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Toxodon (from Ancient Greek τόξον ( tóxon), meaning "bow", and ὀδούς ( odoús), meaning "tooth", in reference to the curvature of the teeth) is an Extinction genus of large ungulate native to South America from the Pliocene to the end of the Late Pleistocene. 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 .
Remains of Toxodon were first collected by Charles Darwin during the voyage of the Beagle in 1832-33, and later scientifically named by Richard Owen in 1837. 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 the northwestern-most region of the continent, inhabiting steppe, savanna and sometimes woodland habitats. It was one of several genera of toxodontids living during the Pleistocene also including Trigonodops, Mixotoxodon (which ranged as far north as the southern United States) and possibly Piauhytherium. Evidence suggests that Toxodon was ecologically plastic and able to adapt its diet to local conditions. While some authors have suggested that Toxodon was semiaquatic, isotopic analysis has supported a terrestrial lifestyle.
Toxodon became extinct as part of the end-Pleistocene extinction event around 12,000 years ago, along with most large mammals across the Americas. The extinctions followed the arrival of humans to South America, who may have been a contributory factor in the extinctions. Several sites have been found suggesting that Toxodon was butchered and possibly hunted by humans.
In his own words, Darwin wrote down in his journal,
Toxodon and its type species, T. platensis, were described in 1837 by Richard Owen based on remains collected by Darwin, in a paper titled " A description of the cranium of the Toxodon platensis, a gigantic extinct mammiferous species, referrible by its dentition to the Rodentia, but with affinities to the Pachydermata and the herbivorous Cetacea", reflecting the many unusual characteristics of its anatomy.Fernicola, J. C., Vizcaino, S. F., & De Iuliis, G. (2009). The fossil mammals collected by Charles Darwin in South America during his travels on board the HMS Beagle. Revista De La Asociación Geológica Argentina, 64(1), 147-159.
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 (which contains equids, rhinoceroses and tapirs) as part of the clade Panperissodactyla, making them true . 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.
Toxodon belongs to Toxodontidae, a large bodied group of notoungulates which first appeared in the Late Oligocene (Deseadan), ~28-23 million years ago, and underwent a great radiation during the Miocene epoch (~23-5.3 million years ago), when they reached their apex of diversity. The diversity of toxodontids, along with other notoungulates began to decline from around the Pliocene onwards, 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. By the Late Pleistocene (Lujanian), the once great diversity of notoungulates had declined to only a few of species of toxodontids (belong to the genera Toxodon, Mixotoxodon, Trigodonops and Piauhytherium, the last possibly being a synonym of Trigodonops) with all other notoungulate families having become extinct.
Cladogram of Toxodontidae, showing the position of Toxodon relative to other toxodontids, after Forasiepi et al., 2014:
The species Toxodon chapalmalensis is known from the Pliocene (Montehermosan-Chapadmalalan) of Argentina, while Toxodon platensis, the type species, is known from the Pleistocene wolf. The validity of other potential species like Toxodon darwini Burmeister, 1866, and Toxodon ensenadensis Ameghino, 1887 from the Early Pleistocene of Argentina is uncertain, and the species Toxodon gezi C. Ameghino, 1917 and Toxodon aguirrei Ameghino, 1917 have been considered junior synonyms of Toxodon platensis by recent authors. Some recent authors have argued that Toxodon gracilis Gervais and Ameghino, 1880, should be recognised as a distinct species from the Pleistocene of the Pampas significantly smaller than T. platensis, with these authors suggesting that T. platensis and T. gracilis represent the only valid species of Toxodon in the Pleistocene of the Pampas region. Other authors have argued that all Pleistocene Toxodon species should be considered synonymous with T. platensis.
