CONICET | Buscador de Institutos y Recursos Humanos

Glyptodontidae (Xenarthra, Cingulata) is a particular group of huge armored herbivores from theCenozoic 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, actually interpreted as defensive structures. In recent years, the microstructure of Glyptodontidae osteoderms has been mainly 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 several individuals 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 or irregular 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 fonamina. The inner surface is characterized by a meshwork of structural fibers macroscopically visible, and several small neurovascular foramina, unlike carapace ostederms 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 exosqueletal structures that composed the cephalic shield are compact structures, composed of primary tissue, with a matrix consisting mainly of mineralized fiber packings, few primaryosteons and some resorption spaces delimited by lamellar tissue, similar to the dermal ossicles present in this genus. Additionally, arrested growth lines arranged parallel to the surface allow us to estimate a relativate age. 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 exclusive of dermal ossicles and partially in the osteoderms of the postero-lateral margin of the dorsal carapace and the caudal armor, all of which have a clear protective/defensive function.

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