Geobiology of Early Triassic coprolites from the Nanzhang Yuan’an Fauna: Implications for fossil phosphatization

Coprolites provide us with valuable archives to decipher predator-prey relationships in ancient ecosystems. Likewise， they can provide a special taphonomic window to examine non-mineralized organisms， aiding a better understanding of fossilization. In this study， detailed geobiological analyses were undertaken on Early Triassic phosphatized coprolites of the Nanzhang-Yuan’An fauna (NYF). Using a combination of scanning electron microscopy， Raman spectrometry， and transmission electron microscopy analyses， three microstructures were commonly discovered within various coprolite morphotypes. The Type-1 microstructures are characterized by microspherulites (0.5-3.5 μm) with dumbbell-shaped structures. The Type-2 microstructures are represented by hollow， capsule-like microstructures (0.5-1.1 μm in length and 0.25-0.7 μm in width) that form colonies. The Type-3 microstructures are snowflake-shaped grains that occur singularly or are organized into patchy aggregates. The size， shape， organization， texture， and mineralogical features together suggest that the Type-1 and Type-2 microstructures were two distinct forms of putative fossilized bacteria， while the Type-3 microstructures were goethite crystals. The widespread presence of these two fossil bacteria in NYF coprolites suggests that they may have facilitated faecal phosphatization through synergistic microbial metabolism.