Case study on Hangenberg Crisis equivalent deposits and associated conodont faunas including Siphonodella progenitors from late Devonian island arc settings (Indert Formation, Shine Jinst region, southern Mongolia)

Abstract

The Late Devonian-early Carboniferous aged Indert Formation near Shine Jinst (southern Mongolia) is subdivided into the Heermorit and Shombon members. A systematic collection of the few marine carbonate-rich levels from the Heermorit Member resulted in a low-diverse conodont assemblage indicating the late Famennian Bispathodus ultimus conodont Biozone at the Tsagaankhaalga-2 section. Supplemental conodont data from the Yamaan Us section support early Tournaisian age for the lowermost limestone beds of the above deposited Shombon Member. In total, twelve species in seven genera are identified and include Apatognathus, Mehlina, Pandorinellina, Polygnathus (including the new taxon Polygnathus communis tomurtogooi n. ssp.), Pseudopolygnathus, transitional forms of Polygnathus-Siphonodella and Siphonodella. The significance and palaeogeographic distribution of the markedly increased occurrence of intergeneric transitional forms before and during the Late Devonian Hangenberg Event and regionally lacking evidence of the end-Famennian Proto-gnathodus kockeli conodont Biozone and associated early protognathodid assemblage below the Devonian/ Carboniferous boundary are discussed.