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

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of large

ungulate Ungulates ( ) are members of the diverse clade Euungulata ("true ungulates"), which primarily consists of large mammals with Hoof, hooves. Once part of the clade "Ungulata" along with the clade Paenungulata, "Ungulata" has since been determined ...

native to

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

from the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Late Pleistocene The Late Pleistocene is an unofficial Age (geology), age in the international geologic timescale in chronostratigraphy, also known as the Upper Pleistocene from a Stratigraphy, stratigraphic perspective. It is intended to be the fourth division ...

. ''Toxodon'' is a member of

Notoungulata Notoungulata is an extinct order of ungulates that inhabited South America from the early Paleocene to the end of the Pleistocene, living from approximately 61 million to 11,000 years ago. Notoungulates were morphologically diverse, with forms re ...

, an order of extinct

South American native ungulates South American native ungulates, commonly abbreviated as SANUs, are extinct ungulate-like mammals that were indigenous to South America from the Paleocene (from at least 63 million years ago) until the end of the Late Pleistocene (~12,000 years a ...

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 The Great American Biotic Interchange (commonly abbreviated as GABI), also known as the Great American Interchange and the Great American Faunal Interchange, was an important late Cenozoic paleozoogeographic biotic interchange event in which land ...

. ''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 Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English Natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...

during the voyage of the Beagle in 1832-33, and later scientifically named by

Richard Owen Sir Richard Owen (20 July 1804 – 18 December 1892) was an English biologist, comparative anatomy, comparative anatomist and paleontology, palaeontologist. Owen is generally considered to have been an outstanding naturalist with a remarkabl ...

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 northeastern-most region of the continent, inhabiting

steppe In physical geography, a steppe () is an ecoregion characterized by grassland plains without closed forests except near rivers and lakes. Steppe biomes may include: * the montane grasslands and shrublands biome * the tropical and subtropica ...

savanna A savanna or savannah is a mixed woodland-grassland (i.e. grassy woodland) biome and ecosystem characterised by the trees being sufficiently widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach th ...

and sometimes woodland habitats. 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 In biology, being semi-aquatic refers to various macroorganisms that live regularly in both aquatic and terrestrial environments. When referring to animals, the term describes those that actively spend part of their daily time in water (in ...

, 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 human interaction with ''Toxodon''.

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 (equivalent to £6.40 in 2018) 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 In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

, ''T. platensis'', were described in 1837 by

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 Cetacea (; , ) is an infraorder of aquatic mammals belonging to the order Artiodactyla that includes whales, dolphins and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively c ...

''", 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. Retrieved from https://revista.geologica.org.ar/raga/article/view/1339

Evolution

''Toxodon'' is a member of

, a group of

that had been part of the fauna of South America since the

Paleocene The Paleocene ( ), or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...

, 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

. 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 Collagen () is the main structural protein in the extracellular matrix of the connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content. Amino acids are bound together to form a trip ...

sequences obtained from ''Toxodon'' as well as from ''

Macrauchenia ''Macrauchenia'' ("long llama", based on the now-invalid llama genus, ''Auchenia'', from Greek "big neck") is an extinct genus of large ungulate native to South America from the Pliocene or Middle Pleistocene to the end of the Late Pleistocene. I ...

'', 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 In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

perissodactyl Perissodactyla (, ), or odd-toed ungulates, is an order of Ungulate, ungulates. The order includes about 17 living species divided into three Family (biology), families: Equidae (wild horse, horses, Asinus, asses, and zebras), Rhinocerotidae ( ...

s (which contains equids, rhinoceroses and tapirs) as part of the clade Panperissodactyla, making them true

s. 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 The Chattian is, in the geologic timescale The geologic time scale or geological time scale (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the pro ...

(

Deseadan The Deseadan () age is a period of geologic time (29.0–21.0 Ma) within the Oligocene epoch of the Paleogene to the Early Miocene epoch of the Neogene, used more specifically within the SALMA classification of South America. It follows the Tingu ...

), ~28-23 million years ago, and underwent a great radiation during the

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

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 The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58 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 The Isthmus of Panama, historically known as the Isthmus of Darien, is the narrow strip of land that lies between the Caribbean Sea and the Pacific Ocean, linking North America, North and South America. The country of Panama is located on the i ...

. By the

Late Pleistocene The Late Pleistocene is an unofficial Age (geology), age in the international geologic timescale in chronostratigraphy, also known as the Upper Pleistocene from a Stratigraphy, stratigraphic perspective. It is intended to be the fourth division ...

(

Lujanian The Lujanian age is a South American land mammal age within the Pleistocene and Holocene epochs of the Neogene, from 0.8–0.011 Mya (unit), Ma or 800–11 tya (unit), tya. It follows the Ensenadan. The age is usually divided into the middle Pleist ...

), 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 A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

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 The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Montehermosan The Montehermosan age is a period of geologic time (6.8–4.0 Mya (unit), Ma) within the Miocene and Pliocene epochs of the Neogene used more specifically with South American Land Mammal Ages. It follows the Huayquerian and precedes the Chapadmalal ...

