The first record ofCooksoniafrom South Americain Silurian rocks of Bolivia

Abstract

Plant megafossils resembling Cooksonia caledonica Lang are described from the Kirusillas Formation in southern Bolivia. Faunal and lithological evidence together with field relationships advocate a late Silurian (Ludlow) age. Palaeocontinental reconstructions suggest that the plants grew on the western edge of Gondwana at 50-60° S and indicate that Cooksonia had spread to higher latitudes soon after its appearance in the Wenlock.Branching is isotomous. Between bifurcations the naked axes are parallel-sided when complete and range between 0.5 mm in diameter proximally and 0.3 mm distally. There is a gradual widening below a sporangium such that the limits of sporangium and axis are difficult to determine (Fig. 2b). Sporangial height is thus estimated at c. 1.3 mm. Two further sporangia and axes lie alongside. All sporangia are 1.5 mm wide and possess a narrow, but well-defined border (0.12 mm maximum width) around the convex margin. Spores were not recovered.

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

“…Such a conclusion has major phytogeographic implications indicating the presence of a possible Lochkovian Laurussian species at high latitudes (c. 60°C) in the uppermost Silurian of Gondwana (Morel et al, 1995) (Fig. 81).…”

“…Olive-green, finely micaceous rusty weathering silty mudstones, siltstones and fine sandstones dip consistently to the south at 70"-80". This is the sequence from where Petriella & Suarez Soruco (1989) first reported plants, and Morel et al (1995) identified the fertile specimen of Cooksonia.…”

“…Geological data for sequences in southern Bolivia. For general geology see Ahlfeld and Branisa (1960), Suarez Soruco (1992) Morel et al, 1995;Iiiiguez Rodriguez & Zalba, 1997;Acefiolaza et al, 1996: 49;Bassett et al, 1996;Bassett, 1997). However, work in progress by M. G. Bassett and D. Poire (pers.…”

Devonian plants from Colombia, with discussion of their geological and palaeogeographical context

“…The most comprehensive reports have been from the Middle and Upper Devonian of western Venezuela (Berry, Edwards and co-authors) and the Silurian/Lower Devonian of Bolivia/Argentina (Morel, Edwards & Iñigez Rodriguez, 1995). This scarcity of fossil evidence has frustrated attempts at global phytogeography during these times (Edwards & Berry, 1991).…”

Silurian (Wenlock–ludlow) Graptolites from Bolivia

“…These were ®rst noted by Petriella and Sua Ârez Soruco (1989). Morel et al (1995) described the ®rst South American record of Cooksonia from the Kirusillas Formation of Negra Muerte near Tarija. Possible algal or other plant remains occur together with the graptolites at Cerro Sikimirani Jatun (locality 4 herein), which have not been recognised previously.…”