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Toxodon
Temporal range: Pliocene–Holocene
Skeleton of Toxodon in Buenos Aires
Scientific classification Edit this classification
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 (from Ancient Greek τόξον (tóxon), meaning "bow", and ὀδούς (odoús), meaning "tooth", in reference to the curvature of the teeth) is an extinct genus of large ungulate native to South America from the Pliocene to the end of the Late Pleistocene.[1][2] 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.[3] Toxodon is a member of the family Toxodontidae, which includes medium to large sized herbivores.[4] Toxodon was one of the largest members of Toxodontidae and Notoungulata, with Toxodon platensis having an estimated body mass of 1,000–1,200 kilograms (2,200–2,600 lb).

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,[5] 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.[6] 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.[3] Several sites have been found suggesting that Toxodon was butchered and possibly hunted by humans.

Taxonomy and evolution

[edit]
Historical restoration of Toxodon platensis from 1913

Charles Darwin, who was in South America as part of the second voyaging expedition of HMS Beagle, was one of the first to collect Toxodon fossils.[7] In September–October 1832 and October 1833, Darwin collected several isolated teeth as well as a mandible from various localities in northern Argentina.[7] On November 26, 1833, Darwin paid 18 pence (equivalent to £6.40 in 2018[8]) for a T. platensis skull from a farmer in Uruguay.[9][10] 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.[11][8] 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 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.[7]

Evolution

[edit]

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.[3] Notoungulata represents the most diverse group of indigenous South American ungulates, with over 150 described genera in 13 different families.[12] Notoungulates are morphologically diverse, including forms morphologically distant from Toxodon such as rodent and rabbit-like forms.[3]

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 (which contains equids, rhinoceroses and tapirs) as part of the clade Panperissodactyla, making them true ungulates.[13][14] 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.[15]

Toxodon belongs to Toxodontidae, a large bodied group of notoungulates which first appeared in the Late Oligocene (Deseadan), ~28-23 million years ago,[16] and underwent a great radiation during the Miocene epoch (~23-5.3 million years ago), when they reached their apex of diversity.[17] The diversity of toxodontids, along with other notoungulates began to decline from around the Pliocene onwards,[3] 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.[18] 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.[3]

Cladogram of Toxodontidae, showing the position of Toxodon relative to other toxodontids, after Forasiepi et al., 2014:[19]

†Notoungulata

Species

[edit]

There has not been a recent taxonomic revision of the genus Toxodon, leaving the number of valid species uncertain.[20]

The species Toxodon chapalmalensis is known from the Pliocene (Montehermosan-Chapadmalalan) of Argentina,[21] 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.[22] 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.[20] Other authors have argued that all Pleistocene Toxodon species should be considered synonymous with T. platensis.[23]

Description

[edit]
Size of Toxodon platensis compared to a human

The bodyform of Toxodon and other toxodontids have been compared to those of hippopotamuses and rhinoceroses.[24] Toxodon platensis is one of the largest known toxodontids and notoungulates, with an estimated body mass of approximately 1,000–1,200 kilograms (2,200–2,600 lb),[25] and a body length of around 2.7 metres (8 ft 10 in).[26]

The skull of Toxodon is proportionally large,[25] and triangular in shape when viewed from above.[27] All of the teeth in the jaws are high-crowned (hypsodont).[28] Like other toxodontids, the upper and lower first incisors (I1 and i1) are large and protrude, with the second upper incisors (I2) and lower third incisors (i3) being modified into evergrowing tusks.[29] The upper incisors display an arched shape,[30] while the lower incisors project horizontally forwards at the front of the lower jaw.[28][30] The wide front of the lower jaw with the horizontally-arranged incisors has been described as "spade-like".[30] There is a gap (diastema) between the incisors and the cheek teeth.[31] Like other derived toxodontids, Toxodon had long, ever-growing (hypselodont) cheek (premolar and molar) teeth,[32] 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.[3] 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 trigonids, while T. platensis populations from northwestern Argentina had noticeably slenderer lower molars.[23]

