Talacastograptus, an unusual biserial graptolite, and other Silurian forms from Argentina and Bolivia

Abstract

A Silurian graptolite fauna that consists of Talacaetograptue leanzai Cuerda, Rickards and Cingolani, Metaclimacograptus? robusius Cuerda et al., and scarce normalograptid remains is described. It was collected in the lower part of the Lipeón Formation, Jujuy Province, República Argentina. Studied sections are in the Zapla and Puesto Viejo ranges, a few kilometres to the east-southeast of San Salvador de Jujuy city, in the Subandean Ranges. The taxa are identified for the first time from the Lipeón Formation, suggesting él correlation with the lower part of the La Chilca Formation, San Juan Precordillera. The graptolites indicate that the base of the Lipeón Formation is of acuminaius biozonal age (early Llandovery), and that the middle and upper part of the lower member is aiauus Biozone age (early Llandovery). Talacastograptus leanzai specimens from the Lipeón Formation show an internal zig-zag list structure described herein for the first time, which led to other workers to classify them as retiolitids. The genus Tolacastograptus, previously recorded in the Precordillera, and here in the Subandean Ranges, seems to be an endemic form of the western Gondwanan border during the early Llandovery. Specimens of Monograptus sp. from the Kirusillas Formation, near Tarabuco, Bolivia, are also described. A middle Ludlow age suggests a correlation with the upper part of the Lipeón Formation, which has proven barren of graptolites at the present