Together with false killer whales, orcas are the only marine mammals that prey on other cetaceans. Credit: Getty Images/imageBROKER RF. 
Part of the study is based on the analysis of the fossil remains of a member of the Delphinidae family, found in 2020 on the island of Rhodes, Greece, by amateur palaeontologist Polychronis Stamatiadis. The fossil was identified by the authors as a new species, and named Rododelphis stamatiadisi. Phylogenetic analysis showed that it is closely related to false killer whales, and lived about 1.5-1.3 million years ago. Strikingly, the remains of its last meal were also found near the mandibles: five otoliths belonging to a relative of cod, Micromesistius poutassou.
The study also examines Orcinus citoniensis, the only known ancestor of the modern killer whale. Its fossil was discovered in Tuscany in 1883 by Giovanni Capellini, but this was the first time it was thoroughly analysed. Studying the evolutionary trends of delphinids’ behaviour is challenging, explains Bianucci, because of the rarity of fossils and their recent evolutionary radiation (i.e., the increase in the number of new species). “We have been fortunate enough to come across the fossilized remains of the ancestors of both cetacean species that feed on large prey, such as blue whales,” he notes. “Besides, we had the luck to determine that the last meal of R. stamatiadisi was a fish about 30 cm in size”.
The skull of Orcinus citoniensis, the fossil killer whale found in Cetona (Tuscany), displayed at Museo Giovanni Capellini, Università di Bologna. Credit: G. Bianucci.
The team investigated the body size, the shape, size, wear and number of teeth, the width of the mandibles and other details of the skull, and found that neither species were specialized in hunting very large preys, nor did they feed by sucking the prey into their mouth like pilot whales do. Most likely, their feeding traits were intermediate between pilot and false killer whales, eating medium and large sized fish rather than other mammals.
The study supports the idea that killer whales and false killer whales evolved the ability to hunt other marine mammals quite late in the Pleistocene period (from 2.5 million to 11,700 years ago). That would mean that these predators were not the main reason why baleen whales evolved to become larger, as some scientist had speculated.
According to Raffaele Sardella, a paleontologist at Sapienza University in Rome who was not involved in this study, “this research contributes to remove predation as one of the causes for the evolution of gigantism in baleen whales”.
