Estudo sistemático de gastrópodes neógenos, com ênfase ao gênero Tryonia (STIMPSON, 1865), Formação Solimões, Estado do Amazonas

Abstract

The Solimões Formation is a geologic unit composed of neogene deposits, situated in brazilian Western Amazonia. These deposits are correlated with Pebas (Peru and Colombia), Curaray (Ecuador) and Urumaco Formations (Venezuela), forming the Pebas system. The lithology of Solimões Formation is mainly composed of clay and sand. Regarding to the palaeoecology of the unit, the majority of previous papers point to a freshwater (fluviolacustrine) or brackish water environment. The Formation is also known for being richly fossiliferous, consisting of palinonology, vertebrate and invertebrate material, especially ostracods and mollusks. The studies about mollusks in this area are few, especially those one regarding to gastropods, which are remote. Therefore, this paper intents to contribute to the taxonomic study of gastropods from 1 AS 34-AM (on the Jutaí riverbank) borehole, as well as Morada Nova and Aquidabã outcrops (on the Juruá riverbank), in the Amazonas state, besides helping in the palaeoenvironmental and biostratigraphic interpretations of the study unit. The gastropod analyses of the outcrop samples allowed the register of ?Pachychilidae family and Onobops (Cochliopidae) genus in Aquidabã, as well as Planorbidae and Hemisinus kochi (Thiaridae) in Morada Nova. The 1 AS 34-AM showed a great abundance and diversity of gastropods. Two families were identified in this borehole: Thiaridae and Cochliopidae. The Cochliopidae is the most diverse and abundant in the samples, represented at least by two genus: Dyris and Tryonia, being the last one the subject of the present study. The systematic study of Tryonia genus allowed the identification of Tryonia scalarioides scalarioides species, beyond three other species which stayed in opened nomenclature. The presence of Tryonia genus in the studied samples infers an environment with a low salinity and a low energetic flow. The presence of T. s. scalarioides between the 121.09 and 130.85 m of depth from the 1 AS 34-AM borehole allowed a correlation between this interval and MZ8-12 biozones, defined in previous papers as Middle – Late Miocene. The register of Hemisinus kochi in Morada Nova at MN2 and MN5 levels suggests a correlation with MZ8 biozone, Middle Miocene. Given this, it is possible that 1 AS 34-AM borehole and Morada Nova outcrop were, at least in part, contemporary. However, a specific taxonomic identification of other gastropod groups from the study area is necessary for a more precise result.