The thoracic vertebrae of Toxodon have elongate neural spines, which likely anchored muscles which supported the large head.[26] The legs of Toxodon are relatively short, with their bones being robust.[33] The hindlimb is considerably longer than the forelimb, resulting in the back being elevated and the shoulder, neck and head being relatively low.[28] The ulna has a strongly backwardly projecting olecranon process similar to that of rhinos, suggesting that the front leg was held extended when standing.[26] The (distal) part of the femur closest to the foot shows a pronounced medial trochlear ridge, which likely 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.[34] There are three functional digits on each foot,[33][31] which are tipped with hoof-like phalanges.[35]

  • Mount at Museo de La Plata, Argentina
    Mount at Museo de La Plata, Argentina
  • Skull in front-on view
    Skull in front-on view
  • Skull in oblique view
    Skull in oblique view
  • Skeleton in side-on view
    Skeleton in side-on view
  • Skull in side-on view
    Skull in side-on view


Distribution

[edit]

Toxodon had a widespread distribution in South America east of the Andes, ranging from northern Argentina and Bolivia to the western Amazon on the Peru-Brazil border, to Northeast Brazil.[36] Although some authors suggest that the distribution of Toxodon extended into Venezuela,[29] other authors suggest that the related Mixotoxodon (which ranged as far north as the southern United States) was the only toxodontid present in the region during the Pleistocene.[37]

Palaeobiology

[edit]
Life restoration of Toxodon platensis (background centre-left) in a Pleistocene Brazilian landscape, alongside the giant ground sloth Eremotherium laurillardi (foreground right), the glyptodonts Glyptotherium and Panochthus, the pampathere Holmesina paulacoutoi (midground centre-left) and the armadillo Pachyarmatherium brasiliense (foreground left)

Although some authors have suggested that Toxodon was semiaquatic based on the similarity of some aspects of its anatomy to hippopotamuses, this has been disputed by other authors, and analysis of oxygen isotope ratios (which differ between terrestrial and aquatic animals) suggests a terrestrial lifestyle for Toxodon.[38][6] As such, it has been suggested that Toxodon was probably more ecologically comparable to rhinoceroses.[6]

Toxodon is suggested to have been capable of moving at considerable speed.[30] Toxodon is believed to have been ecologically plastic and have had a wide niche breadth,[6] with its diet varying according to local conditions,[39] 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 Chaco.[36] Within the Brazilian Intertropical Region (BIR), T. platensis was a mixed feeder;[40] seasonal variations in the BIR had little impact on the diet of T. platensis.[41] Although Toxodon is thought to have inhabited open landscapes like steppe and savannah,[42][43] in some areas like the southwestern Brazilian Amazon, it is suggested to have inhabited woodland.[44][45]

Like living animals of similar size, it has been suggested that Toxodon probably only gave birth to a single offspring at a time.[46]

T. platensis bones have been found displaying signs of disease like osteomyelitis and spondyloarthropathies.[47] 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.[48]

Tracks probably attributable to Toxodon have been reported from eastern Pernambuco in Northeast Brazil.[33]

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.[49]

Extinction

[edit]

Toxodon and the other remaining toxodontids became extinct at the end of the Late Pleistocene around 12,000 years as part of the end-Pleistocene extinction event alongside almost all other large animals in South America.[3] mid-Holocene dates for Toxodon and Pampas other megafauna have been questioned and are suggested to be the result of contamination.[50] These extinctions followed the first arrival of humans in the Americas, and it has been suggested human hunting may have been a casual factor in the extinctions.[3] Several sites record apparent interactions between Toxodon and humans. Remains of Toxodon from the Arroyo Seco 2 site in the Pampas are associated with unambiguously butchered megafauna, but it is unclear if the Toxodon itself was actually butchered or the remains were naturally transported to the site.[51] At the Paso Otero 5 site in the Pampas of northeast Argentina, burned bones of Toxodon alongside those of numerous other extinct megafauna species are associated with Fishtail points (a type of knapped stone spear point common across South America at the end of the Pleistocene, suggested to be used to hunt large mammals[52]). The bones of the megafauna were probably deliberately burned as fuel. No cut marks are visible on the vast majority of bones at the site (with only one bone of a llama possibly displaying any butchery marks), which may be due to the burning degrading the bones.[53] Various remains of Toxodon platensis in the collection of the Museum national d'Histoire naturelle collected from the Pampas region in the 19th century including a femur, an iliac fragment, a tibia, as well as a mandible (the latter of which has been radiocarbon dated to around 13,000 years ago), have been found to display cut marks indicative of butchery.[54]

