Mediterranean pliocene globorotalia : a biometrical approach

Abstract

Globorotalia assemblages are studied from Pliocene deposits in Crete (Greece), Italy, the Gulf of Mexico region, and New Zealand. In each assemblage counts and measurements were performed on 10 test characters. The taxonomy is based on (1) biconvex or planoconvex shape, (2) "growth" patterns of size ratios, (3) frequency distributions of discrete characters on the test. Attention is given to the treatment of such quantitative data. The assemblages have been collected in five groups named after typologically defined species: (1) margaritae group, (2) crassaformis group, (3) puncticulata group, (4) bononiensis group, and (5) inflata group. Each group is illustrated by means of a number of scanning electron microscope photographs; some attention is drawn to wall structures. The puncticulata, bononiensis, and inflata groups occur in stratigraphic order; the ranges do not overlap. In Crete an incidental morphological transition was observed between the bononiensis group and the inflata group. In the Mediterranean the association of the margaritae group and the puncticulata group is restricted to the Lower Pliocene (Tabianian). The puncticulata group ranges slightly higher in the stratigraphic column than does the margaritae group. The upper part of the range of the puncticulata group, the total range of the bononiensis group, and the lower part of the range of the inflata group are indicative of the Middle-Upper Pliocene (Piacenzian). The crassaformis group occurs throughout the Pliocene. In the Gulf of Mexico region the puncticulata and bononiensis groups were not observed. The occurrence of the margaritae group and the beginning of the inflata group may be used for stratigraphic correlations. The crassaformis group displays trends, but it may be doubted if the zigzag path of this group is stratigraphically useful, except perhaps on a local scale. Assemblages of a New Zealand G. crassaformis bioseries show no resemblance to our crassaformis group.