Enigmatic fossils from the Upper Silurian of Bolivia: evidence for marine productivity in high-latitude Gondwana

Abstract

Specimens collected recently from the Las Carmelitas facies of the Kirusillas Formation exposed in the Cochabamba area, Bolivia and dated as Ludlow (Late Silurian) by graptolites, show bedding surfaces almost completely covered by coalified compressions and impressions. The majority comprise mixtures of fragmented amorphous, unidentifiable material (‘debris’), but some horizons are dominated by well-defined morphological entities with a particular type or combination of types confined to a particular layer. In all, five morphotypes have been circumscribed, but their affinities, based on a comprehensive review of the possibilities, remain conjectural. Algal affinities are suggested tentatively for abundant, unbranched, narrow, strap-shaped fossils lacking any further diagnostic features and the numerous coalified discs or three-dimensionally preserved spheres (?leiosphaerid acritarchs) occurring isolated in the matrix or, less often, in clusters. Following a discussion on the recognition of faecal pellets in the fossil record, it is postulated that elliptical, spiral or simply segmented structures may be the excreta of planktonic or benthic metazoans, although the latter seems less likely in view of the proposed anoxic depositional environment of the shales that lack any bioturbation. Such coalified remains, whether from primary producers or grazers, indicate high productivity at high palaeolatitudes in the late Silurian continental seas in this region of peri-Gondwana.