''Arctotherium'' ("bear beast") is an extinct genus of 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 ...

short-faced bears endemic to Central and

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 ...

. ''Arctotherium'' migrated from

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

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 ...

, following the 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 ...

during the late

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Tremarctinae The Tremarctinae or short-faced bears is a subfamily of Ursidae that contains one living representative, the spectacled bear (''Tremarctos ornatus'') of South America, and several extinct species from four genera: the Florida spectacled bear ( ...

subfamily of bears, otherwise known as the short faced bears, which also includes ''

Arctodus ''Arctodus'' is an extinct genus of short-faced bear that inhabited North America during the Pleistocene (~2.6 Year#mya, Mya until 12,800 years ago). There are two recognized species: the lesser short-faced bear (''Arctodus pristinus'') and the gia ...

'' (North American short faced bears) and ''

Tremarctos ''Tremarctos'' is a genus of the monophyletic bear subfamily Tremarctinae, endemic to Americas from the Pliocene to recent. The northern species, the Florida short-faced bear, became extinct 11,000 years ago.B. Kurten & E. Anderson (1980): Plei ...

'' (the Floridian and modern

spectacled bear The spectacled bear (''Tremarctos ornatus''), also known as the South American bear, Andean bear, Andean short-faced bear or mountain bear and locally as jukumari ( Aymara and Quechua), ukumari ( Quechua) or ukuku, is a species of bear native to ...

). Tremarctinae originate with their common ancestor, ''

Plionarctos ''Plionarctos'' is an extinct genus of short-faced bear endemic to North America from the Miocene to the Pliocene. Taxonomy Described by Childs Frick in 1926, ''Plionarctos'' is the oldest known genus within the subfamily of the short-faced b ...

,'' in the Middle Hemphillian (earliest

Late Miocene The Late Miocene (also known as Upper Miocene) is a sub-epoch of the Miocene epoch (geology), Epoch made up of two faunal stage, stages. The Tortonian and Messinian stages comprise the Late Miocene sub-epoch, which lasted from 11.63 Ma (million ye ...

, ~10 Ma) of North America; ''Plionarctos'' is last recorded in the early Blancan (Early Pliocene, ~3.3 Ma). Around 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 ...

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Tremarctines, along with other

ursids The Ursid (URS) meteor activity begins annually around December 17 and runs for over a week, until the 25th or 26th. This meteor shower is named for its radiant point, which is located near the star Beta Ursae Minoris (Kochab) in the constellat ...

, experienced an explosive radiation in diversity, as C4 vegetation (

grasses Poaceae ( ), also called Gramineae ( ), is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos, the grasses of natural grassland and species cultivated in ...

) and open habitats dominated, the world experienced a major temperature drop and increased seasonality, and a faunal turnover which extinguished 60–70% of all Eurasian faunal genera, and 70–80% of North American genera.Correspondingly, recent genetic studies suggest that the mean divergence dates for ''Arctotherium'', ''Arctodus'' and ''Tremarctos'' was 4.8 Ma, and between ''Arctotherium'' and ''Tremarctos'' at 4.1 Ma. Notably, all three genera are first recorded from the

Blancan The Blancan North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 4,750,000 to 1,806,000 years BP, a period of .North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

, with the first possible record of ''Arctotherium sp.'' being a tooth found in the Cuscatlán Formation of

El Salvador El Salvador, officially the Republic of El Salvador, is a country in Central America. It is bordered on the northeast by Honduras, on the northwest by Guatemala, and on the south by the Pacific Ocean. El Salvador's capital and largest city is S ...

, dated to the latest

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58 ''Arctotherium'' is genetically closer to the

(''Tremarctos ornatus''), than to ''

'' of North America, implying the two extinct forms evolved large size in a convergent manner. Recent research suggests that ''Arctotherium'' either emerged from the ''

'' genus or was a sister lineage to ''Tremarctos''.

South America

The oldest dated confirmed remains of ''Arctotherium'' in

are those of the gigantic ''A. angustidens'' from Buenos Aires,

Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...

. What the evolutionary history of ''Arctotherium'' was beforehand, particularly regarding its sudden significant size, is unclear, though ursids are believed to have been part of the second phase of the Great Biotic Interchange, which is believed to have begun 1.8 Ma. ''A. angustidens'' remains have been dated to between 1Ma to 0.7 Ma of the Pleistocene, which corresponds with the

Ensenadan The Ensenadan age is a period of geologic time (1.2–0.8 Ma) within the Early Pleistocene epoch of the Quaternary used more specifically with South American Land Mammal Ages. It follows the Uquian and precedes the Lujanian The Lujanian age is a ...

period (although the younger dates are uncertain). ''A. angustidens'' went extinct at the start of the

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 ...

(~700,000 years ago), replaced by medium-sized ''Arctotherium'' species. The first recorded successor species was ''A. vetustum'' (

Middle Pleistocene The Chibanian, more widely known as the Middle Pleistocene (its previous informal name), is an Age (geology), age in the international geologic timescale or a Stage (stratigraphy), stage in chronostratigraphy, being a division of the Pleistocen ...

), then shortly thereafter by the more robustly built ''A. bonariense'' (

Middle Middle or The Middle may refer to: * Centre (geometry), the point equally distant from the outer limits. Places * Middle (sheading), a subdivision of the Isle of Man * Middle Bay (disambiguation) * Middle Brook (disambiguation) * Middle Creek ...

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 ...

), along with ''A. tarijense'' (

to the

Early Holocene In the geologic time scale, the Greenlandian is the earliest age or lowest stage of the Holocene Epoch or Series, part of the Quaternary. Beginning in 11,650 BP (9701 BCE or 300 HE) and ending with the 8.2-kiloyear event (c. 8200–8300 B ...

