Abstract
In this paper the stratigraphy of the Neogene deposits in the Khania Province,
Crete, Greece, is described. Special attention is paid to the evolution and taxonomy
of foraminiferal genera assigned previously to the family Planorbulinidae.
This partial revision of the Planorbulinidae is based not only on our Cretan
material, but also on samples from
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all over the world, in age ranging from Late
Cretaceous to Recent. It is shown that at least part of the genera assigned to the
Planorbulinidae probably have no relationship with Planorbulina, the nominal
genus.
The following taxa are discussed:
Hellenocyclina REICHEL (Upper Cretaceous), with the speCIes H. beotica
REICHEL, H. visserae (HOFKER) and H. charentensis n.sp.;
Linderina SCHLUMBERGER(Middle Eocene, Upper Eocene), with the species
L. brugesi SCHLUMBERGER,L. buranensis NUTTALL & BRIGHTON (= L. bihilensis
SILVESTRI = L. nuttalli SILVESTRI), L. rajasthanensis SINGH (= L. kirtharensis
SINGH = L. bikanerensis SINGH = L. kolayatensis SINGH), L. paronai OSIMO
(? = L. floridensis COLE);
Planolinderina n.gen. (Upper Oligocene - Lower Miocene), with the species
P. escornebovensis n.sp., P. plana (HERON-ALLEN & EARLAND) and P. inaequilateralis
(HERON-ALLEN & EARLAND);
Planorbulinella CUSHMAN (Lower? Miocene - Recent), with the species P.
trinitatensis (NUTTALL), P. zelandica FINLAY, P. larvata (PARKER & JONES),
P. rokae n.sp., P. astriki n.sp., and P. caneae n.sp.
The H ellenocyclina species probably belong to a single evolutionary lineage.
The data on the age of our Linderina populations are rather controversial if a
single lineage is assumed. The species placed in Planolinderina probably belong
to two, more or less synchronously evolving, lineages. In the case of Planorbulinella
it is assumed that there are three lineages, two of them being more or less
parallel.
All of these lineages developed according to the principle of nepionic acceleration.
Their evolution is shown on the basis of measurements and counts on the
early chambers. The two most important parameters are the length of the early
spiral and the size of the initial chambers.
However, both parameters show a negative correlation, and as a consequence - if this correlation is very strong - random influences on the size of the early
chambers will cause changes in the length of the spiral as well. Such influences
may cause nepionic acceleration to become less distinct and less useful for stratigraphic
correlation. This becomes most apparent in Planolinderina.
In Planorbulinella and Hellenocyclina nepionic development is furthermore
complicated by the presence of recurrences of the one-aperture stage after orbitoidal
growth had started.
The results of these studies indicate that the family Planorbulinidae, as
generally accepted nowadays, may comprise several completely unrelated genera.
For the present it seems more correct to place each genus mentioned in a separate
family. Hence, the following new families are proposed: Hellenocyclinidae, Linderinidae,
Planolinderinidae, and Planorbulinellidae.
Apart from these "planorbulinellid" genera, attention is paid to the evolution
of Heterostegina in the Neogene of Crete, SW France and Italy. Biometrical
methods applied to features thought to change in the course of time do not
yield satisfa:ctory results. Evolutionary trends advanced by PAPP & KUPPER
(1954) and HOTTINGER(1966) are analyzed and partly rejected. The evolutionary
pattern, though understood, does not allow a broad application of the
genus to biostratigraphic correlations. Consequently, it cannot be used for an
age determination of the Cretan Neogene deposits.
On the basis of the evolution of Planorbulinella it is shown that the marine
sedimentation on Crete started during Tortonian time. Although not studied in
detail and not described in this paper, the planktonic foraminifera indicate that
a continuous marine sedimentation has gone on up into the Pliocene.
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