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Whales originated from aquatic artiodactyls in the Eocene epoch of India

Abstract

Although the first ten million years of whale evolution are documented by a remarkable series of fossil skeletons, the link to the ancestor of cetaceans has been missing. It was known that whales are related to even-toed ungulates (artiodactyls), but until now no artiodactyls were morphologically close to early whales. Here we show that the Eocene south Asian raoellid artiodactyls are the sister group to whales. The raoellid Indohyus is similar to whales, and unlike other artiodactyls, in the structure of its ears and premolars, in the density of its limb bones and in the stable-oxygen-isotope composition of its teeth. We also show that a major dietary change occurred during the transition from artiodactyls to whales and that raoellids were aquatic waders. This indicates that aquatic life in this lineage occurred before the origin of the order Cetacea.

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Figure 1: Osteology of Indohyus and cross-sections of long bones of Eocene cetartiodactyls.
Figure 2: Phylogeny of artiodactyls, cetaceans and archaic ungulates.
Figure 3: Plot of the ratio of the thickness of the medial tympanic wall to that of the lateral tympanic wall against the natural logarithm of the width across occipital condyles, showing that the ratio in Indohyus is similar to that in cetaceans.
Figure 4: Bivariate plot of δ 18 O and δ 13 C values for enamel samples of early and middle Eocene mammals from India and Pakistan.
Figure 5: Skeletal reconstruction of Indohyus.

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Acknowledgements

We thank the late F. Obergfell for presenting us with the sediment blocks containing Indohyus fossils collected by A. Ranga Rao for preparation and study; D. S. N. Raju and N. Raju for facilitating our research; B. Armfield, R. Conley and A. Maas for fossil preparation; J. Dillard for preparing Fig. 5; and J. Geisler and J. Theodor for providing additional information about their cladistic analyses. Laboratory research was funded by the National Science Foundation (NSF) – Earth Sciences (grants to J.G.M.T. and M.T.C.). Collaborative work was funded by the Indian Department of Science and Technology (to S.B.) and the NSF – International Division (to J.G.M.T.) under the Indo-US Scientific Cooperation Program. Laboratory analyses were supported by the Skeletal Biology Research Focus Area of Northeastern Ohio Universities College of Medicine.

Author Contributions J.G.M.T. was responsible for anatomical and systematic study, and scientific synthesis, L.N.C. for systematic and bone density study, M.T.C. for the study of stable isotopes, and S.B. and B.N.T. for geological study and collecting of Indohyus and comparative fossil samples.

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Correspondence to J. G. M. Thewissen.

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Thewissen, J., Cooper, L., Clementz, M. et al. Whales originated from aquatic artiodactyls in the Eocene epoch of India. Nature 450, 1190–1194 (2007). https://doi.org/10.1038/nature06343

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