A Risso’s dolphin caught up in fishing line. Dolphin numbers in the Indian Ocean may have dropped by more than 80% in recent decades, with an estimated 4 million small cetaceans caught as “by-catch” in commercial tuna fishing nets since 1950, according to a study

Photograph: Andrew Sutton/Central Studio

Pygmy killer whale (Feresa attenuata)

Despite commonly being called “killer whales” modern orcas are actually the largest living members of the oceanic dolphin family. Their ancestors are thought to have diverged from other dolphins between 10 and 5 million years ago -- and surprisingly their closest relatives are the much smaller snubfin dolphins found in Australasia.

Living during the Pliocene (5-2 mya) in the Mediterranean, Orcinus citoniensis was an early member of the orca lineage, and was probably a transitional form between their early dolphin ancestors and the modern Orcinus orca.