Genus-level T. platensis T. burmeisteri T. chapalmalensis T. ensenadensis T. gracilis Toxodon (meaning "bow tooth" in reference to the curvature of the teeth) is an extinct genus of large ungulate native to South America from the Pliocene to the end of the Late Pleistocene.

Both Darwin and Owen were puzzled by Toxodon's unusual anatomical features, including its long, ever-growing cheek teeth.

Toxodon has been found across much of South America, excluding southern Patagonia, the Andes and northeastern-most region of the continent,[5] inhabiting steppe, savanna and sometimes woodland habitats.

Charles Darwin, who was in South America as part of the second voyaging expedition of HMS Beagle, was one of the first to collect Toxodon fossils.

Having heard of some giant's bones at a neighbouring farm-house on the Sarandis, a small stream entering the Rio Negro, I rode there accompanied by my host, and purchased for the value of eighteen pence the head of the Toxodon."

[11][8] Since Darwin discovered that the fossils of similar mammals of South America were different from those in Europe, he invoked many debates about the evolution and natural selection of animals.

In his own words, Darwin wrote down in his journal, Lastly, the Toxodon, perhaps one of the strangest animals ever discovered: In size it equaled an elephant or megatherium, but the structure of its teeth, as Mr. Owen states, proves indisputably that it was intimately related to the Gnawers, the order which, at the present day, includes most of the smallest quadrupeds: In many details it is allied to the Pachydermata: Judging from the position of its eyes, ears, and nostrils, it was probably aquatic, like the Dugong and Manatee, to which it is also allied.

[3] Analysis of collagen sequences obtained from Toxodon as well as from Macrauchenia, a member of another indigenous South American ungulate order, Litopterna, found that notoungulates and litopterns were closely related to each other, and form a sister group to perissodactyls (which contains equids, rhinoceroses and tapirs) as part of the clade Panperissodactyla, making them true ungulates.

[13][14] This finding has been corroborated by an analysis of mitochondrial DNA extracted from a Macrauchenia fossil, which yielded a date of 66 million years ago for the time of the split from perissodactyls.

[17] The diversity of toxodontids, along with other notoungulates began to decline from around the Pliocene onwards,[3] possibly as a result of climate change, as well as the arrival of competitors and predators from North America during the Great American Interchange following formation of the Isthmus of Panama.

†Andinotoxodon bolivariensis †Dinotoxodon paranensis †Toxodon platensis †Gyrinodon quassus †Ocnerotherium intermedium †Hoffstetterius imperator †Posnanskytherium desaguaderoi †Pisanodon nazari †Pericotoxodon platignathus †Calchaquitherium mixtum †Mixotoxodon larensis †Paratrigodon euguii †Trigodon gaudri There has not been a recent taxonomic revision of the genus Toxodon, leaving the number of valid species uncertain.

[33] The hindlimb is considerably longer than the forelimb, resulting in the back being elevated and the shoulder, neck and head being relatively low.

[28] The ulna has a strongly backwardly projecting olecranon process similar to that of rhinos, suggesting that the front leg was held extended when standing.

[26] The (distal) part of the femur closest to the foot shows a pronounced medial trochlear ridge, which likely served along with the patella (kneecap) to allow the knees to be locked when standing akin to the stay apparatus of living horses as an energy saving mechanism.

[35] Toxodon had a widespread distribution in South America east of the Andes, ranging from northern Argentina and Bolivia to the western Amazon on the Peru-Brazil border, to Northeast Brazil.

[50] At the Paso Otero 5 site in the Pampas of northeast Argentina, burned bones of Toxodon alongside those of numerous other extinct megafauna species are associated with Fishtail points (a type of knapped stone spear point common across South America at the end of the Pleistocene, suggested to be used to hunt large mammals[51]).