Paleobiology of the Itajaí Basin (Ediacaran, Santa Catarina, Brazil): evolutionary, paleoecological and taphonomical implications

Abstract

New studies on the geologic history of Earth are increasingly revealing the complex relationships among evolutionary patterns, diversification of major groups, and environmental changes. In the fossil record of the Ediacaran Period (635–541 Ma), the appearance of macroscopically complex organisms and animals resulted in major modifications in the environment, setting the stage for the subsequent “Cambrian explosion” and the development of Phanerozoic-style ecosystems. The Ediacaran biota can be found in several localities throughout the world, and researchers on this subject are able now to divide it in mainly three Assemblages: Avalon, White Sea and Nama. Compared to other deposits around the world, studies on South American Ediacaran fossils are still in their early stages, but already show a great potential to understand the evolution of early complex life. In this context, this work aimed to investigate the taxonomy, taphonomy and age of the poorly known fossils from the Itajaí Basin (Santa Catarina, southern Brazil), as well as its relationships to other coeval fossiliferous strata. For that, a multi-approach investigation was conducted, using diverse techniques, such as: stereomicroscopic investigation; petrography; U/Pb radiometric dating (SHRIMP); scanning electron microscopy (SEM) and dispersive energy spectroscopy (EDS); computerized microtomography (µCT); and Raman and infrared spectroscopy. The results of this work are presented here in three main published and/or submitted articles regarding: [I] the description of the Itajaí Biota and its age; [II] the taphonomic characteristics of the ‘Ediacaran-style’ preservation at the Itajaí Basin; and [III] the nature of microbial filaments. In Article 1, it is reported the presence of Ediacaran soft-bodied organisms, such as Palaeopascichnus, discoidal forms (Aspidella and Nimbia), as well as abundant microbial mat features (e.g., reticulated tufts, Arumberia, and wrinkles). Moreover, the age of the Itajaí Basin is further constrained by U/Pb radiometric dating of volcanic tuffs, giving a depositional age ca. 563 Ma and setting this Ediacaran biota as one of the oldest in Gondwana. In Article 2, it is presented a detailed investigation on the microbial filaments and their possible affinities with giant filamentous sulfide-oxidizing bacteria, such as the modern Beggiatoa. This has great implications for understanding the biotic and geochemical context during Ediacaran times. In Article 3, it is provided a model to the fossilization mechanisms of the Itajaí Basin, in which the micro and macrofossils would have been preserved by volcanic and microbial activity. This work shows for the first time robust evidences for the role of clay authigenesis in the early diagenesis of fossils. In sum, this thesis provides a comprehensive take on the paleobiology, taphonomy, and age of the Ediacaran Itajaí Biota, showing that these deposits are of key importance in studies on early macroscopic complex lifeforms of the fossil record.