Why did trilobites decline in shallow-water habitats during the Ordovician Period?

During the Ordovician Period, the decline of trilobites in shallow-water habitats was primarily driven by the Ordovician radiation, a period of intense biological diversification.12

As new marine invertebrates—such as articulate brachiopods, gastropods, and cephalopods—emerged and expanded into shallow-water environments, they occupied ecological niches previously dominated by trilobites and inarticulate brachiopods. This process forced the older, Cambrian-era fauna to retreat, causing trilobites to become increasingly rare in shallow settings and restricted primarily to deeper-water habitats as the period progressed. This shift was part of a broader trend where biological communities became more complex, with new species exploiting unutilized niches and expanding into diverse habitats like reefs and crinoid gardens.13