THE EARLY DEVELOPMENT OF CONTINENTAL ECOSYSTEMS REVISITED

Life on the Late Devonian continents reflected the evolution of continental ecosystems through the Ordovician and Silurian. Initially, soil crust and polsterland associations apparently occupied the the full range of terrestrial habitats from upper littoral to upper supralittoral. The phototrophs were likely cryptophytic cyanobacteria (with hemiocryptophytic lichens and eoembryophytes in wetter sites) and the consumers, micro- and meiofaunal, primarily endedaphic, bacteria, protists, and ecdyozoans. An abundance of light and carbon dioxide (perhaps 15X PAL) favored photoproduction, but the inability of the phototrophs to access deep-seated water and nutrients, and exposure of both producers and consumers to physical adversity restricted ecological suites, guilds, modes, and webs. Less is known of aquatic ecosystems (except that some included arthropods and jawless fish); this paucity of evidence suggests similar ecological limitations reflecting the effect of the terrestrial ecosystems on the availability of resources and incidence of adversity in aquatic ones. The rarity of fish may reflect the effect on their respiration of the acidity of freshwater habitats associated with the high level of atmospheric carbon dioxide. The character of continental ecosystems shifted radically through the later Silurian and in the Devonian with the evolution of tracheophytes. Since the latter accessed water and nutients from deeper in the soil and established a canopy well above the surface, they increased the amount and duration of photoproduction and reduced physical adversity and thus induced progressive, self-generating change in aquatic as well as terrestrial ecosystems to the condition in Late Devonian continental ecosystems.