References

[edit]

Further reading

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Toxodon

View on Grokipedia
from Grokipedia
Toxodon is an extinct genus of large-bodied, herbivorous mammals in the order Notoungulata, a diverse group of endemic South American ungulates that originated in the early Paleocene and persisted until the late Pleistocene. Toxodon thrived from the late Pliocene to the early Holocene, approximately 3 million to 12,000 years ago. The type and most well-known species, Toxodon platensis, was first scientifically described in 1837 by anatomist Richard Owen based on a fossil skull collected by Charles Darwin in Uruguay during the voyage of the HMS Beagle. Members of Toxodon were robust, rhinoceros-like creatures adapted to a browsing and grazing lifestyle, with a body length of up to 3 meters, a shoulder height of about 1.5 meters, and a weight exceeding 1 tonne, comparable to that of an American bison or African black rhinoceros. Their build resembled a cross between a hippopotamus and a rhinoceros, featuring a large head, short limbs, a barrel-shaped torso, and three-toed feet suited for traversing varied terrains. The most distinctive feature was their dentition: high-crowned (hypsodont), ever-growing cheek teeth with curved profiles, and an enlarged upper incisor functioning as a tusk-like structure, all indicative of a diet consisting of abrasive grasses, leaves, twigs, and other tough vegetation. Fossils of Toxodon are widespread across South America, from the Pampean region of Argentina to sites in Uruguay, Brazil, and Bolivia. Paleobiological evidence suggests they inhabited open grasslands, where they could access a mix of terrestrial plants. As one of the last surviving notoungulates, Toxodon coexisted with early human populations in South America toward the end of the Pleistocene, and its extinction around 12,000 years ago is attributed to a combination of rapid climate shifts at the Pleistocene-Holocene boundary and anthropogenic pressures, such as hunting, contributing to the broader Quaternary megafaunal die-off.

Discovery and Taxonomy

Discovery and Naming

The discovery of Toxodon fossils occurred during Charles Darwin's voyage on the HMS Beagle from 1831 to 1836, when he collected specimens from Pleistocene deposits along the coast of Uruguay and Argentina, including a notable skull purchased from a local farmer near Montevideo for 18 pence. These early finds, excavated from coastal cliffs at sites like Punta Alta near Buenos Aires, represented some of the first evidence of South America's extinct megafauna and were shipped back to England for study. Darwin's collections highlighted the abundance of large mammal remains in these strata, sparking interest in their affinities to modern forms. In 1837, British anatomist Richard Owen formally named the genus Toxodon platensis based on the cranial material from Darwin's Uruguayan skull, initially classifying it within the Rodentia due to similarities in dentition with rodents like the capybara. The name Toxodon derives from the Greek words tóxon (bow) and odous (tooth), alluding to the curved, arc-like structure of its molars. Owen's description, published in the Zoology of the Voyage of H.M.S. Beagle, marked the first scientific recognition of a notoungulate, though its broader affinities remained debated. Throughout the 19th century, additional expeditions expanded knowledge of Toxodon, with Owen describing postcranial elements, including limb bones, from Buenos Aires localities in the 1840s, contributing to early reconstructions of its robust, rhinoceros-like build. Argentine paleontologist Florentino Ameghino played a pivotal role through his extensive fieldwork in Patagonia and the Pampas, collecting numerous Toxodon specimens. His 1889 work Contribuciones al Conocimiento de los Mamíferos Fósiles de la República Argentina integrated these finds into a framework of autochthonous evolution, solidifying Toxodon's place as an endemic South American lineage distinct from northern mammals.

