The earliest fossil cetacean with borings: narrowing the spatiotemporal gap between Cretaceous marine reptiles and late Cenozoic whales.

Borings of the extant bone-eating worm have previously been found in Cenozoic cetaceans and Cretaceous marine reptiles. The stratigraphically youngest Cretaceous example is from the Maastrichtian, and, until now, the oldest Cenozoic example was from the Oligocene. This leaves a substantial temporal and taxonomic gap between examples from both hosting tetrapod groups. Here, we report nine fossil cetacean specimens with ( bioerosion), identified via CT scans. These include a late Eocene occurrence of the basilosaurid from the eastern USA, which represents the earliest known Cenozoic occurrence of borings, narrowing the temporal gap between occurrences of in Cretaceous marine reptiles and Cenozoic whales. These specimens also include the first bearing fossil cetaceans from the northwestern Atlantic, expanding the Cenozoic biogeography of . Six ichnospecies of are found in these cetacean fossils, including one newly described ichnospecies. The high morphological diversity of in these Cenozoic specimens is broadly consistent with that of the Late Cretaceous, with several ichnospecies now known from both time intervals. Surviving lineages of other large marine vertebrates, such as turtles, crocodyliforms and fish, likely acted as suitable resources for across the Cretaceous-Paleogene boundary, bridging both the temporal and taxonomic gap.