The return of tetrapods to a marine habitat constitutes one of the most significant ecological shifts, occurring multiple times during the last 250 million years. In the course of the evolution of marine tetrapod lineages, numerous behavioral and physiological changes are observed, accompanied by drastic morphological transformations. This process has been recorded in many groups, including extinct Mesozoic reptiles and Cenozoic mammals, but for a vast majority of these groups, either all the members are extinct, or the modern, surviving forms, are fully marine. Among modern marine mammals, cetaceans (whales and dolphins) and sirenians (dugongs and manatees) are indeed fully aquatic, spending their entire life underwater, whereas pinnipeds (true seals, fur seals, sea lions, and walruses) constitute the best living example of amphibious mammals, illustrating some crucial early steps of a successful return to the sea. Although feeding occurs in the water, extant pinnipeds still have to return to land to rest, to give birth, and, for some of them, to mate. Therefore, besides numerous morphological adaptations to an aquatic life (limbs modified into flippers, reduction of outer ear, hydrodynamic body...), pinnipeds retain some degree of terrestrial locomotion ability, interestingly varying from one pinniped group to the other. Fur seals, sea lions, and walruses have a quadrupedal stance on land, whereas true seals (or Phocidae) lost this ability and move forwards thanks to vertical undulations of the trunk. Underwater, contrasting with the forelimbs propulsion of fur seals and sea lions (subaquatic flight), true seals swim by pelvic oscillations (forelimbs being mostly used for steering).
In addition to these unique locomotor specializations, true seals are the most diverse pinniped family, with 18 extant species predominantly occupying polar and subpolar marine areas of both hemispheres. True seals also display a tremendous size and weight range (from 45 kg for the freshwater Baikal seal to 3200 kg in adult males of the elephant seal) and diverse feeding techniques (deep diving teuthophagous elephant seals, macroraptorial leopard seals, filter-feeding crabeater seals, etc.) that further highlight the high potential of this family as an evolutionary model for the early steps of the conquest of a new environment and, more specifically, for studying adaptive features at the interface of terrestrial and aquatic lifestyles.
Although the locomotion capabilities and habitat of some other fossil pinnipeds were investigated in detail, only a few extinct phocids were analyzed from a functional anatomy point of view. Furthermore, previous studies focused mostly on the external bone features and no extinct phocid has ever been studied for inner bone organization. In addition, contrary to several other marine mammal groups for which stable isotope analyses were performed to elucidate the habitat and diet of fossil species, till now only the teeth of a few modern pinnipeds were sampled for stable isotope content, demonstrating strong correlations between carbon and oxygen isotope ratios and prey types and environments.
Besides a single and debated late Oligocene record from South Carolina (USA), the fossil record of true seals starts in middle Miocene (16-12 million years ago) deposits from the North Atlantic realm and the Paratethys, with undisputed members of both subfamilies Monachinae and Phociinae. The most closely related fossil pinniped recorded in the northeastern Pacific, the ancestor of all phocids, is proposed to have entered the North Atlantic either via a northern seaway (Bering Strait) or via the Panama Seaway, sometime during the Neogene.
Although illustrated by numerous fossil finds, the Neogene history of true seals is still poorly known and fragmentary at the level of systematics, paleoecology, stratigraphic range, phylogeny and paleobiogeography. Indeed, XIXth century studies of the European material failed to solve systematic and taxonomic issues related to the fragmentary state of most specimens.
In summary, to elucidate the first steps of the iterative evolutionary shift of tetrapods to an aquatic environment, pinnipeds constitute the fundamental model, as this group is the only major clade with amphibious modern representatives and a rich fossil record. True seals are the most diversified family among pinnipeds, and are best adapted to the marine environment. However, they have only been poorly investigated at the level of functional anatomy and bone histology. Finally, rich European collections of fossil true seals were not reviewed in detail for more than130 years, further emphasizing the high potential of this project to elucidate various aspects of the evolutionary history of true seals.
The main objective of this paleontological PhD project is to determine the crucial Neogene steps in the evolutionary history of true seals (Phocidae), by studying the phylogenetic relationships, locomotor abilities, feeding techniques, habitat, and paleobiogeography of taxa intermediary between the ancestral late Oligocene pinnipeds and the amphibious modern species. The applicant will reassess the substantial RBINS phocid collection together with the new discoveries from the North Sea Basin, the Paratethys and the east coast of North America. A comparative study of a fossil phocid collection from Peru will be included. The final aim of the research is to answer a series of long-standing and debated questions related to the evolution of the family, and contribute to the further elucidation of a crucial, past ecological move: the return of the tetrapods to a marine habitat.