Classification and Species

Toxodon belongs to the kingdom Animalia, phylum Chordata, class Mammalia, order Notoungulata, suborder Toxodontia, superfamily Toxodontoidea, family Toxodontidae, and genus Toxodon. The type species is Toxodon platensis Owen, 1837, which is the most widespread species, known from numerous Pleistocene localities across South America. While several species have been named within the genus, primarily distinguished by temporal distribution, size, and subtle cranial features such as molar lobe proportions and incisor morphology—including T. burmeisteri Giebel, 1866 (a robust Pleistocene form), T. chapalmalensis Ameghino, 1908 (from late Pliocene deposits), T. ensenadensis Ameghino, 1887 (early Pleistocene), T. expansidens Cope, 1889 (noted for larger size), and T. gracilis Gervais, 1855 (a smaller Pleistocene form)—their validity remains debated. Recent revisions recognize T. platensis as the primary species, with T. gracilis potentially valid, while others are often treated as synonyms or variants due to overlapping morphology and stratigraphic ranges; for example, T. intermedius Roth, 1899, is rejected as distinct. Molecular and morphological studies suggest Notoungulata, including Toxodon, may be closely related to perissodactyls, but a comprehensive systematic revision of the genus is needed. As a late-surviving genus, Toxodon persisted into the late Pleistocene and early Holocene within Toxodontidae, outlasting many earlier members of the family.

Evolutionary History

Origins of Toxodontidae

The Toxodontidae family emerged during the Late Oligocene, approximately 28–23 million years ago, in South America as part of the broader radiation of Notoungulata following the isolation of the continent after the Paleocene. This diversification occurred within the Deseadan South American Land Mammal Age (SALMA), marking the initial expansion of toxodontids from smaller, more primitive notoungulates into medium- to large-sized herbivores adapted to emerging grassland environments. Early toxodontids, such as Proadinotherium from the Deseadan stage, represent transitional forms with features bridging smaller, browser-like ancestors, including protohypsodont molars suited for mixed vegetation diets. By the Early Miocene (Santarcrucian SALMA, ~17.5–16.3 million years ago), genera like Adinotherium appeared, exhibiting further advancements such as Y-shaped central fossettes on molars and a shift toward more robust postcranial elements for weight-bearing locomotion. Fossil evidence for these ancestral toxodontids comes primarily from Patagonian sites, including Quebrada Fiera in Mendoza Province, Argentina, where postcranial remains like astragali and calcanea indicate early adaptations for terrestrial browsing in open woodlands. The family reached its diversification peak during the Miocene (~23–5 million years ago), with numerous genera evolving amid environmental changes driven by Andean uplift, which created diverse habitats including expanding arid grasslands and altered rainfall patterns. This period saw key adaptations such as increased body sizes—ranging from medium (around 100 kg) to very large (>1,000 kg)—and the development of hypsodont (high-crowned) teeth capable of withstanding abrasive vegetation like grasses, reflecting a dietary shift from browsing to grazing. Fossils from Santarcrucian deposits in Patagonia, such as those of Adinotherium, further document this radiation, with over 20 genera across the family by the late Miocene, underscoring the adaptive success of toxodontids before the Pliocene.

