Steinkern spiders: A microbial mat-controlled taphonomic pathway in the Oligocene Aix-en-Provence Lagerstätte, France

The Aix-en-Provence Formation is an Oligocene (22.5 Ma) Lagerstätte in southern France that contains an abundance of soft-bodied fossils preserved in exceptional detail. Many taxa have been described from this formation, including insects, spiders, fishes, and plants, suggesting a diverse ecosystem in a subtropical, brackish, lacustrine paleoenvironment. Fossil spiders from this deposit are preserved as compression fossils and internal and external molds. Recently, compression fossils of spiders from Aix-en-Provence were hypothesized to be a product of a taphonomic pathway based on diatoms and sulfurization. Here, we examine fossil spiders preserved as molds to uncover a second taphonomic pathway based on microbial mats. Evidence of microbial mats include wrinkles, pustular textures, and possible microbial mat chips on the bedding surfaces as well as a matrix fabric that contains possible microbial sheaths and bacterial spherules. The evidence presented here supports prolific microbial mat communities during deposition of the Aix-en-Provence Formation, and suggests that they are likely responsible for the moldic preservation of the spiders. Our work shows that the paleoenvironment of the Aix-en-Provence Formation promoted at least two possible taphonomic pathways that resulted in the differing modes of preservation observed.