Osteosclerosis - an increase in bone density. 

As the early ancestors of whales (Eocene cetaceans
Representative Eocene cetaceans.

Eocene cetaceans. Thewissen, J., Williams, E. (2002). The Early Radiations of cetacean (Mammalia): Evolution pattern and developmental correlations. Annual Review of Ecology and Systematics, 33, 73-90.

) started to move from the land to the sea many of their bones grew denser and heavier. This is thought to have helped combat buoyancy and allowed them to walk better in water.

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