Crustacean Microcoprolites from the Luoping Biota: Evidence of Primary Producers in the First Modern-Type Marine Ecosystems

Modern-type marine ecosystems became established in the Triassic following the devastation of the Permian-Triassic mass extinction. Attention has been focused on higher levels of these food chains, but the primary levels have not been established in detail. Here, we present evidence of microcoprolites from the Luoping biota (early Anisian, Middle Triassic) that lophogastrid shrimps were feeding on cyanobacteria, evidence for the primary producers and consumers in the system. Microscopic study of the microcoprolites, using SEM, EDS elemental analysis, micro-CT and FIB-SEM scanning, and Raman testing, revealed that crescent-shaped, symmetrically arranged canals in microcoprolites indicate a possibility that these micron-sized pellets belong to the ichnospecies Palaxius isp. produced by crustaceans. Comparison of the microcoprolites, with their preserved canals, with fossilized intestines from lophogastrid shrimps confirms their source, based on similarities in ultrastructure and chemical composition. Our discovery confirms the rise of Modern-type zooplankton, represented by lophogastrids and other arthropods, replacing extinct Palaeozoic marine invertebrate taxa while the phytoplankton may still have been primitive and represented by cyanobacteria.