Sloths

Sloths
Hoffmann's two-toed sloth ( Choloepus hoffmanni ), a representative of the two-toed sloth
Systematics
without rank: Synapsids (Synapsida) Class : Mammals (mammalia) Subclass : Higher mammals (Eutheria) Superordinate : Sub-articulated animals (Xenarthra) Order : Tooth arms (pilosa) Subordination : Sloths
Scientific name
Folivora
Delsuc , Catzeflis , Stanhope & Douzery , 2001

The sloths (Folivora, also Tardigrada or Phyllophaga) form a suborder of the toothless mammals (Pilosa) and are related to the anteaters and the armadillos ( secondary joint animals ). The three available scientific names are widely used in specialist publications , but the latter two are also used by other animal groups. There are six recent species, which are divided between the two genera of the two-toed sloth ( Choloepus ) and the three-toed sloth ( Bradypus ). These species living today are rather small animals of the suborder, which are on average 50 cm long and weigh around 5 kg. They mostly inhabit tropical rainforests in South and Central America , where they prefer to stay in the treetops and feed on leafy vegetable food. The sloths are best known for their way of life hanging with their backs down in the branches, their very slow movements and the long periods of rest. The latter two properties are caused by an extremely low metabolism , which results from the low-energy leaf food.

In addition to the two genera that exist today - including extinct species - at least 90 more are known, making the sloth one of the most diverse groups of articular animals. These genera often included much larger members than those living today, including giant sloths weighing 4 to 6 tons. A predominant part of the extinct sloths lived largely on the ground - however, they used quite diverse habitats, in addition to forests and open landscapes to desert-like areas, high mountain ranges and coastal areas. The diet was also significantly more varied. The origin of the sloth dates back to the Oligocene over 30 million years ago. In the transition from the Pleistocene to the Holocene around 10,000 years ago, most of the large species of sloth died out.

Research into sloths began as early as the late 18th century. During this time the first recent species were described and only a few years later the first fossil representatives were found. The first high phase of research took place in the first half of the 19th century, when Charles Darwin discovered numerous fossil finds on his trip to South America. The idea that the tree-dwelling sloths of today are more closely related to each other and that they face the ground-dwelling sloths also established itself. It was only in the second half of the 20th century that this could be refuted - initially through anatomical examinations and later through molecular genetic analyzes. Accordingly, the three-toed sloths belong to the monotypical family of the Bradypodidae, while the two-toed sloths belong to the likewise monotypical family of the Choloepodidae . Therefore, the adaptations of today's sloths to the way of life in the trees are to be seen as a convergent development.

anatomy

General appearance and size

Brown-throated sloth ( Bradypus variegatus )

Today's tree-dwelling sloths reach a total length of 42 to 80 cm and weigh between 2 and 11 kg. The head is very short, the face rather round. The small eyes are wide apart, but look forward, the round nose is clearly flattened, the ears are small and hidden in the fur. The tail only exists as a stubby rudiment . Typically, the limbs are very long, the front longer than the rear, which is particularly evident in the three-toed sloth . The two current genera differ in the number of visible fingers . Both groups have three toes each on their hind legs . The fingers and toes have large, crescent-shaped, curved claws and are about the same length.

The extinct sloths partly differed in their appearance from the representatives living today and exceeded them in their body dimensions - apart from a few very small representatives - sometimes considerably. Above all, the forms from the Pleistocene reached extreme dimensions. Sun pointed Scelidotherium a length of 2.7 m and a weight of 800 kg. Glossotherium was 3.25 m long and weighed a good 1.5 t. The relatives of the largest known genera, Megatherium and eremotherium , but reached a total length of up to m to 6 and a weight of approximately 4 to 6 t, making it next to from Eurasia immigrant trunk animals , the largest land-dwelling mammals that time in America showed. These mostly ground-dwelling sloths, like many of their ancestral predecessors, had short and strong limbs and also had a longer tail.

Skull and dentition features

Skull of the Hoffmann two-toed sloth ( Choloepus hoffmanni )

The skulls of today's two- and three-toed sloths are on average 10.8 and 6.7 cm long and generally clearly rounded and short. The largest skulls within the sloth were owned by Eremotherium and Megatherium with dimensions between 65 and 86 cm. In general, the sloths have a very diverse skull design. There were forms with long, narrow skulls such as Nothrotherium and Scelidotherium or with short and wide snouts, such as Lestodon . General characteristics of the sloth are a greatly reduced middle jawbone and an elongated upper jaw . One of the most striking features of the skull is the incompletely developed zygomatic arch , a characteristic that also occurs in the anteater . Partly tempted by this characteristic that even when the unspecified related pangolins is shown to include a rather insectivorous diet of the earliest sloth representatives, but all previously discovered fossil forms are identified as herbivores. Only some of the largest representatives of the sloths such as Megatherium , Megalonyx and Paramylodon had fully developed zygomatic arches again. Furthermore , a laterally flattened bone outgrowth occurs at the anterior arch attachment on the cheekbone . This serves as a starting point for the masseter muscle of the masticatory muscles and enables pronounced forward and backward chewing movements. A similar bone outgrowth can be found in the Glyptodontidae , relatives of the armadillos , but this has a different structure there and also leads to a different attachment of the masticatory muscles, so that both bone outgrowths have arisen independently. Also typical is the occurrence of a septomaxilla (os nariale) in the anterior nasal cavity area. This ossification occurs mostly only in monotremes and older Mesozoic mammals.

Skull of Nothrotherium

There are also special features in the dentition structure, which differs from that of the higher mammals . Most sloths have no front teeth, i.e. the incisors . As a rule, five teeth are formed in the upper jaw and four in the lower jaw, for a total of 18. Only a few lines such as the developed Nothrotheriidae have reduced the foremost tooth. As a rule, two types of teeth can be distinguished in sloths: The rear teeth are homodontic , so that when they are fully developed it is not possible to differentiate between premolars and molars ; they are usually referred to as molar-shaped . In the Bradypodidae , i.e. the three-toed sloths, the foremost tooth is small and chisel-like in design (so-called anterior chisel-shaped tooth or "front chisel-like tooth"). The Megalonychidae as well as the two-toed sloths and the Mylodontidae have transformed this into a canine-like one, which is therefore called caniniform ; it clearly exceeds the other teeth in height. In the closed dentition, the upper and lower canine-shaped teeth are exactly the opposite of those of the other higher mammals, that is, the lower one rests behind the upper one and not the other way around. It was therefore assumed that the foremost tooth in each case does not correspond homologously to the actual canine tooth . The Megatheriidae show an exception , in which all teeth are designed in the same way. Investigations into the formation of teeth in the embryonic stage provided information about the more precise dentition structure of the sloths. If the mineralization sequence of the sloth's teeth corresponds to that of the other higher mammals, the rearmost tooth in each case forms the first molar, and the three teeth in front therefore correspond to the premolars. In the lower jaw, the premolars also include the foremost caniniform ( choloepus ) or chisel-shaped ( bradypus ) tooth. The foremost tooth in the upper jaw would be identical to the canine according to the sequence of the tooth system. In the embryonic state, incisors also form, but they are then reabsorbed very quickly. An additional characteristic of the sloth's teeth is the lack of tooth enamel : the teeth mainly consist of different layers of dentin , including the particularly hard orthodentine . In addition, the chewing surfaces of the rear teeth do not have the pattern of various cusps and ridges characteristic of mammals - however, two transverse ridges ( lophen ) are formed, which enable the food to be crushed. Another special feature is that there is no change of teeth from deciduous to permanent dentition. Rather, the teeth grow their entire life, with young teeth first being round and later getting their more angular shape. It is possible, however, that the upper canine-shaped tooth of the two-toed sloths changes at a very early stage of individual development .

