A new record of early Silurian land plant spores from the Paraná Basin, Paraguay (Malvinokaffric Realm)

Abstract

The first conclusive evidence for Silurian rocks in the Paraná Basin, Brazil, was reported in 1985. This evidence was based on organic-walled microfossils, principally spore tetrads, and associated phytoplankton (‘acritarchs’ and prasinophytes), the first recovered from the Vila Maria Formation on the northeastern rim of the Basin. The spore assemblage was typical of Gray's Microfossil Assemblage Zone I; size-frequency data for the tetrads suggested an early Silurian (early Llandovery; Rhuddanian) age. We now document a new MA Zone I occurrence of spore tetrads together with a few single trilete spores, from the southwestern rim of the Paraná Basin, Paraguay. The single spores are among the earliest known to have normally dissociated from a tetrahedral tetrad. Size-frequency data for the tetrads suggest a late Llandovery, pre-C5 (mid-Telychian) age. These spores, together with phytoplankton and chitinozoans, were recovered from beds laterally equivalent to the Vargas Peña Shale at the type locality from which Llandovery diplograptids and monograptids are known. Llandovery age rocks on the southwestern and northeastern borders of the Paraná Basin, c. 1400 km apart, suggest that early Silurian rocks were extensively distributed throughout southern Brazil and adjacent Paraguay. Close conformity between the age reference obtained with graptolites and spore tetrads again demonstrates the stratigraphic utility of early Silurian spores in providing reliable age determinations for otherwise unfossiliferous rocks. Spore tetrad assemblages available from the Malvinokaifric Realm (South America, Africa, Arabia) represent a distinct phytogeographic unit contrasted with spore tetrad assemblages from the North Silurian Realm of North America and parts of Europe.