GLOWING SPIDERS: THE TAPHONOMIC PATHWAY OF THE AIX-EN-PROVENCE FOSSIL-LAGERSTÄTTE

The Fossil-Lagerstätte of Aix-en-Provence, France is an Oligocene (23 Ma) lacustrine deposit in which an abundance of soft-bodied fossils, including spiders, are preserved; however, there is no research explicitly investigating the taphonomic pathway(s) responsible for this assemblage. This may in part be due to the fact that the color of the fossils is very close to that of the matrix, resulting in ghostly fossils that can be challenging to image. Fluorescence microscopy was used to obtain better images of Aix fossil spiders and to better understand the potential taphonomic pathways in this formation. The fossils are preserved in extremely thin marlstone laminations, and often as part and counterpart. Three main modes of preservation are recognized: compression fossil with original cuticle, high relief internal molds and impressions, and low relief calcite replacements.

Fossil spiders were imaged with UV and blue-violet light. About 20% of the fossils emit fluorescent light in the red, orange, or blue spectra, and for some fossils, different body regions of a single specimen will emit different colors. The differences in fluorescence are likely related to the mode of preservation and the taphonomic pathway. It has been suggested that the laminations of Aix fossils are the result of microbial mats. This is supported by a wrinkled texture observed on the surface of many of the specimens. Microbial mats have previously been shown to play crucial roles in preservation, and the taphonomic pathway for the Aix spiders may include a microbial death mask, in which microbes precipitate minerals like calcite around the site of a dead organism. This hypothesis helps explain the different modes of preservation and possible organic matter preserved within mineral replaced cuticle, as calcite strongly fluoresces and commonly replaces biological material during fossilization. Crystal lattice defects and organic compounds can also cause fluorescence emission in different spectral regions. Continuing research will help delineate the chemistry of these fossils, as well as explore the possibility that some of the signal is related to preserved remains of biosynthesized compounds, as living spiders possess fluorescent biological components including cuticle, guanine, and spider hemolymph, which contains fluorophores.