Skeletal features

Skeleton of the two- toed sloth ( Choloepus didactylus )

Significant skeletal features exist primarily in the area of ​​the spine. The neck of the three-toed sloth comprises eight to ten vertebrae , which means that the animals have a very flexible head. The two-toed sloths, on the other hand, only have five to seven cervical vertebrae, so their necks are significantly shorter. It is assumed that the one to three following the seven cervical vertebrae in the three-toed sloths represent forwardly displaced ribless thoracic vertebrae, while in the two-toed sloth the two anterior thoracic vertebrae represent rib-bearing cervical vertebrae. In all fossil sloths of which the cervical spine is known, only seven vertebrae have so far been detected. The number of thoracic vertebrae varies in the individual species of sloth and is 16 in the three-toed sloth, 24 in the two-toed sloth and 22 in the extinct Hapalops . Especially on the lumbar vertebrae and the posterior thoracic vertebrae, additional joint surfaces occur, which are located on the lateral articular processes and are called xenarthric joints (secondary joints or xenarthrals). These connect the previous with the following articular processes and thus additionally stabilize the back. Today's sloths only have a short, stubby tail, consisting of 4 to 5 vertebrae; However, some extinct had long tails with significantly more caudal vertebrae.

Skeleton of Glossotherium

Individual peculiarities can also be found in the musculoskeletal system. As a special adaptation to their way of life, the recent sloths have extremely long and slender limbs, with the front of the three-toed sloth being about one and a half times as long as the rear; in the two-toed sloths the ratio is much more balanced. The fossil sloths often had shorter and more robust legs. Especially in the ground-dwelling sloths, the front legs were usually shorter than the rear legs. Typical of all sloths is the appearance of a third trochanter as a muscle attachment point on the shaft of the thigh bone , which is less pronounced than in the related armadillos and its location varies greatly. The tibia and fibula are not fused together, the large Megatheriidae are an exception here. The three-toed sloths have three (rays II to IV) on their front feet, the two-toed sloths two clawed rays (rays II and III), most of them have the same length. There are three rays on each of the hind feet. The other toe rays are reduced in length and have no function. The number of claw-guiding rays in extinct sloths living on the ground fluctuates. The construction of the rear foot is important here. Some representatives touched down with the whole foot and thus had a plantigrade locomotion ( foot walkers ). In others, the hind foot was twisted to the side, so that they appeared with the fifth (outer) ray and the heel bone ( pedolateral ), a form of movement that developed independently in the individual sloth lines several times.

hide

Fossilized remains of fur from Mylodon

Two layers can be clearly distinguished in the fur of the three-toed sloth: The short and very dense undercoat and the long, straw-like outer coat . The two-toed sloths, on the other hand, only have the top coat. A special characteristic of the hair of the sloth is the lack of the medulla. The hairline runs from the belly to the back and thus opposite to that of other mammals. This allows the rainwater to run off better, which is an evolutionary adaptation to the predominantly hanging way of life in the branches of the trees. Furthermore, the hair of the two-toed sloth has 3 to 9 longitudinal ribs and grooves on the outer surface, which run along the entire length. This is unique among mammals. This does not occur with the three-toed sloth; their hairs have small air spaces under the cuticle . An unusual camouflage has been developed to protect sloths : Algae settle in the hair , which the animals benefit from between the leaves as a green-changing color - an effect that is particularly evident in the rainy season .

Fossilized finds of the soft tissue in some caves in North and South America have shown that fur was also covered in extinct forms, such as the Mylodon , Megalonyx and Nothrotheriops . The traditional hair is usually long and shaggy, and colored lighter or darker brown. It can be assumed that numerous fossil sloths had fur in this way. It is discussed whether the extremely large representatives, such as Megatherium and Eremotherium , which are common in the tropical regions, were also hairy. Here, the huge body sizes and the associated and necessary thermoregulation indicate that, as with other large, modern herbivores, such as elephants , rhinos and hippos , there was no visible fur covering, but the skin was more or less bare. Some of the large ground sloths such as the Mylodon or Glossotherium also had disc or lens-shaped bone plates ( osteoderms ) that were formed in the skin. With a diameter of 0.5 to 3 cm, these had a small size and, in contrast to similar formations in the armadillos and glyptodons, had a simple, compact structure. Since they are usually found in isolation, they were most likely formed randomly in the skin and not firmly fused together as in the armor of today's armadillos.

Internal organs

In contrast to other mammals that specialize in leaf nutrition, sloths digest their food in the anterior gastrointestinal area. The stomach consists of three partially separate chambers that are connected to the gastric porter. When completely filled, it reaches up to a third of the total body weight of an animal. The lungs represent a special feature, which in both genera does not show any clear separation into individual wings, which may be related to the structure of the rib cage. The spleen of the three-toed sloth is irregularly tubular, but of the two-toed sloth it is flat-triangular. The kidneys are located very deep in the abdominal cavity, the adjoining urinary bladder is 12 cm in diameter and stores up to 1 liter of urine . In young animals it takes up almost half of the peritoneum cavity . Only two-toed sloths have a gallbladder - but these are very small, three-toed sloths have not developed any. In contrast to numerous mammals that do not belong to the primates , the uterus has a simple design, but this differs markedly from some representatives of the armadillos as close relatives within the articular animals .

Sense organs

In general, the sloths orient themselves through the sense of smell and touch . The visual sense of today's sloths is underdeveloped due to the lack of the ciliary muscle , which makes the animals myopic . The strongly convexly curved cornea and the very thick eye lens lead to a poor resolution of the field of view. However, the eyesight of the young is better than that of the adult. Since the lens of the two-toed sloth is significantly narrower compared to the diameter than that of the three-toed sloth, it is also less efficient.

Also the hearing is less well developed. However, the ear of today's sloths is designed for a rather low frequency range between 0.3 and 30 kHz, which is mainly due to the structure of the cochlea . Adult animals are mainly activated by sounds of 2 to 8 kHz, which the young animals also take advantage of when they are separated from the mother. The call sounds of young two-toed sloths are on average lower than those of young three-toed sloths. Since the acoustic sense obviously only plays a minor role, only a few call sounds are known to the sloths. By comparing the inner ear of the recent species with extinct forms, a similar frequency range could be determined for Glossotherium and Lestodon , among others. The optimal hearing frequency was between 1.7 and 2.4 kHz, while the upper limit was 15.3 to 16.5 kHz was reached.

distribution

The sloths are mainly found in central and northern South America and parts of Central America, as well as on some islands in the Caribbean . They inhabit tropical rainforests of the plains and mountain forests, they rarely use more open landscapes. Higher altitudes over 2400 m are only occasionally visited. Both of today's genera do not tolerate cooler temperatures. Due to the lack of an undercoat, the two-toed sloths are more sensitive to the cold and have a higher thermoneutrality (24 ° C) than the three-toed sloths (18 ° C). However, the fur of the two-toed sloth is significantly denser at higher altitudes.

