Caracterização Morfológica de Escamas (Dentículos Dérmicos) de Tubarões da Formação Pirabas, Pará, Brasil

Abstract

The Pirabas Formation (Oligo-Miocene) is the unit that best represents the Brazilian marine Cenozoic because it presents a rich fossil assemblage of invertebrates, vertebrates and plants (ACKERMANN, 1976), representing environments that vary from transitional (mangroves/lagoons) to marine (coastal or outer shelf) (GÓES et al, 1990; ROSSETII; GÓES, 2004). The vertebrates represent the least abundant and diverse group and are represented by bony and cartilaginous fish, mammals, reptiles, and birds. Among fishes, sharks are best represented with about 12 taxa described so far (SANTOS; TRA V ASSOS, 1960; SANTOS; SALGADO, 1971; COSTA, 2005). Due to the cartilaginous constitution of their skeletons, the record of sharks is restricted to the hardest parts of their bodies, such as teeth, scales and spines (CAPETTA, 1987). Scales are not as diagnostic of species as teeth (REIF, 1985), but can assist in paleoenvironmental interpretations when collections are stratigraphically controlled. The present study aims at the morphological identification of shark scales and their morphodynamic functions contributing to the paleoenvironmental interpretation of the sedimentary deposits of the Pirabas Formation. The samples for the present study were collected at Mine B17, belonging to CIBRASA/SA, in the municipality of Capanema. Of these, 230 g of each stratigraphic horizon sampled (B 1, B1A, B2 and B2A) were used. From the 113 scales found, 20 morphotypes were identified according to Reif's criteria (1985). The most abundant morphotype (CASL), which presents the crown with a rounded shape and smooth surface, normally occurs in sharks from coastal marine environments associated with the substrate. In the present study this morphotype was found in all sampled levels, inferring, preliminarily, that the Pirabas Formation in these horizons, represents a marine and coastal depositional environment.