Paleozoic lake faunas: Establishing aquatic life on land

Lakes are important archives for continental records of paleoenvironmental as well as paleoclimatic change. They also record a unique macroevolutionary pattern that occurred when faunas invaded the continental realm. In order to document that pattern, we compiled a database of over one hundred and six lake basins from the Neoproterozoic to Permian.Looking at this critical time in lacustrine evolution, several patterns emerge. For the Neoproterozoic through Silurian time, there is a paucity of documented lake deposits and lake faunal records. However, the records that are available indicate that lakes were oligotrophic and their nutrient cycling regimes primitive. It is not until the establishment of land plants in the Silurian that lakes began to respond with higher diversities and more complex physical and chemical conditions. During the Devonian–Carboniferous periods, diversity was on the rise as trophic structure became more complex. 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, as roles changed and faunal elements became established. 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.