The origin of the sloth is most likely to be found in South America. Central and North America were reached in several waves of propagation. Sloths were found on the Caribbean islands for the first time in the early Oligocene around 34 million years ago, and they settled in North America in the Upper Miocene around 8 million years ago. Immigration to both regions probably took place via small islands. With the closure of the Isthmus of Panama and the creation of a land bridge between South and North America in the Pliocene around 4 million years ago and the ensuing Great American Faun Exchange , Central and North America were further colonized by sloths. In the Pleistocene , they reached their greatest extent over both continents. Thus came Mylodon ago almost to the tip of South America, while Megalonyx in the Far North of North America in Alaska has been demonstrated. The sloths used a variety of habitats throughout their tribal history . Like today they were to be found in dense forests, but also in savannas and steppes up to deserts , as well as in high mountains and on the seashore.

Way of life

General and locomotion

Hoffmann's two-toed sloth ( Choloepus hoffmanni ) hanging in the branches

The recent two- and three-toed sloths are both diurnal and nocturnal and are strictly solitary as well as tree-dwelling ( arboricol ), where they often stay in the tree canopy. In the trees they usually move forward with their backs hanging down (suspensory locomotion). Today's sloths are adapted to this way of life thanks to their long limbs and curved claws that act as hooks, as well as highly flexible joints on the arms and legs and a short and rounded chest with a highly flexible collarbone - sternum connection. In addition, the unusual posture and the tree-climbing movement required a restructuring of the muscles, especially in the area of ​​the front limbs, which provide the drive for this type of locomotion. This enables energy-saving hanging and shackling. The movements are generally very slow and the speeds achieved are a few meters per minute. However, the animals rarely cover more than a few dozen to a hundred meters in search of food within a day. The trees are very rarely abandoned. Today's sloths move awkwardly on the ground and crawl forward there with the forearms and the soles of their hind legs. However, they are very good swimmers.

Live reconstruction of Hapalops climbing upright in the branches

The extinct sloths as a whole used a far greater variety of habitats and had a more extensive range of modes of locomotion. Some of the fossil forms today were also arboreal, which can be assumed for part of the sloths of the Caribbean islands in particular . Other representatives such as Diabolotherium from the Pleistocene of the Peruvian Andes may have developed the ability to climb in rocky mountain areas, although originally a tree-dwelling way of life was not excluded here. The earliest representatives with a tree-climbing locomotion include Hapalops and Schismotherium from the time of the Lower and Middle Miocene. However, there was a big difference to today's tree sloths. So these fossil sloth forms had rather short and strong limbs, which suggests a locomotion in the trees more like today's anteaters or pangolins , which climb upright. In addition, the massive musculature of the forelegs resulting from the robust bone structure is connected to the very broad forefeet and a strongly expanded upper joint of the ulna , the olecranon , which does not exclude a digging way of life. This can also be assumed for some of the forms from the Pleistocene, which show a similar construction of the front legs and forefeet, such as Scelidotherium and Glossotherium . Finds of fossil grave structures 0.8 to 1.8 m wide and up to 40 m long in the region around Mar del Plata in eastern Argentina are interpreted as the remains of these animals. In some cases, these also have scratch marks, the spacing of which corresponds to those of the claws of the sloth. With a weight of 1.5 t Glossotherium would be one of the largest known burrowing animals.

Live reconstruction of Nothrotheriops , a ground sloth

However, numerous other sloths were purely ground-dwellers and mostly moved on four feet. This is especially true for the largest representatives such as Megatherium and Eremotherium , whose forelegs differ somewhat from that of the burrowing sloth with a shorter and differently oriented olecranon, so that they consequently could not muster the necessary strength to dig in the ground. A partially bipedal locomotion was assumed on the basis of Ichnofossils , but this seems skeletal-morphologically rather improbable. However, the large ground sloths were able to stand up on their hind legs, using the strong tail as a support, as is still the case today with armadillos and anteaters. This was also made possible by the fact that most of the body weight lay on the hind legs. In an upright position, the large ground sloths could graze in the canopy of trees or pull up branches with their free front legs. A special form of locomotion has developed with Thalassocnus . A semi-aquatic way of life can be inferred from skeletal modifications, for example on the tail and hind legs. This species of sloth inhabited the coastal areas of the Pacific from today's Peru to Chile.

Diet and Metabolism

White-throated sloth ( Bradypus tridactylus ) feeding

Today's sloths feed almost exclusively on leaves ( folivor ), only the two-toed sloths occasionally eat fruits, insects and smaller vertebrates . Trees, less often lianas, dominate among the leaf food . Individual individuals specialize in certain tree species, but today's sloths as a whole eat many different leafy plants. The sloths have developed special adaptations for food intake, such as a thick, flexible tongue and flexible lips for plucking the leaves. The hyoid bone and the ossified, adjacent cartilage act as muscle attachment points for the tongue and throat.

Because leaves are a fibrous and nutrient-poor food, it takes a long time to digest in the stomach, which can take up to 150 hours. Accordingly, the sloths also have the lowest metabolic rates of all mammals for their size , which is around 40 to 45% of those of similarly large animals. Therefore, sloths move slowly through the branches by hand. Muscle contraction is 3 to 6 times slower than in a house cat . Depending on the season, the animals are inactive for up to 20 hours a day, which they spend sleeping or resting. Feces and urine are only deposited about every eight days . While two-toed sloths sometimes simply drop their excrement from the tree, three-toed sloths regularly descend to the ground to defecate. Apart from any tree changes, this is the only activity the sloths do at ground level. The body temperature is also, unusually for mammals, more variable: In active phases it rises and, depending on the environment, is between 32.7 to 35.5 ° C, at night, during cooler times of the day or during sleep it can be rapid sink. The low metabolism is partly explained by the reduction in the risk of ingestion of toxic substances found in the leaves. In order to obtain important supplements that the sloths cannot take from their low-energy leaf diet, a special symbiosis has developed with some moths from the Chrysauginae group that live in their fur. These “sloth moths” lay their eggs in the excrement of the sloths, on which the larvae feed, and after metamorphosis they look for a new sloth host. The moths emit nitrogen and phosphorus compounds , among other things, into the fur, which in turn promotes the growth of the algae living in the fur . Since the sloths eat the algae while grooming, they also take up these compounds. The higher concentration of algae in the fur further increases the camouflage effect of the animals. The symbiosis is stronger in the three-toed sloth than in the two-toed sloth.

Coprolites of Nothrotheriops in the Rampart Cave (USA)

The diet of the extinct sloths has been studied quite well, after which they can also be regarded as herbivores. Various methods are used to reconstruct nutritional behavior. Anatomical studies of some tree-dwelling sloths on the Caribbean islands, such as Neocnus , revealed a predominantly leaf-eating diet due to a similarly built chewing apparatus to today's representatives. In the case of the ground-dwelling sloths , two groups can be distinguished based on the structure of the snout, especially the symphysis of the lower jaw. Shapes with a narrow symphysis like Megatherium had a correspondingly narrow snout, whereby the muscle attachment points of the facial muscles allow a flexible, pointed upper lip to appear. Analogous to today's herbivores with a similarly shaped upper lip, these animals fed on soft vegetable foods such as leaves. Representatives with a broad symphysis had a wide mouth equipped with bulging lips. These include Lestodon and Glossotherium , both of which were therefore grass-eaters, comparable to today's white rhinoceros . In between there are some intermediate forms, such as Mylodon or Scelidotherium , in which a diet based on a mixed vegetable diet is assumed. Fossil food remains are in the form of coprolites , which are found en masse in some caves in North and South America. For Megatherium , a foliage-based diet could be demonstrated, with a preference for seaweed , Fabiana or Junellia plants. Those from Nothrotheriops provided evidence of no fewer than 72 plant genera, often including xerophytes . Mylodon, on the other hand, was predominantly herbivorous. Further results in the reconstruction of the diet were provided by isotope analyzes , which revealed grass-based food in open landscapes for Lestodon and Glossotherium . Overall, the results of the various analysis methods indicate that the extinct sloths have a diverse diet.

