BONE DENSITY MODIFICATIONS. “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.”
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0118409