Unusual trilobite biofacies from the Lower Ordovician of the Argentine Cordillera Oriental: New insights into olenid palaeoecology

Abstract

The study of biofacies has proven to be relevant in the understanding of trilobite palaeoecology, palaeobiogeography and macroevolution. The widespread Olenid biofacies is one of the best known, and is usually interpreted as occuring in dysoxic environments. Tremadocian successions of the Argentinian Cordillera Oriental bear a diverse and long-studied olenid-dominated fauna. Based on cluster analysis, five distinct biofacies are defined for the middle Tremadocian (Tr2 stage slice), distributed from shelf (below storm wave base) to lower-shoreface settings (above fair-weather wave base). Ordination shows biofacies along two gradients, a bathymetrical one and another related to oxygen content. All of them are dominated both taxonomically and ecologically by olenids. This detailed quantitative palaeoecological study challenges current views suggesting instead that the Olenidae dominated a broad range of environments, from oxygenated shallow-marine to dysoxic deep-marine. Comparisons with largely coeval trilobite records from geodynamically and palaeobiogeographically disparate sites suggest that siliciclastic sedimentation appears as the most influential controlling environmental factor upon olenid distribution and dominance. Further comparisons across different climatic belts show that siliciclastic input controlled trilobite diversity gradients, even more than latitude. From an autoecological viewpoint distribution of traditional olenid morphotypes shows no relation to depth or to oxygen content, and at least some members of the group appear to have had the possibility of coping with low oxygen content, rather than being restricted to oxygen-deficient environments. The analysis performed herein, together with recent research on the group, demonstrate that factors controlling olenid distribution are more complex than previously envisaged. □Biofacies, diversity, Olenidae, palaeoecology, Tremadocian, trilobite.