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

EDWARDS, D.; AXE, L; BASSETT, M; POIRÉ, D.G.; MOREL, E.M.; CINGOLANI, C.A.

Paleozoic Developments

Geological Society, London

Lugar: Geological Society, London; Año: 2008; p. 1 - 23

Enigmatic fossils from the Upper Silurian of Bolivia: evidence for abundant marine productivity in high latitude circum polar Gondwana D. Edwardsa, L. Axea, M.G. Bassettb, D.G. Poirec, E.M. Morelc, d, C. A. Cingolanic a  School of Earth, Ocean and Planetary Sciences, Cardiff University, Main Building, Park Place, Cardiff, CF 10 3YE, UK; b  Department of Geology, National Museum of Wales, Cathays Park, Cardiff, CF10 3NP, UK; c  CIG (UNLP-CONICET), calle 1 n. 644, 1900-La Plata, Argentina; d  Com. Invest. Cientificas Prov. Bs. Aires and Departmento Científico de Paleobotánica, Museo de La Plata, Paseo del Bosque s/n, 1900-La Plata, Argentina. e-mail: EdwardsD2@cardiff.ac.uk Running header:  (High latitude) Upper Silurian marine productivity Key words:  acritarchs, algae, cyanobacteria, eggs, faecal pellets, phytoplankton, Precambrian, zooplankton Abstract Specimens recently collected from the Las Carmelitas Formation exposed in the Cochabamba area, Bolivia and dated as Ludlow (Upper 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 tentatively suggested 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 organisms, although the latter seems less likely in view of the proposed anoxic depositional environment of the shales that lack any bioturbation.  Such coalified remains, be they from primary producers or grazers, indicate high productivity at high palaeolatitudes in the upper Silurian and it is suggested that it stems from blooms stimulated by eutrophication of continental seas following deglaciation. Assemblages from Bolivia have provided the best evidence to date for Silurian land vegetation at high latitudes on Gondwana.  Pridoli examples from the Tarija area of southern Bolivia include taxa (e.g. Cooksonia) at a similar grade of organisation to those found in coeval strata of Laurussia (Edwards et al. 2001) but a more reliably dated and far more diverse Ludlow assemblage (Toro et al. 1997) from the more northerly Cochabamba area is reported to include taxa such as Rhynia, Zosterophyllum, Drepanophycus that occur much later at lower palaeolatitudes.  Such a discrepancy demanded further investigation and, as part of a collaborative multidisciplinary venture between palaeobotanists and sedimentologists from La Plata, Argentina and Cardiff, Wales, new collections of both plant and animal fossils were made at a number of localities to the east of Cochabamba.  A major objective of the project was to relate the organisms not only to the depositional environments that preserved them but also to reconstruct communities in (potentially stressed) habitats associated with high latitudes (Bassett et al. in prep.).  Coalified axial fossils that could unequivocally be assigned to land plants were found at only one locality and were extremely fragmentary and sterile.  However, at a number of localities in the Ludlow Carmelitas Formation, bedding surfaces were sometimes almost completely covered by minute strands, spheres, comminuted debris, thread-like strands and coalified compressions with consistent, but frequently unfamiliar, morphology.  Much greater attention has been paid to such shapes in the Precambrian than in younger rocks (Hofmann 1994) although undoubtedly some of the organisms must have persisted into the Phanerozoic, and may well be present in the Silurian, albeit as members of far more complex ecosystems.  Such similarities will be explored further here as part of a more comprehensive investigation of the entire assemblage.