Are biogeographic provinces discrete or gradational: a test in the Late Ordovician of Laurentia

Provinces are the standard method of characterizing the spatial distributionof communities in ecology and biogeography. Provinces do not always exhibitclear boundaries or homogenous and compellingly distinct compositions. Similarprovinces with spatially meaningful compositional overlap may be divisions along a biogeographic gradient. Four provinces have been recognized in the Late Ordovician of Laurentia: Appalachian, Southern, Midcontinent, and Western. These provinces correspond to geochemically distinct water masses based on isotopic analysis, and have been documented for several taxonomic groups. Spatial trends in the Jaccard similarities and ordination scores of faunal communities in these provinces presented here suggest these provinces are divisions of a continent-scale gradient driven by siliciclastic input and associated environmental effects of the Taconic Orogeny. Gradients can be useful in considering faunal distribution in terms of influential environmental factors and vice versa. Observation of a gradient where provinces have been inferred suggests gradients are insufficiently documented in paleobiogeography.