Revisão da ordem Hybodontiformes Patterson, 1966 (Chondrichthyes) da Formação Alcântara, Bacia de São Luís, Cretáceo do Maranhão: importância sistemática, biogeográfica e bioestratigráfica

Abstract

The sharks of the Hybodontiformes order present their first record dating back to the end of the Devonian period. Since then, they have established themselves as a dominant group in the marine environment. However, during the Mesozoic, possibly due to competition with neoselachians in the marine environment, they migrated and conquered freshwater environment. It was in the freshwater environment that this group became significantly more diverse. Among the different taxa that compose this order, some are known from the northern Gondwana region, during the Cretaceous. The present study aims to review the Hybodontiformes at the bonebed Laje do Coringa locality, an outcrop of the Alcântara Formation (Upper Albian - Cenomanian) in the São Luís Basin, Maranhão state, Northeast Brazil. For this purpose, numerous specimens including oral teeth (ten specimens), cephalic spines, and dorsal fin spines were morphologically and chemically analyzed and described. All specimens were found disarticulated and show signs of wear. Parto of the oral teeth described in this study represent the first occurrence of Aegyptobatus cf. kuehnei and Distobatus cf. nutidae, taxa originally described in the Bahariya Formation of Egypt, outside the African continent. Among the cephalic spines (ten specimens), six had a T-shaped base, a characteristic attributed to species of Hybodontiformes in the family Hybodontidae, and four had an arrow-shaped base, a characteristic attributed to the family Lonchidiidae. As for the dorsal fin spines (three specimens), they exhibit longitudinal ribs arranged on the side of the spine. However, the presence of posterior denticles was observed in only two of the dorsal fin spines. Although the three dorsal fin spines present subtle differences among them, it was not possible to establish taxonomic definitions, as dorsal fin spines are generally not considered diagnostic characters. The results of chemical analyses conducted on six cephalic spine specimens showed that they are mainly composed of Calcium Phosphate and contain Aluminum Silicate, interpreted as a result of fossil diagenetic impregnation. The record of Hybodontiformes in the São Luís Basin had previously been attributed to Tribodus cf. limae. However, our study confirms that Tribodus limae Brito and Ferreira (1989) is only recorded in the Early Cretaceous, Aptian/?Albian (from the Santana and Quiricó formations, respectively in the Araripe and Sanfranciscana basins), while the specimens from the fossil assemblage of the Alcântara Formation (Neoalbian-Cenomanian), morphologically quite different from T. limae, belong to other taxa with known African affinities in the northern part of Gondwana.