Paleoecology of archaeocete whales throughout the Eocene: Dietary adaptations revealed by microwear analysis

Archaeocete whales originated from within terrestrial artiodactyls around the Paleocene-Eocene boundary and gave rise to modern toothed whales (Odontoceti) and baleen whales (Mysticeti) at the end of the Eocene. The cetacean transition from life on land to life in water is documented by archaeocete fossils reflecting stages of increasing aquatic adaptation throughout the Eocene. During this time, feeding and diet in cetaceans supposedly changed from herbivory or omnivory to piscivory and carnivory. It is still not fully understood whether an early dietary shift was the reason for cetaceans to abandon land, or whether aquatic feeding evolved later, at a semiaquatic stage. To assess archaeocete diet and its evolution through time, we analyzed dental enamel microwear of an early Eocene pakicetid, middle Eocene protocetids, and middle-to-late Eocene basilosaurids. We quantified wear features on shearing facets of upper and lower cheek teeth using low-magnification stereoscopic microwear analysis. Results were compared with the microwear of extant fully-aquatic odontocetes, semiaquatic pinnipeds, and terrestrial carnivores. The microwear of archaeocetes strongly resembles that of modern pinnipeds and, to a lesser extent, odontocetes that include cephalopods, crustaceans, bivalves and gastropods, as well as marine birds and mammals besides fish in their diet. The degree of aquatic adaptation in archaeocetes seems to be reflected in their diet. Aquatic feeding is already indicated for the late early Eocene terrestrial Pakicetus inachus since its microwear already resembles that of the middle Eocene semiaquatic protocetids. Middle-to-late Eocene fully-aquatic basilosaurids turn out to be separated from the protocetids and P. inachus in all parts of the study and apparently ingested more or larger hard items. We conclude that cetaceans were not primarily piscivorous, and that piscivory and carnivory, where present in modern cetaceans, evolved from a very diverse diet in Eocene archaeocetes that included various kinds of invertebrates and vertebrates. Inclusion of mammals in the cetacean diet is not a recent dietary specialization but might be as old as the middle Eocene, implied by a microwear signal in Qaisracetus arifi similar to those of the modern hyena and the killer whale. Consumption of mammal meat was very likely well-established in middle-to-late Eocene forms (heavy gouging and extremely destructive macroscopic wear in Basilosaurus isis).