Megalonychid ground sloth
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Ground sloths are a diverse group of
extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
sloths in the
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
ian
superorder Order ( la, ordo) is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and ...
Xenarthra. The term is used to refer to all extinct sloths because of the large size of the earliest forms discovered, compared to existing tree sloths. The Caribbean ground sloths, the most recent survivors, lived in the Antilles, possibly until 1550 BCE. However,
radiocarbon dating Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon. The method was dev ...
suggests an age of between 2819 and 2660 BCE for the last occurrence of ''
Megalocnus ''Megalocnus'' ("great sloth" in Greek) is a genus of extinct large ground sloths that were native to Cuba during the Pleistocene and Holocene epochs. They were among the largest of the Caribbean sloths (Megalocnidae), with individuals estimate ...
'' in Cuba. Ground sloths had been extinct on the mainland of North and South America for 10,000 years or more. They survived 5,000–6,000 years longer in the
Caribbean The Caribbean (, ) ( es, El Caribe; french: la Caraïbe; ht, Karayib; nl, De Caraïben) is a region of the Americas that consists of the Caribbean Sea, its islands (some surrounded by the Caribbean Sea and some bordering both the Caribbean Se ...
than on the American mainland, which correlates with the later colonization of this area by humans. Much ground sloth evolution took place during the late
Paleogene The Paleogene ( ; British English, also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period, geologic period and system that spans 43 million years from the end of the Cretaceous Period million yea ...
and
Neogene The Neogene ( ), informally Upper Tertiary or Late Tertiary, is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period million years ago ( Mya) to the beginning of the present Quaternary Period Mya. ...
of South America, while the continent was isolated. At their earliest appearance in the fossil record, the ground sloths were already distinct at the family level. The presence of intervening islands between the American continents in the Miocene allowed a dispersal of forms into North America. A number of mid- to small-sized forms are believed to have previously dispersed to the Antilles. They were hardy as evidenced by their diverse numbers and dispersals into remote areas given the finding of their remains in Patagonia (
Cueva del Milodón Natural Monument Cueva del Milodón Natural Monument is a Natural Monument located in the Chilean Patagonia, northwest of Puerto Natales and north of Punta Arenas. The monument is situated along the flanks of Cerro Benitez. It comprises several caves a ...
) and parts of Alaska. Sloths, and xenarthrans as a whole, represent one of the more successful South American groups during the
Great American Interchange The Great American Biotic Interchange (commonly abbreviated as GABI), also known as the Great American Interchange and the Great American Faunal Interchange, was an important late Cenozoic paleozoogeographic biotic interchange event in which lan ...
. During the interchange, many more taxa moved from North America into South America than in the other direction. At least five genera of ground sloths have been identified in North American fossils; these are examples of successful immigration to the north.


Families

Paleontologists assign more than 80 
genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...
of ground sloths to multiple families.


Megalonychidae

The megalonychid ground sloths first appeared in the Late Eocene, about 35 million years ago, in Patagonia. Megalonychids first reached North America by island-hopping, prior to the formation of the
Isthmus of Panama The Isthmus of Panama ( es, Istmo de Panamá), also historically known as the Isthmus of Darien (), is the narrow strip of land that lies between the Caribbean Sea and the Pacific Ocean, linking North and South America. It contains the country ...
. Some lineages of megalonychids increased in size as time progressed. The first species of these were small and may have been partly tree-dwelling, whereas the Pliocene (about 5 to 2 million years ago) species were already approximately half the size of the huge
Late Pleistocene The Late Pleistocene is an unofficial Age (geology), age in the international geologic timescale in chronostratigraphy, also known as Upper Pleistocene from a Stratigraphy, stratigraphic perspective. It is intended to be the fourth division of ...
'' Megalonyx jeffersonii'' from the last ice age. Some West Indian island species were as small as a large cat; their dwarf condition typified both tropical adaptation and their restricted island environment. This small size also enabled them a degree of arboreality.J.L. White (1993) ''
Megalonyx ''Megalonyx'' (Greek, "large claw") is an extinct genus of ground sloths of the family Megalonychidae, native to North America during the Pliocene and Pleistocene epochs. It became extinct during the Quaternary extinction event at the end of the ...
'', which means "giant claw", was a widespread North American genus that lived past the close of the last (Wisconsin) glaciation, when so many large mammals died out. Remains have been found as far north as Alaska and the Yukon. Ongoing excavations at Tarkio Valley in southwestern Iowa may reveal something of the familial life of ''Megalonyx''. An adult was found in direct association with two juveniles of different ages, suggesting that adults cared for young of different generations. The earliest known North American megalonychid, '' Pliometanastes protistus'', lived in the southern U.S. about 9 million years ago and is believed to have been the predecessor of ''Megalonyx''. Several species of ''Megalonyx'' have been named; in fact it has been stated that "nearly every good specimen has been described as a different species".Harrington (1993) A broader perspective on the group, accounting for age, sex, individual and geographic differences, indicates that only three species are valid (''M. leptostomus'', ''M. wheatleyi'', and ''M. jeffersonii'') in the late Pliocene and Pleistocene of North America, although work by McDonald lists five species. Jefferson's ground sloth has a special place in modern paleontology, for Thomas Jefferson's letter on ''Megalonyx'', read before the American Philosophical Society of Philadelphia in August 1796, marked the beginning of vertebrate paleontology in North America. When
Lewis and Clark Lewis may refer to: Names * Lewis (given name), including a list of people with the given name * Lewis (surname), including a list of people with the surname Music * Lewis (musician), Canadian singer * "Lewis (Mistreated)", a song by Radiohead ...
set out, Jefferson instructed Meriwether Lewis to keep an eye out for ground sloths. He was hoping they would find some living in the Western range. ''Megalonyx jeffersonii'' was appropriately named after Thomas Jefferson.


