Unveiling the taphonomy of elusive natural tank deposits: A study case in the Pleistocene of northeastern Brazil
Abstract Fossiliferous natural tanks are a singular type of sedimentary deposit restricted to northeastern Brazil, which usually preserves a diversified late Pleistocene vertebrate fauna. Their fossil assemblages are taxonomically well known, but their taphonomy is poorly understood. This work presents a detailed taphonomic analysis of a tank deposit from Jirau site (Ceara State, Northeastern Brazil) to improve understanding of the processes that influenced fossil deposition and preservation in natural tanks and to shed light on paleoecological and paleoenvironmental aspects related to the late Pleistocene vertebrate fauna from Northeastern Brazil. Tank deposits from different localities were also compared to figure out potential taphonomic patterns among them. Additionally, comparisons with other Brazilian Pleistocene deposits and other debris-flow deposits bearing vertebrates were undertaken. The study revealed the predominance of the megamammal Eremotherium laurillardi in the deposit, which we attributed to its abundance in late Pleistocene biocoenosis as well as bone resistance. Scarcity of non-mammalian vertebrates is likely related to their paucity in late Pleistocene biocoenosis. Time-averaging was evidenced by the co-occurrence of different weathering, petrographic and color patterns, and may have affected the ontogenetic profile of the accumulation relative to the original biocoenosis. Analyses of weathering stages, tooth and trample marks, indicate wide exposition of the thanatocoenosis before burial. Abrasion and petrographic patterns allow reworking in the assemblage to be inferred. Permineralization and substitution are the fossilization processes observed. Taphonomy indicates that floods deposited clasts and bioclasts in the tank under an arid or semiarid climate in a debris-flow regime. Comparison with other tank deposits revealed several similar taphonomic features and differences related to sedimentology and stratigraphy. Taphonomic similarities suggest rather homogeneous environmental conditions in the northeastern Brazil during the late Pleistocene, as previously suggested by taxonomic studies. Comparison with other Brazilian Pleistocene deposits shows that tanks have more temporal fidelity than caves, coastal and some fluvial deposits. The Jirau vertebrate accumulation was produced by the reworking and transport of skeletal elements accumulated in a nearby thanatocoenosis and is similar to other better known debris-flow hosted vertebrate accumulations.