The skull of Toxodon is proportionally large, and triangular in shape when viewed from above. All of the teeth in the jaws are high-crowned (hypsodont).Scott WB. Mammalia of the Santa Cruz Beds. Volume VI, Paleontology. Part II, Toxodontia. In: Scott WB, editor. Reports of the Princeton University Expeditions to Patagonia, 1896–1899. Stuttgart: Princeton University, E. Schweizerbart'sche Verlagshandlung (E. Nägele); 1912. pp. 112-116 Like other toxodontids, the upper and lower first (I1 and i1) are large and protrude, with the second upper incisors (I2) and lower third incisors (i3) being modified into evergrowing tusks. The upper incisors display an arched shape,S.F. Vizcaino, R.A. Farina, J.C. Fernicola " Young Darwin and the ecology and extinction of Pleistocene South American fossil mammals" Revista de la Asociacion Geologica Argentina, 64 (2009), pp. 160-169 while the lower incisors project horizontally forwards at the front of the lower jaw. The wide front of the lower jaw with the horizontally-arranged incisors has been described as "spade-like". There is a gap (diastema) between the incisors and the cheek teeth.E. Anderson Who's who in the Pleistocene: a mammalian bestiary P.S. Martin, R.G. Klein (Eds.), Quaternary Extinctions: a Prehistoric Revolution, The University of Arizona Press, Tucson, AZ (1989), pp. 64 Like other derived toxodontids, Toxodon had long, ever-growing (hypselodont) cheek (premolar and molar) teeth, with the name Toxodon deriving from the curved shape of the upper molars, which are bowed inwards towards the midline of the skull to fit in the upper jaw. Evergrowing cheek teeth are unknown in any living ungulates (though they occur in the extinct rhinoceros Elasmotherium), but are present in some other mammal groups like and . The surface of the cheek teeth is primarily composed of . The mandibular molars of T. platensis exhibited significant morphological variability dependent on geographic location, which was likely related to different diets across space; specimens from Mesopotamia (a region of northeast Argentina just west of Uruguay) exhibit highly robust trigonid, while T. platensis populations from northwestern Argentina had noticeably slenderer lower molars.
The thoracic vertebrae of Toxodon have elongate neural spines, which likely anchored muscles and ligaments which supported the large head. The legs of Toxodon are relatively short, with their bones being robust. The hindlimb is considerably longer than the forelimb. While William Berryman Scott suggested in 1912 that Toxodon habitually held its neck and head relatively low, a 1994 study suggested that Toxodon instead likely held its head in a more raised posture.Fariña RA, Alvarez F (1994) La postura de Toxodon: un nueva reconstrucción. The Acta Geol Leopold 17:565-571 (in Spanish with English abstract) Although Toxodon has been historically reconstructed in museum mounts with bent limbs, the ulna of the forelimb has a strongly backwardly projecting olecranon process similar to that of rhinos, suggesting that the front legs and likely the hind legs were held extended straight beneath the body when standing. The (distal) part of the femur closest to the foot shows a pronounced medial trochlear ridge, which has been suggested to have 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.Shockey BJ. 2001. " Specialized knee joints in some extinct, endemic, South American herbivores" Acta Palaeontologica Polonica 46:277–88 There are three functional digits on each foot, which are tipped with hoof-like Phalanx bone.
Toxodon is suggested to have been capable of moving 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, and an almost completely C4 dominated grazing diet in the Gran Chaco. Within the Brazilian Intertropical Region (BIR), T. platensis was a mixed feeder; seasonal variations in the BIR had little impact on the diet of T. platensis. Although Toxodon is thought to have inhabited open landscapes like steppe and , in some areas like the southwestern Brazilian Amazon, it is suggested to have inhabited woodland.
Like living animals of similar size, it has been suggested that Toxodon probably only gave birth to a single offspring at a time.
T. platensis bones have been found displaying signs of disease like osteomyelitis and spondyloarthropathies. The teeth of Toxodon often display enamel hypoplasia (loss of tooth enamel) in the form of grooves and pits, which is likely due to their evergrowing nature and/or environmental stresses.
Tracks probably attributable to Toxodon have been reported from eastern Pernambuco in Northeast Brazil.
Isotopic analysis suggests that Toxodon may have been predated upon by the large sabertooth cat Smilodon populator, the apex predator of South American ecosystems during much of the Pleistocene.
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