Laurussian land-plant diversity during the Silurian and Devonian: mass extinction, sampling bias, or both?

Abstract

In phytogeographic data sets, the number of assemblages or floras from each interval may provide a test of the influence of sampling intensity on land-plant diversity. Using a data set of Silurian and Devonian compression-impression plant genera from Laurussia and the Acadian terrain, regression of five measures of land-plant diversity (total diversity, mean genus richness of floras, median assemblage diversity, most diverse assemblage, and standing diversity at interval boundaries) against the number assemblages or floras from thirteen intervals suggests that sampling bias influences all of the diversity measures to some extent, including within-habitat measures. The standing diversity of land plants at interval boundaries, the measure least influenced by sampling (r = 0.65, p = 0.05), increased steadily from the Middle Silurian to the late Givetian/early–middle Frasnian boundary, fell sharply in the early–middle Frasnian and remained low throughout the late Frasnian–middle Famennian. Standing diversity rose dramatically in the late Famennian and Strunian (latest Devonian): the Frasnian–Famennian extinction event may have affected land plants. The standing diversity of Silurian and Devonian microspore genera at interval boundaries mirrors that of compression-impression genera: neither record supports a land-plant diversity equilibrium during the Devonian.