Independent adaptation to riverine habitats allowed survival of ancient cetacean lineages

The four species of "river dolphins" are associated with six separate great river systems on three subcontinents and have been grouped for more than a century into a single taxon based on their similar appearance. However, se veral morphologists recently questioned the monophyly of that group. By usi ng phylogenetic analyses of nucleotide sequences from three mitochondrial a nd two nuclear genes, we demonstrate with statistical significance that ext ant river dolphins are not monophyletic and suggest that they are relict sp ecies whose adaptation to riverine habitats incidentally insured their surv ival against major environmental changes in the marine ecosystem or the eme rgence of Delphinidae.