Diachronic dispersal of Declinognathodus and the timing of the mid-Carboniferous marine crisis: Evidence from ammonoid-conodont co-occurrence

The mid-Carboniferous biotic crisis occurred near the boundary between the Mississippian and Pennsylvanian subsystems, evidenced by a significant faunal turnover. This crisis interval is traditionally identified through biostratigraphic zones based on the rapid dispersal of marine ammonoids and conodonts. However, strata from the Millaró section, Cantabrian Mountains, Spain, and the Iset section, Middle Urals, Russia, have yielded coeval associations of the Serpukhovian ammonoid Deleshumardites-Delepinoceras Genozone alongside the conodont Declinognathodus , a genus traditionally regarded as exclusively lower Bashkirian. A Serpukhovian age for the studied Spanish faunas is supported herein by regional biostratigraphy and radiometric dating. In the Middle Urals, Serpukhovian-type ammonoids coexisted with Bashkirian conodonts of the Declinognathodus inaequalis Zone. Declinognathodus species have been reported in the upper Serpukhovian strata of Belgium, England, Spain, and the South Urals. These are interpreted as pre-adapted survivors of the latest Serpukhovian biotic crisis episode. The recovery of deep-water ammonoid and conodont faunas in the early Bashkirian was primarily characterized by the diversification and dispersal of the Homoceratidae and Idiognathodontidae, respectively. Initially, some species exhibited limited geographic expansion. The diversification of the D. noduliferus group and Isohomoceras , specifically the first appearance datum of D. inaequalis , marks a globally recognized interval near the mid-Carboniferous boundary. Subsequently, species displayed diachronic inter-basinal migrations during transgressive events. Mississippian “dead clade walking” taxa characterized a Bashkirian survival interval, occurring in parallel with the dispersal of new taxa during the C2 glacial episode.