Toxodon
Toxodon, meaning "bow", and ὀδούς is an extinct 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 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.
Taxonomy and evolution
, who was in South America as part of the second voyaging expedition of HMS Beagle, was one of the first to collect Toxodon fossils. In September–October 1832 and October 1833, Darwin collected several isolated teeth as well as a mandible from various localities in northern Argentina. On November 26, 1833, Darwin paid 18 pence for a T. platensis skull from a farmer in Uruguay. In his book covering the expedition, 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." The skull had been propped up against a fence and been used as target practice for throwing stones by local children, who had knocked out its teeth. 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,
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.
Evolution
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, and had evolved in isolation in South America, 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.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 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 from perissodactyls.
Toxodon belongs to Toxodontidae, a large bodied group of notoungulates which first appeared in the Late Oligocene, ~28-23 million years ago, and underwent a great radiation during the Miocene epoch, 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, the once great diversity of notoungulates had declined to only a few of species of toxodontids 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:
Species
There has not been a recent taxonomic revision of the genus Toxodon, leaving the number of valid species uncertain.The species Toxodon chapalmalensis is known from the Pliocene of Argentina, while Toxodon platensis, the type species, is known from the Pleistocene. 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.
Description
The bodyform of Toxodon and other toxodontids have been compared to those of hippopotamuses and rhinoceroses. Toxodon platensis is one of the largest known toxodontids and notoungulates, with an estimated body mass of approximately, and a body length of around.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. Like other toxodontids, the upper and lower first incisors are large and protrude, with the second upper incisors and lower third incisors being modified into evergrowing tusks. The upper incisors display an arched shape, 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 between the incisors and the cheek teeth. Like other derived toxodontids, Toxodon had long, ever-growing cheek 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, but are present in some other mammal groups like wombats and rodents. The surface of the cheek teeth is primarily composed of dentine. 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 exhibit highly robust trigonids, 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. 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 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 to allow the knees to be locked when standing akin to the stay apparatus of living horses as an energy saving mechanism. There are three functional digits on each foot, which are tipped with hoof-like phalanges.