Conchostráceos (Crustacea) da Formação Quiricó, Cretáceo Inferior da Bacia Sanfranciscana: novos registros e suas implicações bioestratigráficas e paleoecológicas

Abstract

The Sanfranciscan Basin is a sedimentary deposit that holds significant importance for Paleontology due to its rich fossil record, the diversity of which is still not well understood. Among the groups found in the basin, vertebrates such as sauropod and theropod dinosaurs, and invertebrates like conchostracans stand out, which are the focus of this dissertation's research. Conchostracans are crustaceans known for their ability to partially retain growth lines on their bivalve carapaces during the molting process, making them valuable for biostratigraphy and paleoenvironmental indicators. Given their impact and relevance to the regional paleontology of the basin, new specimens of conchostracans collected in the cities of João Pinheiro and São Gonçalo do Abaeté, MG, where Cretaceous rocks of Valanginian-Aptian age from the Quiricó Formation are exposed, were analyzed for taxonomic characterization, paleoenvironmental refinement, and biostratigraphic correlation. The material was observed, photographed, and variable features in the shape of the fossil carapaces were obtained through linear morphometry, including basic measurements such as length and width, with the aid of a magnifying glass. Additionally, a Scanning Electron Microscopy analysis was conducted to obtain more detailed information about microscopic aspects of the valves. These methods are essential for the classification of conchostracans, as they can reveal variations among specimens. Subsequently, the fossils were described and compared to taxa from the Mesozoic of Brazil and other parts of the world to establish their taxonomic classification. For paleoenvironmental characterization, the results were compared with species records from other points in the basin and other parts of the world, especially South America. New features, particularly in terms of carapace ornamentation, allowed for the description of a new species, named Krossiestheria quiricoensis gen. et sp. nov., classified in the Fushunograptidae family. Other specimens were attributed to Palaeolimnadiopsis freybergi and P. reali, both previously recorded in the basin. The relatively consistent size of growth bands on the valves of some fossils, among other characteristics, indicates a paleoenvironment of calm waters with a warm climate and an alkaline pH range, which may have resulted in relatively high salinity. Due to similarities with other taxa, K. quiricoensis supports an age that extends to the Aptian for the Quiricó Formation. These findings add new data regarding the past life diversity in the Sanfranciscan Basin, increasing the fauna record and providing information about the paleoenvironment.