)''.'' While the smallest but most widespread species, ''A. wingei'', is only confirmed from the

and

, the species' more tropical disposition is thought to greatly limit fossilisation. That, along with ''A. wingeis more ancestral position in ''Arctotherium'', suggests an origin in the

. Within ''Arctotherium'', two clades are thought to exist- ''A. bonariense'' and ''A. tarijense'' have been described as the most derived species of the genus, whilst ''A. vetustum'' and ''A. wingei'' are regarded the most archaic, even more so than ''A. angustidens''. Of these successor species, ''A. tarijense'' and ''A. wingei'' are by far the most successful when taking into account temporal & geographic range, and the frequency of fossil finds. A separate highland form of ''Arctotherium'' is also suggested to have existed at the end of the Pleistocene, consisting of the type ''A. wingei'' specimen from Tarija and another ''A. wingei'' individual from El Rodeo, in contrast with the larger and more robust Brazilian ''A. wingei'' specimens. Curiously, while ''Arctotherium'''s known species dramatically shrank in size after ''A. angustidens'', ''

'' underwent the opposite transformation, transitioning from the medium-sized ''A. pristinus'' to the gigantic ''A. simus'' by the end of the Pleistocene. Except for an extraordinarily large specimen of ''A. angustidens'', the largest specimens of ''A. simus'' and ''A. angustidens'' are said to be comparable to one another, and match the absolute upper size limit (~1000kg) of a terrestrial carnivore (based on the more restrictive energy base for a carnivorous diet).

Cladogram

Below is a cladogram exploring the relationships between species of ''Arctotherium''.

Description

Taxonomy

''Arctotherium'' was named by

Hermann Burmeister Karl Hermann Konrad Burmeister (also known as Carlos Germán Conrado Burmeister) (15 January 1807 – 2 May 1892) was a German Argentine zoologist, entomologist, herpetologist, botany, botanist, and coleopterologist. He served as a professor at ...

in 1879.

Tremarctinae The Tremarctinae or short-faced bears is a subfamily of Ursidae that contains one living representative, the spectacled bear (''Tremarctos ornatus'') of South America, and several extinct species from four genera: the Florida spectacled bear ( ...

(and therefore ''Arctotherium'') appeared to have disproportionately shorter snouts compared to most modern bears, hence the name "short-faced" was given to them. This apparent shortness is an illusion caused by the deep snouts and short nasal bones of tremarctine bears compared with ursine bears; ''Arctotherium'' had a deeper but not a shorter face than most living bears. Although known from only partial cranial and dental remains elsewhere, the ''A. wingei'' skeletons identified from Hoyo Negro in Mexico are the most complete known for its species, the ''Arctotherium'' genus and all extinct Tremarctine bears. The Hoyo Negro specimens confirm that the ''A. wingei'' had a high degree of intraspecific morphological variation.

Diagnostics

Size can be a useful indicator in differentiating between species of ''Arctotherium'', but cranial and dental features need to be examined for a definite identification. The upper canine is very similar between species of ''Arctotherium'', differing mainly in size. The canine of ''A. wingei'' is the smallest among the species. The lower canine of ''A. wingei'' presents two enamel ridges as in ''A. angustidens'' and ''A. tarijense'', while in ''A. vetustum'' and ''A. bonariense'' there are three ridges. In ''A. vetustum'', the distal ridge is very small and the mesial ridge is small, while in ''A. angustidens'' and ''A. tarijense'' both ridges are large.

Skull

In comparison with other studied Tremarctine bears (''A. angustidens'', ''A. bonariense'', ''T. floridanus'' & ''T. ornatus'') ''Arctotherium tarijense'' exhibited major morphological differences in the anatomy of its

auditory bulla The tympanic part of the temporal bone is a curved plate of bone lying below the squamous part of the temporal bone, in front of the mastoid process, and surrounding the external part of the ear canal. It originates as a separate bone (tympanic b ...

, showing some similarities with ursine bears. Researchers suggest that this morphology gave ''A. tarijense'' a relative high vestibular sensitivity and possibly greater agility, and therefore a better ability to explore diverse habitats. The canalis semicircularis lateral also suggests ''A. tarijense'' had a 40° head posture, which being higher than ''A. angustidens'' (32°) and ''A. bonariense'' (24°) could infer a greater capacity for long distance vision in ''A. tarijense''.

Postcranial

The shape of the

elbow joint The elbow is the region between the upper arm and the forearm that surrounds the elbow joint. The elbow includes prominent landmarks such as the olecranon, the cubital fossa (also called the chelidon, or the elbow pit), and the lateral and the ...

suggests the possibility of semi-arboreal locomotion for '' Arctodus sp''., ''Arctotherium bonariense'', and ''A. wingei'', but the size of the elbow joint does not. As the medial epicondyle is particularly expanded in these species, it is likely that (as for the

giant panda The giant panda (''Ailuropoda melanoleuca''), also known as the panda bear or simply panda, is a bear species endemic to China. It is characterised by its white animal coat, coat with black patches around the eyes, ears, legs and shoulders. ...

) the fossil ''Arctodus'' and ''Arctotherium'' retained this feature in relation to their higher degree of forelimb dexterity. As these genera convergently evolved towards an increased body size, this high degree of proximal dexterity may have been advantageous for a

scavenging Scavengers are animals that consume dead organisms that have died from causes other than predation or have been killed by other predators. While scavenging generally refers to carnivores feeding on carrion, it is also a herbivorous feeding be ...

lifestyle, and therefore retained in the Tremarctinae in spite of its evolution towards greater size. ''A. bonariense'' is believed to have convergently evolved several adaptations with ''

Arctodus simus ''Arctodus'' is an extinct genus of short-faced bear that inhabited North America during the Pleistocene (~2.6 Year#mya, Mya until 12,800 years ago). There are two recognized species: the lesser short-faced bear (''Arctodus pristinus'') and the gia ...

