river dolphins are thought to have relictual distributions, that is, their ancestors originally occupied marine habitats, but were then displaced from these habitats by modern dolphin lineages. many of the morphological similarities and adaptations to freshwater habitats arose due to convergent evolution; thus, a grouping of all river dolphins is polyphyletic. amazon river dolphins are actually more closely related to oceanic dolphins than to south asian river dolphins. "isthminia panamensis" is an extinct genus and species of river dolphin, living 5.8 to 6.1 million years ago. its fossils were discovered near piña, panama.