It is unclear whether the extinct sloths had a similarly low metabolism as the recent ones. Amino acid tests on the bones of Nothrotheriops showed an average body temperature of 34.4 ° C, which is relatively low for an animal of more than 250 kg body weight, which is why it probably reacted sensitively to cooler temperatures. In contrast, it is believed that some very large ground sloths, such as the Mylodon , which inhabited subarctic regions during the Pleistocene , were less sensitive to cold due to their extreme body mass.

Reproduction and Mortality

The reproductive behavior of today's sloths has not been adequately researched. Females reach sexual maturity after three to four years, males a little later. For the two-toed sloth, it is assumed that females can reproduce all year round; most species of the three-toed sloth have a seasonal dependency. After a gestation of 6 (three-toed sloths) to 11.5 (two-toed sloths) months, a single young animal weighing 190 to 450 g is born. The birth also takes place in a hanging position, then the young animal clings to the belly of the mother animal until it is independent. The young animals begin to eat solid food after a few weeks and are weaned after one to two months. The life expectancy of sloths in the wild is unknown, but it is likely to be comparable to that of armadillos and anteaters. A captured male capable of reproduction was at least 12 years old. Some sloths can live to be over 30 years old in captivity, while a two-toed sloth in the Halle Zoo was around 50 years old, the highest documented age to date. The main predators are big cats , birds of prey and snakes . About half of all adult animals are killed by these predators when they descend from the trees.

Systematics

External system

Internal systematics of secondary articulated animals according to Gibb et al. 2015  Xenarthra     Pilosa     Folivora    Bradypodidae      Choloepodidae   Vermilingua    Cyclopedidae      Myrmecophagidae   Cingulata    Dasypodidae      Chlamyphoridae

The sloths (Folivora) form a suborder within the order of the tooth arms (Pilosa). The anteaters (Vermilingua) can still be assigned to this, so both represent a closer family group. The tooth arms in turn are part of the hierarchy of the secondary articulated animals (Xenarthra), which also include the armored secondary articulated animals (Cingulata) with today's armadillos (Dasypoda) include. The secondary articulated animals represent one of the four main lines of the higher mammals , which stand opposite the other three (collectively as Epitheria ) as a sister group . The eponymous xenarthric joints (secondary joints, also xenarthral joints ) on the articular processes of the posterior chest and the lumbar vertebrae can be seen as a common feature of all articular animals . The only exception to this are the Glyptodontidae within the Cingulata, whose lumbar vertebrae are fused together as a unique feature . Another, but varying, characteristic is the different shape of the dentition compared to most other higher mammals. While anteaters have no teeth at all, the armored articulated animals and the sloths are not formed, while the rear have a homodontic structure and more than none Tooth enamel feature. Very early in the history of research, the secondary articulated animals were considered an independent and natural group that only occurs in South America to some parts of North America. The origin of the articular animals is unknown, the oldest fossil finds come from the Paleocene more than 56 million years ago from South America and represent representatives of the armadillos. With the help of molecular genetic studies, it was possible to determine the splitting off of the secondary animals from the other higher mammals as early as the lower Cretaceous , about 103 million years ago. The armadillos separated from the common line with the toothed arms at the beginning of the Paleocene about 65 million years ago, the splitting of the anteaters and sloths then took place about 58 million years ago.

Internal system

Internal systematics of the sloths according to Varela et al. 2019 (based on skeletal anatomical features)  Folivora     Eufolivora     Megatherioidea    Megatheriidae (†)      Nothrotheriidae (†)      Megalonychidae  Mylodontoidea    Mylodontidae (†)      Orophodontidae (†)      Scelidotheriidae (†)      Bradypodidae

Today there are two genera of sloths, the two-toed sloth ( Choloepus ) with two species and the three-toed sloth ( Bradypus ) with four species. Including the extinct forms, at least 400 species are currently known in over 90 valid genera, which makes the sloth one of the most diverse groups within the secondary articulated animals, alongside the armadillos and the glyptodons. These are usually incorporated into five to ten families, whose representatives were present in America from the Oligocene to the Pleistocene and beyond, and which also form closer or more distant relationships.

For reasons of skeletal anatomy, several major lines can be differentiated. The superfamily of the Megatherioidea represents an important group . These unite the gigantic, ground-living sloths. The structure of the rear teeth, the chewing surfaces of which have two transverse cutting edges, can be cited as a common feature. The group includes the Megalonychidae , which are characterized by the modification of the foremost tooth, which has a canine ( caniniform ) or incisiform shape and is separated from the rear molar-like teeth by a long diastema . The Megalonychidae in the classical view based on skeletal features include small and large members of the sloth, including larger ground sloths of the continental American mainland, smaller representatives of the West Indies and the two-toed sloths ( Choloepus ), which are the only members of this family to have no fossil record while all others are only known to be fossil or sub-fossil. Due to the deep embedding of the two-toed sloths in a descent group of extinct Caribbean sloths, as evidenced by the skeletal features, numerous scientists favored a derivation of the former from the latter. The Megatheriidae produced the largest sloth representatives, but especially the earliest forms were comparatively small. They were generally characterized by monotonous teeth, as the foremost teeth were not reshaped in the shape of a canine. The third family are the Nothrotheriidae , whose representatives had a rather smaller physique. They also had distinctively long and narrow skulls, each with canine-like front teeth. However, later forms had reduced this, so that there were only four teeth in the upper and three in the lower jaw per jaw section. Some forms such as Schismotherium or Pelecyodon from the Lower Miocene cannot be assigned to an exact family and partly belong to the parent group of the Megatherioidea. Even younger representatives such as Hiskatherium and possibly also Diabolotherium are currently only classified as members of the Megatherioidea without a precise family assignment. The superfamily of the Mylodontoidea is regarded as the second large group of forms . In addition to the Megatheriidae, the Mylodontidae were the largest sloth forms. Typical for the developed representatives were high-crowned teeth with flat chewing surfaces as an adaptation to a more grassy food. The rear teeth showed a round or oval cross-section, the front teeth were shaped like canines. The Scelidotheriidae are sometimes only considered a subfamily of the Mylodontidae. In addition to a similar set of teeth, their skulls were determined by their elongated shape. The Orophodontidae are also closely related to the Mylodonts, but they are a side branch that was extinct again in the Oligocene. According to individual views, however, these could also be more closely related to the Megatherioidea. The last group includes that of the Bradypodidae and is monotypic . It includes the three-toed sloth ( Bradypus ), the second, more recent genus, which, like the two-toed sloth, has no fossil record. These are characterized by the short, rounded skull and a chisel-like front tooth. The Bradypodidae form the sister taxon of all other sloth families. A crucial clue to this is the design of the middle ear , where there is a tympanic bladder in the three-toed sloth and a tympanic ring in the two-toed sloth and most floor sloths. For this reason, all sloth families , excluding the Bradypodidae, are grouped together in the higher group of Eufolivora (the name “Eufolivora” was established in 2019 and replaces the term “Eutardigrada” coined by Timothy J. Gaudin in 2004 ).

