Microstructure of the cephalic shield of Glyptodon Owen, 1839 (Xenarthra, Cingulata, Glyptodontidae)

LUNA, CARLOS ALBERTO; CUADRELLI, FRANCISCO; BARASOAIN, DANIEL; ZURITA, ALFREDO EDUARDO; CHIARELLI, FRANCISCO; CUARANTA, PEDRO

Trelew, Chubut, Argentina

Congreso; XXXV Jornadas Argentina de Paleontología de Vertebrados; 2022

Museo Paleontológico Egidio Feruglio

Glyptodontidae (Xenarthra, Cingulata) is a particular group of large armored herbivores from the Cenozoic of South America, characterized by the presence of a cephalic shield, rigid dorsal carapace and caudal armor. Among them, Glyptodon Owen, 1839 stands out by the presence of some particular exosqueletal features, including isolated dermal ossicles and "spine"-like osteoderms along the carapace margin. In recent years, the microstructure of Glyptodontidae osteoderms has been analyzed with the aim of finding new characters of systematic value. However, these studies have been focused on the dorsal carapace but other structures, such as the cephalic shield, have received little attention. In the present contribution we analyze the microstructure of the dermal structures that composed the cephalic shield of Glyptodon reticulatus Owen from Late Pleistocene of Argentina (MAMM-PV-048, CTES-PV-7590) in order to define its histogenesis and possible function. From a macroscopic viewpoint, the components of the cephalic shield of Glyptodon are polygonal with rough surface and variable size. Also, the thickness decreases from the central area to the margins of the cephalic shield. Sutures between adjacent elements are lax so that they are not closely attached, contrary to those of other glyptodonts, like Neosclerocalyptus and Panochthus. Within the cephalic shield of Glyptodon, the dermal structures of the central area have better defined articular surfaces than those of the peripheric area, having a well defined central figure surrounded by a flat area with some piliferous foramina. The inner surface is characterized by a meshwork of structural fibers macroscopically visible, and several small neurovascular foramina, unlike carapace osteoderms in which there is one large foramen surrounded several small foramina. Unlike the osteoderms of the dorsal carapace, characterized by a diploe structure and marked extensive secondary reconstruction, the dermal bones that composed the cephalic shield are compact, composed of primary tissue, with a matrix consisting mainly of mineralized fiber packings, few primary osteons and some resorption spaces delimited by lamellar tissue, similar to the dermal ossicles present in this genus. Five lines of arrested growth arranged parallel to the surface are observed. The histological characteristics described allow us to interpret a metaplastic origin, like that of the dermal ossicles which are very similar. The compactness degree of the cephalic shield is similar to that of the dermal ossicles, osteoderms of the postero-lateral margin and caudal edge of the dorsal carapace and caudal armor, for which a hypothetical defensive function is inferred.