Euendoliths versus ambient inclusion trails from Early Cambrian Kuanchuanpu Formation, South China

Abundant microstructures have been discovered in small skeletal fossils (SSFs) and embryo-like fossils collected from the Lower Cambrian Kuanchuanpu Formation (ca. 535 Ma) in Xixiang County, Shaanxi Province, China. These involve two co-occurring structures: (1) long, unbranched cylindrical filaments, which are comparable to phosphatic casts of microborings constructed by euendolithic cyanobacteria (Endoconchia lata) in morphology and preservation pathway; and (2) meandering micro-tubes or grooves on fossil moulds (and steinkerns) of a wide range of sizes and morphological diversities, perceived as ambient inclusion trails (AITs). Herein, we also report a new occurrence of organic carbon spherules as AIT-propelled material, which is rare in comparable fossils. From direct comparison of endolith fossils and AITs, we propose a mechanism to account for their notably different preservation, and further attempt to offer an explanation for their co-occurrence. Their differential preservation suggests a chronological, taphonomic sequence of their formation. We interpret that E. lata microborings formed prior to phosphate sedimentation, whereas AITs are likely generated in a later phase of (or after) phosphorite precipitation but before calcareous re-cementation. Dissecting the sequence of formation of these structures, in conjunction with detailed morphological observations, assists in distinguishing true biologically produced endoliths from otherwise abiogenically produced microstructures.