TAPHONOMIC CONTROLS ON EARLY JURASSIC LAGERSTÄTTEN AND THEIR RELATIONSHIP TO THE TOARCIAN OCEANIC ANOXIC EVENT

Konservat-Lagerstätten – rare but exceptional fossil deposits with weakly or non-biomineralized tissues and articulated skeletal elements – provide windows into the evolution of life, such as the morphologies and ecologies of organisms as well as paleocommunity structures and phylogenetic relationships. The question of what environmental conditions and processes promoted the preservation of soft tissues has been a topic of research for several decades. Marine Konservat-Lagerstätten vary in abundance across geographic space and time due to changing global and local environmental conditions over time; for example, are more common in the Paleozoic than during the Mesozoic. That said, Lagerstätten from Jurassic and Cretaceous open marine settings appear to occur in stratigraphic intervals coincident with global anoxic events. Analyzing the mineralogy of these fossils can shed light on how they were preserved (i.e., their taphonomic pathway) and whether these anoxic events might exert global controls on Lagerstätten preservation.

Here, we investigated the taphonomy of fossils from three coeval Early Jurassic Konservat-Lagerstätten: the Strawberry Bank Lagerstätte, UK (a shallow marine lagoonal deposit), as well as the Ya Ha Tinda Lagerstätte, Canada, and Posidonia Shale Lagerstätte, Germany (both deposited in deep marine settings that often experienced dysoxia/anoxia). Scanning electron microscopy and energy dispersive X-Ray spectroscopy were used to obtain elemental composition and microstructure of the fossils. These approaches reveal the presence of phosphatized skeletons and soft tissues in all three deposits; therefore, we posit that phosphatization was the primary mode of exceptional preservation during the fluctuating oxygen conditions of the global Toarcian Oceanic Anoxic Event. Despite their geographic separation and unique paleoenvironmental settings, these three Early Jurassic Konservat-Lagerstätten illustrate highly comparable taphonomic pathways. From our observations, we infer that global changes in seawater chemistry associated with the Toarcian Oceanic Anoxic Event was the primary contributor to exceptional fossilization in the Early Jurassic, whereas regional influences had minimal effect on the preservation of these Lagerstätte fossils.