Biogeographical patterns and ecological differentiation of Olenidae across the Cambrian-Ordovician transition

The palaeogeographical distribution of olenid trilobites shifted markedly from the Miaolingian (Guzhangian) to the Ordovician, reaching cosmopolitan range around the Cambrian–Ordovician boundary. Using genus-level occurrences, cephalic geometric morphometrics, and multivariate analyses, we examine the processes underlying this expansion. Olenids initially flourished in dysoxic, siliciclastic settings, achieving broad distribution and high morphological disparity around the Cambrian–Ordovician boundary. Their expansion was enabled by strong dispersal capacity, ecological plasticity, and environmental opportunity, allowing them to occupy habitats inhospitable to most other faunas. During the Furongian, biogeographical structure emerged, with mid-latitude siliciclastic regions hosting taxonomically and morphologically diverse assemblages, while low-latitude carbonate settings held modal morphologies, reduced disparity, and higher endemism, largely restricted to dysoxic deepwater facies. Tremadocian faunas became globally homogenized, reflecting intense late Furongian dispersal; while disparity increased, indicating broader ecological tolerance. As Ordovician ecosystems became more complex and competition intensified, olenids became increasingly restricted to stressed or marginal habitats, persisting only as minor components, except for triarthrinids, which underwent a minor Middle-Upper Ordovician radiation driven by derived ecological strategies that allowed them to persist in increasingly complex marine ecosystems. Overall, ecological dynamics—rather than palaeogeographical constraints—primarily shaped olenid evolution, governing both their expansion and decline.