''Toxodon'' (meaning "bow tooth" in reference to the curvature of the teeth) is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of South American mammals from the Late Miocene to early

Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togeth ...

epochs ( Mayoan to Lujanian in the SALMA classification) (about 11.6 million to 11,000 years ago). It is a member of Notoungulata, one of several now extinct orders of hoofed mammals indigenous to South America distinct from living perissodactyls and artiodactyls. It was among the largest and last members of its order, and was probably the most common large hoofed mammal in South America of its time.

Taxonomy

''Toxodon'' was one of the last members of Notoungulata, a group of ungulates that had been part of the fauna of South America since the Paleocene. ''Toxodon'' was a member of Toxodontidae a large bodied group including similar, vaguely rhinoceros like forms. 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, Analysis of collagen sequences obtained from ''Toxodon'' as well as from '' Macrauchenia'' found that South America's native notoungulates and litopterns form a sister group to perissodactyls, 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 Ma for the time of the split with perissodactyls.

Evolution

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 in body length, with an estimated weight up to and about high at the shoulder and resembled a heavy rhinoceros, with a short and vaguely

hippopotamus The hippopotamus ( ; : hippopotamuses or hippopotami; ''Hippopotamus amphibius''), also called the hippo, common hippopotamus, or river hippopotamus, is a large semiaquatic mammal native to sub-Saharan Africa. It is one of only two exta ...

-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 teeth of ''Toxodon'' have no roots and are ever-growing (euhypsodont) like those of rodents and lagomorphs, and often exhibit

enamel hypoplasia Enamel hypoplasia is a defect of the teeth in which the enamel is deficient in quantity, caused by defective enamel matrix formation during enamel development, as a result of inherited and acquired systemic condition(s). It can be identified as m ...

Palaeobiology

It was initially believed to have been amphibious, but after examining the proportions of the

femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates ...

and

tibia The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it conn ...

, as well as the position of its head, below the top of the spinal column, palaeontologists realized that it had features similar to terrestrial animals such as elephants or rhinoceroses. The fossils are also usually found in arid and semi-arid areas, typically an indication of a primarily terrestrial life. ''Toxodon'' would have had a very unusual gait, due to its peculiar proportions. It may have galloped to escape predators, but like a rhino, it probably relied more on its size as protection. ''Toxodon'' is believed to have been ecologically plastic, 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 In agriculture, grazing is a method of animal husbandry whereby domestic livestock are allowed outdoors to roam around and consume wild vegetations in order to convert the otherwise indigestible (by human gut) cellulose within grass and ot ...

diet in the Chaco.

Extinction

''Toxodon'' became extinct at the beginning of the Holocene 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 have been interpreted to be the result of butchery, suggesting that human hunting was a contributing factor to extinction. Toxodon platensis

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:''Toxodon''
at Fossilworks.org ;Holocene * Abismo Ponto de Flecha, Brazil ;Pleistocene * San José, Fortin Tres Pozos, Chaco and

Luján Formation Luján, Luxan or Luhan, a Spanish surname and given name, might refer to: Tony Lujan Jazz musician.Composer Artist Trumpet And Flughelhorn Born 1956 Albuquerque NM. World Renowned. Extensive Resume. N M Music Hall Of Fame 2020 Over 500 Recording ...

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the List of South American countries by area, second-largest ...

* Tarija and Ñuapua Formations, Bolivia * Brazil * Paraguay * Sopas and Dolores Formations, Uruguay ;Miocene-Pliocene ( Montehermosan) * Monte Hermoso Formation,

;Miocene *

Ituzaingó Formation The Ituzaingó Formation ( es, Formación Ituzaingó), in older literature also described as Entre Ríos or Entrerriana Formation, is an extensive geological formation of Late Miocene ( Tortonian, or Huayquerian in the SALMA classification) age in ...

, then described as Entrerriana Formation,