Phylogenetic Relationships

Toxodon is classified as a derived member of the Toxodontidae family within the suborder Toxodontia of Notoungulata, specifically placed in the subfamily Toxodontinae. Cladistic analyses consistently recover Toxodontidae as a monophyletic group, supported by shared cranial features such as the morphology of the crista intermedia in the upper molars. Within Toxodontinae, Toxodon forms a clade with other late-occurring genera, including Mixotoxodon and Xotodon, indicating a sister-group relationship characterized by advanced hypsodont dentition and robust postcranial adaptations for browsing in open habitats. The phylogenetic affinities of Notoungulata, including Toxodon, have been debated since the 19th century, with early classifications linking them to condylarths or other ungulate groups based on superficial resemblances in limb structure and dental wear patterns. However, modern morphological phylogenies, incorporating extensive cranial and dental datasets, suggest closer relations to Afrotheria rather than to laurasiatherian ungulates, evidenced by shared traits like the absence of a postorbital bar and specific auditory bulla configurations. These afrotherian affinities are bolstered by analyses of hard-tissue features unique to both groups, such as the entoglenoid process morphology, though some studies caution that delayed dental eruption patterns in notoungulates do not fully align with afrotherian models. Key cladistic studies from the 2010s, utilizing parsimony-based methods on up to 133 discrete characters across 50 notoungulate genera, affirm the monophyly of Toxodontia and position Toxodon as a late-branching taxon within Toxodontidae, emerging during the Pliocene radiation. These revisions highlight the paraphyly of earlier groupings like Notohippidae and emphasize the role of dental hypsodonty gradients in resolving internal relationships. Due to the extinction of notoungulates by the end of the Pleistocene, no molecular data are available, leaving phylogenies reliant on morphological evidence from fossils. Recent research from 2023 to 2025 integrates diversification dynamics into notoungulate phylogenies, modeling Toxodon as part of a Miocene-Pliocene radiation driven by Andean uplift, which increased habitat heterogeneity and speciation rates while later elevating extinction pressures through competition and climatic shifts. These birth-death models show net diversification peaks aligning with Toxodon's temporal range, underscoring the influence of orogenic events on toxodontid evolution without altering core morphological cladograms.

Physical Description

Overall Morphology

Toxodon platensis was one of the largest members of the Toxodontidae family, reaching a body length of approximately 2.7 to 3 meters, a shoulder height of about 1.5 meters, and an estimated body mass of 1,000 to 1,500 kilograms, making it comparable in size to a large rhinoceros such as Rhinoceros unicornis. The animal possessed a robust, heavy build characterized by a stocky torso and short, pillar-like legs adapted for supporting its substantial weight on varied terrains. Its limbs were sturdy and columnar, with the forelimbs slightly shorter and more voluminous than the hindlimbs, featuring a robust scapula and humerus that provided strong structural support. The feet were tridactyl, bearing three functional toes, and the overall posture was semi-digitigrade to semi-plantigrade, facilitating stability rather than high-speed locomotion. Evidence for sexual dimorphism in Toxodon platensis is limited, with some variation in incisor size potentially indicating differences between males and females (larger outer incisors in males), though further fossil material is needed to confirm this.

Skull and Dentition

The skull of Toxodon is notably large and triangular in dorsal view, measuring approximately 60 cm in length, with a dolichocephalic structure that emphasizes its elongated form. A prominent sagittal crest runs along the midline of the cranium, providing robust attachment sites for the temporalis and masseter muscles to support powerful jaw adduction. This crest is particularly developed in adult specimens, contributing to the overall robustness of the cranial architecture adapted for forceful mastication. The dentition of Toxodon features hypsodont cheek teeth that are ever-growing and enveloped in thick cementum, enabling prolonged functionality despite heavy attrition. Upper molars exhibit a triangular occlusal outline with transverse lophs that facilitate efficient grinding of tough plant material, while the lower premolars are highly molarized, displaying similar lophid patterns to the molars for enhanced shearing and milling capabilities. Canines are small and reduced in size relative to the incisors and cheek teeth, and the upper incisors are enlarged, with the second upper incisor particularly prominent and functioning as a tusk-like structure. The absence of a diastema results in a continuous dental arcade from incisors to molars, promoting a seamless occlusal surface. The powerful masseter muscles, anchored via the sagittal crest, enable strong lateral jaw movements suited to processing abrasive forage. Slight variations in dental dimensions occur across species, with T. platensis exhibiting proportionally larger molars compared to other taxa, reflecting potential differences in body size or dietary pressures. Fossil specimens often display pronounced tooth wear, including enamel hypoplasia and microstructural defects, indicative of exposure to highly abrasive diets that accelerated occlusal erosion. These pathologies underscore the adaptive limits of the ever-growing dentition in coping with environmental stressors.

