Toxodon

Toxodon; Temporal range: Late Miocene-Holocene (Mayoan-Lujanian);
	Skeleton of Toxodon in Buenos Aires
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	†Notoungulata
Family:	†Toxodontidae
Subfamily:	†Toxodontinae
Genus:	†Toxodon; Owen, 1837
Type species
	†Toxodon platensis; Owen, 1837
Other species
	†T. burmeisteri Giebel, 1866; †T. chapalmalensis Ameghino, 1908; †T. ensenadensis Ameghino, 1887; †T. expansidens Cope, 1886; †T. gracilis Gervais and Ameghino, 1880;
Synonyms
	Genus-level Dilobodon Ameghino, 1886; Chapalmalodon Pascual, 1957; Chapadmalodon Tonni et al., 1992 (lapsus calami); T. platensis T. angustidens Owen, 1846; T. owenii Burmeister, 1866; T. gervaisii Gervais & Ameghino, 1880; T. aguirrei Ameghino, 1917; T. gezi Ameghino, 1917; T. burmeisteri T. paradoxus Ameghino, 1882; T. protoburmeisteri Ameghino, 1887; T. bilobidens Ameghino, 1887; T. chapalmalensis Chapalmalodon chapalmalensis Pascual, 1957; T. chapadmalensis Cione & Tonni, 1995 (lapsus calami); T. chapalmalalensis Oliva & Cerdeno, 2007 (lapsus calami); T. ensenadensis T. giganteus Moreno, 1888; T. elongatus Roth, 1898; T. gracilis T. voghti Moreno, 1888;

Toxodon (meaning "bow tooth" in reference to the curvature of the teeth) is an extinct genus of South American mammals from the Late Miocene to early Holocene epochs (Mayoan to Lujanian in the SALMA classification) (about 11.6 million to 11,000 years ago).[1][2] It is a member of Notoungulata, one of several now extinct orders of South American native ungulates 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.[3] 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."[4] 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!

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.[5][6] 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.[7]

Evolution

The species Toxodon chapalmalensis is known from the Pliocene (Montehermosan-Chapadmalalan) of Argentina,[8] 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:[9]

†Notoungulata

†Pampahippus arenalesi

†Rhynchippus spp.

	†Scarrittia canquelensis

	†Leontinia gaudri

†Toxodontidae

†Proadinotherium leptognathum

†Adinotherium spp.

	†Nesodon taweretus

	†Nesodon imbricatus

†Palyeidodon obtusum

†Hyperoxotodon speciosus

	†Nonotherium henningi

	†Xotodon spp.

†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

Description

Mount at Museo de La Plata, Argentina

Toxodon was about 2.7 m (8 ft 10 in) in body length, with an estimated weight up to 1,415 kg (3,120 lb)[10] and about 1.5 m (4 ft 11 in) high at the shoulder and resembled a heavy rhinoceros, with a short and vaguely hippopotamus-like head.[11] 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.[12] It had short stout legs with plantigrade feet with three functional relatively short toes.[12] The hind limbs are longer and raised higher than the front limbs, giving a sloped appearance to the body.[12] Like horses, it had a stay apparatus allowing the knees to be passively locked while standing.[13]

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.[14] 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.[15]

Palaeobiology

Toxodon skull in front view

Toxodon is suggested to have been capable of running at considerable speed.[16]

Toxodon is believed to have been ecologically plastic, with its diet varying according to local conditions,[17] 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.[18]

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.[19] 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.[20]