Discussion on 'Tectonic and environmental controls on Palaeozoic fluvial environments: reassessing the impacts of early land plants on sedimentation'. Journal of the Geological Society, https://doi.org/10.1144/jgs2016-063

The first-order importance of tectonic and environmental controls for terrigenous sediment supply has rarely been questioned, but the role of vegetation in the modification of ancient alluvial signatures has been observed since the mid-20th century (Vogt 1941). Studies of sparsely vegetated rivers (Schumm 1968) and alluvial stratigraphic variation (Cotter 1978; Davies & Gibling 2010) led to observations of (1) plant modulation of alluvial signatures and (2) Palaeozoic facies shifts (PFS): unidirectional changes to facies diversity and frequency, in stratigraphic alliance with the plant fossil record. One PFS is the Siluro-Devonian appearance of mud-rich, architecturally complex alluvium, traditionally ascribed to meandering rivers, and sedimentologically distinct from pre-vegetation strata (Davies & Gibling 2010; Long 2011). Using selected secondary data, Santos et al. (2017) dispute the correlation of these observations using three key points, as follows. (1) The mid-Palaeozoic was typified by orogenic assembly of low-gradient equatorial continents and elevated sea-level, which led to tropical weathering (abundant fine sediment) and extensive alluvial plains. This drove the PFS by promoting river meandering independently of vegetation. (2) Meandering does not require vegetation; this is shown by examples in Precambrian deposits, on other planets, and in ‘non-vegetated’ deserts. Meandering rivers were more abundant than the pre-vegetation rock record suggests, owing to selective bypass and deflation of fine material. (3) Early Siluro-Devonian (meaning Ludlow–Early Devonian) land plants were too small, their biomass and cover too limited, and their wetland habitat too narrow to have stabilized meandering channels, influencing landscape little more than earlier microbial communities. We contest the conclusions and method of the paper, and deal with each point in turn

Palaeozoic landscapes shaped by plant evolution

Fluvial landscapes diversified markedly over the 250 million years between the Cambrian and Pennsylvanian periods. The diversification occurred in tandem with the evolution of vascular plants and expanding vegetation cover. In the absence of widespread vegetation, landscapes during the Cambrian and Ordovican periods were dominated by rivers with wide sand-beds and aeolian tracts. During the late Silurian and Devonian periods, the appearance of vascular plants with root systems was associated with the development of channelled sand-bed rivers, meandering rivers and muddy floodplains. The widespread expansion of trees by the Early Pennsylvanian marks the appearance of narrow fixed channels, some representing anabranching systems, and braided rivers with vegetated islands. We conclude that the development of roots stabilized the banks of rivers and streams. The subsequent appearance of woody debris led to log jams that promoted the rapid formation of new river channels. Our contention is supported by studies of modern fluvial systems and laboratory experiments. In turn, fluvial styles influenced plant evolution as new ecological settings developed along the fluvial systems. We suggest that terrestrial plant and landscape evolution allowed colonization by an increasingly diverse array of organisms