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Today, sloths are a poorly known group ofherbivorous mammals represented by only twogenera and six species which are restricted to thetropical forests of South and Central America (Fig. 1).Their fossil record is long, ranging from the Eocene tothe present, and reveals a previously far more diverse,widespread group. Some 90 or more genera of slothsare known from the fossil record of South, Centraland North America as well as the Caribbean andAntarctica, making them a large and importantgroup of fossil mammals. While sloths were oncedivided into two distinct groups (ground sloths andtree sloths), abundant evidence now shows that treesloths are not a natural group. Furthermore, certainsloths traditionally regarded as ‘ground sloths’ werequite probably capable of climbing. A number ofassumptions that one might make about fossil sloths– such as that they were all terrestrial, or physicallybig – have been challenged by recent discoveries.

Sloth stereotypes

The stereotypical fossil sloth is Megatheriumamericanum from Pleistocene South America, aspecies first described by George Cuvier in 1812 for askeleton collected in Argentina. M. americanum washuge (Fig. 2), reaching 6 m in length and weighingmore than 4 tonnes (it was exceeded in size only byits close relative Eremotherium), but by no means wasgiant size true of all fossil sloths. The majority offorms seem to have been similar in size to a large bear(approximately 2.5 m long and 800 kg) and manywere smaller. Some were less than 1 m long andNeocnus toupiti from Pleistocene Haiti was evensmaller than any of the extant tree sloths.

Almost as important in the history of vertebratepalaeontology as Megatherium is the North Americansloth Megalonyx, conventionally stated to have beennamed by Thomas Jefferson in 1799. Alas, thishistorical nugget is not correct (the genus wasactually named by Richard Harlan in 1825) but it istrue that Jefferson imagined Megalonyx to be a giantlion-like cat, and to have still survived in the NorthAmerican wilderness.

Relatives and origins

Sloths are part of the placental mammal groupXenarthra (also containing armadillos andanteaters), a group identified by the presence ofunique accessory articulations on their vertebraecalled xenarthrales. Within Xenarthra, sloths seem tobe more closely related to anteaters than toarmadillos. Exactly how xenarthrans are related toother placental mammals is controversial. Radicalrearrangements of the placental mammal family treehave recently been suggested by molecular studiesand, according to these, xenarthrans might either bemost closely related to Afrotheria (that includeselephant shrews, aardvarks, seacows and elephants),or Boreoeutheria (that includes primates, rodents,carnivorans and ungulates).

Exactly which taxonomic name should be used for

Fossils explained 51Sloths

Darren NaishSchool of Earth &

Environmental Sciences,

University of Portsmouth,

Portsmouth, PO1 3QL, UK.

darren.naish@port.ac.uk

Fig. 1. Representatives of thetwo living sloth genera. a.Bradypus. b. Choloepus. Bothinhabit South and CentralAmerica and achieve a totallength of 40–80 cm. Despitetheir strong overall similarity,the two belong to radicallydisparate branches of the slothfamily tree (see Fig. 9).

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sloths is mildly controversial, with most workersusing Tardigrada Brisson, 1762 for the group.However, the name tardigrade is more usuallyapplied to the invertebrates also known as waterbears. Partly as a consequence of this problem, someworkers have argued that Phyllophaga Owen, 1842should be used instead.

Sloths have conventionally been regarded asexclusively American. However, the recent discoveryof an Eocene sloth from Seymour Island, Antarctica,reveals a wider distribution of the group and raisesthe possibility that xenarthrans did not originate inSouth America as has long been thought. As early asthe Oligocene, sloths had colonized the GreaterAntilles as far east as Puerto Rico, apparently via ashort-lived land-span (known as the Greater Antilles–Aves Rise, or ‘GAARlandia’) connected to north-western South America. Although it is often statedthat sloths only migrated to North America once thePanamanian Isthmus had formed about 2.5 millionyears before present, the oldest North Americansloths (the mylodontid Thinobadistes and themegalonychid Pliometanastes) are actually from theLate Miocene, and thus predate the land bridge byseveral million years. Presumably these sloths swamto North America! By the end of the Pleistocene therewere sloths as far north as Alaska.

