NUEVOS RESULTADOS FITOPALEOECOLÓGICOS DE LA FORMACIÓN ARROYO TOTORAL, PÉRMICO INFERIOR, PROVINCIA DE LA RIOJA

Ruben Cúneo, Ana Archangelsky

Abstract


"Recently processed information on lower Permian plant assemblages from NW Argentina (eastem Paganzo basin) is given. A rich taphocoenosis of gondwanic pteridophytes and gymnosperms occurs in marginal facies of the lacustrine interval of the Arroyo Totoral Formation at a single locality. Biostratinomic processes that affected the original plant parts, in particular their incorporation to the sedimentary process, are analyzed. Comparisons with modem analogues are carried out. Based on autoecological, synecological and palaeoenvironmental features, two plant communities have been recognized: a hydrohygrophytic one composed of monotypic populations of equisetaleans (Phyllotheca) that grew aIong the lake shoreline being seasonally influenced by the falling and rising of the water body. In the corresponding taphocoenosis frequent Phyllotheca stems in growth position can be observed. Transitional to sites with a lower water table, a hygro-mesophytic to meso-xerophytic plant community composed of gymnosperms developed, probably colonizing the granitic substrate of the surrounding hills as well. Cordaitaleans, glossopterids and conifers are the main components of this community with a variety of genera and species. The results obtained from this local analysis were linked to previous data from a number of localities belonging to the same formation. Thus, lateral variations of the plant communities have been inferred, in particular the participation of an understorey vegetation of shrubby fems and herbaceous Botrychiopsis that show a more restricted distribution. Vegetational attributes of the intramontane Malanzán-SoIca-Anzulón sub-basin (paleovalley) during the early Permian are compared with coeval and geographicaIly associated examples. AII of them are recognized as part of a NW Argentina phytogeographic subunit which is separate from a southem Brazilian subunit. However, both are regarded as forming part of a major phytogeographic "nothosouthamerican" unit differentiated from a
Patagonian one in the earliest Permian West Gondwana context."

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