TAXONOMY & NOMENCLATURE * *How Do We Know This? Like living animals, fossil remains of once-living animals
classified and grouped according to their relationships
each other and to their ancestors.
(Castañeda & Miller 2004) (DeMuizon et al 2004) (Flynn et al 2005) (Gaudin 2004) (McDonald 1996) (McDonald & De Iuliis 2008)
(McKenna & Bell 1997) (McKenna et al 2006) (Steadman et al 2005)
Describer (Date):Paramylodon harlani - Owen 1840 Nothrotheriopsshastensis - Sinclair 1905 Megalonyx jeffersonii - Wistar 1822 Kingdom: Animalia Phylum: Chordata Class: Mammalia Magnorder: Xenarthra (living and extinct armadillos, anteaters, and sloths; extinct glyptodonts) Order: Pilosa (anteaters and sloths) Suborder: Tardigrada (includes all sloths) Family: Megatheriidae (family is extinct) Family: Mylodontidae (family is extinct) Genus: Paramylodon Species:Paramylodon harlani Family: Nothrotheriidae (family is extinct) Genus: Nothrotheriops Species:Nothrotheriops shastensis Family: Megalonychidae (family has extinct and living members) Genus: Megalonyx (extinct) Species:Megalonyx jeffersoni Genus: Choloepus - Two- toed Sloth(Living) Family: Bradypodidae - Three-toed Sloth Genus: Bradypus - Three- toed Sloth(Living) Family: Myrmecophagidae - includes living Collard and Giant Anteater (family has extinct and living members) Family: Cyclopedidae - includes living Silky Anteater (family has extinct and living members)
Taxonomic History and Nomenclature
Giant ground sloths were some of the first animals to be recognized as extinct
In 1796 George Cuvier described a ground sloth skeleton of Megatherium ("Great Beast") from South America
Cuvier saw characters in the bones and teeth of these fossils that were shared with the group of living animals such as tree sloths, anteaters and armadillos
Sloths, both living and extinct, armadillos, anteaters, and extinct glyptodonts are now termed xenarthrans ("strange joints" referring to the extra points of articulation in their vertebrae)
The diversity of sloths in the fossil record is high; over 100 genera are recognized (McKenna
& Bell 1997) with 19 genera in late Pleistocene sediments (Steadman et al 2005)
McKenna and Bell (1997) established a new sloth Infraorder, Megatheria, with the nothrotheres in a separate tribe, Nothrotheriini
DeMuizon et al (2004) formally recognized nothrotheres in their own family, Nothrotheriidae; this convention is followed by most researchers today
Mckenna et al (2006) suggest Tardigrada should be used for "the most recent common ancestor of Bradypus and Choleopus plus all of its descendants".
Earliest record of a possible sloth is of a single tooth from Eocene rocks on Seymour Island, near Antarctica; other sloth fossils also from Eocene rocks, in Patagonia (McDonald & De Iuliis 2008)
The divergence of the Bradypus and Choloepus lineages may be ancient, dating back as much as 40 million years. (Gaudin 2004)
Sloths are known to have been in Patagonia 35 million years ago (Oligocene) (McDonald & I2008)
By 8 or 9 million years ago (Late Miocene) sloths had dispersed into islands in the Caribbean and North America.
Pliometanastes is the earliest known ground sloth in North America,
Dated to 8 million years ago in Nuevo Leon, Mexico. (Castañeda & Miller 2004)
Found in 7 million year-old (Miocene) Florida rocks.
Megalonyx recorded from 4.8 million year old (Early Pliocene) rocks in Mexico.(Flynn et al 2005)
This date is earlier than known previously and suggests the so-called Great American Biotic Interchange (= North American species going south and South American species moving north) saw South American species like Megalonyx arriving well before North American animals dispersed south.
Megalonyx survived until between 11-12,000 years ago. (Steadman et al 2005).
Nothrotheriops survived until 10,400 to 11,480 years ago. (Steadman et al 2005)
90% of sloth genera became extinct at the end of the Pleistocene (Steadman et al 2005)
Dwarf megalonychid sloth species the size of a cat survived the megafauna extinction on West Indian islands
These remains have been dated at 4,400 years ago, just after the arrival of humans on those islands.
Two-Toed Tree Sloth (Choloepus) may be descended from the extinct megalonychid sloths from the West Indies (Gaudin 2004)
Three-toed Tree Sloth (Bradypus) may be a sister-taxon to all other sloths.
All giant ground sloths are extinct today.
DISTRIBUTION & HABITAT * *How Do We Know This? Scientists use knowledge of the earth's rocks, global plate tectonic movements,
and the chemical process of fossilization
to make sense of fossil distribution patterns and ancient habitats.
