Importance of Foraminifera for Zoogeographical Division of Arctic Seas in Late Jurassic and Early Cretaceous Times: ABSTRACT

Abstract

The discovery of abundant and varied benthonic Foraminifera in marine deposits of the Pechora basin and in western and middle Siberia has made it possible End_Page 2487------------------------------ to distinguish separate water bodies in this region. The following principles and criteria are the bases of constructing zoogeographical divisions into smaller subdivisions or districts: (1) the degree and the rank of endemism; (2) endemism of species; (3) the areal extents of species, genera, and families; and (4) the coefficient of the community (Kc = the ratio of the quantity of species, genera, and families characteristic of a district, to the total number of species, genera, and families). Endemic families and genera are absent. High values of Kc are found at the family level. Thus the very great similarity of the Late Jurassic middle Volzhian fossil complexes of the Pechora basin, and western and middle Siberia (Kc = 0.70-0.77), and the endemism of species (22-48%) indicate that these districts belong to the North Boreal subregion of the Boreal region. Values of Kc, and the areal distribution and endemism of species made it possible to recognize the Pechora-West Ural (Pechora basin, West Siberian basin) and North Siberian (Ust'-Yenisey district, middle Siberia) provinces in middle and early Volzhian times. In late Volzhian time a climatic change took place. The ensuing temperature decrease of the marine water caused a narrowing of the d stribution of almost all species in middle Siberia and an increase in agglutinated Foraminifera (Ammodiscidae, Ataxophragmiidae, Trochamminidae). The basins of the Pechora, western Siberia, and middle Siberia districts were part of the Boreal region, the boundary of which shifted southwestward. In Berriasian (Early Cretaceous) time, the water area of the Soviet Arctic zone was included in the Boreal region with the western and northern Siberia provinces. End_of_Article - Last_Page 2488------------