Abstract

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.

中文翻译：

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欧洲北部俄罗斯伊兹马河流域的中侏罗世角吻龙和无脊椎动物

摘要

首次从Timan-Pechora地区的Bajocian‒Bathonian河床描述了地层上重要的Retroceramus属的大量双壳类动物，沿伊兹马河及其右支流Dreshchanka河露出来。总共发现了7个分类单元，其中4个在物种级别上被识别（其中2个是新分类单元），另外3个是在公开术语中。最早的后角ami汇合发生在晚巴约期带状mon石指数Arctocephalites arcticus的发生以下，在缺少炸药的地层中。该组合允许将该部分的这一部分与下（？）Bajocian相关。这样的假设是基于两个retroceramid种类的标识，先前从俄罗斯only-亚洲部的下巴柔报道R.  （R.）细长信贷基金，1960年和R.（R.）参考 1962年，隆格斯豪森·科什（Lunnershauseni Kosh）。无am石层上部含有一个新物种R.（R.）alliensis Zakharov，sp.。十一月，以及一个新的特殊的工会联合身份Mezounio zverkovi扎哈罗夫（Zakharov）等。十一月。有时会聚积。上覆的上巴霍亚人Arctocephalites arcticus mon虫带产生了一些R.（R.）startensis Polub。，1992，R.（R.）elongatus Kosh。，1960和R.（？）inopinatus Zakharov，sp。的标本。十一月 继承其特征不在于菊和大概的上述部分对应于巴柔/巴通边界区间包含R.  （R.）比照 eximius（von Eichw。）。此外，还有一个R.（R.）sp。indet。在下巴东地区的Arcticoceras ishmae区被发现。