A mixed isograptid-didymograptid graptolite assemblage from the Middle Ordovician of west Gondwana (NW Bolivia): Implications for graptolite paleoecology

Abstract

Black shales of the Coroico Formation are part of a thick succession of Lower and Middle Ordovician strata that were deposited in the Cordillera Oriental foreland basin along the margin of West Gondwana. The basin was inhabited primarily by a cool-water, Atlantic-type graptolite fauna. Newly discovered material from rocks that crop out near the town of Consata in NW Bolivia include unexpected warm-water or Pacific-type elements such as Parisograptus caduceus and Pseudotrigonograptus within an assemblage dominated by abundant pendent Didymograptus specimens, Cryptograptus schaeferi, and diplograptaceans such as Oelandograptus oelandicus and Hustedograptus bulmani n. sp. Parisograptus caduceus has generally been considered to be restricted to oceanic depths below those of the epipalagic realm that occupied continental shelves, whereas the remainder of the assemblage is more characteristic of relatively shallow water, epicratonic sites. We interpret this mixed assemblage to be the Atlantic Province, West Gondwanan equivalent of the off-shore isograptid biofacies that is much more widely known from low latitude sites around the globe. These results suggest that in this region of West Gondwana, the properties of local water masses (productivity and physical features such as temperature, salinity, or oxygenation) strongly influenced graptolite species distribution and led to biofacies differentiation among coeval assemblages. It also suggests that some isograptids inhabited the epipelagic biotope in mid to high latitude regions. The new species Hustedograptus bulmani is described herein.