The origins of the killer whale ecomorph

The killer whale (Orcinus orca) and false killer whale (Pseudorca crassidens) are the only extant cetaceans that hunt other marine mammals, with pods of the former routinely preying on baleen whales >10 m in length and the latter being known to take other delphinids.(1-3) Fossil evidence for the origins of this feeding behavior is wanting, although molecular phylogenies indicate that it evolved independently in the two lineages.(4) We describe a new extinct representative of the killer whale ecomorph, Rododelphis stamatiadisi, based on a partial skeleton from the Pleistocene of Rhodes (Greece). Five otoliths of the bathypelagic blue whiting Micromesistius poutassou are associated with the holotype, providing unexpected evidence of its last meal. The evolutionary relationships of R. stamatiadisi and the convergent evolution of killer whale-like features were explored through a broad-ranging phylogenetic analysis that recovered R. stamatiadisi as the closest relative of P. crassidens and O. orca as the only living representative of a once diverse Glade. Within the clade of Orca and kin, key features implicated in extant killer whale feeding, such as body size, tooth size, and tooth count, evolved in a stepwise manner. The tooth wear in Rododelphis and an extinct species of Orcinus (O. citoniensis) are consistent with a fish-based diet, supporting an exaptative Pleistocene origin for marine mammal hunting in both lineages. If correct, predation by the ancestors of Pseudorca and Orca did not play a significant role in the evolution of baleen whale gigantism.

Automated summary

The killer whale and false killer whale are the only extant cetaceans that hunt other marine mammals, and their feeding behavior evolved independently. A new extinct representative of the killer whale ecomorph was discovered, showing the stepwise evolution of killer whale-like features and the adaptive origin of hunting behavior in both lineages during the Pleistocene.