Os xenacantídeos da formação Teresina, bacia do Paraná, de Angatuba-SP

Abstract

The Teresina Formation is a Permian Paraná Basin sedimentary unit composed of mudstone, siltstone, shale, limestone, and sandstone. Their rocks were deposited in coastal to offshore environments, with variable salinity and under storms influence. Its fossil content is diverse, consisting of plant remains, palynomorphs, sponge spicules, ostracods, bivalve molluscs, coprolites, stromatolites, labyrinthodont amphibians, and fish remains. Bone beds are common, and mainly composed of paleonisciforms and chondrichthyans teeth and scales, generated by high-energy events, such as storms. In these accumulations, xenacanthid shark teeth are frequently found and are identified by their characteristic diplodont pattern. These sharks inhabited Devonian to the Triassic freshwater environments and are found in Paraná Basin Permo-Triassic units, including the Teresina Formation. Nine species of xenacanthids are known from the Teresina and Corumbataí formations (chronocorrelated units) deposits [Mooreodontus moorei (Woodward, 1889), Xenacanthus santosi Würdig-Maciel 1975, Xenacanthus angatubensis Ragonha 1984, Xenacanthus camaquensis Ragonha 1984, Xenacanthus ferrazensis Ragonha 1984, Xenacanthus santaritensis Ragonha 1984, Xenacanthus taquaritubensis Ragonha 1984, Wurdigneria obliterata Richter, 2005, Xenacanthus ragonhai? Pauliv et al., 2014]. However, in the last four decades, just a few studies have been carried out on the taxonomy of Teresina/Corumbataí formations xenacanthids. Considering the presence of nomen nudum among the known species and the existence of taxonomic inconsistencies, the aim of this work is (a) to identify fossil shark teeth from Teresina Formation rocks collected in the region of Angatuba-SP; (b) search for possible new species not yet described; and (c) evaluate chemical preparation methods for removing fossils from the rocks. In the phase of removing the fossils from the rocks, experiments were developed to understand which chemical preparation method would be most efficient in breaking down the rocks and release the fossils without any damages. Acetic acid, hydrogen peroxide and distilled water were used. After removing the fossils, the xenacanthid teeth that could be identified were described and separated into morphotypes. Subsequently, comparisons with the species known from the Corumbataí and Teresina formations and identification have been done. The experiments demonstrated that distilled water and hydrogen peroxide are not efficient in disaggregating samples and detaching fossils. The opposite was shown in reactions with acetic acid, which generated greater amount of particulate matter and the release of the fossils. Acetic acid also did not destroy fish teeth within seven days. In the analyzed samples four species were recognized (Xenacanthus angatubensis, Xenacanthus camaquensis, Xenacanthus ragonhai ? and Mooreodontus moorei), two of them considered nomen nudum. Additionally, two indeterminate Xenacanthidae were found, which possibly represent unknown especies described for Teresina Formation. Among the samples, some intraspecific morphological variations that had not been previously recorded in the literature were observed, which must be related to the ontogeny, position of the teeth in the jaw, or taphonomic process. The results obtained show that there is a need to carry out a detailed taxonomic review of these sharks.