New data on the glyptodontid Trachycalyptoides from the late Miocene Bolivian locality of Achiri

Among armored xenarthrans, Glyptodontidae appears as one of the most peculiar and also characteristic groups of South American mammals that inhabited this continent since at least the early Eocene until the end of the Pleistocene. Their evolutionary history is particularly well documented in Patagonia, Central and North America, but less so in the tropics and central South America. In Bolivia during the Miocene epoch, this family is only recorded at the Laventan locality of Quebrada Honda, represented by cf. “Asterostemma”, “Propalaehoplophorus” andinus, and two sclerocalyptine species, the ?Colloncuran locality of Nazareno represented by “Propalaehoplophorus” and ?Neothoracophorus, occurring in Choquecota, and the late Mayoan–early Chasicoan localities of Chokorasi and Achiri by a single species, Trachycalyptoides achirense. The late Miocene vertebrate locality of Achiri discovered in the early 70’s by the French paleontologist Hoffstetter, was explored by several teams during the subsequent decades. Its mammalian fauna is particularly diverse with more than 20 taxa, including metatherians (i.e., Borhyaenidium), notoungulates (e.g., Hoffstetterius), litopterns, rodents (e.g., Prolagostomus), as well as xenarthrans sloths (e.g., “Xyophorus”) and cingulates (e.g., Trachycalyptoides). Trachycalyptoides was erected by Saint-Andre in 1996 on the basis on two dermal armors and caudal tubes, an incomplete skull, and a hemimandible. The abundant material collected by our team during recent fieldwork, and the preliminary revision of the material housed in the museums of La Paz and Paris, has allowed us to gather new information, specifically on the dentition, dorsal carapace, and caudal tube on this peculiar glyptodontid. In Trachycalyptoides the presence of three lobes is well marked on certain upper and lower molariforms, especially on the most posterior teeth (fourth to eighth molariforms), whereas their presence is doubtful on the third teeth, and absent on the first and second teeth. The general structure of the osteoderms corresponds to a pentagonal or hexagonal polygon with a “rosette” pattern (a central figure surrounded by peripheral figures) on the exposed surface. The central and peripheral figures are elevated and separated by a sulcus. The subcircular central figure is slightly concave in the center. The peripheral figures are small and in a variable number according to region (ranging from 3–4 to 12–15), with foramina at the intersection with the sulcus of the central figure. The caudal tube is conical and elongated and its apex is relatively acute. The ventral face is slightly convex and the dorsal face is flat. The distal portion of the tube is formed by two large right and left osteoderms. The latter are roughly quadrangular, rectangular proximally and quadrate distally. The osteoderms of the caudal tube lack peripheral figures. The molariforms, dorsal carapace, and caudal tube of Trachycalyptoides show affinities with glyptodonts that have a simplified trilobate pattern in the anteriormost molariforms, a dorsal carapace formed by osteoderms with a “rosette” pattern, and a caudal tube formed by osteoderms without peripheral figures like Cochlops, Palaehoplophorus, Trachycalyptus, and Lomaphorus.