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.
Highlights
Many amniote groups made the evolutionary transition from a fully terrestrial to a semi- to fully aquatic life. This required major morphological and physiological changes that are best developed in the most specialized aquatic forms, like extant cetaceans and sirenians, which live totally independent of the terrestrial environment
We address the lifestyle of early cetaceans by analysis of microanatomical features in postcranial elements of the three more derived archaeocete families, Remingtonocetidae, Protocetidae, and Basilosauridae, extending research by Buffrénil et al [12], Madar [13,14] and Gray et al [15]
We focused our study on archaeocetes from the three more derived archaeocete families— Remingtonocetidae, Protocetidae, Basilosauridae—illustrating a wide spectrum of the diversity of this group after its earliest stages
Summary
Many amniote groups (e.g. sauropterygians, squamates, cetaceans, sirenians, pinnipeds) made the evolutionary transition from a fully terrestrial to a semi- to fully aquatic life. Middle Eocene Remingtonocetidae have skeletons indicating that they were long-bodied, with a long cranial rostrum, short limbs, fused sacral vertebrae, and a powerful tail [16,17] They are considered an early aquatic radiation with distinct specializations [6,11], and are sometimes interpreted as amphibious with an otter-like or gavial-like mode of swimming [6,18]. We document much more of the internal structure of bone from various parts of the skeleton in archaeocete specimens belonging to three of the five known families This enables a more substantial discussion of skeletal specialization observed in these taxa and offers greater constraint when discussing behavioral and ecological implications
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