title: Transition of Eocene whales from land to sea: evidence from bone microstructure.
abstract: Cetacea are secondarily aquatic amniotes that underwent their land-to-sea transition during the Eocene. Primitive forms, called archaeocetes, include five families with distinct degrees of adaptation to an aquatic life, swimming mode and abilities that remain difficult to estimate. The lifestyle of early cetaceans is investigated by analysis of microanatomical features in postcranial elements of archaeocetes. We document the internal structure of long bones, ribs and vertebrae in fifteen specimens belonging to the three more derived archaeocete families--Remingtonocetidae, Protocetidae, and Basilosauridae--using microtomography and virtual thin-sectioning. This enables us to discuss the osseous specializations observed in these taxa and to comment on their possible swimming behavior. All these taxa display bone mass increase (BMI) in their ribs, which lack an open medullary cavity, and in their femora, whereas their vertebrae are essentially spongious. Humeri and femora show opposite trends in microanatomical specialization in the progressive independence of cetaceans from a terrestrial environment. Humeri change from very compact to spongious, which is in accordance with the progressive loss of propulsive role for the forelimbs, which were used instead for steering and stabilizing. Conversely, hind-limbs in basilosaurids became strongly reduced with no involvement in locomotion but display strong osteosclerosis in the femora. Our study confirms that Remingtonocetidae and Protocetidae were almost exclusively aquatic in locomotion for the taxa sampled, which probably were shallow water suspended swimmers. Basilosaurids display osseous specializations similar to those of modern cetaceans and are considered more active open-sea swimmers. This study highlights the strong need for homologous sections in comparative microanatomical studies, and the importance of combining information from several bones of the same taxon for improved functional interpretation.
authors.affiliation: UMR 7179 CNRS/Muséum National d'Histoire Naturelle, Département Ecologie et Gestion de la Biodiversité, Paris, France; Steinmann Institut für Geologie, Paläontologie und Mineralogie, Universität Bonn, Bonn, Germany.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Houssaye
authors.affiliation: European Synchrotron Radiation Facility, Grenoble, France.
authors.forename: Paul
authors.initials: P
authors.lastname: Tafforeau
authors.affiliation: Sorbonne Universités, CR2P-CNRS, MNHN, UPMC-Paris 6, Département Histoire de la Terre, Muséum National d'Histoire Naturelle, Paris, France.
authors.forename: Christian
authors.initials: C
authors.lastname: de Muizon
authors.affiliation: Department of Earth and Environmental Sciences and Museum of Paleontology, University of Michigan, Ann Arbor, Michigan, United States of America.
authors.forename: Philip D
authors.initials: PD
authors.lastname: Gingerich
mesh_terms: D000818:Animals; D005075:Biological Evolution; D005580:Fossils; D014907:Whales / Q000033:anatomy & histology / Q000145:classification / Q000235:genetics