Sloths in the Holocene

Ground sloths were contemporaries of humans forthousands of years, and archaeological evidenceindicates that sloths like Megalonyx were hunted byPalaeoindian people. The discovery in 1888 of piecesof Mylodon hide inside an apparently walled-inArgentinian cave called variously Cueva Eberhardt,Cueva de Milodon or Cueva Ultima Esperanza, has ledto the proposal that these sloths had been keptcaptive by people, and perhaps farmed. However, thesupposed wall turned out to be part of the collapsedcave roof, and radiocarbon dating showed thathumans and sloths did not use the cave at the sametime. Ground sloths on the American mainland do

Fig. 2. Skeleton of the huge Pleistocene megatheriid Megatheriumamericanum (total length c. 6 m). Note the bulge on the lower jawthat housed the long tooth roots. Photograph used with permission,courtesy of the Natural History Museum, London.

Fig. 3. Life restorations of a diversity of fossil sloths. a.Eremotherium, a Pleistocene megatheriid from both South and NorthAmerica. b. Hapalops, a Miocene megatherioid from South America.c. Megalocnus, a megalonychid from Pleistocene Cuba andHispaniola. d. Mylodon, a Pleistocene mylodontid from SouthAmerica. e. Nothrotheriops, a Pleistocene nothrotheriid from NorthAmerica. Only drawn approximately to scale.

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not seem to have survived to more recently thanabout 8600 years before present but it appears thattwo sloths from Haiti, Synocnus comes and Parocnusserus, survived to as recently as 500 years beforepresent. Their remains have been discovered inassociation with pottery and the bones of domesticpigs.

A minority view among zoologists today is thatone ground sloth species may survive, the enigmaticmapinguari of the Amazon. While the mapinguari’scoarse reddish fur, armour-studded skin and massivehand claws make it sound something like a sloth,attributed to it are a number of fantastic features thatcast doubt on its reality. These include the presenceof a mouth-like organ on its belly that secretes anoxious gas and a frightening voice that sounds like ajet engine.

Sloth morphology

A typical fossil sloth can be imagined as a ratherbear-shaped, shaggy-furred mammal withparticularly powerful forelimbs, a barrel-shapedribcage, a stout tail, prominent curved hand and footclaws and a markedly broad, robust pelvis (Fig. 7).

Sloth skulls are diverse in form and range from thedeep and broad, snub-faced morphology seen inBradypus and derived megalonychids, to the elongatealmost horse-like skulls of megatheriids and others(Fig. 4). Some megalonychids had a domed craniumresulting from marked enlargement of the sinuseswithin the frontal bones. The sloth palate is roughand covered in pits and grooves and there aredistinctive deep laminae that descend ventrally fromthe pterygoid bones. The tip of the sloth mandible isusually spout-shaped and there is a foramen,representing an external opening of the mandibularcanal, on the side of the lower jaw. In sloths withparticularly long-rooted teeth there is a distinct bulgeon the ventral margin of the lower jaw (Fig. 2).

Sloths have peculiar teeth. They do noit possessdeciduous teeth, having instead a single set of open-rooted teeth that grow continuously throughout life.As such, the lack of a replacement dentition hasmade it difficult to compare sloth teeth with those ofother mammals. Incisors are absent, and it is notreally possible to distinguish between the similarpremolars and molars. The living tree sloth Choloepus,as well as some mylodontids, megalonychids andnothrotheriids, possess canine-like teeth which aredistinctly separated from the other teeth. In thePleistocene megalonychid Megalocnus (Fig. 3c) fromCuba, and in certain other genera, the two mostanterior upper jaw teeth are more like incisors thancanines.

Sloth teeth lack enamel and are composed instead

of two different kinds of dentine plus an outer layer ofcementum, the softer dentine forming the innermostregion of the tooth. The teeth in a sloth’s upper jawdo not occlude with those in its lower jaw in thestandard ‘one to one’ mammalian way. Instead, eachupper jaw tooth occludes with two lower jaw teeth.Furthermore, when sloth teeth erupt they are devoidof the cusps and basins normally seen in mammalianteeth and are simple and cylindrical in form. Somefossil sloths had squarish or sub-rectangular teethand, in these forms, transverse ridges and adjacentvalleys are particularly prominent.

