Universita di Siena, Italy

Despite the biostratigraphic and chronostratigraphic importance of the Globigerinoides trilobus-Praeorbulina evolutionary lineage in the upper Burdigalian to Langhian interval, some uncertainties concern the taxonomic interpretation of the intermediate stages of this lineage, in particular the generic attribution of the species sicanus De Stefani 1952 and its relationship with Globigerinoides bisphericus Todd 1954 and Praeorbulina glomerosa curva (Blow 1956). In this study, we present: 1) a review of the intermediate stages of the Globigerinoides trilobus-Praeorbulina evolutionary lineage focussing on the different taxonomic concepts of the species bisphericus, sicanus and glomerosa curva which according to Blow (1956; 1969) and Jenkins et al. (1981) gave rise to two different lines of evolution, and 2) our concepts based on the study of assemblages from several Mediterranean successions which have been compared with those of mid- and low-latitude Atlantic Ocean. On the basis of several diagnostic characters, we identified three morphotypes (Morphotypes 1, 2 and 3) within the populations transitional from G. trilobus to Praeorbulina. In our assemblages, Morphotype 3 and Morphotype 2 are the most representative ones and are considered as Globigerinoides sicanus sensu Blow (1956; 1969) and Kennett and Srinivasan (1983) and as a variant of G. sicanus, respectively. In fact, the taxonomic concepts of Blow, which take both the population variability and the gradual stratigraphic evolution of the biocharacters into account, can be better applied to our assemblages than those of Jenkins et al. (1981), which are mainly based on the characteristics of the holotypes without considering the variability of the species. Accordingly, the evolution of Praeorbulina via G. sicanus (senior synonym of G. bisphericus) proposed by Blow (1956; 1969) can be followed in the studied assemblages and the Praeorbulina datum is represented by the first appearance of P. glomerosa curva. On the basis of our data the evolution of P. glomerosa curva from G. sicanus lasts more than one myr. The main evolutionary changes within G. sicanus populations leading to Praeorbulina are the appearance of specimens with three apertures and the gradual increase in the outline sphericity. Near-spherical individuals of G. sicanus with three apertures can be considered very close to P. glomerosa curva, which in turn is characterized by a spherical outline, at least four apertures with the primary aperture nearly undistinguishable from the supplementary ones and a close umbilicus. Ultimately, our re-examination of the intermediate stages between G. trilobus and Praeorbulina results in a re-evaluation of the evolution proposed by Blow (1956; 1969) with significant bio- and chronostratigraphic implications.