Unveiling an ignored taphonomic window in the Early Cambrian Chengjiang Biota

The exceptionally preserved soft-bodied fossils from the Chengjiang Biota of southwest China have proved to be critical for our understanding of the biodiversity and ecology of the Cambrian explosion of animals. Almost all previously reported soft-bodied Chengjiang fossils are from the event mudstone beds (EB) of the Yu'anshan Formation. Here, we present the first comprehensive investigation of the soft-bodied fossils from the interbedded background mudstone beds (BGB), based on collections made across the past decade, and characterize their preservation. Soft internal cellular organs/tissues, such as digestive tracts and glands, cardiovascular systems, nervous systems, and musculature, are predominantly preserved as carbonaceous compressions, in contrast to their iron oxide-rich preservation in the EB. Our work highlights two distinct preservation modes within a single Burgess Shale-type Lagerstaette, possibly due to the complex interplay of decay, kerogenization, and pyritization.