'' and ''

Agriotherium ''Agriotherium'' is an extinct genus of bears whose fossils are found in Miocene through Pleistocene-aged strata of North America, Eurasia, and Africa. The earliest species, ''A.'' ''aecuatorialis'' evolved during the early Late Miocene, around ...

'' / ''

Huracan Huracán (; ; , "one legged"), often referred to as ''U Kʼux Kaj'', the "Heart of Sky", is a Kʼicheʼ people, Kʼicheʼ Maya mythology, Maya god of wind, storm, fire and one of the creator deities who participated in all three attempts at crea ...

'', such as proportionally longer limbs, very large body size, short broad rostrums, premasseteric fossa on the mandible, and possible carnassial shears. The postcranial skeleton of ''A. tarijense'' is unknown.

Size

''Arctotherium'' consists of one early giant form, ''A. angustidens'', and several succeeding smaller species, which were within the size range of modern

bear Bears are carnivoran mammals of the family (biology), family Ursidae (). They are classified as caniforms, or doglike carnivorans. Although only eight species of bears are extant, they are widespread, appearing in a wide variety of habitats ...

s. ''Arctotherium'' species ranged between a variety of sizes, both between species and individuals of the same species. The sole remaining Tremarctine bear, the

, exhibits strong sexual dimorphism, with adult males being 30–50% larger than females. Various attempts to calculate each species' body mass have been made; for example, a 2006 study calculated the mean weight of two species, ''A. bonariense'' at ~ (hypothetical typical weight range = ), and ''A. tarijense'' at ~ (range of ). According to a 2009 study, the weight ranges for ''Arctotherium'' were calculated as follows- ''A. wingei'' at , ''A. vetustum'' at , ''A. tarijense'' at , ''A. bonariense'' between , and ''A. angustidens'' at . The study considered each end figure as the maximum hypothetical weight. Further studies calculated an ''A. tarijense'' specimen's weight (MACN 971) at , and ''A. wingei'' specimens from the Brazilian intertropical region at ~. An extraordinarily large specimen of ''A. angustidens'' recovered in 2011 from

Buenos Aires Buenos Aires, controlled by the government of the Autonomous City of Buenos Aires, is the Capital city, capital and largest city of Argentina. It is located on the southwest of the Río de la Plata. Buenos Aires is classified as an Alpha− glob ...

shows an individual estimated, using the humerus, to weigh between . However, the authors consider the upper limit as improbable, and say that is more likely. An estimated standing height for this ''A. angustidens'' individual is between . It would still make the species the largest bear ever found, and contender for the largest carnivorous land mammal known.

Distribution and habitat

Almost all ''Arctotherium'' species appear to be largely restricted to the

Southern Cone The Southern Cone (, ) is a geographical and cultural subregion composed of the southernmost areas of South America, mostly south of the Tropic of Capricorn. Traditionally, it covers Argentina, Chile, and Uruguay, bounded on the west by the Pac ...

, particularly

, with the richest records being in the

Buenos Aires Province Buenos Aires, officially the Buenos Aires Province, is the largest and most populous Provinces of Argentina, Argentine province. It takes its name from the city of Buenos Aires, the capital of the country, which used to be part of the province an ...

. The exceptions are the

Tarija formation The Tarija Formation is a Pleistocene-age geologic formation found near Tarija, Bolivia. Background Fossils of numerous large herbivorous mammals have been discovered, including toxodonts like ''Toxodon platensis'', glyptodonts, ground sloth ...

in southern Bolivia where three species have been recovered, a possible record of ''A. vetustum'' in

Brazil Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...

, unassigned ''Arctotherium'' sp. postcranial remains from

Rio Grande do Sul Rio Grande do Sul (, ; ; "Great River of the South") is a Federative units of Brazil, state in the South Region, Brazil, southern region of Brazil. It is the Federative units of Brazil#List, fifth-most populous state and the List of Brazilian s ...

, a

Blancan The Blancan North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 4,750,000 to 1,806,000 years BP, a period of .El Salvador El Salvador, officially the Republic of El Salvador, is a country in Central America. It is bordered on the northeast by Honduras, on the northwest by Guatemala, and on the south by the Pacific Ocean. El Salvador's capital and largest city is S ...

, and ''A. wingei,'' which almost exclusively inhabited a more northern range. By the

, ''A. tarijense'' held domain over the open and semi-arid

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

Patagonia Patagonia () is a geographical region that includes parts of Argentina and Chile at the southern end of South America. The region includes the southern section of the Andes mountain chain with lakes, fjords, temperate rainforests, and glaciers ...

n habitats east of the

Andes The Andes ( ), Andes Mountains or Andean Mountain Range (; ) are the List of longest mountain chains on Earth, longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range ...

, inhabiting

, Patagonian Chile, southern Bolivia, and

Uruguay Uruguay, officially the Oriental Republic of Uruguay, is a country in South America. It shares borders with Argentina to its west and southwest and Brazil to its north and northeast, while bordering the Río de la Plata to the south and the A ...

, although ''A. bonariense'' may have also been contemporary in Late Pleistocene Uruguay. ''A. tarijense'' has been described as having a very low density of fossil material in Patagonia. On the other hand, ''A. wingei'' spanned across the northern, more mixed/forested and tropical parts of the continent, throughout the tropical savanna forests of

Bolivia Bolivia, officially the Plurinational State of Bolivia, is a landlocked country located in central South America. The country features diverse geography, including vast Amazonian plains, tropical lowlands, mountains, the Gran Chaco Province, w ...

Venezuela Venezuela, officially the Bolivarian Republic of Venezuela, is a country on the northern coast of South America, consisting of a continental landmass and many Federal Dependencies of Venezuela, islands and islets in the Caribbean Sea. It com ...

