Title: Hypersalinity drives convergent bone mass increases in Miocene marine mammals from the Paratethys
Abstract: Pachyosteosclerosis—a condition that creates dense, bulky bones—often characterizes the early evolution of secondarily aquatic tetrapods like whales and dolphins 1 de Buffrénil V. Canoville A. D'Anastasio R. Domning D.P. Evolution of sirenian pachyosteosclerosis, a model-case for the study of bone structure in aquatic tetrapods. J. Mamm. Evol. 2010; 17: 101-120 Crossref Scopus (73) Google Scholar , 2 de Buffrénil V. de Ricqlès A. Ray C.E. Domning D.P. Bone histology of the ribs of the archaeocetes (Mammalia: Cetacea). J. Vertebr. Paleontol. 1990; 10: 455-466 Crossref Scopus (86) Google Scholar , 3 Houssaye A. Tafforeau P. de Muizon C. Gingerich P.D. Transition of Eocene whales from land to sea: evidence from bone microstructure. PLoS ONE. 2015; 10: e0118409 Crossref PubMed Scopus (50) Google Scholar but then usually fades away as swimming efficiency increases. 4 de Buffrénil V. Schoevaert D. On how the periosteal bone of the delphinid humerus becomes cancellous: ontogeny of a histological specialization. J. Morphol. 1988; 198: 149-164 Crossref PubMed Scopus (64) Google Scholar 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 Simon D. Palcu D. Meijer P. Krijgsman W. The sensitivity of middle Miocene paleoenvironments to changing marine gateways in Central Europe. Geology. 2019; 47: 35-38 Crossref Scopus (17) Google Scholar 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. 6 Gol'din P. Startsev D. A systematic review of cetothere baleen whales (Cetacea, Cetotheriidae) from the Late Miocene of Crimea and Caucasus, with a new genus. Pap. Palaeontol. 2016; 3: 49-68 Crossref Scopus (14) Google Scholar ,7 Gol'din P. Haiduc B.S. Kovalchuk O. Górka M. Otryazhyi P. Brânzilǎ M. Pǎun E.I. Barkaszi Z. Tibuleac P. Rǎțoi B.G. The Volhynian (late Middle Miocene) marine fishes and mammals as proxies for the onset of the Eastern Paratethys re-colonisation by vertebrate fauna. Palaeontol. Electronica. 2020; 23: a43 Google Scholar