Ordovician cephalopod evolution in the Central Andean Basin

Abstract

Although long known from the Ordovician of the Central Andean Basin (CAB) in NW Argentina (NWA), cephalopods are relatively rare and hitherto, poorly understood. Recent and ongoing investigations are gradually increasing our knowledge of the diversity and palaeobiogeographical affinities of these faunas as more material is discovered. This is a preliminary report on the changing diversity and affinities of cephalopod faunas from the Ordovician of the Central Andean Basin. Currently, the oldest cephalopods belong to small and abundant Ellesmeroceras, accompanied by a few specimens of probable Bassleroceras in the Tremadocian (Tr1) of NWA. Cephalopod occurrences of this age are restricted to the paleotropical belt and consist predominantly of ellesmeroceratids. During the middle Tremadocian (Tr2), there was a marked increase in cephalopod diversity. One conspicuous element is the cyrtocerinid Saloceras, whose affinities are peri-Gondwanan, especially with Avalonia. Unpublished material from the middle Tremadocian includes rioceratids and isolated endosiphuncles probably of proterocameroceratids and/or dissidoceratids. Endoceratids were reported from the upper Tremadocian (Tr3) of Parcha. During the Floian, the cyrtocerinids become very important, with the origin of new taxa, and are accompanied by the proterocameroceratid Protocyptendoceras fuenzalidae, which together belong to the Peri-Gondwana Realm. Other taxa that need revision were also reported, as Robsonoceras, Purmamarcoceras, and Clarkoceras. In addition, unpublished protocycloceratids, endoceratids and rioceratids increase the cephalopod diversity of the Floian of NWA. No cephalopods are currently known from the Dapingian of the CAB in Argentina, but large endoceratids are present in the Darriwilian Capillas Formation of Jujuy Province. In the same strata, cyclostomiceratids, baltoceratids and protocycloceratids have been reported and also need revision. Further north, in Bolivia, during the Floian, cyrtocerinids endemic to the CAB occur together with rioceratids endemic to peri-Gondwana at that time. In Perú, cyrtocerinids, orthoceratids, and possible endoceratids are present in probable Middle Ordovician sediments. Although erratic, and based on incomplete data, there appears to be a progressive increment of diversity since the middle Tremadocian, with a peak during the Floian (Fl2-3), and another in the Darriwilian. The apparent origination of several cyrtocerinid taxa in the CAB suggests a degree of endemicity, but other taxa appear to comprise elements of a peri-Gondwanan fauna, parts of which may have links with Avalonia and also with Baltica. Future work on recently collected material is expected to expand the number of cephalopod taxa known from the CAB, and further clarify their paleobiogeographic affinities and patterns of diversity.