, and into

(

Belize Belize is a country on the north-eastern coast of Central America. It is bordered by Mexico to the north, the Caribbean Sea to the east, and Guatemala to the west and south. It also shares a maritime boundary with Honduras to the southeast. P ...

and

Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...

Yucatán Peninsula The Yucatán Peninsula ( , ; ) is a large peninsula in southeast Mexico and adjacent portions of Belize and Guatemala. The peninsula extends towards the northeast, separating the Gulf of Mexico to the north and west of the peninsula from the C ...

). A possible ''A. wingei'' specimen has also been found in

northwest Argentina The points of the compass are a set of horizontal, radially arrayed compass directions (or azimuths) used in navigation and cartography. A ''compass rose'' is primarily composed of four cardinal directions—north, east, south, and west—each ...

Diet

''A. angustidens''

Using carbon isotopes, ''A. angustidens diet has been posited to be omnivorous with a preference towards large quantities of meat. Beyond the scavenging of mega-herbivore carcasses, the type of tooth wear present amongst ''A. angustidens'' specimens, in addition to the frequency of broken teeth from most specimens (especially at older ages), suggests the active predation of large vertebrates, including but not limited to

horses The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 milli ...

tapirs Tapirs ( ) are large, herbivorous mammals belonging to the family Tapiridae. They are similar in shape to a Suidae, pig, with a short, prehensile nose trunk (proboscis). Tapirs inhabit jungle and forest regions of South America, South and Centr ...

camelids Camelids are members of the biological family Camelidae, the only currently living family in the suborder Tylopoda. The seven extant members of this group are: dromedary camels, Bactrian camels, wild Bactrian camels, llamas, alpacas, vicuñas, ...

, macraucheniids,

glyptodont Glyptodonts are an extinct clade of large, heavily armoured armadillos, reaching up to in height, and maximum body masses of around 2 tonnes. They had short, deep skulls, a fused vertebral column, and a large bony carapace made up of hundreds o ...

giant ground sloths Ground sloths are a diverse group of extinct sloths in the mammalian superorder Xenarthra. They varied widely in size with the largest, belonging to genera '' Lestodon'', ''Eremotherium'' and ''Megatherium'', being around the size of elephants. G ...

, toxodontids, and

gomphotheres Gomphotheres are an extinct group of proboscideans related to modern Elephant, elephants. First appearing in Africa during the Oligocene, they dispersed into Eurasia and North America during the Miocene and arrived in South America during the Ple ...

by ''A. angustidens''.

''A. bonariense'' & ''A. tarijense''

''A. bonariense'' and ''A. tarijense'' had a typical prey weight of , with a maximum of . ''A. tarijense'' occasionally hunted camelids and horses as a supplement to scavenging, smaller prey and herbivory. Carbon isotope studies from southernmost Patagonia suggest that ''A. tarijense'' was a particularly active scavenger. Although carnivory increased the further south ''Arctotherium'' lived, carbon isotopes suggest that ''A. tarijense'''s prey weight limit peaked at 300 kg, leaving the (subadult and younger) mega-mammals, such as the gomphotheres, giant ground sloths, and toxodontids, to ''Smilodon populator'' and giant jaguars. ''

Smilodon fatalis ''Smilodon'' is an extinct genus of felids. It is one of the best known saber-toothed predators and prehistoric mammals. Although commonly known as the saber-toothed tiger, it was not closely related to the tiger or other modern cats, belon ...

'', ''Arctotherium bonariense'', ''

Canis nehringi ''Canis nehringi'' is an extinct species of canid. ''Canis gezi'', a poorly known small wolf from the Ensenadan age of South America, appears to have given rise to ''Canis nehringi'', a Lujanian age species from Argentina. The species was first ...

,''

maned wolves The maned wolf (''Chrysocyon brachyurus'') is a large caninae, canine of South America. It is found in Argentina, Brazil, Bolivia, Peru, and Paraguay, and is almost extinct in Uruguay. Its markings resemble those of a red fox, but it is neither a ...

, and

human Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

s would have also joined this predator guild at various stages of the

. However, a fragmented ''Arctotherium'' c.f. ''tarijense'' tooth from Baño Nuevo-1 cave in southern Chile preserves cavities, which could be interpreted as a consequence of consuming carbohydrate-rich foods such as fruit or honey. A further microwear analysis attempt of the tooth in 2015 was complicated by hard plant and bone consumption causing similar damage to teeth in omnivores.

''A. vetustum'' & ''A. wingei''

Along with clues from various teeth of ''A. wingei'', carbon isotope studies suggest that ''A. wingei'', at least in the Brazilian intertropical region, were highly herbivorous, specialising in C3 vegetative matter such as fruits and leaves. However, the diet of ''A. wingei'' was not necessarily orthodox, with carnivory likely peaking in times of resource instability. Isotope spikes from the Brazilian Intertropical Region also indicate the occasional consumption of the ground sloth ''Nothrotherium'' ''maquinense'' (hypothesized as a preference for younger individuals and opportunistic scavenging), and ''A. wingei'' itself, which could represent cannibalism for juveniles or cubs, as observed in

American black bear The American black bear (''Ursus americanus''), or simply black bear, is a species of medium-sized bear which is Endemism, endemic to North America. It is the continent's smallest and most widely distributed bear species. It is an omnivore, with ...

s and

polar bear The polar bear (''Ursus maritimus'') is a large bear native to the Arctic and nearby areas. It is closely related to the brown bear, and the two species can Hybrid (biology), interbreed. The polar bear is the largest extant species of bear ...

s. Additionally, several bite marks on recovered fossils of herbivores, such as ''

Glossotherium ''Glossotherium'' is an extinct genus of large mylodontid ground sloths of the subfamily Mylodontinae. It represents one of the best-known members of the family, along with ''Mylodon'' and '' Paramylodon''. Reconstructed animals were between ...