Megatheriidae

The megatheriid ground sloths are relatives of the megalonychids; these two families, along with the family Nothrotheriidae, form the infraorder Megatheria. Megatheriids appeared later in the Oligocene, some 30 million years ago, also in South America. The group includes the heavily built '' Megatherium'' (given its name 'great beast' by
Georges Cuvier Jean Léopold Nicolas Frédéric, Baron Cuvier (; 23 August 1769 – 13 May 1832), known as Georges Cuvier, was a French natural history, naturalist and zoology, zoologist, sometimes referred to as the "founding father of paleontology". Cuvier ...
) and '' Eremotherium''. The skeletal structure of these ground sloths indicates that the animals were massive. Their thick bones and even thicker joints (especially those on the hind legs) gave their appendages tremendous power that, combined with their size and fearsome claws, provided a formidable defense against predators. The earliest megatheriid in North America was ''Eremotherium eomigrans'' which arrived 2.2 million years ago, after crossing the recently formed Panamanian land bridge. With more than five tons in weight, 6 meters in length, and able to reach as high as , it was larger than an
African bush elephant The African bush elephant (''Loxodonta africana'') is one of two extant African elephant species and one of three extant elephant species. It is the largest living terrestrial animal, with bulls reaching a shoulder height of up to and a body ...
bull. Unlike relatives, this species retained a
plesiomorphic In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorphy, apomorphy, and ...
extra claw. While other species of ''Eremotherium'' had four fingers with only two or three claws, ''E. eomigrans'' had five fingers, four of them with claws up to nearly a foot long.


Nothrotheriidae

Recently recognized, ground sloths of
Nothrotheriidae Nothrotheriidae is a family of extinct ground sloths that lived from approximately 17.5 mya—10,000 years ago, existing for approximately . Previously placed within the tribe Nothrotheriini or subfamily Nothrotheriinae within Megatheriidae, they ...
are often associated with those of the Megatheriidae, and together the two form the superfamily Megatherioidea. The most prominent members of the group are the South American genus '' Thalassocnus'', known for being aquatic, and ''
Nothrotheriops ''Nothrotheriops'' is a genus of Pleistocene ground sloth found in North America, from what is now central Mexico to the southern United States. This genus of bear-sized xenarthran was related to the much larger, and far more famous ''Megatherium ...
'' from North America. The last ground sloths in North America belonging to ''Nothrotheriops'' died so recently that their
subfossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...
dung has remained undisturbed in some caves. One of the skeletons, found in a
lava tube A lava tube, or pyroduct, is a natural conduit formed by flowing lava from a volcanic vent that moves beneath the hardened surface of a lava flow. If lava in the tube empties, it will leave a cave. Formation A lava tube is a type of lava ca ...
(cave) at
Aden Crater Aden Crater is a small shield volcano located in Doña Ana County, about southwest of Las Cruces, New Mexico. It is located in the northwest part of the Aden-Afton basalt field, which is part of the central area of the Potrillo volcanic field ...
, adjacent to
Kilbourne Hole Kilbourne Hole is a maar volcanic crater, located west of the Franklin Mountains of El Paso, Texas, in the Potrillo volcanic field of Doña Ana County, New Mexico. Another maar, Hunt's Hole, lies just two miles south of Kilbourne Hole. Kilbour ...
, New Mexico, still had skin and hair preserved, and is now at the Yale Peabody Museum. The largest samples of ''Nothrotheriops'' dung can be found in the collections of the Smithsonian Museum. Another ''Nothrotheriops'' was excavated at
Shelter Cave ''Shelter Cave'' is an archaeological and paleontological site located in Doña Ana County, New Mexico. Description The site is a rock shelter well up on the western side of Bishop's Cap, an outlier of the Organ Mountains. It lies about 450  ...
, also in
Doña Ana County Don (; ; pt, Dom, links=no ; all from Latin ', roughly 'Lord'), abbreviated as D., is an honorific prefix primarily used in Spain and Hispanic America, and with different connotations also in Italy, Portugal and its former colonies, and Croatia ...
, New Mexico.


