Upper Ordovician (Sandbian) gastropods from redeposited boulders in the Don Braulio Formation, Argentine Precordillera

Abstract

The glacigenic boulders from the Hirnantian Don Braulio Formation of the Argentine Precordillera contain a well preserved shelly fauna allowing to a better knowledge of brachiopods and bivalves, but gastropods remain undescribed. The purpose of this paper is to give a preliminary list of gastropods and to evaluate their paleobiogeographic affinities. The following taxa have been recognized: Tropidodiscus sp., Raphistoma sp., Clahtrospira aff. C. subconica Hall and Tetranota bidorsata Hall. Other contemporary coarse-grained boulders have yielded Bucania sp., Cyclonema sp., Clathrospira sp., Trochonema? sp., bellerophontids, platyceratids, Hormotoma-like forms, murchisonids and lophospirids indet. This material is not well preserved enough for accurate identifications. The genus Tropidodiscus Meek and Worthen has been proposed as a potential paleobiogeographical marker for the Mediterranean Province, and consequently for cold water environments. Tetranota bidorsata Hall and Clathrospira subconica Hall are common taxa in the Upper Ordovician Trenton Limestone. Such a mixture of faunas is consistent with the paleogeographic pattern of rhynchonelliform brachiopods, which display Mediterranean, Anglo-Baltic and Laurentian affinities. The studied gastropod assemblage includes mainly widespread taxa, lacking the marked endemism showed by coeval bivalves.