Brachiopod faunal exchange through an epioceanic-epicontinental transitional area from the Early Jurassic South-Iberian platform system

Abstract

The La Mola region (eastern External Betic Zone) can be regarded as one of the easternmost complete Jurassic successions of the Betic Cordillera in the Iberian Peninsula, but the paleogeographical setting of their outcrops remains widely discussed. Analysis of brachiopod assemblages from the Lower Jurassic improves the accuracy of previous paleogeographical data, enabling dentification of a mainly epioceanic transitional area in which influences of epicontinental habitats are also detected. Assemblage 1, mainly with a Mediterranean affinity but also sharing several constituents with the African and Northeastern Iberian basins, typifies the Sinemurian-Pliensbachian transition. Assemblage 2, as a whole, shows a transitional character between epioceanic and epicontinental habitats; it is subdivided into two successive and interrelated sub-assemblages: Ass. 2a (Demonense-Tenuicostatum Zones) reveals a free connection with the epioceanic Subbetic area, whereas Ass. 2b (uppermost Pliensbachian-lower Toarcian) shows a closer relationship with epicontinental environments. Assemblage 3 (uppermost Spinatum-basal Serpentinum Zones) is commonly recorded in the peri-Iberian epicontinental platform system integrated within the NWEuropean bioprovince, but it can also be regarded as a marginal assemblage that is widespread in the westernmost Tethyan margin prior to the Early Toarcian extinction event. Assemblage 2 constitutes a suitable index for assessing the paleobiogeographical affinity of the La Mola region, as Ass. 2a is progressively replaced by Ass. 2b, thus triggering the arrival of epicontinental taxa to the more intra-epioceanic Subbetic environments, inferring a possible connection through the La Mola transitional slope. Consequently, this region enabled a faunal mixing and exchange between both environments, and La Mola likely remained as an area that would facilitate migration and an effective dispersal seaway or, at least, did not constitute an ecological filter-barrier for brachiopods. Biostratigraphical data from brachiopods and ammonites are correlated for the first time in La Mola,refining and calibrating biochronostratigraphical gaps in the pre-Domerian deposits where biochronological markers are usually scarce, and around the Pliensbachian-Toarcian boundary, a crucial timespan in which the Early Toarcian extinction event took place.