Thalassinidean shrimp ecosystem engineering and trophic amensalism as drivers of molluscan community collapse during the Miocene

Ecosystem engineers are organisms that fundamentally reshape their habitats, creating distinct beneficiary and victim taxa within ecological communities. While their positive roles are well-documented, their potential to exert negative, destructive effects is less explored in the fossil record. This study investigates the intricate relationship between thalassinidean decapods (producing Thalassinoides and Glossifungites ichnofabrics) and molluscan communities within the Upper Miocene Dar Pahn Unit, Makran, SE Iran. Through integrated sedimentological, taphonomic, and ichnological analysis, we identify a clear pattern: the proliferation of large, complex thalassinidean burrow networks is consistently associated with the collapse of dense, suspension-feeding bivalve communities, such as Dosinia and Anadara . We propose a model of “trophic amensalism via bioturbation,” whereby the intense sediment reworking by burrowing axiidean and/or gebiidean shrimps does not merely coexist with these molluscs but actively excludes them. The engineering activity fluidizes the substrate, smothers infauna, clogs filtration apparatuses, and destabilizes the seafloor, rendering it uninhabitable for most molluscs. This study provides a deep-time validation of a powerful and potentially widespread ecological process, demonstrating that the trace fossil record can archive fundamental regime shifts driven by negative species interactions. Furthermore, our findings bridge paleoecology and modern biogeochemistry, suggesting that such engineer-driven community turnovers likely had significant, yet overlooked, consequences for benthic-pelagic coupling and carbon cycling in ancient marine environments.