SEM-EDS AND RAMAN SPECTROSCOPY INTEGRATION: PALEOMETRY TO DECIPHER THE FOSSILDIAGENESIS OF JURASSIC SPINICAUDATA FROM THE PARANÁ BASIN

The abundant fossil record of Spinicaudata (crustaceans) in continental environments since the Devonian presents a paradox, since two factors unfavor their preservation: the predominantly chitinous carapace, degradable in oxic environments, and the low biomineralization that does not guarantee a broad fossil record. To investigate this enigma, we analyzed dozens of fossil carapaces from the Linha São Luiz Geosite (Faxinal do Soturno-RS), in a Jurassic fluvial-lacustrine redbed package of the Paraná basin, which exhibit three distinct visual classes of preservation: (I) translucent iridescent, (II) opaque white, and (III) metallic black. We applied an integrative paleometric approach, combining morphological analysis by Scanning Electron Microscopy (SEM) in Secondary Electron (SE) and Backscattered Electron (BSE) modes, elemental mapping by Energy Dispersive Spectrometry (EDS), and molecular characterization by Raman Spectroscopy (RS) with power variations for better detection. Class I revealed a wavy surface texture and heterogeneous composition with Ca, P, Si, Al and Fe distributed in a correlated manner, in addition to intense phosphate peaks with intermediate crystallinity. Class II showed polyhedral and dendritic clusters with higher intensity in the phosphate peak (~960 cm⁻¹) and presence of hematite in the matrix. Class III presented a lumpy texture, with Fe, Ca and P, strong phosphate peak, goethite signals, conferring metallic luster. The different powers (5%, 15% and 25%) applied to RS allowed the identification of organic matter peaks in isolated black spots in classes II and III, although original chitin was not detected. In general, the carapaces exhibit laminar organization and concentrations of P and Ca in the fossil, contrasting with Si and Al in the matrix. The preservation classes may be related to the amount and degree of crystallization of biomineralized apatite (detected by comparing RS peaks), with II having more crystalline apatite, which may reflect environmental conditions with higher concentrations of PO₄³⁻ in the water. Another factor is bacterial induction during degradation that may stimulate the deposition of goethite, similar to the processes identified in plants ferrified by microaerophilic bacteria in the same outcrop. In summary, the preservation of Spinicaudata seems to be conditioned by the biomineralization of the carapace, by environmental and eodiagenetic processes.