Systematics, phylogeny and feeding behavior of the oldest killer whale: a reappraisal of Orcinus citoniensis (Capellini, 1883) from the Pliocene of Tuscany (Italy)

Reaching body lengths of 9 m, killer whales (Orcinus orca) are the top mammalian predators of the present-day oceans. These distinctive, cosmopolitan dolphins feature an extremely broad and diverse trophic spectrum, ranging from mollusks like octopuses and squids to other toothed whales and even baleen whales. Due to the lack of fossils that can be confidently assigned to Orcinus or close relatives thereof, the evolutionary origin of extant killer whales has rarely been addressed. Here, we provide an updated and thorough reappraisal of the systematics, phylogeny and feeding behavior of Orcinus citoniensis from the Pliocene of Tuscany (Italy), the only fossil species of killer whale currently known on the basis of diagnostic material. Our systematic and phylogenetic reassessment confirms that O. citoniensis is a bona fide species of Orcinus that mostly differs from the extant O. orca by virtue of a smaller body size (ca. 3.5 m). Besides Orcinus spp., the subfamily Orcininae is here recognized as a relatively early-branching clade of delphinids that also includes the Italian Pliocene species “Tursiops” osennae, Hemisyntrachelus pisanus and Hemisyntrachelus cortesii. Our morphofunctional analysis supports a mainly piscivorous diet for O. citoniensis. In particular, the degree of apical tooth wear observed on the holotype is consistent with that shown by the extant generalist type of Atlantic killer whales while clearly differing from members of the cetacean-eating specialized types. The prominence of fish in the diet of O. citoniensis is further supported by the fine and shallow microwear features on the dentine exposed at the apical portion of the tooth crown. The emergence of Orcinus as one of the highest trophic level predators of the global oceans, especially at mid and high latitudes, may have involved some process of exaptation, well into the Pleistocene, when large eurytrophic sharks that used to be common and widespread in Pliocene times either became extinct or underwent a severe reduction of their biogeographic ranges.