'' and '' Equus'', are suggested to have been inflicted by scavenging short-faced bears across

South America. According to a 2021 study, the maximum prey for ''A. wingei'' would be around its own bodyweight (~).

Paleobiology

Hibernation

Three ''A. angustidens'' individuals were discovered in a

paleoburrow A paleoburrow is an underground shelter excavated by extinct paleo-vertebrate megafauna that lived in the prehistoric era. Most paleoburrows are likely made by giant armadillos and large ground sloths, depending on their size. Thousands of examp ...

together (postulated to have been a mother with adolescent cubs), which opens the possibility that ''A. angustidens'' used dens for hibernation. In contrast with the spectacled bear's tropical and temperate habitat,

Argentina's seasonal and often harsh climate suggests quasi-hibernation would have been an effective strategy for survival, as ursine bears do today. ''A. angustidens'' is thought to have reoccupied caves excavated by

Xenarthra Xenarthra (; from Ancient Greek ξένος, xénos, "foreign, alien" + ἄρθρον, árthron, "joint") is a superorder and major clade of placental mammals native to the Americas. There are 31 living species: the anteaters, tree sloths, and ...

, such as the mylodonts ''

'' and ''

Scelidotherium ''Scelidotherium'' is an extinct genus of ground sloth of the family Scelidotheriidae, endemic to South America during the Late Pleistocene epoch. It lived from 780,000 to 11,000 years ago, existing for approximately . Description It is charac ...

'', and the pampatheriid ''

Pampatherium ''Pampatherium'' is an extinct genus of xenarthran that lived in the Americas during the Pleistocene. Some species went extinct right at the Pleistocene-Holocene border. Taxonomy Pampatherium was one of the first fossil mammals reported from S ...

''. As suitable

s are rare before the

, it has been suggested that predation and competition for dens by the newly arrived eutherian carnivores, especially by ''A. angustidens'', increased the rate of xenarthran cave excavations. Other ''Arctotherium'' species such as ''A. tarijense'' (southern Chile) and ''A. wingei'' (Brazil and Venezuela) have been recovered from caves and are also hypothesized to have utilised dens.

Relationship with other bears

'' does not appear in the South American fossil record until the

Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...

, suggesting that the extant

descends from an independent, later dispersal event from North America to that of ''Arctotherium'', possibly after ''A. wingei'' became extinct in

the Americas The Americas, sometimes collectively called America, are a landmass comprising the totality of North America and South America.'' Webster's New World College Dictionary'', 2010 by Wiley Publishing, Inc., Cleveland, Ohio. When viewed as a sin ...

. The modern

may have hybridised with ''Arctotherium'' as they migrated southwards into South America. However, the ranges of the spectacled bear, a specialist of highland Andean forests, and the more lowland & open-environ adapted ''Arctotherium'', could have co-existed in South America. Researchers suggest an altitude of 1,800 meters (such as in Andean valleys) represented a buffer zone between the genera.

Trackways

Footprints from Pehuén-Có (Argentina) under the

ichnotaxon An ichnotaxon (plural ichnotaxa) is "a taxon based on the fossilized work of an organism", i.e. the non-human equivalent of an artifact. ''Ichnotaxon'' comes from the Ancient Greek (''íchnos'') meaning "track" and English , itself derived from ...

''Ursichnus sudamericanus'' were assigned to ''Arctotherium tarijense'' due to their Late Pleistocene age. Similar in dimensions & body mass to a male spectacled bear (13.8cm long & 13.2 cm wide manus, 22cm long & 11.8cm wide foot), the tracks show an inward rotation of 10°, with the strides being 65cm in length. Long claws are visible, and digit III is longer than the others; the heel impression is absent in the manus.

Paleopathology

Beyond the scavenging of mega-herbivore carcasses, the type of tooth wear present amongst ''A. angustidens'' specimens, in addition to the frequency of broken teeth from most specimens (especially at older ages), suggests the active predation of large vertebrates, including but not limited to

, macraucheniids,

, toxodontids, and

by ''A. angustidens''. Of the dentition known from later ''Arctotherium'' species, only one specimen of ''A. bonariense'' exhibits the same cracked teeth which ''A. angustidens'' had, although extreme wear of the occlusal molar surface is common throughout the genus. A microwear analysis attempt of ''Arctotherium'' c.f. ''tarijense'' in 2015, suggests the consumption of hard plant and bone consumption caused similar damage to teeth in omnivores. The pathologies found on a huge specimen of ''A. angustidens'', being multiple deep injuries which had long healed despite infection, demonstrate a lifestyle of conflict.

Paleoecology

The

carnivora Carnivora ( ) is an order of placental mammals specialized primarily in eating flesh, whose members are formally referred to as carnivorans. The order Carnivora is the sixth largest order of mammals, comprising at least 279 species. Carnivor ...

ns that invaded South America probably quickly adopted the predatory niches formerly occupied by the native typical South American groups such as

metatheria Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as wel ...

sparassodonts Sparassodonta (from Greek to tear, rend; and , gen. , ' tooth) is an extinct order of carnivorous metatherian mammals native to South America, related to modern marsupials. They were once considered to be true marsupials, but are now though ...

and phorusracids that had largely gone extinct shortly prior to their arrival. ''Arctotherium angustidens'' appears first and was endemic to the Ensenadan faunal stage, while all subsequent species are only found in the proceeding Lujanian faunal stage. ''Arctotherium'' is believed to have been adapted to the open areas of arid and semi-arid habitats. After ''A. angustidens'' became extinct at the end of the Ensenadan, two forms begin to appear in the fossil record. The ''A. bonariense'' / ''A. tarijense'' species complex was composed of adaptable, cosmopolitan omnivores, whereas ''A. vetustum'' & ''A. wingei'' were largely herbivorous. However, as ''A. vetustum'' and ''A. wingei'' are the most archaic species of ''Arctotherium'', their lineage must have existed before the emergence of ''A. angustidens'' in the Enseadan period of the Early Pleistocene. Although ''A. angustidens'' was highly carnivorous, it has been suggested that as a diverse carnivore guild became established in South America, the ''Arctotherium'' genus began to revert to more classic ursid diets as the ecosystem matured in the Middle Pleistocene.

