UNEXCEPTIONAL FOSSILS, EXCEPTIONAL INSIGHTS: USING PROTEROZOIC CARBONACEOUS FOSSILS TO UNDERSTAND THE BURGESS SHALE TYPE PRESERVATION

The carbonaceous compression taphonomic pathway remains poorly understood, although it is responsible for the preservation of some of the most paleontologically significant fossils found in the rock record, such as the Chengjiang and Burgess Shale biotas. While paleontologists are reluctant to destructively analyze these rare and unique fossils, analyzing more common carbonaceous compressions can serve as a proxy for their exceptionally preserved counterparts and may provide a broader understanding of the Burgess Shale type preservation. This study examines, in both plane view and cross section, several abundant carbonaceous compression taxa including Chuaria and Vendotaenia fossils from the Neoproterozoic Doushantuo and Denying Formations in South China, as well as acritarchs from the Mesoproterozoic Ruyang Group of North China.

Observation and analysis of the fossils using traditional light microscopy, environmental scanning electron microscopy, and energy dispersive x-ray spectroscopy reveal complexities to carbonaceous compressions previously undescribed. Chuaria fossils from differing lithologies are seen in cross section to be composed of a layer of probable aluminosilicates concentrated to one side of a thinner carbonaceous layer. Smaller Chuaria fossils are preserved as a carbonaceous layer, or are completely casted in pyrite framboids. Vendotaenia fossils are preserved as a carbonaceous lamination with a sulfur-rich layer concentrated to one side of the compression. Ruyang Group acritarchs are also associated with probable aluminosilicate layers, similar to those seen with Chuaria fossils. This variation in preservation suggests that while there may be only a limited set of conditions that allow fossils to be preserved as carbonaceous compressions, there may be multiple pathways that lead to these conditions. In addition, while carbonaceous compressions can be preserved simply as a carbonaceous layer, very often they are preserved in conjunction with other taphonomic pathways as well, such as with clay coats or pyritization.