First fossil record of stomach content for a baleen whale: fish remains associated to a Cetotheriidae from the Miocene of the Pisco Formation (Peru)

During their evolutionary history, ancestors of modern baleen whales progressively replaced their teeth with horny structures known as baleens, thus abandoning a primitive raptorial feeding in favor of various bulk-filter feeding strategies. Among extant mysticetes, Balaenidae and Neobalaenidae slowly skim for plankton near the sea surface, Eschrichtiidae forage for benthic invertebrates on shallow sea floors, and Balaenopteridae engulf huge amounts of prey and water while swimming fast (a feeding performance considered to be the largest biomechanical event that ever existed on Earth). Based on morphofunctional analyses of fossil skeletons, some forms of Cetotheriidae s.s. (a cosmopolitan family of mysticetes ranging from the Miocene to the Pleistocene) were proposed to feed like balaenopterids. In the middle-late Miocene Peruvian beds of the Pisco Formation exposed at Cerro Colorado (the type locality of the giant raptorial sperm whale Livyatan melvillei), fossil skeletons referred to a 5-8 m long cetotheriid sharing similarities with P. nana represent about a fifth of the whole vertebrate assemblage. This whale mainly differs from P. nana in having a larger size and more reduced or absent contact between the premaxillae and the anteroposteriorly elongated nasals. In 2014, an almost complete and fully articulated skeleton of this cetotheriid was partially excavated for taphonomic study. An accurate preparation in the field revealed the presence of tightly packed fish remains between the posterior ribs, a location most likely occupied by the caudal lobe of the forestomach of the whale. Detailed taphonomic observations were made in the field and several hundred of photos of the exposed bones were taken in order to elaborate an orthogonal 3D image of the entire skeleton. Small samples of the intercostal aggregate of bones and scales were investigated through a high resolution micro-CT scanning, revealing a complex architecture of fragmentary, deeply mingled clupeid fish remains. All the identified fossil elements (including a well-diagnostic opercle) are fully compatible with the extant pacific sardine, Sardinops sagax. This discovery represents the first fossil record of a stomach content for a mysticete and the first direct evidence that the edentulous mysticetes (Chaeomysticeti) have a long history as fish eaters. Since only Balaenopteridae are currently known as piscivores among modern mysticetes, this fossil record may suggest that some members of the extinct Cetotheriidae already experimented engulfment feeding. Anyway, other feeding strategies (e.g., suction feeding) could account for epipelagic fish consumption. Further specimens of the Cerro Colorado cetotheriid will be examined during next fieldwork seasons in order to clarify its feeding ecology. This work also reports the most ancient record of the genus Sardinops in the Southern Hemisphere, occurring near the Serravallian peak of productivity and cooling phase. It is noteworthy that the most ancient (and most robust) evidence of piscivory by a cetotheriid whale from the Southern Hemisphere corresponds to the first large-scale expansion of Sardinops, the latter event likely resulting from enhanced primary productivity. Acknowledgements. This research was supported by a grant of the Italian Ministero dell’Istruzione, dell’Università e della Ricerca (PRIN Project 2012YJSBMK) and by a National Geographic Society Committee on Research Exploration grant (9410-13).