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Epipubic bones in eutherian mammals from the Late Cretaceous of Mongolia


An important transformation in the evolution of mammals was the loss of the epipubic bones. These are elements projecting anteriorly from the pelvic girdle into the abdominal region in a variety of Mesozoic mammals, related tritylodonts, marsupials and monotremes but not in living eutherian (placental) mammals1,2,3. Here we describe a new eutherian from the Late Cretaceous period of Mongolia, and report the first record of epipubic bones in two distinct eutherian lineages. The presence of epipubic bones and other primitive features suggests that these groups occupy a basal position in the Eutheria. It has been argued that the epipubic bones support the pouch in living mammals1,3,4, but epipubic bones have since been related to locomotion and suspension of the litter mass of several attached, lactating offspring5. The loss of the epipubic bones in eutherians can be related to the evolution of prolonged gestation, which would not require prolonged external attachment of altricial young. Thus the occurrence of epipubic bones in two Cretaceous eutherians suggests that the dramatic modifications connected with typical placental reproduction3,6,7 may have been later events in the evolution of the Eutheria.

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Figure 1: Dentition of the type specimen (PSS-MAE 102) of Ukhaatherium nessovi.
Figure 2: Detail of the skeleton (PSS-MAE 105) of Ukhaatherium nessovi showing the pelvic girdle and epipubic elements.
Figure 3: Detail of the skeleton (PSS-MAE 105) of Ukhaatherium nessovi showing the pelvic girdle and epipubic elements.
Figure 4: Lateral view of the skeleton (PSS-MAE 131) of cf.
Figure 5: Lateral view of the skeleton (PSS-MAE 131) of cf.
Figure 6: Detail of the skeleton (PSS-MAE 131) of cf.
Figure 7: Detail of the skeleton (PSS-MAE 131) of cf.

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We thank A. Davidson for preparation of the fossils and E. Heck for the illustrations. This work was supported by the National Science Foundation, the Jaffe Foundation, the National Geographic Society, the Eppley Foundation, and the James Carter Memorial Fund and the Frick Laboratory Endowment of The American Museum of Natural History.

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Correspondence to Michael J. Novacek.

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Novacek, M., Rougier, G., Wible, J. et al. Epipubic bones in eutherian mammals from the Late Cretaceous of Mongolia. Nature 389, 483–486 (1997).

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