Fig. 4. Skulls from various representative sloths. a. Eremotherium, a Pleistocene megatheriid known fromboth South and North America. b. Nematherium, a mylodontid from Early Miocene South America. c.Scelidotherium, a scelidotheriine mylodontid from the Pliocene and Pleistocene of South America. d.Thalassocnus, a nothrotheriid from the Miocene and Pliocene of the Pacific coast of South America. e.Acratocnus odontrigonus, a choloepodine megalonychid from the Pleistocene of Puerto Rico. f.Acratocnus simorhynchus, an even shorter-snout choloepodine, from the Pleistocene of Hispaniola. Not toscale, redrawn from various sources.

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The forelimbs of most sloths are about subequal inlength to the hindlimbs, the most prominentexceptions being the long-armed tree sloths of thegenus Bradypus. Mylodontids had a particularlyprominent process on the ulna. Recent studies haveshown that the length of this process, the olecranon,relative to the rest of the ulna is a good indicator ofdigging ability in mammals as it provides theattachment area for the triceps, the main muscleused in digging. Forelimb bone strength inmylodontids was also high and shows that theforelimbs were resistant to impact with the ground.Furthermore, the wide, straight and relatively flatclaws of these sloths resemble those of livingmammals that dig (Fig. 5). Accordingly, mylodontidsseem to have been proficient diggers that unearthedroots and tubers and they may even have constructedburrows.

Sloths are amazingly diverse and unusual in handmorphology (Fig. 6). Among megatheriids, primitivespecies of Eremotherium had five digits (albeit it with ashort thumb and a fifth digit with only one phalanx)while later E. laurillardi was tridactyl, possessing onlydigits III–V (Fig. 6c).

The sloth pelvis is massive and broad (Fig. 7) andunusual in that that ischia are connected to thevertebral column (in most tetrapods only the ilia areconnected), a feature that sloths share with all otherxenarthrans with the sole exception of Cyclopes, thepygmy anteater. The femur in fossil sloths varies fromrobust to very robust, with the femora of giantmegatheriids being almost rectangular (Fig. 7). Thetibia in most fossil sloths is proportionally short and isalso massively constructed. As is true of the hand,some sloth groups reduced the number of toes withonly three present in some megatheriids (Fig. 8).

Mummified sloth skin preserved in the arid cavesof Chile, Argentina, Arizona and Nevada providesexcellent information on ground sloth skin and fur.Small bony ossicles were embedded in the skin of themylodontids Mylodon, Glossotherium andParamylodon, and probably also in Eremotherium, butare definitely not present in the mummified skin ofNothrotheriops. The fur itself was either yellowish orreddish brown.

Locomotion and posture

The configuration of the ground sloth foot and ankleindicates that most of these animals were plantigrade(that is, they placed the entire surface of the foot onthe ground). However, it was argued as early as the1840s that at least some ground sloths walked with apedolateral foot posture: that is, with most of theweight supported by the outer margins of the feet.This bizarre configuration meant that the dorsal

surface of the foot faced laterally (Fig. 8).The centre of gravity in the ground sloth body and

the strength of their hindlimb bones, pelvis andvertebrae indicate that at least some forms couldwalk bipedally and fossil trackways confirm this.Most sloths have hands and hand claws that appearwell suited for the manipulation of foliage and therobust tail seen in most fossil sloths suggests that theymay have sat in a tripodal posture when foraging andeating. The tripodal abilities of ground sloths haveproved inspirational to palaeontologists working onother fossil tetrapod groups.

Living tree sloths are good swimmers so it seemsreasonable to assume that ground sloths were too.However, a few fossil sloths reveal morphologicalfeatures which indicate that they were habitual,rather than occasional, swimmers and amphibioushabits have been suggested for both scelidotheriinemylodontids and nothrotheriids. As discussed below,one group of nothrotheriid seems to have been trulysemi-aquatic.

