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Transition of Eocene whales from land to sea: evidence from bone microstructure.

Houssaye A, Tafforeau P, de Muizon C, Gingerich PD - PLoS ONE (2015)

Bottom Line: 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.

View Article: PubMed Central - PubMed

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.

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.

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Left rib 4 of Basilosaurus isis WH-074.A-C, transverse sections from the proximal, middle, and distal portions of the rib; medial is at the left and posterior at the top. D, longitudinal section from the middle of the rib, in anterior view; medial is at the left and posterior at the top. Scale bars equal 1 cm.
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pone.0118409.g013: Left rib 4 of Basilosaurus isis WH-074.A-C, transverse sections from the proximal, middle, and distal portions of the rib; medial is at the left and posterior at the top. D, longitudinal section from the middle of the rib, in anterior view; medial is at the left and posterior at the top. Scale bars equal 1 cm.

Mentions: The proximal third of the rib is strongly compact (CI = 95.2). The transverse section shows a very compact cortex with a limited spongious medullary area (Fig. 13A). The latter increases in size distally (Fig. 13B–C)). Compactness remains high at midshaft (CI = 87.9; Fig. 13B). Here the medullary area is strongly off-center, which is evident in both transverse and longitudinal sections (Fig. 13B, D). In the most distal part of the rib, the spongiosa occupies most of the section but is dense and surrounded by a thick layer (especially laterally) of compact cortical bone (Fig. 13C), so that the rib remains strongly compact (CI = 84.9). LAGs are observed in the compact cortex, where remodelling is thus probably limited.


Transition of Eocene whales from land to sea: evidence from bone microstructure.

Houssaye A, Tafforeau P, de Muizon C, Gingerich PD - PLoS ONE (2015)

Left rib 4 of Basilosaurus isis WH-074.A-C, transverse sections from the proximal, middle, and distal portions of the rib; medial is at the left and posterior at the top. D, longitudinal section from the middle of the rib, in anterior view; medial is at the left and posterior at the top. Scale bars equal 1 cm.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4340927&req=5

pone.0118409.g013: Left rib 4 of Basilosaurus isis WH-074.A-C, transverse sections from the proximal, middle, and distal portions of the rib; medial is at the left and posterior at the top. D, longitudinal section from the middle of the rib, in anterior view; medial is at the left and posterior at the top. Scale bars equal 1 cm.
Mentions: The proximal third of the rib is strongly compact (CI = 95.2). The transverse section shows a very compact cortex with a limited spongious medullary area (Fig. 13A). The latter increases in size distally (Fig. 13B–C)). Compactness remains high at midshaft (CI = 87.9; Fig. 13B). Here the medullary area is strongly off-center, which is evident in both transverse and longitudinal sections (Fig. 13B, D). In the most distal part of the rib, the spongiosa occupies most of the section but is dense and surrounded by a thick layer (especially laterally) of compact cortical bone (Fig. 13C), so that the rib remains strongly compact (CI = 84.9). LAGs are observed in the compact cortex, where remodelling is thus probably limited.

Bottom Line: 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.

View Article: PubMed Central - PubMed

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.

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.

Show MeSH
Related in: MedlinePlus