Pachyosteosclerosis—a condition that creates dense, bulky bones—often characterizes the early evolution of secondarily aquatic tetrapods like whales and dolphins^1–3 but then usually fades away as swimming efficiency increases.⁴ Here, we document a remarkable reversal of this pattern, namely the convergent re-emergence of bone densification in Miocene seals, dolphins, and whales from the epicontinental Paratethys Sea of eastern Europe and central Asia. This phenomenon was driven by imbalanced remodeling and inhibited resorption of primary trabeculae and coincided with hypersaline conditions—the Badenian salinity crisis—that affected the Central Paratethys between 13.8 and 13.4 Ma.⁵ Dense bones acting as ballast would have facilitated efficient swimming in the denser and more buoyant water and hence were likely adaptive in this setting. From the Central Paratethys, pachyosteosclerosis subsequently spread eastward, where it became a defining feature of the endemic late Miocene whale assemblage.