Distribution and Fossil Record

Geographic Range

Toxodon fossils are primarily known from eastern South America, encompassing a broad area from the Pampas region of northern Argentina northward to southern Bolivia, with additional records in Uruguay, Paraguay, and northeast Brazil. This distribution is confined to the eastern side of the Andes, reflecting the barrier posed by the Andean uplift during the Pleistocene, and excludes western regions as well as southern Patagonia. The genus inhabited diverse landscapes, including open grasslands and wooded savannas, across this extensive territory. Key fossil localities include the Luján Formation in Buenos Aires Province, Argentina, where abundant remains of Toxodon platensis have been recovered from fluvial and alluvial deposits; the Sopas Formation in northern Uruguay, yielding skeletal elements associated with late Pleistocene megafauna assemblages; and the Tarija Valley in southern Bolivia, particularly the Tarija Formation, which has produced numerous toxodontid specimens indicative of a rich local fauna. Over 100 sites across these countries have documented Toxodon remains, highlighting the genus's widespread occurrence and the intensity of paleontological exploration in the region. Evidence suggests provinciality between northern and southern populations, with subtle morphological variations in dental elements, such as lower molar shape, potentially arising from geographic isolation or local environmental influences. For instance, specimens from northwestern Argentina exhibit slenderness in molars compared to those from the Pampas or southern Brazil, though overall size remains consistent across populations. These differences underscore regional adaptations within the species Toxodon platensis. Toxodon showed no significant northward dispersal as part of the early Great American Biotic Interchange, with migrations into Central America occurring only in the late Pleistocene. Recent analyses in 2024 of isolated teeth from southern Brazil, including the Touro Passo Formation in Rio Grande do Sul State, have further documented the genus's presence and expanded understanding of its northeastern range limits.

Temporal Range

Toxodon first appeared during the Late Pliocene, approximately 3.6 million years ago, in the Chapadmalalan stage of the Pampean region in Argentina, marking the initial diversification of the genus within Toxodontidae. Fossils from this period are documented in stratigraphic units such as the upper levels of the Raigón Formation in Uruguay, which correlate with the upper Pliocene–lower Pleistocene boundary. The genus reached peak abundance during the Middle to Late Pleistocene, spanning the Ensenadan (approximately 780,000 to 240,000 years ago) and Lujanian (approximately 240,000 to 12,000 years ago) stages/ages, when it became a dominant component of South American megafaunal assemblages. Abundant remains are preserved in formations including the Arroyo Seco in the Argentine Pampas, representing Lujanian deposits, and the Ituzaingó Formation in Uruguay and Entre Ríos Province, Argentina, which encompass Ensenadan to Lujanian sediments. Biostratigraphically, Toxodon co-occurs with other late Pleistocene megafauna, such as the ground sloth Megatherium americanum, in assemblages defining the Megatherium Biozone of the Lujanian stage, indicating shared habitats across pampas and Andean foreland environments. There are no confirmed Holocene records, though some early Holocene dates remain disputed due to potential reworking or contamination. Radiometric dating supports this temporal span, with electron spin resonance (ESR) analyses of Toxodon teeth yielding ages from the Middle Pleistocene to the terminal Late Pleistocene, and uranium-series (U/Th) methods corroborating the latest occurrences around 12,000 years ago prior to the genus's extinction.

