Fossil hints that orcas don’t have a long history of killing whales

A 1.4-million-year-old fossil relative of killer whales had teeth that suggest it ate small fish rather than large marine mammals

Life 7 March 2022

Artistic reconstruction of Rododelphis stamatiadisi

Artistic reconstruction of Rododelphis stamatiadisi

Rossella Faleni

A 1.4-million-year-old ancestor of orcas and false killer whales seems to have dined on small fish – which suggests its descendants adapted to hunt larger prey such as other dolphin and whales relatively recently.

“This is an extraordinary fossil that helps qualify the origin of the unique diet of killer whales and false killer whales,” says Giovanni Bianucci at the University of Pisa, Italy. “It’s difficult to give a precise date, but these cetaceans seem to have evolved to eat marine mammals less than a million years ago.”

Orcas (Orcinus orca) – also known as killer whales – and false killer whales (Pseudorca crassidens) are the only living species of cetaceans that feed on other marine mammals. When they evolved to do so is unclear because cetacean fossils are exceptionally rare, says Bianucci.

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So Bianucci was pleasantly surprised to learn that a private collector had found a skeleton of a Pleistocene-epoch false killer whale on the Greek island of Rhodes, in 2021. The specimen was donated to the Stamatiadis Museum of Mineralogy and Paleontology on the island.

Analyses of the specimen revealed that the fossil was new to science, says Bianucci. His team named it Rododelphis stamatiadisi in honour both of Rhodes and of the collector, Polychronis Stamatiadis.

The scientists noted that the R. stamatiadisi fossil had a body about 5-metres long, which is similar to modern false killer whale – but it had smaller teeth, says Bianucci. In addition, its teeth lacked the deep grooves seen in killer whale and false killer whale teeth – which are adaptations important for crunching tough mammal bones.

This suggests that the species probably hunted smaller prey – more evidence for which comes from the fossilised remains of blue whiting fish (Micromesistius poutassou) found within the ancient dolphin. M. poutassou – which still exists today – is a 30-centimetre-long fish. It was clearly the R. stamatiadisi’s last meal, says Bianucci.

Fitting R. stamatiadisi into cetacean genealogy, Bianucci and his colleagues determined that the animal shared a common ancestor with modern false killer whales – and that this ancestor descended from the same line that orcas did.

Significantly, early members of the orca group also seem to have hunted small prey rather than dolphins and whales. The group includes a 4-million-year-old species (Orcinus citoniensis) found fossilised in Italy in the 19th century, which also lacked mammal-crunching teeth, says Bianucci.

The findings hints that cetacean-hunting predators evolved independently in both killer and false killer whales during the second half of the Pleistocene, beginning about 1.3 million years ago, says Bianucci.

The find is important, he says, because it might help overturn a popular theory that baleen whales, such as blue and humpback whales, became gigantic in size in order to avoid attack by killer whales. The baleen whales were already huge by about 3 million years ago – long before Bianucci believes the orcas and false killer whales began hunting them.

Journal reference: Current Biology, DOI: 10.1016/j.cub.2022.02.041

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