The diversity of fossil sloths

Recent work indicates that the living three-toedsloths (Bradypus) are most closely related to all othersloths, a group named Eutardigrada (Fig. 9). Three-toed sloths are superficially similar to two-toed sloths(Choloepus), a remarkable case of convergentevolution. An interesting consequence of thisdiscovery is that Bradypus has a ghost lineageextending back to the Eocene at least. In theclassification used here, Eutardigrada consists of fourfamily-level groups, the mylodontids, megatheriids,nothrotheriids and megalonychids. A fifth family-level group, Orophodontidae, has been named forsloths that are probably part of Mylodontidae.

Mylodontids, best known for the Pleistocene SouthAmerican genus Mylodon, appear to be the mostprimitive eutardigrades and seem mostly to havebeen medium-sized ground-dwelling sloths, some ofwhich were accomplished diggers. They include thescelidotheriines, a Pliocene–Pleistocene groupcharacterized by broad molar teeth, the Miocene–Pleistocene lestodontines, and Paramylodon, wellknown thanks to the well-preserved remainsdiscovered in the Californian Rancho la Brea tar pits.

Nothrotheriids include the amphibious sloths ofthe genus Thalassocnus and the Shasta ground slothNothrotheriops (Fig. 3e). Nothrotheriops becameextinct about 11 000 years ago and is wellrepresented by hundreds of bones discovered in thesouth-western USA. The abundant dung produced bythis sloth has been discovered and analyzed.Nothrotheriids have long been a controversial groupto classify, but it now appears that Nothrotheriidae

Fig. 5. Arm of the mylodontidGlossotherium from PleistoceneSouth America. This genusshares with several othermylodontids wide and relativelystraight claws, and robustforelimb bones that possessmassive muscle attachment sitesand high strength values. Thesefeatures suggest proficientdigging abilities.

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was most closely related to Megatheriidae (Fig. 9).One of the most surprising discoveries made in

sloth palaeontology concerns a nothrotheriid lineagefrom the Pacific coast of South America. These sloths,all belonging to the genus Thalassocnus, aretremendously abundant within the Miocene andPliocene marine sediments of the Pisco Formationand are associated with a fossil fauna of fish, whales,penguins and other sea creatures. The terrestrialenvironment adjacent to the former shoreline of theregion appears to have been a barren desert, and thusthese sloths were apparently coastal animals that atemarine vegetation. Thalassocnus natans, the firstspecies to be described, exhibited snout bone featuressuggesting the presence of a prehensile lip like thatseen in living seacows and hind limb and tail bonefeatures recalling those seen in swimming rodents

and other amphibious mammals. Perhaps it rested onthe land and, two or three times during the day,swam out to feed on kelp or sea grass. One member ofthis group, T. carolomartini, carried these trends to anextreme by having a markedly elongate snout, abroad spatulate tip to its lower jaw, and postcranialbones that resembled those of a sea-lion more thanthose of a sloth. It may therefore have spent muchmore time swimming at sea than T. natans. Incredibleas it may seem then, from the late Miocene into theearly Pliocene, the beaches and shorelines of Peruwere home to hundreds of amphibious sloths.

Megatheriids include the most famous fossil sloth,Megatherium, as well as its giant relativeEremotherium (Fig. 3b). Megatherium first appeared inthe early Pliocene of Bolivia and there is someindication that different Megatherium speciesinhabited different palaeo-environments, with someknown only from high altitude regions and othersfrom temperate lowland environments.

Finally, megalonychids include the arboreal two-toed sloths (Choloepus), and hence are still aroundtoday. Most of the group’s key features are related totheir canine-like teeth. Basal megalonychids likeMegalonyx were large, terrestrial sloths (body length

Fig. 6. Several sloth hands,chosen to depict the diversity insloth digital formula. a.Pleistocene mylodontidGlossotherium robustum, inwhich the hand is pentadactyl.b. Pleistocene megatheriidMegatherium americanum, inwhich the thumb is absent. c.Pleistocene megatheriidEremotherium laurillardi, inwhich digits I and II have beenlost. Not to scale, redrawn fromvarious sources.

