A NEW GENUS OF TIRONUCULIDAE (BIVALVIA, P ALAEOTAXODONTA) FROM THE ARENIG OF NORTHWESTERN ARGENTINA

The Early-Middle Arenigian Acoite Formation of Northwestern Argentina contains richly fossiliferous beds composed mainly by bivalves belongi ng to the Family Tironuculidae. On the basis of the hinge morphology t he new genus Natasia is proposed. The abundance of specimens permit th orough analysis of their internal and external features and their rang e of ontogenetic variations. This analysis shows that in the early sta ges the hinge is composed by taxodont, chevron-like teeth, changing in the adult to an actinodont type. Likewise, the shell outline changes from trigonal to nearly rectangular shape.