Details of Uvigerina development in the Cretan Mio-Pliocene

Abstract

Neogene uvigerinids with uniserial chambers were investigated biometrically. They were obtained from some 400 closely spaced samples from Upper Miocene and Lower Pliocene sediments in sections on the island of Crete (Greece). In each assemblage counts and measurements were carried out on a number of characteristics of the test of some fifty individuals. Five of these characteristics were believed to give the best information about the changes in the morphology in Uvis;erina. These five are the total length and the maximum breadth of the test, the numbers of the uniserial and of the biserial chambers, and a factor describing the shape of the uniserial chambers, and a factor describing the shape of the uniserial chambers. In a few samples we also measured the diameter of the protoconch, and a number of specimens was dissolved stepwise to obtain data about the early growth stages. Univariate, bivariate and multivariate statistical methods were applied to get more insight in the huge amolint of data. For some samples we measured and calculated the oxygen and carbon isotopic ratios. No obvious pattern of sustained change is observed for any of the parameters; instead we meet with large fluctuations and no consistent shift in the parameter mean values. Two morphotypes could be separated, a thick and a thin one, but small numbers of intermediate individuals are present in many of the samples, and a group of samples in the lowermost Pliocene contains only intermediate morphotypes. The separate groups of thick and thin uvigerinids show no sustained changes either, but a pattern of statistically significant fluctuations without a perceivable net change. In our opinion the morphotypes are ecophenotypes of a single species. The thick Uvigerina proliferates in laminated sediments in the Upper Miocene. These sediments are probably deposited under nutrientrich and oxygen-minimum conditions in stagnant bottom waters in (semi-) isolated basins. The thin types are never numerous; they are the normal marine forms, which tolerate low nutrient levels. Both types are indifferent to high salinities. In the Pliocene we only find the thin type in our lowermost samples. Higher in the sections these uvigerinids change fluctuatingly, but gradually into homeomorphs of the Miocene thick type, at about the level where the sediments change as well. Open marine marls pass gradually into an alternation of grey and brown clays. The latter clays were probably deposited under oxygen-mmlmum conditions, comparable to those of the Upper Miocene laminated marls, and the thick type once again becomes very numerous. Still higher in the Pliocene sections the thick uvigerinids are replaced by thin ones, but it is not understood in which way this replacement took place. Whether there was a direct descendance in situ or a sudden immigration of the thin uvigerinids from other parts of the Mediterranean cannot be decided. Morphologically the thin uvigerinids in the highest Pliocene samples are not different from those in the lowest Miocene sections. We have no good explanation for the large, statistically significant, fluctuations in the mean values of the parameters in the separate groups. An environmental control is likely. However, a contribution from random processes cannot be precluded; the staggered course of the development might be the result of a random walk of the succession of many asexual generations without correction by sufficient sexual interludes. If the changes in the morphology of Uvigerina are primarily dependent upon the environment, they can be used for a facies correlation, which will have a limited value within separate basins only. Uvigerinids cannot be used for a time-bound zonation over larger distances. Taxonomically we consider our thick and thin uvigerinids as subspecies of Uvigerina cylindrica (d'Orbigny). The thin type is named Uvigerina cylindrica cylindrica (d'Orbigny), the thick one Uvigerina cylindrica gaudryinoides Lipparini, and intermediate assemblages are given a hyphenated notation.