Central America

The oldest known specimen of ''Arctotherium'' consists of a

baby tooth Deciduous teeth or primary teeth, also informally known as baby teeth, milk teeth, or temporary teeth,Fehrenbach, MJ and Popowics, T. (2026). ''Illustrated Dental Embryology, Histology, and Anatomy'', 6th edition, Elsevier, page 287–296. are ...

(dp4 molar) found in the Río Tomayate locality of Cuscatlán Formation of

, along with a partial ''

Borophagus ''Borophagus'' ("gluttonous eater") is an extinct genus of the subfamily Borophaginae, a group of canids endemic to North America from the Middle Miocene epoch through the Early Pleistocene epoch 12—1.8 Annum, Mya. Evolution ''Borophagus'', ...

'' skull, dated to the latest

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58However, ''Arctotherium'' fossils in Central America only subsequently found in the Late Pleistocene of the

, with ''A. wingei'' finds from

Quintana Roo Quintana Roo, officially the Free and Sovereign State of Quintana Roo, is one of the 31 states which, along with Mexico City, constitute the 32 administrative divisions of Mexico, federal entities of Mexico. It is divided into municipalities of ...

, Mexico and

. ''A. wingei'' is believed to have restricted the range of ''

Tremarctos floridanus ''Tremarctos floridanus'' is an extinct species of bear in the family Ursidae, subfamily Tremarctinae. ''T. floridanus'' became extinct at the end of the last ice age, 11,000 years ago. Its fossils have been found throughout the Southeastern U ...

'' at the

, with ''Tremarctos'' only being able to migrate into the

Neotropics The Neotropical realm is one of the eight biogeographic realms constituting Earth's land surface. Physically, it includes the tropical terrestrial ecoregions of the Americas and the entire South American temperate zone. Definition In biogeog ...

once ''A. wingei'' went extinct. Subsequently, ''T. floridanus'' remains from Central America (and the dubious '' Panthera balamoides'') are believed to represent ''A. wingei''. ''A. wingei'' has been recovered from cenotes & cave systems across the

Yucatán Yucatán, officially the Free and Sovereign State of Yucatán, is one of the 31 states which, along with Mexico City, constitute the 32 federal entities of Mexico. It comprises 106 separate municipalities, and its capital city is Mérida. ...

; the Hoyo Negro also preserves ground sloths ('' Nohochichak xibalbahkah'', ''

Nothrotheriops shastensis ''Nothrotheriops'' is an extinct genus of ground sloths endemic to North America during the Pleistocene to early Holocene (~2.5 Mya until 10,000 years ago). The genus contains two species, ''N. shastensis'' and ''N. texanus'', the former of whic ...

''), ''

Cuvieronius ''Cuvieronius'' is an extinct New World genus of gomphothere which ranged from southern North America to western South America during the Pleistocene epoch. Reaching a shoulder height of and a body mass of , it was on average shorter but compara ...

'',

Baird's tapir The Baird's tapir (''Tapirus bairdii''), also known as the Central American tapir, is a species of tapir native to Mexico, Central America, and northwestern South America. It is the largest of the three species of tapir native to the Americas, a ...

, ''

'', ''

Protocyon ''Protocyon'' is an extinct genus of large canid endemic to South and North America from Middle to Late Pleistocene living from 781 to 12thousand years ago. Description ''Protocyon'' was a hypercarnivore, suggested by its dental adaptations. Li ...

'', puma,

bobcat The bobcat (''Lynx rufus''), also known as the wildcat, bay lynx, or red lynx, is one of the four extant species within the medium-sized wild cat genus '' Lynx''. Native to North America, it ranges from southern Canada through most of the c ...

collared peccary The collared peccary (''Dicotyles tajacu'') is a peccary, a species of artiodactyl (even-toed) mammal in the family Peccary, Tayassuidae found in North America, North, Central America, Central, and South America. It is the only member of the gen ...

, and

white-nosed coati The white-nosed coati (''Nasua narica''), also known as the coatimundi (), is a species of coati and a member of the family Procyonidae (raccoons and their relatives). Local Spanish names for the species include ''antoon'', ''gato solo'', ''pi ...

from a tropical forest, while Extinction Cave contains

armadillo Armadillos () are New World placental mammals in the order (biology), order Cingulata. They form part of the superorder Xenarthra, along with the anteaters and sloths. 21 extant species of armadillo have been described, some of which are dis ...

, '' Panthera atrox'', jaguar, puma, collared peccary, '' Palaeolama mirifica'',

red brocket deer The red brocket (''Mazama americana'') is a species of brocket deer from forests in South America, ranging from northern Argentina to Colombia and the Guianas. It also occurs on the Caribbean island of Trinidad (it also occurred on the island of ...

, ''Bison'' sp., ''

Equus conversidens ''Equus conversidens'', or the Mexican horse, was a Pleistocene species of horse, now extinct, that inhabited North America. The holotype of ''Equus conversidens'', a partial palate, was unearthed in Pleistocene deposits northeast of Mexico Ci ...

