Paleoecologia dos moluscos bivalves dulcícolas da Formação Serra da Galga, Cretáceo Superior, Bacia Bauru

Abstract

The Serra da Galga Formation (Upper Cretaceous, Bauru Basin) is a recently proposed stratigraphic unit laterally correlated with the Marília Formation and previously considered part of it. The rocks of this formation are primarily composed of sandstones, conglomerates, paleosols, and calcretes, representing facies characteristic of deposition in a proximal to medial distributive fluvial environment. This unit is rich in vertebrate fossils, predominantly crocodylophorms and dinosaurs (sauropods and theropods), with less frequent occurrences of turtles, fish, and frogs. Among the invertebrates (bivalve and gastropod mollusks, ostracods, and conchostracans), bivalve mollusks have been known since 1974, first described by Prof. Sergio Mezzalira. However, studies on this group have since focused on taxonomic issues rather than taphonomic and paleoecological analyses. Nevertheless, the paleoecology of macroinvertebrates is widely recognized as a valuable tool for interpreting paleoenvironmental conditions. It can contribute to understanding the depositional environment of the Serra da Galga Formation. With this aim, bivalve fossil deposited in the scientific collection of the Complexo Cultural e Científico de Peirópolis – CCCP-UFTM, in Uberaba, Minas Gerais, are under study. Paleoecological analyses will be based on morphofunctional assessments, allowing the identification the lifestyle and ecological preferences of the studied organisms. Additionally, genera with living representatives will be compared with extant species and their ecology (mode of life, ecological preferences, and habitat). The results suggest a greater diversity than previously assumed and at least two distinct assemblages. At Point 1 (UTM 23K212117; 7816715), four morphotypes (F, G, I, and the genus Diplodon Spix 1827) were found, while at Point 2 (UTM 23K-211571; 7817115), five morphotypes were identified (Anodontites pricei Mezzalira 1974, Anodontites freitasi Mezzalira 1974, Florenceia peiropolensis Mezzalira 1974, Sancticarolis tolentinoi Mezzalira 1974, and morphotype M). The exclusivity of morphotypes in different locations indicates distinct environmental conditions. These mollusks are associated with proximal contexts within the distributive system, predominantly preserved in fine sandstones, likely related to interchannel environments where floodplain deposits alternate with crevasse channel deposits. They are parautochthonous elements of infaunal or semi-infaunal communities that lived in shallow, calm water conditions. Finally, among the elements found in the Serra da Galga Formation, at least five morphotypes represent taxa that have yet to be formally described, highlighting the need for further taxonomic studies.