The Vertebrate Invasion of Fresh Water

The origin and early evolution of the vertebrates took place in exclusively marine conditions. From the Middle Cambrian chordate Pikaia, through the Upper Cambrian, Ordovician and early Silurian records, there is little evidence of non-marine vertebrate faunas. With the progressive closing of the Iapetus Ocean, there developed from Wenlock times possible brackish vertebrate faunas, dominated by thelodonts, anaspids and cephalaspids, suggesting the formation of a Palaeozoic equivalent of the Tertiary Sarmatic Sea, also noted for the evolution of endemic brackish and fresh water faunas. The end of Silurian (Pridolian) times was marked by the suturing of the Old Red Sandstone continent and the establishment of the vertebrates, thelodonts, heterostracans and cephalaspids in fluvial regimes. The amphiaspid heterostracans colonized non-marine habitats during Lochkovian or Pragian times near the northwestern margins of Angaraland. A separate invasion of fresh waters was accomplished during Wenlock or Ludlovian times by the galeaspids, a group of cephalaspidomorphs, known only from South China. The colonization of fresh waters marked perhaps one of the most important advances in the evolution of the physiology of the vertebrates. The jawless vertebrates, microphagous detrital feeders, heralded the invasion of predators, the benthonic placoderms and nectonic acanthodian fishes.