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

Abstract

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.

Microbially mediated fossil concretions and their characterization by the latest methodologies: a review

“…Optical (light) Microscopy helps in visualizing the fine details of an object by creating a magnified image through a series of glass lenses. Optical microscopy has been widely used in the study of concretions (e.g., Iniesto et al, 2015 ; Dias and Carvalho, 2022 ; Downen et al, 2022 ; Lin et al, 2023 ).…”

“…This technique can be used to identify structures in fixed and living biological samples ( Lichtman and Conchello, 2005 ; Sanderson et al, 2014 ), and helps in understanding the viability of living and dead bacterial cells, biofilms, EPS and calcium carbonate precipitates ( McMullan, 1995 ; Vernon-Parry, 2000 ). This technique has been recently utilized for investigation of internal architecture in steinkern spider fossils from Oligocene Aix-en-Provence Lagerstätte in southern France ( Downen et al, 2022 ), which identified prolific microbial mat communities as likely being responsible for the preservation of infilled spider molds.…”

Description of a new fossil genus of conifer sawfly (Hymenoptera: Diprionidae) revealed by UV light

“…1.5 m thick of very delicate and thin laminites. Its stratigraphy was extensively studied by Nel (1991) and Peinado (2002) who indicated that the laminites were generated by microbial mat activities, a result that was later confirmed by Downen et al (2022) when studying fossil Arachnida. The layer which has yielded the new fossil originates is very particular, being very rich in Silica, green-colored, with numerous dragonfly larvae, marine fishes, and Siphonophora, suggesting a putative marine influence at least in this Western part of the paleolake (Nel et al, 1987;Nel, 1991).…”