Physics.org posted an article by Jon Tennant with this title. It includes the following paragraph:
“But did you know that all marine mammals descended from common land-dwelling ancestors? It might be difficult to see that by looking at modern species alone, but that’s where the fossil record comes in handy. An accurate picture of their evolution is crucial for helping us to understand the structure of increasingly threatened aquatic ecosystems.
By looking in detail at the fossilised ancestors of marine mammals in order to understand their ecology, we can see that terrestrial mammals returned to the seas millions of years ago – this makes them secondarily aquatic. A major part of this involved the morphological and behavioural adaptations required to become specialised oceanic feeders. Anyone who’s ever tried to eat underwater will know exactly what we mean.”