A rise in global oxygen content allowed for the evolution of large, predatory fish that were able to exploit the shallow tidal areas and swamplands as top predators. Several groups evolved to fill these niches; the most successful were the elpistostegalians. In such environments, they would have been challenged periodically by low oxygen levels in the water. In comparable modern aquatic environments like shallow
eutrophic lakes and swampland, modern
lungfish and some genera of
catfish also rely on the more stable, atmospheric source of oxygen. Being shallow-water fishes, the elpistostegalians evolved many of the basic adaptions that later allowed the tetrapods to become terrestrial animals. The most important ones were the shift of main propulsion apparatus from the tail fin to the pectoral and pelvic fins, and a shift to reliance on lungs rather than gills as the main means of obtaining oxygen. Both of these appear to be a direct result of moving to an inland freshwater mode of living. Fossils of
Panderichthys are dated to the
Givetian (around 385 million years old) or the
Frasnian (around 380–375 million years old). Thus, the oldest bone remains of elpistostegalians appear in the late Middle Devonian or in the early Late Devonian. However,
a series of trace fossils from the early Middle Devonian of Poland suggests that tetrapod-limbed animals may have existed as early as in the
Eifelian, around 395 million years ago. ==Traits==