Isotemnus

Isotemnus is an extinct genus of notoungulate belonging to the family Isotemnidae. It lived from the Late Paleocene to the Middle Eocene of what is now Argentina.

Description
This genus was smaller than Thomashuxleya and Periphragnis, and did not exceed 50 kilograms in weight. Its build was comparable to a modern peccary, with a body relatively massive and strong and sturdy legs. Compared to other Eocene notoungulates, like basal Notohippidae and Notostylopidae, Isotemnus had an humerus whose distal part had a high medial trochlear crest, while the bicipital radial tuberosity was almost unexistant. The astragalus had a broad and low trochlea with a short neck. The calcaneus had rectangular fibular facets, and an unusually thick sustentaculum. Several of the distinctive anatomical leg characteristics of Isotemnus could be due to its smaller size ; Periphragnis and Thomashuxleya, while very similar, had different characteristics.

Classification
Isotemnus is the eponymous genus of the family Isotemnidae, a possibly paraphyletic group of notoungulates including the most basal forms of toxodonts. Isotemnus was one of the most archaic and basal of the isotemnids. The type species is Isotemnus primitivus, first described in 1897 by Florentino Ameghino, based on fossil remains found in terrains dating back from the Early Eocene of Argentine Patagonia. Other species were later attributed to the genus, such as I. ctalego (Early Eocene), I. haugi (Early Eocene, initially described as Leifunia haugi), I. latidens (Middle Eocene). Fragmentary remains attributed to Isotemnus were discovered in Late Paleocene formations in Argentina.