'', and ''Smilodon fatalis'' from a mixed grassland / scrub savanna. ''A. wingei'''s association with ''

'' in the Hoyo Negro, another animal previously thought to be endemic to

, suggests a complex relationship of faunal interchange long after the

Southern Cone

''Arctotherium'' only reappears in the fossil record 1 million years ago as ''A. angustidens'', from the

of Argentina in the

. ''A. angustidens'' is the only known species of ''Arctotherium'' from the Early Pleistocene. The first recorded ''Arctotherium'' specimens in South America occur alongside the earliest known South American records of several other

ns: the sabre-toothed cats ''

Smilodon ''Smilodon'' is an extinct genus of Felidae, felids. It is one of the best known saber-toothed predators and prehistoric mammals. Although commonly known as the saber-toothed tiger, it was not closely related to the tiger or other modern cats ...

'' and ''

Homotherium ''Homotherium'' is an extinct genus of Homotherini, scimitar-toothed cat belonging to the extinct subfamily Machairodontinae that inhabited North America, Eurasia, and Africa, as well as possibly South America during the Pliocene and Pleistocene ...

'', the puma (''Puma concolor''), the

jaguar The jaguar (''Panthera onca'') is a large felidae, cat species and the only extant taxon, living member of the genus ''Panthera'' that is native to the Americas. With a body length of up to and a weight of up to , it is the biggest cat spe ...

(''Panthera onca''), some large

canids Canidae (; from Latin, ''canis'', "dog") is a biological family of caniform carnivorans, constituting a clade. A member of this family is also called a canid (). The family includes three subfamilies: the Caninae, and the extinct Borophaginae ...

, and several smaller (<)

mustelids The Mustelidae (; from Latin , weasel) are a diverse family of carnivoran mammals, including weasels, badgers, otters, polecats, martens, grisons, and wolverines. Otherwise known as mustelids (), they form the largest family in the suborder Can ...

felids Felidae ( ) is the family of mammals in the order Carnivora colloquially referred to as cats. A member of this family is also called a felid ( ). The 41 extant Felidae species exhibit the greatest diversity in fur patterns of all terrestria ...

and mephitids.

''A. angustidens''

In the

, ''A. angustidens'' was only rivalled in size by '' Smilodon populator'', with '' Theriodictis platensis'', '' Canis gezi,'' '' Protocyon scagliorum,'' ''

Panthera onca The jaguar (''Panthera onca'') is a large cat species and the only living member of the genus ''Panthera'' that is native to the Americas. With a body length of up to and a weight of up to , it is the biggest cat species in the Americas a ...

'' and pumas rounding out the predator guild in the

Early Pleistocene The Early Pleistocene is an unofficial epoch (geology), sub-epoch in the international geologic timescale in chronostratigraphy, representing the earliest division of the Pleistocene Epoch within the ongoing Quaternary Period. It is currently esti ...

. The extinction of the scavenger-niche specialist

procyonid Procyonidae ( ) is a New World family (biology), family of the order (biology), order Carnivora. It includes the Procyon (genus), raccoons, ringtails, cacomistles, coatis, kinkajous, olingos, and olinguitos. Procyonids inhabit a wide range of e ...

Chapalmalania ''Chapalmalania'' is an extinct genus of procyonid from the Late Miocene or Early Pliocene to Early Pleistocene (Montehermosan to Uquian) of Argentina ( Andalhualá Formation), Venezuela (San Gregorio Formation, Venezuela), and Colombia (Ware For ...

'' during this faunal turnover event is hypothesized as being the gateway for ''A. angustidens''' gigantism. Additionally, the evolution of highly-modified spiny osteoderms in Pleistocene-era ''

Glyptodon ''Glyptodon'' (; ) is a genus of glyptodont, an extinct group of large, herbivorous armadillos, that lived from the Pliocene, around 3.2 million years ago, to the early Holocene, around 11,000 years ago, in South America. It is one of, if not th ...

'' has been attributed to the arrival of large carnivorans such as ''A. angustidens'' in South America.

''A. bonariense'' & ''A. tarijense''

''A. tarijense'' competed against '' Smilodon populator'', giant jaguars (''

Panthera onca mesembrina ''Panthera onca mesembrina'', also known as the Patagonian panther, is an extinct subspecies of jaguar (''Panthera onca'') that was endemic to southern Patagonia during the late Pleistocene epoch. It is known from several fragmentary specimens, ...

''), pumas, ''

Lycalopex The South American foxes (''Lycalopex''), commonly called ''raposa'' in Portuguese, or ''zorro'' in Spanish, are a genus inhabiting South America. Despite their name, they are not true foxes, but are a unique canid genus more closely related t ...

Cerdocyon The crab-eating fox (''Cerdocyon thous''), also known as the forest dog, wood fox, bushfox (not to be confused with the bush dog) or maikong, is an Extant taxon, extant species of medium-sized Canidae, canid endemic to the central part of South ...

Conepatus The hog-nosed skunks belong to the genus ''Conepatus'' and are members of the family Mephitidae (skunks). They are native to the Americas. They have white backs and tails and black underparts. Species Extinct species †'' Conepatus robustus ...

Didelphis ''Didelphis'' is a genus of New World marsupials. The six species in the genus ''Didelphis'', commonly known as Large American opossums, are members of the ''opossum'' order (biology), order, Didelphimorphia. The genus ''Didelphis'' is composed ...

,'' and ''

Dusicyon avus ''Dusicyon avus'' is an extinct species of canid native to South America during the Pleistocene and Holocene epochs. It was medium to large, about the size of a German shepherd. It was closely related to the Falkland Islands wolf ''(Dusicyon aus ...

