Piauhytherium

This animal in general terms resembles a hippopotamus, with a big short snout, a massive body and a large head. The skull measured almost 60 centimetres (24 in) in length, which indicates that Piauhytherium could be as big as a modern black rhinoceros. With regard to its nearest relatives, such as Toxodon, this animal's legs were shorter and thicker, in addition, certain differences in the denture distinguish it from other notoungulates of this period.[1]

Piauhytherium capivarae was described for the first time in 2013, based on a complete skull including a jaw and some postcranial bones found in Serra da Capivara in Piauí, in northeastern Brazil. This animal belonged to a group of notoungulates known as Toxodonta, which comprises numerous herbivores from the Cenozoic of South America, whose better-known representative is Toxodon. (Some remains found in Brazil have been reassigned to Piauhytherium.) Piauhytherium was very similar to Toxodon, but is differentiated in some features of is leg bones and characteristics of the teeth.[1]

The 2018 study that described Falcontoxodon found Piauhytherium, along with Mixotoxodon, Gyrinodon and Falcontoxodon, to be in the same monophyletic clade within Toxodontinae. Below is a parsimony tree establishing the relationships between the genera of Toxodontidae, as proposed by Carrillo et al, 2018.[2] Some authors have suggested that Piauhytherium is a junior synonym of the previously named species Trigodonops lopesi.[3]

The limb bones, which are particularly short and massive, led the authors of the original scientific description to hypothesise that P. capivarae led a semiaquatic lifestyle, similar to that of modern hippopotamus. This hypothesis had already been previously proposed for other toxodontids (including Toxodon), but equally has been refuted by other studies. The discovery of Piauhytherium is remarkable since it increases the degree of diversity of the toxodontids in the final stages of their evolutionary history: in the Upper Pleistocene only Toxodon, Trigodonops and Mixotoxodon were also known.[1]