Swimming with Livyatan: a reconstruction of the vertebrate marine fauna that lived with the giant raptorial sperm whale from the Miocene of Peru

In recent years, the correlation between diatom and cetacean richness patterns through time has been deeply investigated. However, the comparison of diatom and cetacean global data sets faced some criticisms, due to the gaps characterizing the cetacean fossil record, contrasting with the well-known Cenozoic fossil record of diatom. Moreover, other studies pointed out the tight and complex relationships existing between biodiversity, primary productivity, and trophic interactions: changes in one or more of these parameters could have had drastic effects on the stability of past marine ecosystems. For example, it has been observed that the stability of these ecosystems could be dramatically threatened in response to changes in the feeding habits of animals occupying the top of the trophic chains, notably the hypercarnivores. With the aim to provide a contribute to this debate, we undertook an in-depth investigation of Cerro Colorado (middle Miocene, Pisco Formation, Peru), the rich locality where the holotype of the giant raptorial sperm whale Livyatan melvillei and many other well preserved fossil marine vertebrates outcroup from diatom beds. Nowadays, diatoms constitute the base of marine food chains off Peru, and their abundance enriches the biodiversity of fish feeding on them. Therefore, the wealth of fossil vertebrates in the Pisco-Ica desert could be related to the great abundance of food resources that characterized the area during the Neogene. Interestingly, the analysis of worldwide data sets indicated a maximum of diversity for both cetaceans and diatoms during the middle-late Miocene, a time interval including the Livyatan assemblage of Cerro Colorado. During five successive campaigns, all the vertebrate fossils surrounding the Livyatan type locality, on a surface of about 4 km2, were mapped using GPS equipment and preliminarily identified in the field, whereas the most significant specimens were collected for preparation. The study of a large sample of specimens attributed to the beaked whale Messapicetus lead to two publications including preliminary data on paleocology and taphonomy. A detailed geological and stratigraphical survey of the area, combined with satellite images, allowed an evaluation of the stratigraphic position of each specimen. A high vertebrate concentration and diversity were revealed around the stratigraphic level of Livyatan. In addition, taphonomic observations evidenced the absence of abrasion and bioincrustation on bones, the scarcity of breaks, and the prevalence of articulated skeletons. Based on these taphonomic data, the Livyatan assemblage much likely reflects an original and local abundance of life, rather than a condensed deposit and/or an accumulation due to bottom current transport. This high concentration of marine vertebrates provides a broad range of potential prey for the three top predators of the area, the sperm whale Livyatan and the sharks Caracharocles megalodon and Cosmopolitodus hastalis (whose teeth are frequently encountered in Cerro Colorado): two cetotheriid mysticetes, the ziphiid Messapiceus gregarius (represented by males, females, and one calf), two new kentriodontids, the small pontoporiid Brachydelphis mazeasi (including a fetus), another new pontoporiid, at least one other undeterminate small physeteroid, the marine turtle Pacifichelis urbinai, and several seabirds and fishes. With such an exceptional fossil record, questions arise about the trophic interactions between members of the Miocene marine ecosystems. Among these questions, the impact of top predators on the diversity and body size evolution of their prey deserves special attention. Livyatan and the other large macropredators certainly generated a strong predatory pressure on medium-size to large members of the ecosystems off Peru. Looking at mysticetes, the bizygomatic width of the numerous skulls found at Cerro Colorado ranges from 60 to 80 cm, corresponding to an estimated body length between 560 to 720 cm. Such moderately large and fat-rich prey would have provided to Livyatan and large sharks a high amount of calories. The bizygomatic width of most of the mysticetes from Cerro la Bruja and Cerro los Quesos, two other fossil vertebrate-rich localities of the Pisco Formation (slightly younger than those of Cerro Colorado), falls in the range of 120-140 cm, roughly the double of the size of the Cerro Colorado mysticetes. Such an abrupt increase in the body size of mysticetes, also observed at a worldwide scale from middle to late Miocene, was tentatively interpreted as an antipredatory strategy, an hypothesis that should be tested with other fossil-rich Miocene localities.