Mylodontidae

The mylodontid ground sloths together with their relatives the scelidotheriids form the Mylodontoidea, the second radiation of ground sloths. The discovery of their fossils in caverns associated with human occupation led some early researchers to theorize that the early humans built corrals when they could procure a young ground sloth, to raise the animal to butchering size. However, radiocarbon dates do not support simultaneous occupation of the site by humans and sloths.
Subfossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...
remains like coproliths, fur and skin have been discovered in some quantities. The
American Museum of Natural History The American Museum of Natural History (abbreviated as AMNH) is a natural history museum on the Upper West Side of Manhattan in New York City. In Theodore Roosevelt Park, across the street from Central Park, the museum complex comprises 26 inter ...
has exhibited a sample of '' Mylodon'' dung from Argentina with a note that reads "deposited by Theodore Roosevelt".


Scelidotheriidae

The ground sloth family Scelidotheriidae was demoted in 1995 to the subfamily Scelidotheriinae within Mylodontidae. Based on collagen sequence data showing that its members are more distant from other mylodontids than Choloepodidae, it was elevated back to full family status in 2019. Together with Mylodontidae, the enigmatic '' Pseudoprepotherium'' and two-toed sloths, the scelidotheriids form the superfamily Mylodontoidea. ''
Chubutherium ''Chubutherium'' is an extinct genus of ground sloth from the Late Oligocene and Early Miocene of Chubut Province, Argentina. Description This species of ground sloth was assigned to the family Scelidotheriidae, which was subsequently demote ...
'' is an ancestral and very plesiomorphic member of this subfamily and does not belong to the main group of closely related genera, which include '' Scelidotherium'' and ''
Catonyx ''Catonyx'' is an extinct genus of ground sloth of the family Scelidotheriidae, endemic to South America during the Pliocene and Pleistocene epochs. It lived from 2.5 Ma to about 10,000 years ago, existing for approximately . The most recent dat ...
''.


Orophodontidae

The formerly recognized ground sloth family
Orophodontidae Mylodontidae is a family of extinct South American and North American ground sloths within the suborder Folivora of order Pilosa, living from around 23 million years ago (Mya) to 11,000 years ago. This family is most closely related to another f ...
constitutes a rather small but quite distinct group. They have been classified as a distinct mylodontan
superfamily SUPERFAMILY is a database and search platform of structural and functional annotation for all proteins and genomes. It classifies amino acid sequences into known structural domains, especially into SCOP superfamilies. Domains are functional, str ...
Orophodontoidea, the sister taxon to the Mylodontoidea. The taxon is now often disused with genus members reassigned to Megalonychidae and Mylodontidae.


Phylogeny

The following sloth family phylogenetic tree is based on collagen and mitochondrial DNA sequence data (see Fig. 4 of Presslee ''et al''., 2019).


Biology

Sloths that had longer snouts are presumed to have had greater olfactory acuity, but appear to have also had less binocular vision and poorer ability to localize sounds. A number of extinct sloth species are thought to have had hearing abilities optimized for low frequencies, perhaps related to use of infrasound for communication. A 2021 study on isotope proposed '' Mylodon'' and some other ground sloths must have been at least sporadically omnivorous. Many species of ground sloth are considered armored mammals, because of the bone fragments embedded in their skin


Extinction

Radiocarbon dating places the disappearance of ground sloths in what is now the United States at around 11,000 years ago. The Shasta ground sloth (''
Nothrotheriops ''Nothrotheriops'' is a genus of Pleistocene ground sloth found in North America, from what is now central Mexico to the southern United States. This genus of bear-sized xenarthran was related to the much larger, and far more famous ''Megatherium ...
shastensis'') visited Rampart Cave (located on the Arizona side of the Lake Mead National Recreation Area) seasonally, leaving behind a massive stratified
subfossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...
ized dung deposit, and seemed to be flourishing from 13,000 until 11,000 BP, when the deposition suddenly stopped. Steadman ''et al.'' argue that it is no coincidence that studies have shown that ground sloths disappeared from an area a few years after the arrival of humans. Trackways preserved in New Mexico (probably dating from 10 to 15.6 thousand years ago) that appear to show a group of humans chasing or harassing three ''
Nothrotheriops ''Nothrotheriops'' is a genus of Pleistocene ground sloth found in North America, from what is now central Mexico to the southern United States. This genus of bear-sized xenarthran was related to the much larger, and far more famous ''Megatherium ...
'' or '' Paramylodon'' ground sloths may record the scene of a hunt. The tracks are interpreted as showing seven instances of a sloth turning and rearing up on its hind legs to confront its pursuers, while the humans approach from multiple directions, possibly in an attempt to distract it. Those who argue in favor of humans being the direct cause of the ground sloths' extinction point out that the few sloths that remain are small sloths that spend most of their time in trees, making it difficult for them to be spotted. Although these sloths were well hidden, they still would have been affected by the climate changes that others claim wiped out the ground sloths. Additionally, after the continental ground sloths disappeared, insular sloths of the Caribbean survived for approximately 6,000 years longer, which correlates with the fact that these islands were not colonized by humans until about 5500 yr BP. It is difficult to find evidence that supports either claim on whether humans hunted the ground sloths to extinction. Removing large amounts of meat from large mammals such as the ground sloth requires no contact with the bones; tool-inflicted damage to bones is a key sign of human interaction with the animal.