Palaeobiology

Diet and Habitat

Toxodon exhibited a mixed diet as a generalist herbivore, consuming both C₃ browse (such as leaves and shrubs) and C₄ grasses, with the proportion varying by region and time period. Stable carbon isotope (δ¹³C) analyses of tooth enamel reveal a broad range of values from -8.2‰ to 1.3‰ during the Late Pleistocene, indicating dietary flexibility from predominantly browsing to significant grazing. In wetter southern regions like central Argentina, δ¹³C values averaged -5.9‰ ± 2.1‰, suggesting a higher reliance on C₃ plants, while in the more arid northern areas such as northeastern Argentina, values were higher at -3.0‰ ± 3.7‰, reflecting up to 70% C₄ grass consumption. This regional plasticity allowed Toxodon to adapt to local vegetation availability amid environmental changes. Dental microwear and molar morphology further support a diet of tough, abrasive vegetation. Scratches on enamel surfaces, observed in notoungulate relatives including Toxodon, indicate processing of fibrous grasses and leaves, consistent with mixed feeding habits. A 2025 study in Geobios on lower molar shape and size across South American sites (northern Pampa, Mesopotamia, and Buenos Aires) demonstrates geographic dietary plasticity, with molar complexity and wear patterns varying by locality to handle diverse plant toughness, from softer browse in humid areas to harder grasses in open environments. Toxodon inhabited open steppes, savannas, and gallery woodlands across South America during the Pleistocene, thriving in mosaic landscapes that supported herbaceous and woody vegetation. Fossil remains are commonly associated with fluvial deposits from riverine systems and aeolian sediments indicating windy, grassy plains, reflecting preferences for well-drained, seasonally variable terrains. As a generalist, Toxodon occupied a broad ecological niche, overlapping with litopterns like Macrauchenia patachonica and xenarthrans such as ground sloths (Lestodon, Eremotherium), competing for mid- to high-biomass herbaceous resources in shared savanna ecosystems. Seasonal adaptations are inferred from dental tissues, with cementum increments and enamel isotope profiles (δ¹⁸O) recording wet-dry cycles over multiple years. For instance, sequential sampling of Toxodon enamel shows minimal dietary shifts but marked climatic oscillations, enabling behavioral adjustments like migration to wetter areas during dry periods.

Locomotion and Behavior

Toxodon was an ambulatory quadruped characterized by short, robust limbs that limited its speed and favored a walking or grazing gait over rapid running. Its hindlimbs were longer than the forelimbs, supporting a stable posture suited to browsing low vegetation in open habitats. The bone robusticity, particularly in the weight-bearing elements, indicates adaptations for sustained terrestrial locomotion rather than cursorial pursuits. Fossil assemblages containing multiple individuals of Toxodon alongside other megafauna suggest possible gregarious behavior, though direct evidence remains limited. While direct trace fossils like footprints attributable to Toxodon remain undocumented, the co-occurrence of remains in localized bone beds implies social grouping rather than solitary living. No definitive evidence supports long-distance migration, though its broad geographic range across South America during the Pleistocene hints at some mobility in response to environmental shifts. Skeletal features include relatively large orbits, potentially indicating reliance on visual cues for detecting predators or navigating terrain, though quantitative data on eye size is limited. Relative brain size in Toxodon was small compared to body mass and other notoungulates, positioning it among the lowest encephalization quotients in Late Quaternary mammals and suggesting moderate rather than advanced cognitive abilities for problem-solving or complex social interactions. This smaller brain-to-body ratio may have constrained behavioral flexibility in changing environments. As a large herbivore, Toxodon faced predation pressures from contemporary carnivores such as Smilodon populator, which migrated to South America and targeted megafauna. Fossil evidence of healed skeletal pathologies in Toxodon specimens, including fractures and infections, points to survival after injuries possibly inflicted during predatory encounters or intraspecific conflicts. Its robust build and potential group living likely served as defenses, though juveniles and weakened adults remained vulnerable.

