Diversity and paleobiogeographic distribution patterns of early and Middle Ordovician graptolites in distinct depositional environments of South China

An analysis of Ordovician graptolite diversity across several distinct lithofacies belts in South China, which represent nearshore, inner-shelf, outer-shelf, slope, and basin environments, indicates that diversity increases progressively from nearshore toward the outer-shelf, acquiring its maximum in the slope facies and thereafter decreases slightly in basinal facies. When scaled against graptolite zones and more equal duration ‘Time Units’, the graptolite diversity patterns of the Early and Middle Ordovician are broadly similar among the distinct environments of South China but exhibit some important differences. A synthesized graptolite diversity curve from the Early Ordovician to the Sandbian Stage of the Late Ordovician in South China displays a step-wise increase with three peaks: 1) middle Floian (Early Ordovician), 2) late Dapingian-early Darriwilian (Middle Ordovician), and 3) latest Darriwilian (Middle Ordovician), which closely coincide with transgressive events in South China, implying a likely correlation. The biogeographic distribution of four graptolites with different ecological preferences, Pseudisograptus, Isograptus-Parisograptus, Undulograptus austrodentatus, and Corymbograptus turgidus, displays a significant parallelism with the coast or shelf margin, suggesting that their distributions may be controlled by water depth, basin morphology, and the distance from the coast.