Toothed cetacean (Odontoceti) lineages in the Miocene and Pliocene evolved rostra that are proportionally more elongate than any other aquatic mammal or reptile, living or extinct. Their similarities in cranial proportions to billfish may suggest a convergent feeding style, where the rostrum is swept through the water to hit and stun prey. Here we calculated second moment of area from rostral cross sections of these fossil odontocete taxa, as well as from extant ecological analogues, to infer variation in feeding behavior. Our results show that the extremely long rostra of extinct toothed whales vary considerably in functionally relevant measures of shape and likely exhibited a diversity of feeding behaviors, ranging from those similar to modern odontocetes to those convergent with billfish. Eustatic sea-level and temperature maxima of the Miocene likely led to changes in prey characteristics or abundance that enabled the repeated evolution of this extreme morphotype, which later went extinct during late Pliocene climatic deterioration.