An exceptionally preserved Cambrian Psammichnites assemblage as a window into the earliest extensive horizontal-burrowing ecosystem engineers in shallow marine environments

Trace fossils provide unique windows into animal-environment coevolution during the Cambrian Explosion. Here we report an exceptional middle-Cambrian trace fossil assemblage from Xuzhou, China, preserving shallow marine siltstone-to-fine-sandstone substrates with diverse traces recording continuous morphological transitions. Among these, Psammichnites—the largest, longest and most structurally-complex macroscale horizontal burrows, penetrating up to three centimeters depth and first appeared in Fortunian shallow marine environments—is particularly abundant. The ichnotaxonomical consistency, morphological conservatism and continuous occurrences of Psammichnites gigas through the Cambrian make P. gigas from this post-Fortunian locality a window into its early representatives back to the Fortunian. The documented transitional morphologies of Psammichnites enable reconstruction of a model with six vertically-arranged levels. Specimen-based
analyses support Plagiogmus and Olivellites as junior synonyms, identify Taphrhelminthopsis as a preservational variant, and reveal close bauplan similarities with Climactichnites. This relationship clarifies the taxonomic and behavioural diversity of early burrowers and implies anatomical resemblances between shallow-marine engineers and some of the earliest subaerial explorers. Our analysis further reveals that Psammichnites tracemaker was a short-bodied, mollusc-analogous animal potentially with a ventral foot and mouth, rhythmic locomotion, mucus secretion, advanced sensory capabilities and plausible planktonic larvae, representing the essential ecosystem engineering innovations in less-explored, dynamic nearshore environments. By physically and chemically reworking sediments and destabilizing microbial matground, especially at Fortunian, Psammichnites profoundly enhanced sediment-water nutrient exchange at unprecedented fluxes and scales. And their Fortunian
pioneers might have represented the earliest large-scale centimetric-depth horizontal bioturbation, serving as important complements reshaping nearshore benthic ecosystems, before the vertical-burrow-dominated Cambrian shallow-marine substrate revolution.