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South American Permo-Triassic Fish Fauna Turnover and its Bearing on the Mass Extinction Scenario

Final Activity Report Summary - P-T FISH FAUNA (South American permo-triassic fish fauna turnover and its bearing on the mass extinction scenario)

During this project, fieldwork was conducted in the Middle-Upper Permian Parana Basin of southern and central Brazil. Rock and fossil samples were collected from 24 localities and substantial comparative Permo-Triassic fish material was studied in 11 museum collections with international collaboration. Numerous palaeontological and geochemical analyses were undertaken and interpreted in the context of 13 stratigraphic sections logged within the Middle-Upper Parana Basin in Brazil. The Permo-Triassic fish fauna turnover in South America and worldwide was analysed using the newly available data and literature.

These investigations yielded the main following results and conclusions:
There are no fish records from Early Triassic rocks so far in Brazil and the precise date of the upper Permian strata has not been firmly determined. Radiometric datings are sparsely available and based on rare zircons. Palynology seems to suggest that the Changhsingian stage is missing in the Parana Basin. Worldwide across the Permo-Triassic boundary, several chondrichthyan genera that were only known from Devonian rocks elsewhere, persisted into the Late Permian of South America. The world's youngest record for the Class Acanthodii is found in the Teresina/Corumbatai formations in Brazil. Tooth-whorl bearing sharks common in the Permian, were also found outside South America in the Early Mesozoic in seemingly unreduced diversity. Among the bony fishes, the lungfishes and coelacanths fared well across the Permo-Triassic boundary, and the latter group even diversified in particular in the Early Triassic. Taxonomic difficulties in both these groups may have led to misinterpretation of actual diversity, and their distributional patterns can be explained in the light of a worldwide regression close to or at the Permo-Triassic boundary.

In the southern hemisphere, the ray-finned fishes (Actinopterygii) represent the most common fish fossils with a relatively higher number of more primitive genera in the non-marine realm surviving into the Mesozoic. Relatively more advanced genera of ray-finned fishes, whose origin remains problematic due to the gap of preservation across that boundary, occurred (in geological terms) 'suddenly' and worldwide in many marine environments in the Early-Middle Triassic, including South America. At least one archaic ray-finned fish family persisted across the Triassic-Jurassic boundary in non-marine, rather than in the marine realm of both the southern and northern hemispheres.

The exact geological age of the only accessible and palaeontologically relevant impact crater caused by a large asteroid that presumably hit central Brazil near the Permo-Triassic boundary in central Brazil (Araguainha impact crater) is still unknown. The regional effect of the impact itself and gaps in the sedimentary record of this region have been under-studied. However, its claimed direct link with the end-Permian mass extinction event could not be substantiated. Relatively complete fish specimens point to a higher faunal diversity than previously assumed in the Permian of Brazil. The South American Parana Basin was a large epicontinental sea established in the early Palaeozoic that underwent gradual dissecation towards the Late Permian, possibly creating a variety of niches where salinity, temperature and depth varied, favouring the development of an endemic fish fauna at species level but still preserving affinities at genus level with earlier faunas from North America and Europe. Vertebrate remains in this basin are predominantly recovered as fragmentary teeth, scales and spines, and precise identification is problematic due to the sparse fossil record.

Actinopterygians and specialised, endemic holocephalians were remarkably abundant. Statistic analyses of these remains, geochemical and x-ray analyses of paternal sedimentary rock support the interpretation of a highly variable Late Permian palaeoenvironment near the southern rim of the Parana Basin. The main results of this project have been published in scientific journals.