Slow Crawl Across the Salinity Divide: Delayed Colonization of Freshwater Ecosystems by Invertebrates

Ecologically complex communities occupied marine habitats <100 m.y. after the first appearance of multicellular animals during the Ediacaran Period of the late Neoproterozoic. By contrast, freshwater habitats, particularly habitats within the substrate, remained mostly uninhabited for another 200 m.y. This delayed colonization of freshwater substrate ecospace is reflected by the small amount of bioturbation in Upper Carboniferous to Triassic freshwater deposits and in the records of body and trace fossils. Terrestrial insects invaded some freshwater habitats left open by the paucity of immigrants from the ocean during the late Paleozoic, but insects did not inhabit the substrate extensively until later in the Mesozoic. Marine invertebrates were slow to acquire the osmoregulatory capabilities and reproductive and dispersal mechanisms characteristic of successful freshwater animals.