Data from: Evolutionary adaptation to aquatic lifestyle in extinct sloths can lead to systemic alteration of bone structure

Through phenotypic plasticity, bones can change in structure and morphology, in response to physiological and biomechanical influences over the course of individual life. Changes in bones also occur in evolution as functional adaptations to environment. In this study, we report on the evolution of bone mass increase (BMI) that occurred in the postcranium and skull of extinct aquatic sloths. Although non-pathological BMI in postcranial skeleton has been known in aquatic mammals, we here document general BMI in the skull for the first time. We present evidence of thickening of the nasal turbinates, nasal septum, and cribriform plate, and further thickening of the frontals, and infilling of sinus spaces by compact bone in the late and more aquatic species of the extinct sloth Thalassocnus. Systemic bone mass increase occurred among the successively more aquatic species of Thalassocnus, as an evolutionary adaptation to the lineage’s changing environment. The newly documented pachyostotic turbinates appear to have conferred little or no functional advantage and are herein hypothesized as a correlation with or consequence of the systemic BMI among Thalassocnus species. This could in turn be consistent with a genetic accommodation of a physiological adjustment to a change of environment.

生物体可通过表型可塑性（phenotypic plasticity）响应个体生长发育全过程中的生理与生物力学作用，进而发生骨骼结构与形态的改变。骨骼的演化变化亦可作为对环境的功能适应而发生。本研究报道了已灭绝水生树懒的颅后骨骼（postcranium）与头骨（skull）中出现的骨量增加（bone mass increase, BMI）演化特征。尽管水生哺乳动物的颅后骨骼出现非病理性骨量增加已为学界所熟知，但我们首次在此记录了头骨普遍存在的骨量增加现象。我们的研究证据显示，在已灭绝的海懒兽属（Thalassocnus）中，晚期且水生适应性更强的物种的肥厚性鼻甲（pachyostotic turbinates）、鼻中隔（nasal septum）与筛板（cribriform plate）出现增厚，额骨（frontals）进一步增厚，且鼻窦腔（sinus spaces）被密质骨（compact bone）填充。随着海懒兽属物种的水生适应性逐步增强，全身性骨量增加作为该支系应对环境变化的演化适应特征不断出现。此次新发现的肥厚性鼻甲似乎未带来任何显著的功能优势，因此我们推测其与海懒兽类的全身性骨量增加存在关联，或是该现象的副产物。这一结论亦可与“生物体针对环境变化做出生理调整后发生遗传顺应（genetic accommodation）”的演化逻辑相契合。