Chapadmalalan The Chapadmalalan age is a period of geologic time (4.0–3.0 Ma) within the Pliocene epoch of the Neogene used more specifically with South American Land Mammal Ages. It follows the Montehermosan and precedes the Uquian age. Fossil content ...

) of Argentina, while ''Toxodon platensis'', the type species, is known from the

Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

. 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 .Fariña RA, Vizcaíno SF& De Iuliis G. 2012 ''Megafauna: giant beasts of Pleistocene South America''. Bloomington, IN: Indiana University Press. p. 264-265 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. 211-216 Like other toxodontids, the upper and lower first

incisor Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...

s (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, but are present in some other mammal groups like

wombat Wombats are short-legged, muscular quadrupedal marsupials of the family Vombatidae that are native to Australia. Living species are about in length with small, stubby tails and weigh between . They are adaptable and habitat tolerant, and are ...

s and

rodent Rodents (from Latin , 'to gnaw') are mammals of the Order (biology), order Rodentia ( ), which are characterized by a single pair of continuously growing incisors in each of the upper and Mandible, lower jaws. About 40% of all mammal specie ...

s. The surface of the cheek teeth is primarily composed of

dentin Dentin ( ) (American English) or dentine ( or ) (British English) () is a calcified tissue (biology), tissue of the body and, along with tooth enamel, enamel, cementum, and pulp (tooth), pulp, is one of the four major components of teeth. It i ...

e. The

thoracic vertebrae In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebra (anatomy), vertebrae of intermediate size between the ce ...

of ''Toxodon'' have elongate

neural spines Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...

, which likely anchored muscles 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, resulting in the back being elevated and the shoulder, neck and head being relatively low. The ulna has a strongly backwardly projecting

olecranon The olecranon (, ), is a large, thick, curved bony process on the proximal, posterior end of the ulna. It forms the protruding part of the elbow and is opposite to the cubital fossa or elbow pit (trochlear notch). The olecranon serves as a lever ...

process similar to that of rhinos, suggesting that the front leg was held extended when standing. The (distal) part of the femur closest to the foot shows a pronounced medial trochlear ridge, which likely served along with the

patella The patella (: patellae or patellas), also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint. The patella is found in m ...

(kneecap) 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 The hoof (: hooves) is the tip of a toe of an ungulate mammal, which is covered and strengthened with a thick and horny keratin covering. Artiodactyls are even-toed ungulates, species whose feet have an even number of digits; the ruminants with ...

-like

phalanges The phalanges (: phalanx ) are digit (anatomy), digital bones in the hands and foot, feet of most vertebrates. In primates, the Thumb, thumbs and Hallux, big toes have two phalanges while the other Digit (anatomy), digits have three phalanges. ...

Distribution

''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 The Northeast Region of Brazil ( ) is one of the five official and political regions of Brazil, regions of the country according to the Brazilian Institute of Geography and Statistics. Of Brazil's twenty-six states, it comprises nine: Maranhão, ...

. Although some authors suggest that the distribution of ''Toxodon'' extended into Venezuela, 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.

Palaeobiology

A-Pleistocene-meso-megamammals-from-Sergipe-Brazil

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''. As such, it has been suggested that ''Toxodon'' was probably more ecologically comparable to rhinoceroses. ''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 Browsing is a kind of orienting strategy. It is supposed to identify something of relevance for the browsing organism. In context of humans, it is a metaphor taken from the animal kingdom. It is used, for example, about people browsing open sh ...

diet in the Amazon rainforest, mixed feeding C₃ in

Bahia Bahia () is one of the 26 Federative units of Brazil, states of Brazil, located in the Northeast Region, Brazil, Northeast Region of the country. It is the fourth-largest Brazilian state by population (after São Paulo (state), São Paulo, Mina ...

and the

Pampas The Pampas (; from Quechua 'plain'), also known as the Pampas Plain, are fertile South American low grasslands that cover more than and include the Argentine provinces of Buenos Aires, La Pampa, Santa Fe, Entre Ríos, and Córdoba; all o ...

, and an almost completely C₄ dominated

grazing In agriculture, grazing is a method of animal husbandry whereby domestic livestock are allowed outdoors to free range (roam around) and consume wild vegetations in order to feed conversion ratio, convert the otherwise indigestible (by human diges ...

diet in the 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

and

h, in some areas like the southwestern Brazilian Amazon, it is suggested to have inhabited

woodland A woodland () is, in the broad sense, land covered with woody plants (trees and shrubs), or in a narrow sense, synonymous with wood (or in the U.S., the '' plurale tantum'' woods), a low-density forest forming open habitats with plenty of sunli ...

. 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 Osteomyelitis (OM) is the infectious inflammation of bone marrow. Symptoms may include pain in a specific bone with overlying redness, fever, and weakness. The feet, spine, and hips are the most commonly involved bones in adults. The cause is ...

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 Pernambuco ( , , ) is a States of Brazil, state of Brazil located in the Northeast Region, Brazil, Northeast region of the country. With an estimated population of 9.5 million people as of 2024, it is the List of Brazilian states by population, ...

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.

Extinction

''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. mid-Holocene dates for ''Toxodon'' and Pampas other megafauna have been questioned and are suggested to be the result of contamination. 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. 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. 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). 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. 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.