Une nouvelle espèce de Modiolopsis (Bivalvia) Dans L' Arenig (Ordovicien Inferieur) de Daroca (Aragon, Espagne); réflexions sur la denture des bivalves primitifs

Abstract

A new species of Modiolopsis, M. aragonensis, is described from the middle or upper Armorican Sandstone (Arenig, Lower Ordovician) of the area around Daroca (Aragon, Spain). The study of this edentulous genus, the presence of which is thus confirmed in the Lower Ordovician, presents the opportunity to develop some thoughts on the phyletic relations of the primitive bivalves taking their types of dentition into consideration. The comparison of the recently described genera from the Cambrian and the Lower Ordovician apparently shows a rapid prevalence of various palaeoheterodonts. Consequently it can be suggested that the hypothetical archetype unknown and named proterodont could have been edentulous or with a very primitive dentition of undefined type. From such an ancestor which would be probably found in the lowermost Cambrian (Nemkyt-Daldynian) or in the uppermost Proterozoic (Vendian), the palaeotaxodont and palaeoheterodont types could have been sister groups with a rapid diversification of the latter.