Fig. 7. The broad, flaring pelvis and proportionally short, robust lefthindlimb of the giant megatheriid Megatherium americanum. As istypical for xenarthrans, the ischium in Megatherium has a bonyconnection with the tail vertebrae. The human silhouette at the leftgives a sense of scale. Photograph used with permission, courtesy ofthe Natural History Museum, London.

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c. 3 m) but the most derived members of the groupappear to have become increasingly suited forarboreal life, a trend that culminated in Choloepus.Many of these derived megalonychids were endemicforms unique to the Greater Antillean islands andinclude the smallest (Neocnus) and youngest(Synocnus and Parocnus) of fossil sloths. Whilecladistic studies show that megalonychids are theyoungest major sloth clade, the oldest reportedmember of this group is from the Middle Eocene ofSeymour Island, Antarctica. If this Eocene sloth reallyis a megalonychid, then all of the major divergenceswithin sloth phylogeny must have occurred at orprior to this time.

Fossil sloth biology, behaviour and lifestyle

Little direct data is available on fossil sloth behaviourand lifestyle. However, because the living sloths

phylogenetically bracket the fossil ones, we can makesome reasonable inferences about fossil slothbehaviour based on what we know about the livingspecies. Living sloths are long-lived for their size(Choloepus weighs 4–8.5 kg, yet individuals havelived in captivity for more than 30 years), use theirhand claws in self-defence, give birth to a single babythat clings to the mother, and do not appear to beparticularly sociable. Sloths have been described ashaving ‘an extreme tenacity to life’ and are reportedlycapable of surviving injuries that would kill mostother mammals. Conceivably, the same behaviouraltraits were true of fossil sloths.

Reconstructing other aspects of fossil slothpalaeobiology is problematical however because thesimilarities seen in living sloths may be convergentresponses to a similar leaf-eating lifestyle, rather thanattributes common to all sloths. Living sloths arenotoriously slow-moving and sluggish and possessedof an extremely slow metabolism, their metabolicrates being only 40–45 per cent of those expected fora mammal of their body size. All living sloths sharelarge, multi-compartmented stomachs that housesymbiotic cellulose-digesting bacteria. Digestion isslow, with food remaining in the stomach for morethan a month in some cases.

Both two-toed and three-toed sloths havespecialized hairs that encourage the growth of blue-green algae. In Choloepus, the algae grow inlongitudinal grooves in the hairs, while in Bradypusthere are transverse cracks that become progressivelywider with age. During the wetter parts of the year,the algae blooms and the sloths turn green. Moths eatthe algae and birds eat the moths. Again, it remainsunclear whether this is a convergent feature uniqueto living tree sloths, or whether extinct sloths alsogrew algae on their fur. Extinct sloths for which hairis known lack either the longitudinal grooves ortransverse cracks seen in living forms, but theremains of powdery green algae have been discoveredon the hairs of some extinct sloths, so thisrelationship may have been widespread after all.

Because some mylodontids exhibit the forelimbfeatures indicative of good digging abilities, it hasbeen proposed that sloths like Glossotherium andScelidotherium were the creators of the giant burrowspreserved in the Pleistocene sediments of Argentina.These burrows are more than a metre tall, almost2 m wide, and in some cases more than 20 m long.The sides and roofs of these burrows are scored with

Fig. 8. Right foot of the giantmegatheriid Eremotherium.Though this is how the footlooks in lateral view: the lateralsurface of the foot is theanatomical dorsal surface. Onlydigits III-V were present in thissloth, and of these only digit IIIpossessed a claw (this wascurved toward the midline, andits tip is thus not visible inlateral view).

Fig. 9. Cladograms representing the affinities of, and between,sloths. a. The major divergences within Xenarthra, the placentalmammal clade that includes sloths and their relatives. b. Thephylogeny of sloths, simplified to show only the major divergences.Based on Gaudin (2004).

