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Astrapotheria is an order of extinct South American herbivores recorded throughout the continent, from the late Paleocene to middle Miocene. Here we describe Hilarcotherium castanedaii, gen. et sp. nov., an Uruguaytheriinae astrapothere from sediments of La Victoria Formation (middle Miocene) in the Tolima Department, Upper Magdalena Valley, Colombia. H. castanedaii, represented by a partial skull, mandible, and some postcranial remains, is characterized by (1) unique dental formula, with 0/3i, 1/1c, 1/1p, and 3/3 m; and (2) lower canines with subtriangular transverse section at the base. Hilarcotherium differs from the equatorial Uruguaytheriinae genera Xenastrapotherium and Granastrapotherium in (1) having three lower incisors; (2) the diagonal implantation of the lower canines; (3) lower molars with lingual cingulid; (4) the presence of the hypocone in the third upper molar; and (5) the presence of an anterolingual pocket in the fourth upper premolar. Our phylogenetic analysis supports the monophyly of the subfamilies Astrapotheriinae and Uruguaytheriinae. Within the latter, we confirm the monophyly of the neotropical clade (Hilarcotherium, Xenastrapotherium, and Granastrapotherium). H. castanedaii shows some plesiomorphic features such as the aforementioned presence of the i3 and the developed hypocone in the last upper molar. Its estimated body mass (1303 kg) is intermediate among Astrapotheriidae. http://zoobank.org/urn:lsid:zoobank.org:pub:82377B38-8B39-4D71-9117-FD17CB329238 SUPPLEMENTAL DATA-Supplemental materials are available for this article for free at www.

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