Evidence for different shark species feeding on a diminutive right whale and a relative of the beluga in the Early Pliocene of the southern North Sea.

Abstract

Documenting past trophic relationships is crucial to understand deep time changes in the ecology and geographic distribution of large marine predators. Though bite marks on cetacean bones are a useful source of information to assess shark-whale predator-prey relationships, in many cases the identity of the prey and/or predator cannot be precisely determined. In this work, we investigate two cetacean specimens from the Kattendijk Formation (Lower Pliocene, north of Belgium): (i) the partial cranium of a small balaenid (right whale) that we describe and refer to Balaenella brachyrhynus and (ii) the previously described partial skeleton of a monodontid (family of beluga and narwhal), attributed here to the genus Casatia. Shark bite marks observed on the bones of the two specimens are described and the tip of a shark tooth found embedded in each cranium is visualized through micro-CT imaging. A comparison with shark species recorded in the Kattendijk Formation allows for the identification of the author of at least one bite for each cetacean: the bluntnose sixgill shark Hexanchus griseus for B. brachyrhynus and the large lamnid shark Carcharodon plicatilis for Casatia sp. Based on the position and topology of the marks, the H. griseus bite on B. brachyrhynus may have occurred on an upside down, floating carcass, a hypothesis that could indicate a scavenging event. Bite marks on the monodontid cranium suggest an attempt by C. plicatilis to sever the head from the rest of the body and point to the forehead region as a main targeted area. These new Early Pliocene records of shark-cetacean trophic relationships constitute a first step towards the exploration of the possible link between changes through time in the availability of prey in the southern North Sea and the local loss of large predatory sharks, which occurred in the Late Pliocene to Pleistocene time interval.