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Ancient killer whale snacked on fish, sharks, and…other whales

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Posted January 15, 2019

The fossil record of ancient whales is incredible, revealing to us how these sea-swimming giants evolved from their terrestrial ancestors, and adapted to a range of different lifestyles.

Image credit: Voss et al., 2019

Image credit: Voss et al., 2019

One extinct species called Basilosaurus was the largest apex predator of its time, reaching up to 18 metres in length – more than twice the length of a fully-grown killer whale! Now a new study has revealed what one individual consumed for its final meal before death.

The new fossil comes from a place called Wadi Al Hitan, or the ‘valley of whales’ near Cairo, Egypt. They were discovered by the team back in 2010, and identified as belonging to the species Basilosaurus isis.

Map showing the location of the Wadi Al Hitan UNESCO World Heritage Site in the Western Desert of Fayum Province, Egypt. (Voss et al., 2019)

During the late Eocene, around 38-34 million years ago, this locality was about as opposite to how it is now as possible. Much of Egypt was covered by a shallow sea, the remains of which are preserved in the geology and wealth of marine fossils known. Abundant fossils from the valley of whales include fish, sharks, crocodiles, sea snakes, turtles, and a number of different whale species.

One common species of whale found here is known as Dorudon atrox, much smaller than the giant Basilosaurus, growing only up to around 5 metres in length. The sharp, pointed teeth of Basilosauruswould have made light work of Dorudon, and this is well-evidenced by the new fossils. Many of the bones of Dorudon showed signs where they had been crushed or bitten by the teeth of Basilosaurus, and some were even found within their stomach cavity regions! Many of these bite marks were even on the skull bones of Dorudon, showing that they had been actively predated on by the ancient orcas. And possibly also chewed and swallowed alive…

Map of Basilosaurus isis with locations of other vertebrate specimens superimposed. (Voss et al., 2019)

Alongside the bones of at least two individuals of Dorudon were also those of the large shark, Carcarochles sokolow and the bony fish, Pycnodus. So one thing we know is that Basilosaurus at least enjoyed a variable diet, and nothing in the ancient oceans was safe from its jaws.

This hunting behaviour is similar to the modern killer whale (Orcinus orca), which also often predates on other smaller whales and even sharks. They have been known to feed even on the calves or juveniles of humpback whales, but now we know this behaviour goes back many millions of years to the ancestors of modern orcas.

These predator-prey relationships are notoriously difficult to find direct evidence of in the fossil record, and are invaluable in helping to accurately reconstruct past ecological networks. Thanks to these new fossils, we now know that whale-on-whale predation dates back at least 34 million years.

Reference

Voss M, Antar MSM, Zalmout IS, Gingerich PD (2019) Stomach contents of the archaeocete Basilosaurus isis: Apex predator in oceans of the late Eocene. PLoS ONE 14(1): e0209021. https://doi.org/10.1371/journal.pone.0209021

Source: PLOS EveryONE

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