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
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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.
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