Miliammina Gerochi n.sp. - A middle Jurassic rzehakinid (Foraminiferida) from quasi-anaerobic biofacies

Abstract

Miliammina gerochi n.sp. - środkowojurajska otwornica (nadrodzina Rzehakinacea) z biofacji quasi-anaerobowej Miliammina gerochi n.sp. is so far the oldest reported representative of the superfamily Rzehakinacea from Jurassic sediments. Its occurrence is confined to the lower part of Harcygrund Shale Formation in the Pieniny Klippen Belt (Western Carpathians, Poland). This formation is represented by a “black shale” facies developed during the late phase of the Aalenian/Bajocian “regional anoxic/dysoxic event”. Overall biofacies relationships indicate that M. gerochi n.sp., as a benthic agglutinated foraminifer, inhabited normal saline, middle to outer neritic palaeoenvironments under extremely dysoxic or even suboxic, food-rich conditions, and gradually disappeared with improving bottom water oxygenation, enhanced density of macrofaunal bioturbation, increasing sedimentation rate, and probably deepening of the basin. The Miliammina gerochi Assembage marks a very productive phase in the Klippen Basin history. Based on functional morphology and studies of modern Miliammina species, it can be speculated that this Jurassic species preferred an endobenthic microhabitat and probably thrived on/within benthic bacterial mats associated with suboxic conditions. Comparison between different habitats successfully colonized by rzehakinids suggests that since the Jurassic till the present, this group preferred stressed environments rich in food. Evolutionary relationships remain uncertain due to the lack of reported ancestors.