Apparent changes in the Ordovician-Mississippian plant diversity

This study aims to approximate an illustrative vision of the apparent temporal dynamics of land plant diversity during the terrestrialization process (i.e., the invasion of the land by plants). This problem has often been addressed through studying the Silurian-Devonian megafossil-based diversity patterns of land plants (embryophytes). However, the inclusion of the dispersed spore fossil record is essential for characterizing the first steps of this process (i.e., Eoembryophytic flora). Consequently, this new study includes both spore taxa and plant megafossils in order to discern the diversity trajectories from the first phases of early land plant diversification until the stabilization of early Carboniferous forests (i.e., Palaeophytic flora). The diversity patterns of the primary recognized plant lineages (i.e., Lycophyta and Euphyllophyta) have been also traced in order to discern the origin of the major fluctuations of plant megafossil-based diversity. Results reveal that while the dispersed spore diversity curve shows a sustained increase towards the end of the Devonian Period - with a first maximum towards the late Silurian - the megafossil embryophyte diversity curve is characterized by a set of sequential ascending peaks at the Pragian (Early Devonian), Givetian (Mid Devonian), and Visean (Mississippian), with a single significant depletion in the Eifelian (Mid-Devonian). The comparative analysis of the lycophyte and euphyllophyte diversity patterns suggests that the substantial embryophytic diversity changes are driven by diversification within lineages rather than by the rhythm of the appearance of key morphological traits. This evidence (1) advocates for an intrinsic ecological control on the apparent changes in taxic richness and (2) implies that the observed plant diversity dynamics respond to the overlapping of the various expansion phases of the plant lineages involved. (C) 2015 Elsevier B.V. All rights reserved.