title: Gradual adaptation of bone structure to aquatic lifestyle in extinct sloths from Peru.
abstract: Non-pathological densification (osteosclerosis) and swelling (pachyostosis) of bones are the main modifications affecting the skeleton of land vertebrates (tetrapods) that returned to water. However, a precise temporal calibration of the acquisition of such adaptations is still wanting. Here, we assess the timing of such acquisition using the aquatic sloth Thalassocnus, from the Neogene of the Pisco Formation, Peru. This genus is represented by five species occurring in successive vertebrate-bearing horizons of distinct ages. It yields the most detailed data about the gradual acquisition of aquatic adaptations among tetrapods, in displaying increasing osteosclerosis and pachyostosis through time. Such modifications, reflecting a shift in the habitat from terrestrial to aquatic, occurred over a short geological time span (ca 4 Myr). Otherwise, the bones of terrestrial pilosans (sloths and anteaters) are much more compact than the mean mammalian condition, which suggests that the osteosclerosis of Thalassocnus may represent an exaptation.
authors.affiliation: Département Histoire de la Terre, Muséum national d'Histoire naturelle, Sorbonne Universités - CR2P (CNRS, MNHN, UPMC-Paris06), , 57 rue Cuvier, CP38, 75005 Paris, France.
authors.forename: Eli
authors.initials: E
authors.lastname: Amson
authors.forename: Christian
authors.initials: C
authors.lastname: de Muizon
authors.forename: Michel
authors.initials: M
authors.lastname: Laurin
authors.forename: Christine
authors.initials: C
authors.lastname: Argot
authors.forename: Vivian
authors.initials: V
authors.lastname: de Buffrénil
mesh_terms: D000222:Adaptation, Physiological; D000818:Animals; D005075:Biological Evolution; D015519:Bone Density / Q000502:physiology*; D001842:Bone and Bones / Q000033:anatomy & histology* / Q000502:physiology*; D017753:Ecosystem; D005580:Fossils; D010568:Peru; D012896:Sloths / Q000033:anatomy & histology* / Q000502:physiology*
keywords: Xenarthra; aquatic adaptation; bone histology; bone microstructure; exaptation; osteosclerosis