The double skin of the pink fairy armadillo, the peculiar integumentary system of Chlamyphorus truncatus Cingulata (Mammalia, Xenarthra)

Chlamyphorus truncatus is the smallest known armadillo, weighing about 100 grams and measuring less than 15 cm from head to tail. It is endemic to central Argentina and the species is named after its unique carapace. This armadillo has almost completely subterranean habits and, unlike all other subterranean mammals, C. truncatus has a carapace composed of osteoderms and cornified scales. Furthermore, the skin and carapace show unique characteristics compared even with other armadillos. Owing to the reduced development of its eyes, it is important to analyse the skin morphology of this species, as well as specialized sensory organs like vibrissae. The aim of this contribution is to describe the histomorphology of the integumentary system of C. truncatus. For this, histological sections of the skin with and without osteoderms were made, and routine histological and immunohistochemical techniques were performed. Furthermore, microCTs were performed on osteoderms to describe their micromorphology. Our results show that the integumentary system presents unique characteristics compared to other armadillos, such as the peculiar skin under the dorsal shield and the flexibility of almost all of its carapace. Furthermore, the modification of the arrangement of hairs in the first pelvic buckler osteoderms may suggest biomechanical adaptations, such as an improvement in the displacement through the tunnels of sandy soils. The skin without osteoderms presents convergent characteristics with that of other subterranean mammals. Furthermore, the vibrissae conserve the structure of the armadillos (the cavernous sinuses extend throughout the follicle-sinus complex) with a thinning and shortening of the hairs, which are probably linked to passive monitoring of the surrounding environment. Evidently, the adaptations of subterranean mammals to their environment imply structural and functional changes, which are both regressive and progressive in nature.