Evolutive Implications of Megathericulus patagonicus (Xenarthra, Megatheriinae) from the Miocene of Patagonia Argentina

Abstract

In this contribution we describe a partial dentary with teeth and an astragalus referred to the ancient megatheriine Megathericulus patagonicus Ameghino, 1904 (Xenarthra, Tardigrada) recovered from the Collón Curá Formation (Miocene) at Chubut Province, Argentina. Referral of the new specimens is based on metric and morphological similarities with the holotype and specimens that have been referred to this species. Shared features include: 1) overall size, smaller than other Megatheriinae such Eomegatherium Kraglievich, 1926, Pliomegatherium Kraglievich, 1930, and Pyramiodontherium (Rovereto, 1914); 2) shape of the molariforms, molariforms mesiodistally compressed and rectangular in outline; 3) Hypsodonty Index close to 100; 4) position of the navicular facet, two-thirds of the navicular facet projects dorsal to the discoid facet; 5); shape of the ventral portion of the fibular facet, the ventral portion has subtriangular outline and contacts the ectal facet; 6) in the astragalus, presence of a sesamoid facet. Considering the large intraspecific differences exhibited in Megatheriinae, the similarities and few differences between Megathericulus primaevus Cabrera, 1939, and M. patagonicus allow us to consider M. primaevus as a junior synonym of M. patagonicus. The presence of Megathericulus and ‘Xyophorus’ Ameghino, 1887 (Mammalia, Xenarthra, Nothrotheriinae) in some distant paleofauna associations (Cerro Zeballos, Argentina, and Quebrada Honda, Bolivia) would suggest a similar path of dispersion and paleoenvironment requirements of both taxa. The presence of these two genera in mentioned localities supports that the Quebrada Honda fauna as a whole is more similar to the asynchronous high-latitude faunas from Patagonia than to contemporary La Venta Fauna (Colombia).