Diversity patterns of the vascular plant group Zosterophyllopsida in relation to Devonian paleogeography

The Zosterophyllopsida originated in the Silurian and became prominent vascular components of Early Devonian floras worldwide. An updated dataset of zosterophyllopsids at species level is analyzed to compare the taxic composition of five putative paleophytogeographic units, Laurussia, Siberia, northwestern Gondwana, Kazakhstan and northeastern Gondwana (i.e., Australia, China and the Shan-Thai block). The high level of endemicity shown by each unit confirms the phytogeographic differentiation and the occurrence of geographical barriers preventing massive floral exchanges between the corresponding regions for the Late Silurian–Early Devonian time interval. Statistical analyses conducted on the three largest datasets, those corresponding to Laurussia, Siberia and northeastern Gondwana, indicate that the diversity dynamics of the group followed the same pattern in these regions. Almost all taxic diversity measures show that specific diversity was greatest in the middle and late Pragian. Diversity dropped significantly thereafter; however, residual diversity reveals genuine regional patterns. From this, we show that the radiation of the Zosterophyllopsida may have stopped earlier in northeastern Gondwana and Siberia than Laurussia. We propose that the onset of these extinctions resulted from the competitive replacement of the zosterophyllopsids by increasingly diversified lycopsids and basal euphyllophytes whose evolution could have been favored by external factors.