Internal systematics of the sloths according to Presslee et al. 2019 (based on protein analysis)  Folivora     Megalocnoidea    Acratocnidae (†)       Parocnidae (†)        Megatherioidea    Nothrotheriidae (†)       Megatheriidae (†)       Megalonychidae (†)       Bradypodidae   Mylodontoidea    Scelidotheriidae (†)       Choloepodidae      Mylodontidae (†)  The megalocnoid is subdivided according to Delsuc et al. 2019

The anatomically determined relationships cannot be confirmed by molecular genetic or biochemical means. According to genetic data, the separation of the three- and two-toed sloths took place around 30 million years ago in the Oligocene , which, among other things, resulted in a molecular genetic analysis from 2015 in which all recent representatives of the sibling animals were taken into account. The genetic relationships between today's tree-living forms and the extinct large ground sloths remained unexplained for a long time. As early as the turn of the 20th to the 21st century, DNA examinations were carried out, the results of which were consistent with the three-toed sloths clustered more closely with the Nothrotheriidae, while the two-toed sloths with the Mylodontidae . However, these results were contradicted by collagen studies from 2015 that included today's sloths and some ground sloths. As a result of this, today's tree sloths were much closer to each other than to the ground sloths. The problem with this work was that too few fossil species were analyzed and the data obtained remained so ambiguous. Two extensive studies on molecular genetics and protein structure published in 2019 gave a more conclusive picture by considering representatives of almost all fossil families. Accordingly, three large groups can be distinguished within the sloth. One is the Megatherioidea with the Megatheriidae, the Nothrotheriidae and the Megalonychidae, the latter being reduced to the large ground sloths of North and South America. In addition, there are the three-toed sloths in this group. The second large group is represented by the Mylodontoidea, which includes the Mylodontidae, the Scelidotheriidae and the two-toed sloths. As the third major line, the sloths of the West Indies form the Megalocnoidea with the families of the Parocnidae and the Acratocnidae . The diversification of the sloth started very early. The Caribbean sloths had already detached themselves around 35 million years ago in the transition from the Eocene to the Oligocene, the Megatherioidea and Mylodontoidea followed a little later. At least 28 million years ago, most of the known families had differentiated themselves.

Overview of the recent and fossil families of the sloths

The following families belong to the sloths:

• Suborder Folivora Delsuc, Catzeflis, Stanhope & Douzery , 2001

• Superfamily Megalocnoidea Kraglievich , 1923

• Family Acratocnidae Varona , 1974
• Family Parocnidae Delsuc, Kuch, Gibb, Karpinski, Hackenberger, Szpak, Martínez, Mead, McDonald, MacPhee, Billet, Hautier & Poinar , 2019

• Superfamily Mylodontoidea Gill , 1872

• Family Mylodontidae Gill , 1872
• ? Family Orophodontidae Ameghino , 1895
• Family Scelidotheriidae Ameghino , 1889
• Family Choloepodidae Gray , 1871

• Superfamily Megatherioidea Gray , 1892

• Family Megatheriidae Gray , 1821
• Family Megalonychidae Gervais , 1855
• Family Nothrotheriidae Gaudin , 1994
• Family Bradypodidae Gray , 1821

In some classifications, the Scelidotheriidae and the Orophodontidae are classified as subfamilies in the Mylodontidae.

Tribal history

Origins and Development

Fossil remains of sloths are generally quite often, but focus mainly on three time periods, on the one hand the transition from Lower to Middle Miocene (lokalstratigraphisch before 18-16 million years Santacruzium called), on the other hand, the transition from the Upper Miocene to Pliocene (lokalstratigraphisch Huayquerium called ) about 6 to 5 million years ago and the end of the Pleistocene (locally stratigraphically called Lujanian ) 13,000 to 8,500 years ago. A higher number of finds has been handed down from the Middle Oligocene (locally stratigraphically called Deseadum ) about 29 to 26 million years ago, but this largely only affects the representatives of the Mylodonts. In part, these temporary fossil concentrations are explained by gaps in the tradition of the subsequent period, but they can also indicate an increased development of the sloths with a subsequent collapse of the populations due to climatic and geographical changes. In the case of the Santacruzium, most of the sloths are occupied from the southern part of South America, which offered more favorable living conditions at the time. In the further course of the Miocene, however, the unfolding of the Andes and the increasing glaciation of Antarctica caused a considerable deterioration in the climatic conditions in Patagonia with dramatic effects on the living environment at that time. The decline in the diversity of the sloths at the end of the Huayquerium, on the other hand, may be connected with the gradual onset of the Great American Fauna Exchange , while the drastic decline at the end of the Lujanium reflects the Quaternary extinction wave .

The origin of the entire group is unknown. One of the possibly oldest finds could come from Seymour Island about 100 km southeast of the northern tip of the Antarctic Peninsula . This includes a tooth found only about 2 cm long, which is similar to a canine and typically does not have any enamel for sloths . It was discovered in the La Meseta Formation , which consists mainly of sandstones and dates to the Middle to Upper Eocene between 42 and 36 million years ago. Paleobotanical studies have shown that the region was densely overgrown with pseudo-beeches that grew in a temperate to cool climate, comparable to that of today's Patagonia . However, new analyzes of the tooth structure, especially the dentin , revealed deviations from the sloths known today. The tooth is now assigned to a possibly previously unknown mammal. As early as the beginning of the 20th century, individual finds from the Lower and Middle Eocene Patagonia had been referred to various sloths, such as Protobradys and Proplathyarthrus . The latter has been handed down about a phalanx and an ankle bone . Both genera cannot, however, be clearly assigned to sloths according to today's standards.

Oligocene

The first clear fossil record of sloths comes from the Oligocene . The very early genera include Orophodon , Octodontotherium and Paroctodontotherium , some of which are assigned to the Orophodontidae family . As a rule, the finds are limited to tooth and jaw remains. The most important sites include La Flecha in Patagonia and Salla-Luribay in Bolivia . A specialty is Pseudoglyptodon , which has a mixture of characteristics of the sloths and the Glyptodontidae . The dentition, for example, has 5 teeth per upper jaw and four per lower jaw, the foremost canine-like, structured similarly to that of the sloth, but the rear molars have three flap-like formations on the chewing surface instead of two transverse ridges, which is characteristic of the trilobate teeth the glyptodon is. Pseudoglyptodon finds also came to light in Salla-Luribay, which date back to 29 to 26 million years ago. An almost complete skull is again documented from the Tinguiririca fauna of central Chile . Furthermore, the earliest representatives of the Megalonychidae can also be identified at this time , including Deseadognathus using a lower jaw from the Deseado formation in Patagonia, which is an ancestor of the two-toed sloth. Overall, the group of sloths in the late Oligocene already had a fairly high diversity with around a dozen genera spread across several families. This is also expressed in different adaptations, for example in the mode of locomotion from plantigrad to pedolateral or in the type of diet, with the myloodontid Paroctodontotherium , a form that is more specialized in grass, among other things . In addition, the body weight varies considerably from 40 to 250 kg.

