Taphonomy and palaeoecology in the upper Barremian of the SW Iberian chain (Spain): A model to compare taxonomy and diversity of biotas from different coeval basins
Résumé
A detailed study of the floral and faunal diversity of an upper Barremian lentic ecosystem was performed by examining 50,000 microfossils. The study provides an assemblage of 41 species of charophytes, spermatophytes, ostracods, molluscs, and vertebrates (including eggshells). The main goal of this study is to reconstruct the palaeoecology of a fluvio-lacustrine and associated carbonate depositional system, by integrating the sedimentology, taphonomy, and diversity from twelve horizons cropping out in the Buenache de la Sierra and the Las Hoyas basins. The studied area corresponds to the lowermost succession of the Tragacete Formation (Serrania de Cuenca, Iberian chain). Four facies associations (polymictic alluvial; carbonate palustrine and lacustrine; eutrophic lacustrine; and calcarenitic drainage facies) were correlated to the composition of the fossil assemblages, which have yielded a set of taphofacies confirmed by the non-metric multidimensional scaling ordination test of all the horizons. The taphofacies are: (alpha) dominated by bones and plant leaves, (beta) by charophytes-ostracods-bivalves and eggshells, and (gamma) by coal-leaves and bones. The Tragacete depositional system consists of demic and mostly autochthonous (non-catastrophic) fossil associations. A mixture of aquatic and terrestrial plants, freshwater molluscs and ostracods coupled with aquatic and terrestrial vertebrate species fosters a biodiversity typical of depositional environments that are congruent with modern floodplain wetland systems. These floodplain wetlands are characterised by a combination of periodically inundated ponds, small lakes, and floodplain areas with terrestrial habitats. As such, they provide suitable environments for the diversification and colonisation of charophytes, ostracods, gastropods, fish, and an ample herpetofauna. The abundance and diversity of eggshells also suggest the development of nesting areas, whereas the occurrence of charred cheirolepidiacean conifer would indicate intermittent wild fires linked to drought events. The taxonomic breadth of this ecosystem is characterised by the taxonomic distinctness index, which, in turn, becomes a referential model to which other Lower Cretaceous microfossil sites may be compared.