ALBERT

Description: We thank Davies et al. (2017) for their comments and welcome the opportunity to further discuss the role of early land plants in fluvial environments. Critically, Davies et al. (2017) note that although testable hypotheses exist for the possible role of early land plants they remain untested, and thus there is correlation (between an increase in meandering systems and early plants) without any mechanistic causation. In our paper (Santos et al. 2017) we challenged this causation, offering an alternative mechanism for increasing meandering channels based on a unique set of tectonic and environmental controls. It is important to be clear that Santos et al. (2017) focused on this meandering transition and the possible role of early land plants. Davies et al. (2017) conflate our comments regarding the impacts of early land-plant evolution with the longer-term influence of vegetation; there is little doubt over the longer-term role played by vegetation, particularly in stabilizing deeply rooted soils. Fluvial sedimentary successions are strongly influenced by tectonics, climate, sediment flux and sea level, and we question whether early land plants of limited size and restricted occurrence were able to dominate over such controls during terrestrialization (see Leeder 2007). We maintain our principal argument that a series of abiotic factors conspired to induce a worldwide increase in the proportion of meandering rivers, which in turn favoured development of an environment appropriate for land-plant evolution and colonization of the continents.

Description: The apparent increase in occurrence of meandering fluvial channel systems in the Middle Palaeozoic has long been related to the effects of land-plant colonization. However, evidence for meandering channels in non-vegetated settings is shown by pre-vegetation successions on Earth, from the prevalence of meandering channels on Mars, from physical modelling of meandering channels, and from non-vegetated channels in modern desert basins. In addition, early land plants had small dimensions, were limited in their occurrence, and were dependent on environmental factors. Here, we question the capacity of early land plants to impose the major impacts suggested by current models. We propose that the sudden widespread occurrence on Earth of fluvial deposits indicative of the accumulation of meandering river systems in the Middle Palaeozoic was primarily an effect of environmental and tectonic conditions that prevailed during this period. These conditions induced a worldwide increase in the proportion of meandering rivers, which in turn helped favour the appropriate environment for land-plant colonization of the continents. We propose that land plants opportunistically took advantage of an appropriate global environment, which enabled them to thrive in continental environments. Fluvial environments characterized by single-channel systems and stable floodplains facilitated the greening of the land.