A so far unnamed representative of the sloths from Puerto Rico appears enigmatic . Proven on the basis of an upper fragment of a thigh bone from the Juana-Díaz formation near Yauco, this dates back to 34 to 33 million years due to the presence of foraminifera and is therefore somewhat older than Deseadognathus . The size of the find suggests one of the smallest representatives of the fossil sloth, which reached about the size of today's three-toed sloth. The Caribbean islands may have been reached via a short-term, 35 to 33 million existing land bridge in the form of a headland or smaller island groups, which was created by tectonic movements. The find can be interpreted as an indication that the Caribbean sloths went their own development path very early.

Miocene

Skeleton of Hapalops

Only a few fossil records of sloths have survived from the beginning of the Miocene , including Chubutherium , known only from a few skeletal remains, and Holomegalonyx . Both come from the Argentine province of Chubut . Central are the finds from the Santa Cruz Formation in Patagonia from the period 18 to 16 million years ago, the transition from the Lower to the Middle Miocene, which indicate the first comprehensive radiation of the sloths. The fossil material is very extensive and includes not only individual finds but also partially and completely preserved skeletons. To date, almost a dozen genera are known from the formation, representing four large families. So are Megalonychotherium and Eucholoeops to Megalonychidae, Analcitherium to Mylodontidae while Nematherium the Scelidotheriidae represents and Prepotherium and Planops the megatheriidae. Other forms such as Schismotherium can only generally be classified as basic representatives of the Megatherioidea. The position of the very common Hapalops is assessed differently , as it is an early representative of both the Megatheriidae and the Megalonychidae. The sloths of the Santa Cruz Formation were small to medium-sized animals weighing between 35 and 123 kg. The skeletal anatomy differs from today's sloths and is somewhat similar to the anteaters , especially the shorter front limbs refer to a ground-dwelling or burrowing, but sometimes also tree-climbing way of life. The latter differed significantly from that of the recent genera. There were also certain adjustments to the diet. While the Megatherien were mainly adapted to leaf food, the Mylodontidae preferred harder and more fibrous plant food. About the same age as the finds from the Santa Cruz Formation are the remains of sloths from Cuba , which come from the Lagunitas Formation near Domo de Zaza. They include some remains of the skull and elements of the body skeleton and have been described as belonging to the genus Imagocnus , whose body weight was around 200 kg.

Skeleton of Thalassocnus

Only a few fossil finds are known from the Middle Miocene. Almost at the same time as the late section of the Santacruzium, Hiskatherium, a rather small megatherioid, occurs, which was detected with the help of some dentition fragments from Quebrada Honda in Bolivia. Megathericulus can be seen as a representative of the younger Megatheria ( Megatheriinae ) in phylogeny . These more modern megatheries are developing rapidly, but little is known about their earliest members. However, in the Upper Miocene Pyramiodontherium reached an enormous body weight of 2.5 t. In the Middle Miocene, another large family appears with the Nothrotheriidae , which are closely related to the Megatheria. The genus Thalassocnus is significant, as it is characterized by a stronger adaptation to a semi-aquatic to aquatic way of life, which can be recognized above all by the shape of the hind legs and the hind feet. Significant fossil deposits come from the Pisco Formation along the Peruvian and Chilean coasts; the oldest finds date from 7 to 8 million years ago. On the other hand, Mionothropus comes from the western Amazon region , which was described using a partial skeleton and which is closely related to the later Nothrotherium . Outstanding in northern South America is the Urumaco sequence of the Falcón Basin in northern Venezuela, which dates in the transition from the Middle to the Upper Miocene. The sloth community occurring here is very extensive and contains various forms of the Mylodonts with Baraguatherium , Eionaletherium or Bolivartherium as well as members of the Megatheria with Urumaquia and Proeremotherium .

The sloths reached North America for the first time in the Upper Miocene . Since the continent was not yet connected to South America by a land bridge at that time, this occurrence shows that the sloths were able to negotiate open stretches of sea, possibly by migrating from island to island. The earliest records of Pliometanastes , a megalonychid, and the Mylodontid Thinobadistes from California and Florida, both of which are around 8 million years old. While the latter became extinct, is the former became Megalonyx further, the oldest evidence of an age of 6.7 million years ago in the Lemoyne-fauna in Nebraska has been furnished. Even in the late Miocene, Zacatzontli , also a Megalonychidae, appeared in Mexico and Sibotherium , a Megatheriidae, in Costa Rica.

Plio and Pleistocene

Megalonyx skeleton

In the Pliocene , genera can be detected for the first time, which are decisive for the later Pleistocene and also for today's picture, especially the large ground sloth. However, due to the numerous gaps in the tradition, it is sometimes still difficult to directly relate these to the previous forms. Thus, among other things, Glossotherium emerged from the family of the Mylodontidae, Megatherium , the eponymous form of Megatheria and with Proscelidodon and Scelidotherium representatives of the family of Scelidotheriidae . The Miocene forms such as the Nothrotheriide Thalassocnus , however, died out during the Pliocene. During the Pliocene, the closure of the Isthmus of Panama resulted in the formation of a land bridge to North America and, as a result, the Great American Fauna Exchange, so that numerous forms of sloth immigrated to North America. At that time, however, only sparse fossil remains of sloths are known from Central America . This includes the large Meizonyx and a smaller representative of Megalonyx .

In the Pleistocene there was a fairly high variety of different sloth forms. Around two dozen genera from North and South America are known from this period. Some had only a regional distribution, such as the very large Archaeomylodon from the pampas region or Diabolotherium from the Andean regions of Peru, which, as the numerous finds from caves suggest, may have been climbing rocks. Megistonyx from the group of Megalonychidae has come down to us from the northern Andes from an altitude of over 3200 m . Australonyx , who lived in a mosaic of forests and savannahs at the end of the last glacial period, belong to the same family . as well as Nohochichak and Xibalbaonyx both are recorded from underwater caves on the Yucatán . In particular, the large and better-known forms were sometimes quite widespread at that time. The gigantic megatherium, weighing up to 6 t, populated predominantly higher or temperate regions in South America, while its close and similarly large relative Eremotherium populated the tropical and subtropical lowlands as far as the south of North America. The Mylodon , proven over large areas of South America , had the southernmost occurrence of all sloths with finds from the Cueva del Milodón in the Chilean province of Última Esperanza at 51 ° 35 'south latitude and from Tres Arroyos on Tierra del Fuego at 53 ° 23'. The erect up to 2.6 m high megalonyx , on the other hand, reached the northernmost find points in North America at about 68 ° north latitude and has been detected there in Yukon and Alaska, among others . It is also noteworthy that numerous forms underwent massive increases in body size during the course of the Pliocene and Pleistocene. The former representatives of Megalonyx reached a weight of around 184 kg, but later of just under 1.1 t, which means an increase of almost six times.

Extinction of the ground sloth

Skeleton of Megalocnus

Many of the sloths, which today only survived in fossil form, died out at the end of the Pleistocene in the course of the Quaternary extinction waves. For South America, the very young age values ​​for Mylodon come from the Cueva del Milodón in southern Chile and range from 11,330 to 13,630 BP , from the Cueva Chica, only a few kilometers away, from 10,780 to 14,240 BP. Late finds of Eremotherium in Itaituba on the Rio Tapajós , a tributary of the Amazon , date to 11,340 BP, while those of Nothrotherium - determined using a coprolith from the Gruta dos Breiões cave in the Brazilian state of Bahia - were 12,200 BP. For Megatherium , Glossotherium and Lestodon , age data could be determined at the Paso Otero site on the Río Quequén Grande in Argentina at 10,200 to 10,450 BP. Megatherium may have occurred even later, as data from Campo Laborde, also Argentina, at 8000 BP. and thus lie in the Lower Holocene . It would be the most recent evidence of a ground sloth from South America, but more recent dates shift the values ​​to 10,250 to 12,730 years BP. Less data are known for North America. Nothrotheriops remains from Rampart Cave at the Grand Canyon have been determined to be between 10,400 and 11,480 BP. A late record of Megalonyx from Lang Farm in Illinois is similarly old at 11,430 to 11,485 BP .

