HOW LAKES GOT THEIR FAUNAS: THE INITIATION AND DEVELOPMENT OF PALEOZOIC LAKE FAUNAS

Lakes are important archives for continental records of paleoenvironmental, paleoclimatic and macroevolutionary patterns that occurred when faunas invaded the continental realm. In order to document that pattern, we compiled a database of over ninety lake basins from the Neoproterozoic to Permian. Looking at this critical time in lacustrine evolution, several patterns emerged. Lakes during the Neoproterozoic through Silurian time were oligotrophic and their nutrient cycling regimes were primitive. It is not until the establishment of land plants in the Silurian that lakes begin to respond with higher diversities and more complex physical and chemical conditions. During the Devonian-Carboniferous periods, diversity was on the rise as trophic levels became more complex. Globally, CO2 increased while marine Sr decreased, coinciding with the peaking of diversity within lakes. Most lakes of the Devonian and Carboniferous formed along continental margins or in tectonic basins with occasional connection to the marine realm. The faunas from these types of lakes were commonly comprised of mixed marine and freshwater elements and were far more diverse than other, more inland lakes. This “estuary effect” created a gateway or filter through which faunas invaded the continental realm. The early history of lake faunas is one of opportunity and amelioration. The feedback loops created by the establishment of vascular plants altered the nutrient cycle on land and in lakes. All trophic levels were established early, but became increasingly complex throughout the Paleozoic. Groups invading the continents via the “estuary effect” did so numerous times before establishing themselves permanently. This was linked with the episodic re-establishment in marine-freshwater connections along these continental margins. In general, the macroevolutionary history of lake faunas demonstrates a dramatically different diversification pattern than that of the marine. Further study is necessary to understand the intricacies of these patterns and whether or not they continue through the Mesozoic and Cenozoic eras.