Paleobiogeographical zonation of the Northern Eurasian seas in
Kimmeridgian using ostracods
Tesakova, E.M. *, Rogov, M.A. **
*Moscow State University, e-mail: tesakovaa@yandex.ru
**Geological Institute RAS
Introduction. Upper Jurassic ostracods have been often used for
paleobiogeographical zonation of the Western European seas. In the present work for
the first time all information about distribution of Kimmeridgian ostracods in
Northern Eurasia (Western and Eastern Europe and Western Siberia) was
summarized. Detailed scrutiny of ostracods allowed us to distinguish different zones;
biochores were compared with those established according to the analogous
investigation of ammonites. The paleogeographical evolution of the Western
European basin was reconstructed from the Early to Late Kimmeridgian time.
Materials and methods. More than 50 publications were analyzed, as a result a table
with information on the distribution of 350 Early and Late Kimmeridigian ostracod
species in 21 regions was made including comparison with the data from North
America (Nova Scotia). The data on the distribution of 70 genera were then plotted on
the paleogeographical scheme of Northern Eurasia separately for the Early and Late
Kimmeridgian times. For this purpose the lists of ostracods were used. The revision of
generic names was made only for the old Russian publications.
Results. In general, variations of generic diversity in the Kimmeridgian ostracods
allowed us to reveal 6 different sublatitudinal biochores (regions), each characterized
by a certain taxonomic composition. The first, westernmost region is located in Nova
Scotia (13 genera). The second is situated in Western Europe (45 genera).
Assemblage of the Southern Sweden characterizes a single subregion (10 genera). The
third region occupies the Dnieper-Don depression, northwestern Donbass, Belgorod
and Kursk regions (18 genera). Strong reduction in taxonomic diversity could be due
to the diminishing Tethys water inflow and increasing Arctic water influence. It is
noteworthy, that the genus Cytherelloidea indicative of tropical shallow-water
environments was not found eastward from Poland. Further to the east bottom water
temperatures decreased and in the fourth region, located in Povolzh’ye, only 9 genera
were identified, all cold-water or eurythermal ones. The fifth region was established
in the Timan-Pechora area (only 6 genera); the influence of the Tethys water was
minimal there. The sixth region was located in Western Siberia (6 genera). Rare
findings of ostarcods were also recorded in the Kimmeridgian beds of the Kostroma
region, Obshii Syrt region and the Crimea. In the first case, low ostracod abundance
can be explained by insufficient investigation, in the second case, by active arctic
bottom currents, and finally, in the third case, by facial sediment composition
unfavourable for valve preservation. Borders between biochores are similar to those
established on ammonites. Ostracods from the first region and Crimea correspond to
the super-region Tethys-Panthalassa; all the rest correspond to the Panboreal superregion. In the latter, within Boreal-Atlantic region, two biochores are very distinct
according to the ostracod data: the second region corresponds to Western European
province; regions 3, 4 and 5 correspond to the Eastern European province. Region 6 is
allied to the Artic region. Analysis of the generic diversity during different epochs
showed that the most unfavorable environments (decrease in the number of genera)
for ostracods occurred in the Late Kimmeridgian in the central part of the Western
European Sea, while on its margins (Denmark, Holland, southwestern France) the
situation was opposite. Only two brackish-water genera were recorded in the Early
Kimmeridgian of Germany and Poland. In the Late Kimmeridgian time, the number
of genera reached four and their habitat area extended to Holland and southwestern
France, probably due to the shallowing of the central part of the sea. It was also
revealed that the genus Cytherelloidea, which was abundant all over Western Europe
in the Early Kimmiridgian, was restricted to the southwestern France in the Late
Kimmeridgian. Consequently, the shallowing led to the abrupt bottom water cooling
in the central part of the sea and warming in its marginal zones. It allows us to
suppose that cold waters spread over the surface and warm currents moved along the
bottom. The shallowing of certain parts of the sea did not affect distribution of cold
waters, but rather prevented the circulation of warm currents and changed their
directions.
Keywords. Paleobiogeography, paleocurrents, Northern Eurasia, Kimmeridgian.