The extinction of numerous sloths in both North and South America coincided with the end of the last glacial period (110,000 to 10,000 years ago), which was accompanied by dramatic climate changes. It was at this point in time that modern humans first set foot on American soil and very quickly spread from North to South America. One of the earliest records of human activity in South America is the Monte Verde site in Chile, which is around 14,500 years old; Pedra Pintada in central Brazil is a little younger at 13,000 years . However, it is still unclear whether humans are actually responsible for the extinction of the sloths, and evidence that these animals were hunted is extremely rare. One of the few records comes from Quebrada de Quereo in northern Chile, where, in addition to numerous stone artifacts, remains of two individuals of a Mylodontid ( Mylodon or Glossotherium ) were discovered. In the aforementioned Campo Laborde, a Megatherium skeleton was apparently dismantled by early human settlers. Another clue is a processed tooth from Eremotherium from the São José farm in the Brazilian state of Sergipe , but it is unclear here whether the people only picked up the piece or killed the animal themselves. In North America, too, only a few sites are known where human remains and sloth remains are found, such as Kimmswick in Missouri , where the ossified skin shields of paramylodon were uncovered. A possible direct interaction between humans and large ground sloths was published in 2018 on the basis of step seals and tracks in the White Sands National Monument in New Mexico . The Ichnofossils are around 15,000 to 10,000 years old and show various overlapping traces. In several areas where human footprints meet those of ground sloths, the latter change from a dead straight to a circular arrangement, which suggests defensive postures of the animals. In view of the previous rarity of human-sloth interactions, the early American settlers do not need to have hunted the animals directly, changes in the landscape can also have indirectly led to the extinction of numerous lines of sloths.

It is noteworthy that sloths have survived much longer on some islands of the Caribbean than on the American mainland. Remnants of Neocnus from several sites on Hispaniola date to about 5260 to 4840 BP and 5300 to 4970 BP respectively. Are similar to 6350 to 4950 BP finds of young Parocnus from a Erdpechgrube in Cuba, even younger contrast, those of megalocnus with 4190 BP, produced through a tooth from a Abri in Havana . The age dates coincide with the first colonization of the Caribbean islands, but no human remains have been found with the sloths so far.

About the origin of today's sloths

The origin of today's sloths is unknown, fossil material and consequently predecessor forms are not available. The two- and three-toed sloths are with their way of life - almost complete life in trees, leaf-eating nutrition and the resulting metabolism - strongly adapted to the tropical rainforests and dependent on them. Their suspensory movement in the branches is unique. No predecessor or transitional forms are known for this type of locomotion, and within the large group of extinct sloths, no representative had a similar specialization. The ground sloths were adapted to numerous ways of life as a result of different niche use, the locomotion ranges from four-footed to sometimes bipedal running to climbing (in trees or rocks) to swimming, whereby tree-climbing fossil sloths, however, according to their skeleton structure, moved fundamentally different in the branches than the recent ones Species. Because of this and with a view to the fossil record, the earliest sloths may also have lived on the ground and / or climbed trees. The occurrence of today's tree sloths in the regions of America with dense tropical rainforests and their extreme adaptations to the biotope suggest that they also originated there. Genetic studies date the splitting of the two- and three-toed sloths back to the Oligocene around 30 million years ago. This result in connection with individual anatomical differences between the two recent genera leads to the assumption that the suspensory locomotion developed independently in each case. It is possible that the predecessor forms lived in a similar ecological environment during the Miocene or Pliocene, for example the "Proto-Amazonas", and gradually shaped the special way of life. When this actually developed is unclear; The diverse sloth fauna documented at numerous sites in the region, many of which were only discovered in the 21st century, has provided no evidence of suspensory gait to this day. From the lack of information in the fossil report so far, some scientists conclude that the head-hanging mode of locomotion is a rather late formation that may have emerged in the Pliocene.

nomenclature

The current scientific name Folivora for the sloth was introduced in 2001 by Frédéric Delsuc and research colleagues. The name is derived from the Latin words folium for "leaf" and vorare for "devour", thus referring to the animals' preferred food resource. But other names are also in use. The oldest available name is Tardigrada. This is also of Latin origin and means something like tardus ("slow") and gradus ("step"), and is therefore based on the slow movement of the sloth. Lazzaro Spallanzani used the name in a modified form ( il tardigrado ) in 1776 for the tardigrade class (the official correct name as Tardigrada dates back to the 1840s). Only around twenty years later, in 1795, was it officially established for sloths by John Latham and Hugh Davies as part of Johann Reinhold Forster's treatise Zoología indica . As early as 1762, Mathurin-Jacques Brisson had combined the two- and three-toed sloths under the generic name Tardigradus in his work Regnum animale in classes IX distributum sive Synopsis methodica . It is preceded by the compilation Thesaurus from 1734, in which the author Albert Seba referred to the white-throated sloth with Tardigradus . In 1842 Richard Owen introduced the term phyllophaga, which comes from the Greek and is composed of the words φύλλον ( phyllon "leaf") and φαγεῖν ( phageín "to eat"). Like Folivora, phyllophaga relates to diet. However, the name had previously been given in 1826 by Thaddeus William Harris for a genus of beetles . Since both Tardigrada and Phyllophaga are occupied by other animal groups, some scientists prefer the name Folivora for the sloth. However, the International Rules for Zoological Nomenclature do not prohibit higher-ranking homonyms , which is why both names are available and are still used in scientific publications to this day.

Research history

Skeleton reconstruction of Megatherium after Bru de Rámon 1793

The official scientific naming of today's sloths was given in 1758 by Linnaeus , where he combined the two recent genera in one, Bradypus . The term Bradypus had Linnaeus already in 1735 in the first edition of his Naturae Systema used the bulk of the information he took from Albert Sebas work Thesaurus of 1734. Nearly 40 years after Linnaeus' first description was Georges Cuvier , who in 1796 a complete skeleton of Megatherium described . This was discovered in the late 1780s near Luján in eastern Argentina and sent to the Real Gabinete de Historia Natural in Madrid . There was Juan Bautista Bru de Ramón prepared and set up in 1793, the first reconstructed according to scientific standards skeleton of an extinct vertebrate ever, but with an unnatural currently considered to pose. Cuvier found out about this skeleton and so wrote his description of one of the largest mammals in America. His publication is significant in several ways: on the one hand it is Cuvier's first publication on an extinct animal, on the other hand he noticed that a similar animal no longer exists in today's fauna, which was a revolutionary idea at the time. In addition, Cuvier established comparative anatomy with the help of the Megatherium skeleton . Only three years after Cuvier, Thomas Jefferson described the bones of a large animal with long claws from the US state of Virginia , which he assigned to a predator and which were recognized as belonging to Megalonyx in 1825 .

