Microrrestos de vertebrados da formação Quiricó, Cretáceo inferior da Bacia Sanfranciscana (Minas Gerais)

Abstract

The Mesozoic deposits of the Sanfranciscana Basin represent one of the most extensive records of Western Gondwana ecosystems, formed during the separation of South America and Africa and the subsequent opening of the South Atlantic. The fossil record of this basin is particularly concentrated in the Quiricó Formation, which corresponds to a predominantly lacustrine paleoenvironment, under arid climatic conditions, with fossils of algae, plants (palynomorphs and macroflora), invertebrates (ostracods, conchostracans, possible annelids, insects), and vertebrates (fish, lizards, crocodyliforms, and dinosaurs), dated to the Lower Cretaceous (?Valanginian–Early Aptian). Despite recent advances in understanding the paleodiversity of the Quiricó Formation based on macro- and microfossils, there remains a significant number of vertebrate microremains that are underexplored. These microremains, defined as millimeterscale mineralized fragments preserved in isolation within sedimentary systems, have great potential for biostratigraphy and paleoecology studies. Aiming to expand paleontological knowledge of the Quiricó Formation and discuss its taphonomic and paleoecological implications, this dissertation aimed to describe and identify vertebrate microremains from the João Pinheiro region (MG). Samples were collected from mudstones at Fazenda Tereza locality, processed with 35% PA hydrogen peroxide, sieved in meshes of different sizes (1 mm, 250 µm, 150 µm, 90 µm), and sorted under a stereomicroscope. Relevant specimens were selected for optical imaging and scanning electron microscopy (SEM) analysis after metallization. Most of the recovered microremains correspond to fish bone fragments, including remains of the coelacanthiform Mawsonia gigas, as well as teeth and denticles of Actinopterygii and Chondrichthyes. In the study’s first stage, M. gigas bone fragments with taphonomic alterations, characterized by microtunnels and tubes, were analyzed. SEM-EDS, XRD, XRF, and FTIR analyses revealed the presence of fluorapatite and calcite, with textures indicative of microbial action (bacteria or fungi). Comparison with previous studies suggests that the documented microtunnels and tubes are unprecedented. The second stage focused on the taxonomic identification of the microremains, which revealed low diversity, including bony fish of the family Amiidae and hybodontiform sharks of the family Lonchiididae, with low morphological disparity. The results of this dissertation contribute to the understanding of the paleodiversity of lacustrine environments of the Quiricó Formation and report a taphonomic process of microbial origin that is still rarely addressed in the specialized literature.