Protobranch bivalves from the Piauí Formation (Middle Pennsylvanian), Parnaíba Basin, Brazil

Abstract

Dolomitic limestones of the upper Piauí Formation of the Parnaíba Basin, Northern Brazil, preserve a rich and diversified invertebrate fauna of Middle Pennsylvanian age. The fauna shows affinities with the Pennsylvanian Itaituba Formation of the Amazon Basin, Brazil, and with Upper Paleozoic Andean faunas of the Cerro Prieto Formation and Tarma Group of Peru, and the Caño Indio Formation of Venezuela. In North America similar faunas are present in the Mississippian-Pennsylvanian Amsden Formation and in the Desmoinesian of Missouri. The Piauí fauna thus shows a clear "Tethyan" (boreal) affinity in contrast with other Late Paleozoic assemblages from southern South America. Bivalves comprise the majority of the fauna (60%, 31 species), followed by gastropods (17%, 9 species) and brachiopods (17%, 9 species). Other invertebrates including a cephalopod, a trilobite and a bryozoan make up the remainder of the fauna (6%, 3 species). Three species of protobranch bivalves, Polidevcia bellistriata, Nuculavus levatiformis and Solemya (Janeia) radiata are described here in. Obesity and elongation indexes as well as the external ornamentation and muscle scars of the nuculoid species here recovered are consistent with their interpretation as rapid (P. bellistriata) and slow, shallowly burrowing (N. levatiformis) deposit feeders. S. (Janeia) radiata, a slow shallow burrower, deposit feeder, is preserved in life position. This is the first of three publications that aim to ameliorate the present lack of paleontologic and paleobiogeographic knowledge on the Carboniferous of northern South America.