'' in

. Although carnivory increased the further south ''Arctotherium'' lived, ''A. tarijense'''s prey weight limit peaked at 300 kg, leaving the (subadult and younger) mega-mammals, such as the gomphotheres, giant ground sloths, and toxodontids, to ''Smilodon populator'' and giant jaguars. ''

'', ''Arctotherium bonariense'', ''

,''

, and

s would have also joined this predator guild at various stages of the

Eastern South America

In the low-density savanna forests of the Brazilian intertropical region, ''A. wingei'', pumas and

s played a supporting role to the (also likely solitary) '' Smilodon populator'''s dominance of the regional predator guild, avoiding competition with '' Protocyon troglodytes'' in more open savanna. Being smaller and more herbivorous, ''A. wingei'' would have also likely competed with other smaller

ns present in the BIR, such as

jaguarundi The jaguarundi (''Herpailurus yagouaroundi''; or ) is a wild felidae, cat native to the Americas. Its range extends from central Argentina in the south to northern Mexico, through Central America, Central and South America east of the Andes. T ...

, ''

'', ''

Chrysocyon The maned wolf (''Chrysocyon brachyurus'') is a large canine of South America. It is found in Argentina, Brazil, Bolivia, Peru, and Paraguay, and is almost extinct in Uruguay. Its markings resemble those of a red fox, but it is neither a fox nor ...

'', ''

Theriodictis ''Theriodictis'' is an extinct genus of canid endemic to South America during the Middle Pleistocene, and possibly the south-eastern United States. Taxonomy and evolution The type specimen ''T. platensis'' is a sister taxon to Canis gezi. One ...

'', ''

Speothos ''Speothos'' is a genus of canid found in Central and South America. The genus includes the living bush dog, '' Speothos venaticus'', and an extinct Pleistocene species, '' Speothos pacivorus''. Unusually, the fossil species was identified and ...

'', ''

Nasua ''Nasua'' is a genus of coatis of the family Procyonidae. Two additional species of coatis, commonly known as mountain coatis, are placed in the genus '' Nasuella''. Characteristics ''Nasua'' differs from ''Nasuella'' in being larger and ...

'', ''

Procyon Procyon () is the brightest star in the constellation of Canis Minor and usually the list of brightest stars, eighth-brightest star in the night sky, with an apparent visual magnitude of 0.34. It has the Bayer designation α Canis Min ...

'', ''

Eira Eira is a neighborhood in Helsinki, the capital of Finland. History The district dates back to the early 20th century and received its name after Eira Hospital in the neighboring district of Ullanlinna, which in turn took its name from Eira H ...

'', ''

Galictis A grison (, ) is any mustelid in the genus ''Galictis''. Native to Central and South America, the genus contains two extant species: the greater grison (''Galictis vittata''), which is found widely in South America, through Central America to s ...

'', and ''

Leopardus ''Leopardus'' is a genus comprising eight species of Felinae, small cats native to the Americas. This genus is considered the oldest branch of a genetic lineage of small cats in the Americas whose common ancestor crossed the Bering land bridge fr ...

.''

Paleo-ecological reconstructions

Although mostly herbivorous, the modern

is on occasion an active predator. The

has several hunting techniques- principally, the bear surprises or overpowers its prey, mounts its back, and consumes the immobilised animal while still alive, pinning the prey with its weight, large paws and long claws. Alternatively, the bear pursues the prey into rough terrain, hillsides, or precipices, provoking its fall and/or death. After death, the prey is dragged to a safe place (usually a nest over a tree, or a forested area) and consumed, leaving only skeletal remains. These behaviours have been suggested as ''Arctotherium'''s hunting strategies as well. However, although the

is capable of

climbing trees Climbing Trees are a Welsh four-piece "Cymrucana" band based in Pontypridd, South Wales. The group currently features Matthew Frederick (piano/guitar/vocals), Colenso Jones (guitar/bass/vocals), Martin Webb (guitar/bass/vocals) and James Benn ...

, ''Arctotherium'' is thought to be non-arboreal.

Interaction with humans

The remains of ''A. wingei'' in the Hoyo Negro of the

appear to be in association with human remains.

Extinction

The last known records of ''Arctotherium'' are an ambiguous find of ''A. bonariense'' from

(

cf. The abbreviation cf. (short for either Latin or , both meaning 'compare') is generally used in writing to refer the reader to other material to make a comparison with the topic being discussed. However some sources offer differing or even contr ...

/ aff, either ~36,900 or ~14,485 BP of the

Sopas Formation The Sopas Formation is a Lujanian geologic formation in Uruguay. Fossil content The following fossils have been reported from the formation: * '' Antifer''Cueva Del Puma,

Chile Chile, officially the Republic of Chile, is a country in western South America. It is the southernmost country in the world and the closest to Antarctica, stretching along a narrow strip of land between the Andes, Andes Mountains and the Paci ...

, and ''A. wingei'' at 12,850 BP in the

Sistema Sac Actun Sistema Sac Actun (, ) is an underwater cave system situated along the Caribbean coast of the Yucatán Peninsula with passages to the north and west of the city of Tulum. Discovery of a connection to Sistema Dos Ojos in 2018 made it the longest ...

(

, with a possible record of 9,000 BP in Muaco,

. Globally, in the

Quaternary Extinction Event The Late Pleistocene to the beginning of the Holocene saw the extinction of the majority of the world's megafauna, typically defined as animal species having body masses over , which resulted in a collapse in faunal density and diversity acro ...

, extinction favoured 'conservative morphologies' in ursid body plans, such as those found in the

. For example, the more specialised teeth of ''Arctotherium'' could have limited its diet more than the contemporary spectacled bear, and thus have made ''Arctotherium'' more vulnerable to extinction.

References

{{Taxonbar, from=Q2076664 Pleistocene bears Pleistocene carnivorans Prehistoric carnivoran genera Pleistocene mammals of North America Blancan Pleistocene El Salvador Fossils of El Salvador Pleistocene mammals of South America Uquian Ensenadan Lujanian Pleistocene Argentina Pleistocene Bolivia Pleistocene Brazil Pleistocene Chile Pleistocene Venezuela Fossils of Argentina Fossils of Bolivia Fossils of Brazil Fossils of Chile Fossils of Venezuela Fossil taxa described in 1857 Tremarctinae