Data from: Gradual adaptation of bone structure to aquatic lifestyle in extinct sloths from Peru

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.

骨骼的非病理性骨密度增高（osteosclerosis，骨硬化）与肿胀（pachyostosis，骨肥厚），是重返水生环境的陆生脊椎动物（tetrapods，四足类）的骨骼系统所经历的主要改造形式。然而，学界迄今仍缺乏对这类适应性特征演化过程的精准时间校准。本研究以产自秘鲁皮斯科组新近纪地层的水生树懒属（Thalassocnus）为研究对象，评估这类适应性特征的演化时序。该属包含5个物种，分别赋存于不同地质年代的连续含脊椎动物化石层位中，其化石记录展现了随时间推移骨硬化与骨肥厚程度不断升高的特征，为四足类水生适应性特征的逐步演化提供了迄今最为详尽的实证数据。这类反映栖息地从陆生向水生转变的骨骼改造，发生在极短的地质时间跨度内（约400万年）。此外，陆生披毛目动物（树懒与食蚁兽）的骨骼紧实度远高于哺乳动物的平均水平，这表明水生树懒属的骨硬化或许属于一种预适应（exaptation）。