Extinction

Timing and Last Records

Toxodon became extinct during the end of the Late Pleistocene, approximately 12,000 to 10,000 years ago, as part of the broader Quaternary extinction event that affected megafaunal assemblages across the Americas. This timing aligns with a sharp decline in South American megafauna populations, with virtually no reliable records extending into the Holocene. The youngest confirmed records of Toxodon come from terminal Pleistocene sites in southern South America, where radiocarbon dating of bone collagen has provided precise chronologies. In the Argentine Pampas, a key region for late-surviving populations, specimens from Arroyo Seco 2 yield dates of 11,750 ± 70 years BP (calibrated to 13,695–13,457 cal BP) and 11,590 ± 90 years BP (calibrated to 13,486–13,311 cal BP), while Río Luján material dates to 11,920 ± 50 years BP (calibrated to 13,799–13,566 cal BP). At Paso Otero 5 in the Pampas, associated megafaunal remains, including Toxodon, are dated to around 10,440 ± 100 years BP (approximately 12,300 cal BP), indicating persistence until the onset of the Younger Dryas chronozone. Further south in Chile, the Tagua Tagua site has yielded disputed records potentially as young as ~10,500 years ago, though these are based on indirect associations and lack direct radiocarbon dates on Toxodon remains, with most evidence pointing to earlier Late Pleistocene ages. No verified Holocene survival has been documented, as post-10,000 cal BP dates for Toxodon are either contaminated or attributable to reworked sediments. These final appearances coincide with the persistence of other megafauna, such as ground sloths (e.g., Megatherium americanum) and horses (e.g., Hippidion principale and Equus neogeus), before the complete collapse of South American megafaunal communities around 11,600 cal BP. Dating relies primarily on accelerator mass spectrometry (AMS) radiocarbon analysis of bone collagen, which offers high precision for late Pleistocene samples, supplemented by taphonomic indicators of rapid population decline such as clustered last-appearance dates and low stratigraphic overlap in terminal assemblages. Summed probability distributions of these dates reveal a synchronous drop-off across sites, supporting a brief window of final survival rather than prolonged rarity. Evidence suggests regional asynchrony in Toxodon's demise, with northern and central populations vanishing closer to 12,500 cal BP, while southern groups in the Pampas and Patagonia endured slightly longer, up to ~12,000–11,000 cal BP, possibly due to varied environmental refugia. In contrast, central Andean records indicate earlier local extirpations around human arrival, highlighting geographic variability in the extinction pulse.

Causes and Human Impact

The extinction of Toxodon occurred as part of the broader End-Pleistocene megafaunal extinction event in South America, which eliminated approximately 83% of large mammal genera around 12,000 to 10,000 years ago. This wave is attributed to a synergistic interplay between rapid climate cooling during the Younger Dryas period (approximately 12,900 to 11,700 years ago) and the arrival of Paleoindian hunters. The Younger Dryas introduced abrupt cooling and aridification across southern South America, transforming diverse woodlands and browse-rich habitats into expansive arid grasslands, as evidenced by pollen records from the Pampas region showing a marked decline in forest pollen and rise in grass taxa around 12,000 years ago. For Toxodon, a semi-aquatic browser reliant on leafy vegetation near water bodies, this habitat contraction likely reduced forage availability and increased vulnerability to stressors. Direct evidence of human impact on Toxodon includes butchery marks on its bones at key archaeological sites, indicating exploitation by early humans. At Arroyo Seco 2 in Argentina, cut marks on Toxodon platensis remains, dated to approximately 11,750 ± 70 and 11,250 ± 100 radiocarbon years before present (BP), are associated with lithic artifacts used for processing, suggesting intentional hunting or scavenging around 13,500 to 13,000 calibrated years ago. Similarly, at Piedra Museo in Patagonia, Argentina, Toxodon bones are associated with hearths and Clovis-like fishtail projectile points dated to about 11,000 BP, indicating coexistence with Paleoindian groups approximately 13,000 calibrated years ago. These findings align with broader patterns where extinct megafauna, including toxodontids, comprised a significant portion of human subsistence, with cut-marked bones appearing in over 70% of late Pleistocene assemblages across southern South America. The overkill hypothesis posits that Toxodon, as a large (up to 1,500 kg) and behaviorally naive herbivore unused to human predation, was particularly susceptible to efficient hunting by small groups of Paleoindians equipped with atlatls and stone-tipped spears. This model suggests that even low population densities of humans could drive local extirpations through targeted hunting of reproductively slow megafauna, with Toxodon's abundance in open habitats facilitating encounters. While climate alone may not suffice to explain the selectivity for large mammals, the combination amplified pressures, as aridity concentrated surviving populations into huntable refugia. Recent studies from 2024 and 2025 reinforce human agency as the primary driver over climate, citing staggered extinction timings across the Americas that closely track human dispersal waves rather than uniform Younger Dryas effects. For instance, analyses of 18 southern South American sites show megafauna like Toxodon persisting until shortly after human arrival, with dietary reliance on these species evidenced by cut marks in 13 assemblages, supporting overkill amid environmental stress. These findings challenge climate-centric models by highlighting non-synchronous losses, such as later Toxodon survivals in refugia, aligned with regional human expansions around 12,000 to 10,000 years ago.

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