Middle Jurassic Retroceramids and Unionids from the Izhma River Basin, North of European Russia

An abundant collection of bivalves, belonging to the stratigraphically important genus Retroceramus, has been described for the first time from the Bajocian‒Bathonian beds of the Timan-Pechora region, exposing along the Izhma River and its right tributary, the Dreshchanka River. Totally 7 taxa were found, among which 4 were identified to a species level (and 2 of them are new taxa), and other 3—in the open nomenclature. The earliest retroceramid assemblage occurs below the occurences of the Late Bajocian zonal ammonite index, Arctocephalites arcticus , in the strata lacking ammonites. The assemblage allowed to correlate this part of the section to the Lower (?) Bajocian. Such an assumption is based on the identifications of two retroceramid species, previously reported from the Lower Bajocian of the Asian part of Russia only— R. ( R. ) elongatus Kosh., 1960 and R. ( R. ) cf. lungershauseni Kosh., 1962. In its upper part, the ammonite-free interval contains a new species R. ( R. ) alliensis Zakharov, sp. nov., as well as a new peculiar unionid Mezounio zverkovi Zakharov, gen. et sp. nov., sometimes met as accumulations. The overlying Upper Bajocian Arctocephalites arcticus ammonite Zone yielded a few specimens of R. ( R. ) startensis Polub., 1992, R. ( R. ) elongatus Kosh., 1960 and R. (?) inopinatus Zakharov, sp. nov. The above part of the succession which is not characterized by ammonites and probably corresponds to the Bajocian/Bathonian boundary interval contains R. ( R. ) cf. eximius (von Eichw.). Additionally, a single specimen of R. ( R. ) sp. indet. was found in the Lower Bathonian Arcticoceras ishmae Zone.