Hunting of ground sloths


Hunting weapons

Humans are believed to have entered the New World via
Beringia Beringia is defined today as the land and maritime area bounded on the west by the Lena River in Russia; on the east by the Mackenzie River in Canada; on the north by 72 degrees north latitude in the Chukchi Sea; and on the south by the tip ...
, a land bridge which connected Asia and North America during the
last glacial maximum The Last Glacial Maximum (LGM), also referred to as the Late Glacial Maximum, was the most recent time during the Last Glacial Period that ice sheets were at their greatest extent. Ice sheets covered much of Northern North America, Northern Eur ...
. Mosimann and Martin (1975) suggested the first of these nomads descended from hunting families who had acquired the skills to track down and kill large mammals. By this time, humans had developed early hunting weapons, including the Clovis points, which were narrow, carved stone projectiles used specifically for big game. A couple of hundred years later, the atlatl became widely used, which allowed them to throw spears with greater velocity. These inventions would have allowed hunters to put distance between them and their prey, potentially making it less dangerous to approach ground sloths.


Advantages

Certain characteristics and behavioral traits of the ground sloths made them easy targets for human hunting and provided hunter-gatherers with strong incentives to hunt these large mammals. Ground sloths often fed in open fields. Recent studies have attempted to discover the diet of ground sloths through fossils of their dung. Analysis of these coproliths have found that ground sloths often ate the foliage of trees, hard grasses, shrubs, and yucca; these plants were located in areas that would have exposed them, making them susceptible to human predation. Ground sloths were not only easy to spot, but had never interacted with humans before, so would not have known how to react to them. Additionally, these large mammals waddled on their hind legs and front knuckles, keeping their claws turned in. Their movement and massive build (some weighed up to ) imply they were relatively slow mammals. These reasonable after-the-fact inferences from the evidence might explain why ground sloths would have been easy prey for hunters, but are not certain.


Difficulties

While ground sloths would have been relatively easy to spot and approach, big game hunters' weapons would have been useless from farther than thirty feet away. It would have been difficult to take down a ground sloth with a spear-thrower and would have required extensive knowledge of the species. Additionally, the ground sloths' already thick hide was fortified by osteoderms, making it difficult to penetrate. Since ground sloths thrived in an environment filled with large predators, they evidently would have been able to also defend themselves against human predation, so there is no reason to expect that they would have been "easy pickings". When feeding, they had enough strength to use their long, sharp claws to tear apart tree branches; presumably their strength and formidable claws would be dangerous for hunters that attempted to attack them at close quarters.


References


Sources

*; (2): 227–228. * * Harrington, C.R. (1993)
Yukon Beringia Interpretive Center – Jefferson's Ground Sloth
Retrieved 2008-JAN-24. * Hogan, C.M. (2008)
''Cueva del Milodon'', Megalithic Portal
Retrieved 2008-APR-13 * Kurtén, Björn and Anderson, Elaine (1980): ''Pleistocene Mammals of North America.'' Columbia University Press, New York. * McKenna, Malcolm C. & Bell, Susan K. (1997): ''Classification of Mammals Above the Species Level.'' Columbia University Press, New York. * Nowak, R.M. (1999): ''Walker's Mammals of the World'' (Vol. 2). Johns Hopkins University Press, London. * * *


External links


Picture and information about a ground sloth skeleton on display at the University of Georgia's Science Library.







''Eremotherium'' in Florida.





Western Center for Archaeology and Paleontology
Hemet, CA {{Taxonbar, from=Q933583 Clawed herbivores Eocene first appearances Mammal common names Miocene xenarthrans Oligocene xenarthrans Pleistocene xenarthrans Pliocene xenarthrans Prehistoric mammals of South America Prehistoric mammals of North America Prehistoric sloths