Serpukhovian Occurrence of a Bashkirian Conodont, Idiognathoides Asiaticus

The occurrence of various species of Declinognathodus (D. bernesgae, D. praenoduliferus and D. tuberculosus) in the late Serpukhovian (Mississippian) of the Urals, Belgium Dinant-Namur Basin, Cantabrian Mountains, and more rarely in the UK and Türkiye, was proposed in 2025 by Sanz-López et alii, confirming the earlier appearance of this typically Bashkirian genus. These species were considered pre-adapted to the mid-Carboniferous biocrisis, because they diversified during the early Bashkirian. In this study, the occurrence of the three species is confirmed in the middle part of the Millaró Member (Alba Formation) at the La Braña section (Cantabrian Mountains, Spain). They occur just above the level of radiometric date 326.36 ± 0.13 Ma obtained from an ash horizon; furthermore, beds below and and above have yielded late Serpukhovian ammonoids. While abundant Gnathodus and Lochriea specimens are currently under study, the Declinognathodus specimens occur in beds correlated with the Serpukhovian G. postbilineatusZone. A novelty of the La Braña section is the presence of rare specimens showing a short row of nodes ajacent to the ventral part of the caudal parapet, resembling an incipient lobe. These fall within the range of variation of D. intermedius. Previously, this species was only known from the D. noduliferus sensu lato Zone and the I. sinuatus zones of the early Baskhirian in the Naqing section (China). Most significant is the rare occurrence of small Idiognathoides asiaticus specimens in higherbeds, where Declinognathodus is absent. It represents the earliest record of an Idiognathoides species within the Serpukhovian G. postbilineatus Zone. Consequently, this genus originated prior to the mid-Carboniferous biotic crisis but dispersed globally later than the diversification of Declinognathodus in the earliest Bashkirian. Idiognathoides migrated between basins from the middle Chokierian to the basal Kinderscoutian regional substages.