Early tetrapods - the first four-footed vertebrates to truly walk on land - lived in variably salty waters such as those found in estuaries, according to a study published in Nature this week.
Although famous for emerging onto the land, tetrapods were primarily aquatic, with gills and a powerful tail for swimming. The type of water they lived in, however, has been heavily debated. The first fossils were found in sandstones initially thought to have been deposited in freshwater. However, the discovery of tetrapod remains and trackways in sediments derived from brackish (slightly salty) water and from seawater suggest that early tetrapods may have been able to tolerate varying salinities.
Jean Goedert and colleagues analysed 51 early tetrapod fossil specimens alongside associated armoured and lobe-finned fish, found in rocks from the Devonian Period (around 365 million-years ago) of northwest China and east Greenland. Using new methods that enable the measurement of isotope ratios from sulfur as well as carbon and oxygen,the authors could distinguish freshwater animals from their marine counterparts.
After demonstrating their sulphur isotope approach on modern vertebrates, including crocodiles, red-eared terrapins and various fish, the authors determine that the fossil tetrapods and other vertebrates they studied lived in environments that had a mixture of fresh and seawater - such as is found in estuaries and deltas. The authors conclude that it is therefore likely that tetrapods could cope with varying levels of brackishness. This versatility may have helped them survive the extinctions of the Late Devonian, before going on to colonize the land.