Early Ordovician foraminifers from the Lava River Section, northwestern Russia

摘要

Recent studies on the biostratigraphy and fauna of the Lower Ordovician successions of the Russian part of Baltoscandia have shown for the first time that agglutinated foraminifers are very numerous in the deposits of the lower part of the Latorp Russian Regional Stage in the vicinity of St. Petersburg, northwestern Russia. These forms were discovered in a classic section exposed on the left side of the Lava River canyon in the northern outskirts of the village of Vassilkovo. The lower part of the Latorp Regional Stage is represented in this section by unlithified sands and clays of the Lakity Member of the basal pan of the Leetse Formation and corresponds to the upper part of the Hunneberg Regional Stage of Baltoscandia. The clay beds of the Lakity Member encompass the upper part of the Paroistodus proteus and lower part of the Prioniodus elegans conodont zones and correspond to the Tetragraptus phyllograptoides graptolite zone. Foraminifers from the Lakity Member were collected from strata belonging to both of the above mentioned conodont zones and are represented by monothalamous agglutinated foraminiferal tests of a new species of a new genus Lakites ordovicus n. gen., n. sp,, new species of the genus Amphitremoida: A. asperella n. sp., A. laevis n. sp., A. longa n. sp., A. orbicularis n. sp., and A. rugosa n. sp. (family Hippocrepinellidae), a new genus and species Lavella cucumeriformis n. gen., n. sp. of the family Saccamminidae, and one specimen assigned to the genus Arenosiphon? of the family Hippocrepinidae. The finding of representatives of the genus Amphitremoida (emended herein), previously known only from the Middle Ordovician to the Lower Carboniferous (Tournaisian) permits the extension of the range of this genus down into the Lower Ordovician.