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 Lipeon (previously Kirusilla) Formation of southern Bolivia. Field relationships and limited palaeontological data suggest that the rocks are of Ludlow to possibly early Pridoli age (i.e. la te Silurian). The majority of the fossils are sterile coalified compression s or impressions of parallel-sided axes, some with branching typical of Hos tinella. No tracheids have been found and such remains are best described a s rhyniophytoid. Fragments with irregular branching and variable axial diam eters probably belong to algae with some similarities to Hungerfordia and B uthotrephis. Rarely axes terminate in clearly delimited globular or ellipti cal swellings that are interpreted as sporangia, although no spores have be en recorded. The most completely preserved specimens have dichotomous branc hing ending in predominantly elliptical sporangia with distal borders and c losely resemble Cooksonia caledonica. Solitary isolated sporangia are verti cally elliptical (cf. Tarrantia), globose (cf. C. cambrensis, C. hemisphaer ica) or laterally extended (cf. C. pertoni). Those with cup- or funnel shap ed morphologies superficially resemble the rhyniophytoid Steganotheca or dy ad-containing Culullitheca. Thus while it is impossible to compare with con fidence the taxonomic composition of Bolivian assemblages with coeval ones, their overall morphological grade is closer to material collected from cir cum-northern Atlantic localities than from assemblages in Australia and Kaz akhstan/China. Palaeogeographically this translates into floristic similari ties between Gondwanan high latitudes and equatorial Laurussia rather than with low latitude, north-eastern Gondwana or with a low latitude Kazakhstan /Xinjiang micro-palaeocontinent. (C) 2001 The Linnean Society of London.