Colonisation of the Land

The image of the colonisation of the land by plants as a spectacular event, when barren wastes suddenly became verdant triggered by a change in some extrinsic environmental factor, is fading. It derived from the appearance and rapid diversification of vascular plants, unequivocal land colonisers, and, almost simultaneously, the earliest terrestrial arthropods, in the late Silurian and early Devonian. The environmental stimulus usually cited hitherto was a reduction in ultraviolet (UV) radiation due to ozone production when oxygen levels increased. A growing amount of evidence suggests that terrestrialisation was a gradual process, occurring throughout the earliest Palaeozoic, possibly with origins in the Precambrian. As the atmosphere evolved, particularly with respect to partial pressure of oxygen (Po,), so land surfaces were modified as mats of microbes and thallophytes produced substrates more convivial for colonisation by larger plants (macrophytes) and animals (Holland 1984; Retallack 1985). Thus to define the physical parameters of atmosphere and hydrosphere associated with the colonisation of land is to trace their history throughout the early Palaeozoic. Direct evidence is lacking. Considering the hydrosphere, analysis of fluid inclusions in evaporites suggests that the major constituent composition of seawater, including Na+ and Cl-, has changed little over the last 900 Myr. Values from rocks as old as Permian confirm this general observation, but Silurian results are less secure (Holland et al. 1(86). Freshwater systems may have been depleted in minerals prior to the establishment of extensive terrestrial vegetation and associated chemical weathering, even allowing for retention of some minerals within organic