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Palaeoecology and Palaeobiology |
1 Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK
2 Department of Earth Sciences, Memorial University of Newfoundland, St John’s, NL, Canada A1B 3X5
Silicified coniferous wood is commonly found in the Middle Jurassic (Bajocian) Lajas Formation of the Neuquén Basin, west-central Argentina. The wood is preserved in a succession of sandstones, siltstones, mudstones and minor conglomerates that represent deposition as part of tide-dominated deltas and fluvial plains across which large rivers meandered. Most of the wood occurs as dense accumulations in the tidal and fluvial channels. The wood fragments are worn, abraded, and lack both bark and branches, indicating that they were transported prior to deposition. The material is typically 20–30 cm long, with only infrequent examples of larger trunks (c. 80 cm in diameter, 5–6 m long). No trunks were found with root systems attached, and no stumps were found upright and in situ. The fossil wood genus Araucarioxylon dominates the assemblage. Growth rings are largely absent from the specimens, although one sample (from Rhea Gorge) displays highly diffuse and irregularly spaced rings, suggesting that it grew in different conditions from the others studied. Large-scale interpretations for southern Gondwana suggest a seasonally dry climate. However, these fossil wood specimens show no evidence of this, indicating that in this area at least the effects of any seasonal component to the climate may have been over-ridden by factors such as a locally plentiful supply of water and/or the possibility that growth was to some extent controlled by the taxonomic affinity of the trees.
