Deciphering the origin of dubiofossils from the Pennsylvanian of the Paraná Basin, Brazil

Abstract. Minerals are the fundamental record of abiotic processes over time, while biominerals, are one of the most common records of life due to their easy preservation and abundance. However, distinguishing between biominerals and abiotic minerals is challenging because the record is defined by the superimposition and repetition of geologic processes and the interference of ubiquitous and diverse life on Earth's surface and crust. Mineral dubiofossils, located on the threshold between abiotic and biotic, can help resolve this issue. The aim of this contribution is to decipher the origin and history of branched mineralized structures that were previously considered mineral dubiofossils. While this material has different forms and refers to biological aspects, it is difficult to associate it with any known Fossil Group due to the overlapping geological processes that occur in the Rio do Sul Formation (Pennsylvanian of the Paraná Basin), very close to the contact from a sill of the Serra Geral Group – Lower Cretaceous with a proven thermal effect. The samples were described using a protocol that evaluated: 1) morphology, texture, and structure; 2) relationship with the matrix; 3) composition and 4) context, assessing indigeneity and syngenicity, and comparing them with abiotic and biotic products. Despite conducting an extensive comparison with abiotic minerals, as well as controlled, induced, and influenced biominerals, no more probable hypothesis was found, excluding the possibility of it being a controlled biomineral due to its patternless diversity of forms and the purely thermometamorphic origin due to the branched elongated form. The occurrence of these structures suggests a complex history: a syndepositional or eodiagenetic origin of some carbonate or sulfate (gypsum, ikaite, dolomite, calcite, siderite), which may be linked to the presence of microbial mats, could have served as a template for mineralization and possibly mediated mineral growth. Mesodiagenesis could have also modified the occurrence, but the main agent responsible for its formation was the Cretaceous intrusion, which dissolved and replaced the initial mineral and precipitated calcite, resulting in the dubiofossil. Throughout these steps, physical-chemical and biological reactions, aided by the intrinsic characteristics of the matrix, amount of organic matter, and distance from contact with the intrusive body, may have increased the morphological complexity. This material illustrates how dubiofossils can be the result of a complex history and overlapping geological processes. It also highlights the difficulty in differentiating biominerals from abiotic minerals due to the scarcity of biogenicity arguments.