Barremian rudists of the San Lucas formation around San Lucas, Michoacán, SW México
Por:
Pantoja-Alor J., Skelton P.W., Masse J.-P.
Publicada:
1 ene 2004
Resumen:
Despite recent progress with the description of Lower Aptian rudists of the New World, Barremian examples have remained poorly documented. Here we record the rudist fauna of some rudist- and coral-rich biostromes intercalated in the upper Terrero Prieto Member of the predominantly volcaniclastic San Lucas Formation (Valanginian-Barremian) to the north of San Lucas town, near Huetamo in Michoacán State, SW Mexico. The biostromes are assigned to the Barremian on the basis of their contained microfossils and ammonites in surrounding beds. We describe four rudist taxa, Toucasia sp., Offneria sp., cf. O. simplex CHARTROUSSE, and MASSE, and two new primitive species of canaliculate caprinuloidinids, Amphitriscoelus primaevus (previously recorded. though unnamed, from the probably Barremian Benbow Limestone of Jamaica) and Pantojaloria estanciensis. The last two species are interpreted as the chronospecific founders of the phyletic lineages that gave rise, respectively, to the type species of the two genera in the early Aptian. A. primaevus was apparently derived from the Barremian Retha tulae (Felix), from which it differs only by the possession of a few pallial canals. Whether P. estanciensis was derived in turn from A. primaevus, or independently from R. tulae (with parallel evolution of canals) remains unresolved.