Plant assemblages from the Silurian of southern Bolivia and their palaeogeographic significance

Abundant but fragmentary plant fossils are described from two locations in shallow water marine facies of the Lipeón (previously Kirusilla) Formation of southern Bolivia. Field relationships and limited palaeontological data suggest that the rocks are of Ludlow to possibly early P'ídolí age (i.e. late Silurian). The majority of the fossils are sterile coalified compressions or impressions of parallel-sided axes, some with branching typical of Hostinella . No tracheids have been found and such remains are best described as rhyniophytoid. Fragments with irregular branching and variable axial diameters probably belong to algae with some similarities to Hungerfordia and Buthotrephis . Rarely axes terminate in clearly delimited globular or elliptical swellings that are interpreted as sporangia, although no spores have been recorded. The most completely preserved specimens have dichotomous branching ending in predominantly elliptical sporangia with distal borders and closely resemble Cooksonia ca-ledonica . Solitary isolated sporangia are vertically elliptical (cf. Tarrantia ), globose (cf. C. cambrensis, C. hemi-sphaerica ) or laterally extended (cf. C. pertoni ). Those with cup- or funnel-shaped morphologies superficially resemble the rhyniophytoid Steganotheca or dyad-containing Culullitheca . Thus while it is impossible to compare with confidence the taxonomic composition of Bolivian assemblages with coeval ones, their overall morphological grade is closer to material collected from circum-northern Atlantic localities than from assemblages in Australia and Kazakhsta/nChina. Palaeogeographically this translates into floristic similarities between Gondwanan high latitudes and equatorial Laurussia rather than with low latitude, north-eastern Gondwana or with a low latitude Kazakhstan/ Xinjiang micro-palaeocontinent