Toxodon

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 Late Miocene to early Holocene epochs (Mayoan to Lujanian in the SALMA classification) (about 11.6 million to 11,000 years ago). Toxodon is a member of Notoungulata, an order of extinct South American native ungulates distinct from the two living ungulate orders that had been indigenous to the continent for over 60 million years since the early Cenozoic, prior to the arrival of living ungulates into South America around 2.5 million years ago during the Great American Interchange. Toxodon is a member of the family Toxodontidae, which includes medium to large sized herbivores. Toxodon was one of the largest members of Toxodontidae and Notoungulata, with Toxodon platensis having an estimated body mass of 1000-1200 kg.

Toxodon has been found across much of South America, excluding southern Patagonia, the Andes and northeastern-most region of the continent. Evidence suggests that Toxodon was ecologically plastic and able to adapt its diet to local conditions.

Toxodon became extinct as part of the end-Pleistocene extinctions around 12,000 years ago, along with most large mammals across the Americas.

Taxonomy
Charles Darwin was one of the first to collect Toxodon fossils, after paying 18 pence for a T. platensis skull from a farmer in Uruguay. In The Voyage of the Beagle Darwin wrote, "November 26th – I set out on my return in a direct line for Montevideo. 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." 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. How wonderfully are the different Orders, at the present time so well separated, blended together in different points of the structure of the Toxodon!"

Toxodon was described in 1837 by Richard Owen based on remains collected by Darwin.

Toxodon is a member of Notoungulata, a group of South American native ungulates that had been part of the fauna of South America since the Paleocene, over 60 million years ago, prior to the arrival of living ungulates in South America around 2.5 million years ago as part of the Great American Interchange. Notoungulata represents the most diverse group of indigenous South American ungulates, with over 150 described genera in 13 different families. Notoungulates are morphologically diverse, including forms morphologically distant from Toxodon such as rodent and rabbit-like forms. Toxodon was a member of Toxodontidae, a large bodied group including vaguely rhinoceros like forms.

Analysis of collagen sequences obtained from Toxodon as well as from the litoptern (another group of indigenous South American ungulates) Macrauchenia 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. 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 with perissodactyls.

Evolution
The species Toxodon chapalmalensis is known from the Pliocene (Montehermosan-Chapadmalalan) of Argentina, while Toxodon platensis, the type species, is known from the Pleistocene.

In 2014, a study identifying a new species of toxodontid resolved the phylogenetic relations of the toxodontids, including to Toxodon. The below cladogram was found by the study:

Description
Toxodon was about 2.7 m in body length, with an estimated weight up to 1415 kg and about 1.5 m high at the shoulder and resembled a heavy rhinoceros, with a short and vaguely hippopotamus-like head. Because of the position of its nasal openings, it is believed that Toxodon had a well-developed snout. Toxodon possessed a large, barrel shaped body. It had short stout legs with plantigrade feet with three functional relatively short toes. The hind limbs are longer and raised higher than the front limbs, giving a sloped appearance to the body. Like horses, it had a stay apparatus allowing the knees to be passively locked while standing.

The vertebrae were equipped with high apophyses, which most likely supported the massive weight and muscles as well as its powerful head. Toxodon had broad jaws which were filled with bow shaped teeth and incisors. The molar teeth of Toxodon have no roots and are ever-growing (euhypsodont), like the incisors of rodents and lagomorphs, and often exhibit enamel hypoplasia.

Palaeobiology
Toxodon is suggested to have been capable of running at considerable speed. Toxodon is believed to have been ecologically plastic and have had a wide niche breadth, with its diet varying according to local conditions, with an almost totally C3 browsing diet in the Amazon rainforest, mixed feeding C3 in Bahia and the Pampas to almost completely C4 dominated grazing diet in the Chaco. Within the Brazilian Intertropical Region, local climate had little impact on the diet of T. platensis.

T. platensis specimens have been found displaying osteological signs of osteomyelitis and spondyloarthropathies.

Extinction
Toxodon became extinct at the end of the Late Pleistocene around 12,000 years as part of the Quaternary extinction event, alongside almost all other large animals in South America. Previous mid-Holocene dates are now thought to be in error. Remains from the Arroyo Seco 2 site in the Pampas are associated with butchered megafauna, but it is unclear if the Toxodon itself was actually butchered or the remains were naturally transported to the site.

Distribution
Toxodon had a wide distribution in South America during the Late Pleistocene, extending from the Pampas into the Amazon rainforest.

Fossils of Toxodon have been found in:


 * Holocene
 * Abismo Ponto de Flecha, Brazil
 * Pleistocene
 * San José, Fortin Tres Pozos, Chaco and Luján Formations, Argentina
 * Tarija and Ñuapua Formations, Bolivia
 * Brazil
 * Paraguay
 * Sopas and Dolores Formations, Uruguay


 * Miocene-Pliocene (Montehermosan)
 * Monte Hermoso Formation, Argentina


 * Miocene
 * Ituzaingó Formation, then described as Entrerriana Formation, Argentina