Environmental flexibility across dynamic Cambrian ecosystems fueled trilobite success

Trilobites, Paleozoic arthropods, have been viewed as marine animals. Using multiproxy geochemical analyses, we suggest that Cambrian Series 2 to early Miaolingian strata in Morocco, preserving three-dimensional trilobites, record deposition within a river- and wave-influenced delta with variable salinity and episodic freshwater influence, including brackish conditions. These populations persisted through environmental transitions, spanning cool, oxic, low-salinity arid conditions with periodic freshwater input during Cambrian Stage 3-4 to warmer, more humid, increasingly saline, and anoxic settings at the transition to Wuliuan. This transition coincides with an increase in trilobite body-size and the emergence of more stable, deeper, and marine salinity environments that could have contributed to enhanced biomineralization, ecological specialization, and habitat broadening. Smaller-bodied Stage 3–4 trilobites appear adapted to dynamic shelves, whereas larger-bodied Wuliuan forms exploited increasingly stable conditions that may have favored size enlargement and exoskeletal thickening. These patterns suggest a potential link between environmental conditions and body-size distribution, with possible implications for ecological structuring across Cambrian ecosystems.