Boreskov

PP‐32STRATIGRAPHIC AND GEOGRAPHICAL DISTRIBUTION OF CEPHALOPODS ATTHE BAJOCIAN‐BATHONIAN BOUNDARY (MIDDLE JURASSIC) IN EUROPEANRUSSIAMitta V.V., Barskov I.S.Borissiak Paleontological Institute of RAS, Moscow, RussiaUntil recently, the only known Bajocian‐Bathonian boundary deposits on the RussianPlatform were in the region of Don‐Medveditsa dislocations of the Volgograd‐Saratov‐Povolzhye region. The cephalopod assemblages established are very similar in taxonomiccomposition to those of the northern (Northern Caucasus) and western (Sub‐Mediterranean)margins of the Tethys Ocean. Deposits of this interval in the Boreal regions (EasternGreenland, North Siberia) contain completely different ammonoid and belemnoid taxa.These differences led to the recognition of parallel biostratigraphic subdivisions (stages andzones) for the Boreal and Tethyan regions (Callomon, 1993; Meledina, 1994). This hassignificantly complicated the study of phylogenetic and ecological evolution andbiogeographical distribution of the cephalopod communities of different basins. Data fromthe city of Saratov on the association and distribution of ammonites of Tethyan(Parkinsoniidae) and Boreal (Arctocephalitinae, Cardioceratidae) origin in the same sectionallow the Boreal and Tethyan scales to be correlated at the zonal level (Mitta & Seltzer,2002; Mitta et al., 2004). The larger, lower portion of the Middle Jurassic series is dated asthe local Pseudocosmoceras michalskii Zone, which is equivalent to the terminal Bajocianzone of the standard (Tethyan) scale and is characterized by exclusively Peri‐Tethyancephalopods. This zone is overlain by the Oraniceras besnosovi Zone, corresponding to thestandard Gonolkites convergens and Gonolkites macrescens subzones of the lowermostBathonian zone, which contains both Tethyan and Boreal taxa (belemnites and supposedlythe first ammonites). The overlying Arcticoceras ishmae Zone is characterized exclusively byArctic cephalopods. No indications of large gaps in sedimentation have been found in thesection. The continuous sedimentary succession established (Besnosovi and Ishmae Zones) ispart of the same cycle of sedimentation, and paleontological evidence allows the correlationof the Ishmae Zone with the upper half of the Lower Bathonian of the standard scale.Studies in the basin of the Pechora River (at the Izma River), allowed the recognition inthis region of European the earliest beds of the Boreal type (Upper Bajocian), characterized190