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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 femur of Rodhocetus kasranii GSP-UM 3012.A-B, partial longitudinal section in lateral view; proximal is at the top. Limits of the compact cortex (dotted lines), as well as the position of the growth center (GC), are indicated on B; C, transverse section cutting the growth center. Scale bars equal 5mm. Cavities are either filled by sediment (light grey) or by epoxy (black) resulting from bone preparation.
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pone.0118409.g005: Left femur of Rodhocetus kasranii GSP-UM 3012.A-B, partial longitudinal section in lateral view; proximal is at the top. Limits of the compact cortex (dotted lines), as well as the position of the growth center (GC), are indicated on B; C, transverse section cutting the growth center. Scale bars equal 5mm. Cavities are either filled by sediment (light grey) or by epoxy (black) resulting from bone preparation.

Mentions: The femur of Rodhocetus resembles that of Remingtonocetus, although it is more compact. The longitudinal section shows that the inner organization of the bone changes markedly along the diaphysis (Fig. 5A). The growth center, i.e., the point where growth originated, corresponds to the point of the transverse section displaying the thicker remains of the original cones of primary periosteal bone where the cones of endochondral and periosteal origin intersect (Fig. 5B). The growth center is usually located close to the mid-diaphysis, but here it appears clearly proximal (Fig. 5A). Around this point, the bone is strongly compact (CI = 83.6 and 87.4 on two different sections). The open medullary cavity is clearly off-center and surrounded by a cortex displaying numerous fairly small cavities, although they are larger posteriorly (Fig. 5C). Proximal and distal to the open medullary cavity, the micro-organization changes rapidly to more spongious bone.


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 femur of Rodhocetus kasranii GSP-UM 3012.A-B, partial longitudinal section in lateral view; proximal is at the top. Limits of the compact cortex (dotted lines), as well as the position of the growth center (GC), are indicated on B; C, transverse section cutting the growth center. Scale bars equal 5mm. Cavities are either filled by sediment (light grey) or by epoxy (black) resulting from bone preparation.
© Copyright Policy
Related In: Results  -  Collection

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

pone.0118409.g005: Left femur of Rodhocetus kasranii GSP-UM 3012.A-B, partial longitudinal section in lateral view; proximal is at the top. Limits of the compact cortex (dotted lines), as well as the position of the growth center (GC), are indicated on B; C, transverse section cutting the growth center. Scale bars equal 5mm. Cavities are either filled by sediment (light grey) or by epoxy (black) resulting from bone preparation.
Mentions: The femur of Rodhocetus resembles that of Remingtonocetus, although it is more compact. The longitudinal section shows that the inner organization of the bone changes markedly along the diaphysis (Fig. 5A). The growth center, i.e., the point where growth originated, corresponds to the point of the transverse section displaying the thicker remains of the original cones of primary periosteal bone where the cones of endochondral and periosteal origin intersect (Fig. 5B). The growth center is usually located close to the mid-diaphysis, but here it appears clearly proximal (Fig. 5A). Around this point, the bone is strongly compact (CI = 83.6 and 87.4 on two different sections). The open medullary cavity is clearly off-center and surrounded by a cortex displaying numerous fairly small cavities, although they are larger posteriorly (Fig. 5C). Proximal and distal to the open medullary cavity, the micro-organization changes rapidly to more spongious bone.

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