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parallel claw marks. While the idea of large animalscreating or dwelling in giant burrows may seemoutlandish, living bears create cave-like dens and ithas recently been discovered that pygmy hippos alsodig giant burrows. The preservation of sloth bonesand dung in caves suggests that some species soughtrefuge in these shelters, or perhaps entered the cavesto mine for salt. The latter behaviour is widespread inliving mammals, being most famously practised bythe elephants living near Kenya’s Mount Elgon.

Some ground sloth species appear to exhibitpronounced sexual dimorphism: in Eremotheriumlaurillardi presumed males are about 50 per centbigger than presumed females.

Sloth diet

Ground sloths have conventionally been regarded asstrict herbivores that used their powerful forelimbsand large curved claws to manipulate foliage, andevidence from sloth morphology and fossil dungsupports this view. It remains possible, however, thatsome species were omnivorous, occasionallyconsuming carrion and small animals.

One of the most bizarre suggestions made aboutany fossil sloth is the proposal that Megatherium wasan active carnivore, adept at stabbing anddispatching large herbivores. This hypothesis restsentirely on the discovery that the forelimb anatomyof Megatherium would have allowed it to perform bothrapid, powerful forelimb strikes and the carrying ofheavy loads. This idea seems extraordinarily unlikelyhowever as all the morphological details of this giant,slow moving animal are clearly in agreement withthe more conventional concept of it as a herbivore. IfMegatherium and other sloths were capable ofdeploying rapid forelimb strikes, it seems logical thatthey did so to defend themselves from the large andformidable carnivorans they were contemporaneouswith: giant lions, dire wolves, sabre-toothed cats,short-faced bears and so on.

Good evidence for the true diets of fossil slothscomes from their fossil dung. Some sloth dung,despite being 11 000 or so years old, looks and smellsfresh and will even burn. DNA extracted from dungsamples matches that recovered from bones, so insome cases dung can be referred with confidence to asloth species, and this has been done with dungbelonging to Nothrotheriops shastensis. DNA analysisof the plant fragments within the N. shastensis dungshow that this animal fed on an impressive diversityof plant species ranging from pines, grapes and rosesto mints, grasses and lilies, though with differentspecies being favoured according to their availabilityat the time.

Suggestions for further reading

Bargo, M.S., Vizcaíno, S.F., Archuby, F.M. & Blanco,R.E. 2000. Limb bone proportions, strength anddigging in some Lujanian (Late Pleistocene-EarlyHolocene) mylodontid ground sloths (Mammalia,Xenarthra). Journal of Vertebrate Paleontology, v.20,pp.601–610.

Cartelle, C. & De Iuliis, G. 1995. Eremotheriumlaurillardi: the Panamerican late Pleistocenemegatheriid sloth. Journal of VertebratePaleontology, v.15, pp.830–841.

Fariña, R.A. & Blanco, R.E. 1996. Megatherium, thestabber. Proceedings of the Royal Society of London B,v.263, pp.1725–1729.

Gaudin, T.J. 2004. Phylogenetic relationships amongsloths (Mammalia, Xenarthra, Tardigrada): thecraniodental evidence. Zoological Journal of theLinnean Society, v.140, pp.255–305.

Hofreiter, M., Betancourt, J.L., Sbriller, A.P.,Markgraf, V. & McDonald, H.G. 2003. Phylogeny,diet, and habitat of an extinct ground sloth fromCuchillo Curá, Neuquén Province, southwestArgentina. Quaternary Research, v.59, pp.364–378.

Muizon, C. de, McDonald, H.G., Salas, R. & Urbina,M. 2004. The youngest species of the aquaticsloth Thalassocnus and a reassessment of therelationships of the nothrothere sloths(Mammalia: Xenarthra). Journal of VertebratePaleontology, v.24, pp.387–397.

Fig. 10. Skull of themylodontid Glossotherium fromPleistocene South America. Thissloth exhibits a particularlydeep, broad snout and a broadtip to the lower jaw. Photographused with permission, courtesyof the Natural History Museum,London.