SHEDDING NEW LIGHT ON OLD FOSSILS: A SPECTROSCOPIC INVESTIGATION OF CENOZOIC ARACHNIDS

Fossil arachnids are extremely rare in the geological record. This highly incomplete record hampers new insights into the evolution and preservation of this important group. Here we studied the preservation of new arachnids from the Tremembé Fm (ca. 23 Ma), São Paulo State, Brazil. We investigated the samples using scanning electron microscopy coupled with energy dispersive detectors (SEM/EDS), Raman spectroscopy and synchrotron radiation-based µ-x-ray fluorescence (SR- µXRF). The results showed that S and Fe are highly enriched in the fossils compared to the host rocks. Indeed, SEM showed octahedral pyrite microcrystals composing the fossils and occasional pyrite framboids. SR-µXRF also showed higher intensities of trace elements (Ni and Cr) in the fossils. Additionally, radial prisms enriched in calcium and sulfur are often observed as overgrowths on the pyrite crystals, as well as in a dispersed manner in the fossils and host rock. Raman spectroscopy confirmed the identification of pyrite and demonstrated that the Ca and S minerals are gypsum crystals (CaSO₄·2H₂O). Taken together these results indicate that the fossils were preserved in anoxic sediments likely by the influence of sulphate-reducing bacteria. The metabolism of these microorganisms causes the reduction of SO₄^2- and promotes the increase in sulfides in the microenvironment, which react with dissolved Fe²⁺ and lead to the precipitation of iron sulfides. Although the geobiological processes of pyritization are well-known, the preservation of arachnids by pyrite is not a common phenomenon in the fossil record. Furthermore, the different morphologies of the pyrite crystals and their size variations observed in the fossil arachnids may indicate changing conditions in the microenvironment, such as ion diffusion, nucleation rates, pH and Eh. On the other hand, the gypsum minerals are not related to early diagenesis, but result from pyrite oxidation during recent weathering. These newly formed minerals often obscure or destroy the preserved information in our samples, furthering biasing the already low details preserved with pyritization. Understanding the retention of information in the fossil record is at the core of taphonomic studies and these samples will be crucial to provide a more trustworthy record of the arachnid evolutionary history.