A major phase of sloth research began in 1832 when Charles Darwin reached South America on his journey with the HMS Beagle . By 1834, he collected more than 5000 fossil remains, mainly in Argentina, which he sent to the Royal College of Surgeons in England (but only 175 objects of the collection survived the intensive bombing of London in 1941 during the Second World War ). Richard Owen studied the fossils there from 1836, which led to the description of genera such as Glossotherium , Scelidotherium or Mylodon . Owens' work is still one of the standard works on the extinct ground sloth today. It was also largely Owen who separated the tree-dwelling sloths as Tardigrada from the ground-dwelling sloths as Gravigrada and put both together in the taxon Phyllophaga. Especially in the transition from the 19th to the 20th century, numerous sloth forms were discovered and described, for example by important researchers such as the brothers Carlos and Florentino Ameghino or John Bell Hatcher , who made multiple expeditions to South America. The view that today's tree sloths were closely related to one another and that they were separated from the ground sloths continued well into the 20th century. The American zoologist George Gaylord Simpson separated both groups of sloths from one another in his standard work on the classification of mammals in 1945 and referred all of today's sloths to the Bradypodidae family.

The assignment of all sloths living today to a family was largely based on their adaptation to life in the trees, as well as their unusual locomotion and some atypical skull features. Only at the end of the 1960s and the beginning of the 1970s was the mistake recognized, whereupon the two-toed sloths were separated from the Bradypodidae and relegated to the Megalonychidae. The enlarged and canine-shaped foremost tooth, the structure of the zygomatic bone and the lack of an ossified tympanic bubble on the middle ear spoke in favor of this re-examination of the relationship of the two-toed sloths . As a result, the respective adaptations to the head-hanging (suspensory) life in the treetops and the associated anatomical adaptations and overprinting of the body structure of the recent sloths are to be regarded as a convergent development that represents one of the most impressive within the entire class of mammals . In the 1990s, molecular genetic investigations were able to underpin the anatomically based, less close relationship between the two recent species of sloth by including fossil forms. Genetic and biochemical analyzes in 2019 clearly clarified the relationship between the recent and fossil sloths.

Sloths and humans

Actual two-toed sloth

As rainforest inhabitants, today's sloths are particularly threatened by slash and burn , which is carried out on a large scale to create fields and settlements. In addition, there is increased hunting of the animals, on the one hand as a source of food, but on the other hand also for illegal sale as pets, which often affects young animals. According to the IUCN , the two species of the two-toed sloth , the real and the Hoffmann two-toed sloth , are currently not threatened, as are the white-throated and brown-throated sloths from the three-toed sloth group , as all of the species mentioned have a fairly wide distribution area. The ring-necked sloth , which is only found in southeast Brazil , is considered threatened, while the pygmy sloth , known only from a small island off the east coast of Panama , is threatened with extinction. The population trend is unknown for the non-threatened species, but it is falling for the threatened species. It appears problematic that hardly any data are available on the dynamics of the various sloth populations, and in many cases the way of life of the individual species has not yet been researched. However, all species are represented in protected areas.

literature

• Richard M. Fariña, Sergio F. Vizcaíno and Gerardo de Iuliis: Megafauna. Giant beasts of Pleistocene South America. Indiana University Press, 2013, pp. 1-436 ISBN 978-0-253-00230-3
• Alfred L. Gardner (Ed.): Mammals of South America, Volume 1: Marsupials, Xenarthrans, Shrews, and Bats. University of Chicago Press, 2008 (pp. 157-164) ISBN 0-226-28240-6
• Ronald Nowak: Walker's Mammals of the World. Johns Hopkins University Press, Baltimore 1999, ISBN 0-8018-5789-9
• Luciano Varela, P. Sebastián Tambusso, H. Gregory McDonald and Richard A. Fariña: Phylogeny, Macroevolutionary Trends and Historical Biogeography of Sloths: Insights From a Bayesian Morphological Clock Analysis. Systematic Biology 68 (2), 2019, pp. 204-218
• Sergio F. Vizcaíno and WJ Loughry (Eds.): The Biology of the Xenarthra. University Press of Florida, 2008, pp. 1-370 ISBN 978-0-8130-3165-1
• Don E. Wilson and DeeAnn M. Reeder: Mammal Species of the World. Johns Hopkins University Press, Baltimore 2005, ISBN 0-8018-8221-4

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53. ↑ Frédéric Delsuc, Sergio F Vizcaíno and Emmanuel JP Douzery: Influence of Tertiary paleoenvironmental changes on the diversification of South American mammals: a relaxed molecular clock study within xenarthrans. BMC Evolutionary Biology 4 (11), 2004, pp. 1-13
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58. ^ ^{A b} François Pujos, Gerardo De Iuliis and Bernardino Mamani Quispe: Hiskatherium saintandrei, gen. Et sp. nov .: An Unusual Sloth from the Santacrucian of Quebrada Honda (Bolivia) and an Overview of Middle Miocene, Small Megatherioids. Journal of Vertebrate Paleontology 31 (5), 2011, pp. 1131-1149
59. ↑ ^{a b c} François Pujos and Gerardo de Iuliis: Late Oligocene Megatherioidea fauna (Mammalia: Xenarthra) from Salla-Luribay (Bolivia): New data on basal sloth radiation and Cingulata-Tardigrada split. Journal of Vertebrate Paleontology 27 (1), 2007, pp. 132-144
60. ↑ ^{a b} Samantha Presslee, Graham J. Slater, François Pujos, Analía M. Forasiepi, Roman Fischer, Kelly Molloy, Meaghan Mackie, Jesper V. Olsen, Alejandro Kramarz, Matías Taglioretti, Fernando Scaglia, Maximiliano Lezcano, José Luis Lanata, John Southon, Robert Feranec, Jonathan Bloch, Adam Hajduk, Fabiana M. Martin, Rodolfo Salas Gismondi, Marcelo Reguero, Christian de Muizon, Alex Greenwood, Brian T. Chait, Kirsty Penkman, Matthew Collins and Ross DE MacPhee: Palaeoproteomics resolves sloth relationships. Nature Ecology & Evolution 3, 2019, pp. 1121-1130, doi: 10.1038 / s41559-019-0909-z
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80. ↑ Gerardo de Iuliis, Guillermo H. Ré and Sergio F. Vizcaíno: The Toro Negro Megatheriine (Mammalia, Xenarthra): A new species of Pyramiodontherium and a review of Plesiomegatherium. Journal of Vertebrate Paleontology 24 (1), 2004, pp. 214-227
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82. ↑ Gerardo de Iuliis, Timothy J. Gaudin and Matthew J. Vicars: A new genus and species of nothrotheriid sloth (Xenarthra, Tardigrada, Nothrotheriidae) from late Miocene (Huayquerian) of Peru. Paleontology 54 (1), 2011, pp. 171-205
83. ↑ Ascanio D. Rincón, Andrés Solórzano, H. Gregory McDonald and Mónica Núñez Flores: Baraguatherium takumara, Gen. et Sp. Nov., the Earliest Mylodontoid Sloth (Early Miocene) from Northern South America. Journal of Mammalian Evolution 24, 2017, pp. 179-191
84. ↑ Ascanio D. Rincón, H. Gregory McDonald, Andrés Solórzano, Mónica Núñez Flores and Damián Ruiz-Ramoni: A new enigmatic Late Miocene mylodontoid sloth from northern South America. Royal Society Open Science 2, 2015, p. 140256, doi: 10.1098 / rsos.140256
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