Ecological control of the Pelsonian colonisation event (Anisian, Triassic) leading to the first settlement of Tetractinella-beds (Brachiopoda) in the Iberian palaeomargins. Palaeobiogeographical and biostratigraphical significance

Abstract

Marine benthic biota recovery after the Permian/Triassic extinction is still a challenging topic in several fossil clades such as brachiopods. In fact, available data from the Early-Middle Triassic are scarce worldwide and their global record derives from limited localities throughout the Tethys Ocean. This work presents the first record of the Anisian Tetractinella-beds (Brachiopoda, Athyridida) in the peri-Iberian epicontinental platforms established in the westernmost Tethys margins. Their utility as a reliable biohorizon marker is emphasised as they provide an effective tool for age calibration and correlation in the middle Anisian (Pelsonian) successions, also supported by the co-occurring fossil assemblage. In the South-Iberian Palaeomargin, Tetractinella-beds occur in an intensively-burrowed shallow-marine succession. The rich ichnological content has been exhaustively evaluated in order to assess the ecological factors controlling the development of the brachiopod fauna. Three ichnoassemblages are documented dominated by Oravaichnium, Balanoglossites and Thalassinoides, respectively. The gathered faunal, ichnological, taphonomical, and sedimentological data reveal an alternation of sea-level changes influencing the composition of the Anisian marine communities. The first stage consists of near-shore restricted environments sporadically disturbed by short-term shallow-marine interludes, characterised by the presence of Oravaichnium and its associated fauna. Subsequently, the Pelsonian transgression event is recorded (MID-2), documented by a firm-ground surface pointed out by the development of a Balanoglossites surface, followed by the highstand, when shallow epicontinental environments developed in connection with open-marine areas. This phase involved the spreading of Tetractinella and deep-water bivalves. Finally, near-shore restricted conditions were re-established typified by Oravaichnium and the last occurrences of Tetractinella, with the disappearance of the open sea-water fauna. The maximum flooding reached after the Pelsonian transgression facilitated new palaeobiogeographical migration routes, allowing for an Alpine-Germanic dispersion of taxa throughout the North-South epicontinental Tethyan seaway as well as the earliest first faunal migration event from the Sephardic Bioprovince northwards to the South-Iberian palaeomargin.