Arab-Madagascan brachiopod dispersal along the North-Gondwana paleomargin towards the Western Tethys Ocean during the Early Toarcian (Jurassic)

Abstract

Multiple approaches, mainly focused on assessing the depositional environments, paleotemperature, chronostratigraphical and paleobiogeographical data, morphometrical analyses and the study of the internal structure of spiriferinide brachiopods assigned to the genus Calyptoria, have revealed that this brachiopod fauna migrated from their Arab-Madagascan homeland along the North-Gondwana paleomargin into the peri-Iberian epicontinental platform system, in the same well-known spreading episode suffered by the Arab-Madagascan Bouleiceras ammonite fauna over the worldwide-distributed early Toarcian platforms. This brachiopod fauna has been so far overlooked as potential Arab-Madagascan immigrant fauna, as it was integrated together with diverse assemblages well-established in the peri-Iberian basins. Dispersal of Calyptoria stock was conditioned by the interplay of several factors, such as the tectonic framework, the development of epicontinental seas on both Tethyan margins, the early Toarcian transgression, the prevailing ocean current pattern, and their limited ability for dispersal. The revised chronostratigraphical framework of this dispersal episode reveals the coincidence with the thermal maximum recorded prior to the Early Toarcian Mass Extinction Event (ETMEE) in the westernmost Tethys Ocean. The concurrence of these factors played a significant role in the Calyptoria spreading, showing an inter-tropical distribution between 0° and 30° in both hemispheres. The taxonomical updating performed in the light of the current systematic trends support this adaptive strategy carried out by Calyptoria species, consisting of a sudden and practically synchronous dispersal without outstanding morphological changes instead of the diverse evolution of different brachiopod taxa in response to the environmental changes related to the ETMEE. Updating and rearranging of the species attributed to the genus Calyptoria suggest a new systematic scheme for several former attributions, supporting that lower Toarcian occurrences of L. undulata from westernmost Tethyan areas belong to Calyptoria, thus pointing towards the dispersal from the Northern peri-Gondwana seas to the westernmost Tethyan epicontinental platforms. Similar adaptive strategies were linked to other extinction events, supporting a possible pattern in the response of certain brachiopod populations to such biotic crises.