Evolution of Sirenian Pachyosteosclerosis, a Model-casefor the Study of Bone Structure in Aquatic Tetrapods.

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 histological
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 subsequently
regressed in some taxa that do not exhibit hyperplasic
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.