The dermal armor of mylodontid sloths (Mammalia, Xenarthra) from Cueva del Milodón (Última Esperanza, Chile)

• Published in: Journal of morphology (1931) 2021-04, Vol.282 (4), p.612-627
• Main Authors: Toledo, Néstor, Boscaini, Alberto, Pérez, Leandro Martín
• Format: Article
• Language: English
• Subjects: Folivora; ground sloth; integument; megafauna; ossicle; osteoderm; X‐ray
• ISSN: 0362-2525; 1097-4687
• DOI: 10.1002/jmor.21333

Dermal ossifications (osteoderms, dermal ossicles, osteoscutes) appear independently in various tetrapod lineages. In mammals, however, dermal ossifications are only present in some members of Xenarthra. This clade includes Cingulata (armadillos and their relatives), and Pilosa, including Vermilingua (anteaters) and Folivora (sloths). In extant xenarthrans, osteoderms are invariably present in cingulates whereas they are absent in pilosans. Among extinct sloths, however, a limited number of taxa possessed dermal ossifications. Records of mummified skins of ground sloths bearing osteoderms found in Cueva del Milodón (Southern Chile), with a late Pleistocene age, allowed us to analyze their micro‐ and macroscopic morphology. The main goal of this study is to closely examine a portion of a mylodontid skin portion using radiography. The arrangement, morphology and internal structure of the ossicles are analyzed and the results are discussed in the context of previous research. The results we obtained indicate that ossicles vary in shape and size, and the integument has four different patterns of arrangement of the ossicles: that is, areas without ossicles, disorganized ossicles, rows, and mosaic areas. The latter has two variants, with clusters of ossicles forming rosettes or stars. Thin sections of the ossicles allowed us to recognize and describe anatomical features of the bone and its mode of growth. Finally, paleobiological and functional considerations of the dermal armor are discussed along with its phylogenetic and chronological implications. A fragment of mummified skin of the mylodontid sloths (Xenarthra, Folivora) from Cueva del Milodón (Chile), housed in the Museo de La Plata, was analyzed under X‐rays. The dermal ossicles showed a diversity of both morphology and spatial arrangement. Above left, map of South America depicting the location of the Cueva del Milodón (star). Center right, composite image of the analyzed skin and the X‐ray images we performed, and in interpretation of their hypothetical position in life; scale bar equals 20 cm. Below left, a life reconstruction of a mylodontid showing the approximate position of the fragment of skin.