ORIGINAL PAPER Evolution of Sirenian Pachyosteosclerosis, a Model-case for the Study of Bone Structure in Aquatic Tetrapods Vivian de Buffrénil & Aurore Canoville & Ruggero D’Anastasio & Daryl P. Domning Published online: 26 February 2010 # Springer Science+Business Media, LLC 2010 Abstract Osteosclerosis, or inner bone compaction, and pachyostosis, or outer hyperplasy of bone cortices (swollen bones), are typical features of tetrapods secondarily adapted to life in water. These peculiarities are spectacularly exemplified by the ribs of extant and extinct Sirenia. Sea cows are thus the best model for studying this kind of bone structural specializations. In order to document how these features differentiated during sirenian evolution, the ribs of 15 species, from the most basal form (Pezosiren portelli) up to extant taxa, were studied, and compared to those of other mammalian species from both morphometric and histo- logical points of view. Pachyostosis was the first of these two specializations to occur, by the middle of the Eocene, and is a basal feature of the Sirenia. However, it subse- quently regressed in some taxa that do not exhibit hyper- plasic rib cortices. Osteosclerosis was only incipient in P . portelli. Its full development occurred later, by the end of the Eocene. These two structural specializations of bone are variably pronounced in extinct and extant sirenians, and relatively independent from each other, although frequently associated. They are possibly due to similar heterochronic mechanisms bearing on the timing of osteoblast activity. These results are discussed with respect to the functional constraints of locomotion in water. Keywords Sirenia . Bone . Histology . Osteosclerosis . Pachyostosis . Evolution Introduction Since the pioneer works by Kiprijanoff (1881–1883), Nopcsa (1923), Sickenberg (1931), and Nopcsa and Heidsieck (1934), an abundant literature has confirmed that secondary adaptation of tetrapods to an aquatic life always induced modifications of the inner architecture and histological characteristics of bones. As compared to the basal condition of tetrapod skeletal elements, at least in the Amniota (tubular long bones, cancellous short bones, diploë-containing flat bones; see Francillon-Vieillot et al. 1990), this process led to variably pronounced conditions oriented either towards in- crease in bone compactness and volume, or towards light- ening of bone structure (reviewed in Ricqlès and Buffrénil 2001). A general trend appears among marine tetrapods: in the beginning of their evolutionary radiation, when taxa are incipiently adapting to life in water and have not yet devel- oped highly efficient propelling modes, their skeletons display a local or general increase in density and mass, resulting from higher compactness of bone inner architecture (osteosclerosis) and/or from hyperplasy of periosteal cortices (pachyostosis). All lineages went through this initial stage during their evolution (Ricqlès 1989). The evolution of the opposite condition, i.e., osteoporotic-like skeletal lightening, followed this first stage (Ricqlès and Buffrénil 2001; Gray et al. 2007; Madar 2007; see also Buffrénil et al. 1990a), and V. de Buffrénil (*) : A. Canoville Département Histoire de la Terre, UMR 7207 (CR2P), Muséum National d’Histoire Naturelle, Bâtiment de Géologie CC 48, 57 rue Cuvier, Paris F-75005, France e-mail: vdebuff@mnhn.fr R. D’Anastasio Faculty of Medicine (Section of Anthropology), State University of Chieti, Trento e Trieste, Chieti, Italy D. P. Domning Department of Anatomy (Laboratory of Evolutionary Biology), Howard University, Washington, DC, USA J Mammal Evol (2010) 17:101–120 DOI 10.1007/s10914-010-9130-1