number of nepionic chambers (X), arranged in a single spiral, just peculiar in the more primitive populations. Miogypsinoides bantamensis and Miogypsina basraensis are recognized biometrically for the first time in the Central Apennines. Nephrolepidina populations are investigated applying the recently introduced biometry of the fifth stage of the neanic equatorial chamberlets, supplying additional data about parameters and factors that had already demonstrated to have taxonomic-biostratigraphic significance. The biometric analysis of the embryo-nepionic stages was performed as an integration of the data obtained in a previous study, excluding the uppermost samples in which Nephrolepidina is clearly reworked, being characterized by primitive morphology, i.e., almost isolepidine embryo, few adauxiliary chamberlets, and ogival equatorial chamberlets. The neanic data evidence that the "Degree of stolonic distalization" (Factor FD5) and the "Shape Index" (Factor SI5), at the 5th nepionic stage of the population have a very good fitting with the previously evidenced Neanic acceleration, as the overall trend to modify the shape of the equatorial chambers, connected with a gradually more distal position of the radial stolons. Unispiralled Miogypsina, associated with Nephrolepidina at an advanced evolutionary stage, suggest a continuous reworking of primitive miogypsinids, without an involvement of more evolute bispiralled forms. The biostratigraphic assessment of the Venacquaro and Glauconitic Calcarenite formations is improved, assigning the Venacquaro Formation to the Chattian-lowermost Aquitanian (SBZ22B-basal SBZ24) and the base of the Glauconitic Calcatenite to the Aquitanian (SBZ24 p. p.). The investigated sedimentary succession was deposited along a slope to toe of slope characterized by a mixture of taxa displaced from shallow-water environments and reworked primitive specimens eroded along channels.