Ancient killer whales ate fish, not other mammals

The killer whale (Orcinus orca) and the false killer whale (Pseudorca crassidens) are the only living cetaceans, or oceanic dolphins, that feed upon other marine mammals, with pods of orcas even known to aggressively hunt and eat gigantic blue whales. However, the origins of this type of predatory behavior are unclear and fossil records for both of these species are very limited. 

In 2020, a research team led by the New York Institute of Technology (NYIT) discovered the remains of an ancestor of the false killer whale on the island of Rhodes in Greece. The species was named Rododelphis stamatiadisi after the island where it was found and the paleontologist who made the discovery, Polychronis Stamatiadis. The remains represent the first clear fossil evidence for the origins of the false killer whale, which is estimated to have lived during the Pleistocene about 1.5 million years ago. 

The researchers compared the anatomy of Rododelphis to today’s false killer whales and orcas, as well as to Orcinus citoniensis, the orca’s only known ancestor. Based on the dimensions of its skull, Rododelphis was roughly the same size as its modern counterpart, measuring 13 feet in length and weighing about 1,200 pounds. Next to the fossils, the scientists also discovered traces of what has probably been this creature’s last meal: fish bones.

While both Orcinus and Rododelphis had powerful jaw muscles and sharp interlocking teeth, these teeth were smaller and more numerous than those of today’s orcas. Moreover, the teeth of both these ancient whales lacked the rough scratches and chipping commonly caused by eating mammals, suggesting that their diet might have consisted entirely of fish.

These findings contradict the popular theory that large whales evolved such giant bodies to avoid predation. While the first giant whale emerged 3.6 million years ago, this study suggests that ancient dolphins began preying on other marine mammals such as whales much later, probably within the last three million years ago.

“The diversification of the oceanic dolphin family occurred within the last five million years, but fossil evidence from the Pleistocene epoch is exceedingly rare,” said study co-author Jonathan Geisler, an expert in marine mammal evolution at NYIT. “With Rododelphis, we’re now beginning to fill this gap and better understand the repeated evolution of feeding adaptations in oceanic dolphins—in other words, how both orcas and false killer whales separately evolved similar cranial anatomy and the behavior of feeding on other marine mammals.”

Further research and the discovery of more fossils are needed in order to establish with precision when these whales began changing their diet and started feeding upon other marine mammals.

The study is published in the journal Current Biology.

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By Andrei Ionescu, Earth.com Staff Writer