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Abstract

Plant assemblages are described from the Talacasto and Punta Negra Formations, San Juan Province, the Argentine Precordillera. A detailed facies study involving trace fossil assemblages indicates that the plants were buried in shallow low- to high-energy marine environments, but not in turbidites as previously postulated. Apart from a single specimen resembling Cooksonia from the Lochkovian Talacasto Formation, the abundant compression fossils consist of short lengths of smooth axes, occasional defoliated lycopod stems, and very rare isolated sporangia. It is thus impossible to assign any to existing taxa with confidence nor are there sufficient distinguishing characters to erect new ones. Although the possible age of the formations in the Precordillera remains conjectural, the fossils themselves, even preserved in such fragmentary states, indicate much lower grades of organization than seen in coeval assemblages elsewhere, particularly on the Laurussian and South China palaeocontinents. Palaeocontinental reconstructions of Gondwana indicate that the localities are within 30° of the palaeo South Pole. Thus a global latitudinal gradient in vegetation is postulated for the Early Devonian with decreasing disparity and diversity at higher latitudes. An explanation for the latter is explored in terms of light regime and temperature in highly stressed environments.

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