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Marine ecosystems with complex trophic structure began to develop in the early Cambrian. Evidence from exceptional biotas (e.g. Chengjiang in S. China) and other fossil sites indicates a high level of biological interactivity (e.g. prey–predator relationships) and the colonization of a wide range of pelagic and benthic niches by predators, scavengers, detritus feeders and filter-feeders. A chain of biotic innovations and events seems to have played a crucial role in the construction of modern trophic webs: (1) the achievement of complex nervous systems, visual organs and motor functions; (2) the development of predation (e.g. by arthropods and infaunal worms); (3) the colonization of new niches (e.g. water column); and (4) the introduction of new tiers such as the mesozooplankton (e.g. chaetognaths, arthropods). All of them changed the dynamics of the ecosystem (energy flow, transfer of biomass, food source), introduced new selection pressures, triggered a cascade of adaptive behavioural and morphofunctional responses in the trophic layers, and catalysed biological diversification. The role of non-biological factors (e.g. oxygen, water chemistry, carbon cycle, climates) may have been important in the early stages of metazoan evolution and ecospace colonization, but these were probably negligible in the ecosystem build-up process itself. The early Cambrian global re-organization of the marine ecosystem resulted in more complex trophic links and stronger interactions between plankton and benthos. This unprecedented increase of interdependence between organisms, whose dynamics through the Precambrian–Cambrian transition still needs detailed explanation, made the marine ecosystems as a whole, and for the first time in their history, highly vulnerable to environmental perturbations such as climatic variations, changes in ocean chemistry and ultimately human action.

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