(Hoganson & McDonald 2007) (McDonald 1985) (McDonald et al 2004) (Schubert et al 2004)(Spaulding et al 1983)
Paramylodon: widely distributed across U.S., especially in western states (excluding the Colorado Plateau) (McDonald et al 2004)
primarily western U.S. but also in Florida, Texas, Mexico. (McDonald 1985)
Megalonyx : wide distribution in over 150 sites in United States, including Alaska, northwestern Canada, Mexico
Fossils of three taxonomic families of sloths were found at Rancho La Brea asphalt deposits in California.
Not found in dry habitats; preferred areas of open grass or parkland; perhaps associated with permanent water in rivers and lakes from sea level to 2,330 m (7,644 ft) (Spaulding et al 1983) (McDonald 2004)
Nothrotheriops: Found in habitats that are deserts today and were drier in the Pleistocene. (McDonald et al 2004); open savanna scrub land with trees; deciduous forests. (Hoganson & McDonald 2007)
gallery forests along rivers or lakes (Hoganson & McDonald 2007)
One skeleton from Illinois found with fossil pollen representing a spruce forest transitional to a mixed hardwood forest. (Schubert et al 2004)
PHYSICAL CHARACTERISTICS* *How Do We Know This? Careful study of fossil bone or tooth anatomy yields much exact information
about placement and strength of muscles, tendons, ligaments, nerves, and blood vessels.
In rare cases, skin and hair impressions or actual skin or hair is preserved.
Body weight is more difficult to gauge because fat leaves no impression on the skeleton.
(Bargo et al 2000) (Hill 2006) (McDonald 2004, 2006) (Bargo et al 2006) (McNab 1985) (Naples 1989) (Vizcaíno 2008)
Estimated Body Weight:Paramylodon harlani: varied estimates from 1,000 kg (2,205 lb) to 1,089 kg (2,400 lb)
Nothrotheriops shastensis:estimated 250 kg (551 lb) Megalonyx jeffersoni: estimated near 1,000 kg (2,205 lb)
Body Length: Paramylodon harlani: 3 m (9.8 ft.) - bison sized
Nothrotheriops shastensis: - black bear sized
Megalonyx jeffersoni: 3 m (9.8 ft) - bison sized, slightly smaller than Paramylodon
Giant ground sloths were a diverse groups of large, hairy extinct sloths with massive jaws, blunt snouts, and powerful clawed limbs.
One huge North American ground sloth from Florida, Eremotherium, grew up to 6 m (20 ft) in length
Megatherium americanum grew to nearly 11 feet standing height, weighing 6 metric tons (13,228 lb) (Vizcaíno et al 2008)
Forelimbs are shorter than hindlimbs.
Feet are twisted inward when walking ( McDonald 1977)
Living tree sloths are not good analogues for studying physiology, behavior, or morphology of extinct ground sloths; the two groups are very different.
Teeth are high crowned (hypsodont) and open rooted
The extreme height of the teeth may have been necessary for grazing abrasive grass and withstanding large amounts of grit from the soil. (Bargo et al 2006)
Teeth all resemble molars (molariforn) (Bargo et al 2006)
Teeth lack enamel; are composed of dentine held together by cementum. (Bargo et al 2006)
Nothrotheriops has no teeth towards the front of the jaw but Paramylodon and Megalonyx did (Bargo et al 2006)
All ground sloths lack incisors. (Naples 1989)
Hair length of South American mylodontid sloth fossil preserved in dry cave in Argentina: up to 50 mm on back of head, 70-100 mm on flank, 220 mm on rear limbs
Hair length of Nothrotheriops: 45 mm average. (McNab 1985)
Two sizes of skeletal elements; not known which sex is larger. (McDonald 2004) but perhaps the male in Paramylodon harlani. (McDonald 2006)
Other Physical Characteristics
Sloths in the mylodontid family had small simple isolated bony deposits (osteoderms) imbedded in their skin (Hill 2006)
Osteoderms preserve well after the animal dies
Living pangolins and armadillos also have osteoderms, but theirs are highly modified and fit together in complex patterns (Hill 2006)
Many other groups of animals have osterderms, including lizards, some dinosaurs, crocodiles
Three large powerful claws on sloths' hands; probably for digging as well as defense.
Claws of Paramylodon wide, somewhat triangular; presumably for digging
Claws of Megalonyx are somewhat sharp; perhaps for grasping or defense
BEHAVIOR & ECOLOGY* *How Do We Know This? Since direct observation of a fossil animal's behavior isn't possible, paleontologists
comparison and contrast with living animals for guidance. Tracks can sometimes
reveal further clues.
(Fariña and Blanco 1996)(France et al 2007)(McDonald & DeIuliis 2008)(McDonald & Pelikan 2005) (Vizcaíno et al 2001) (Vizcaíno et al 2008)
Descriptions of giant ground sloths in the past emphasized their relationship to tree sloths, with slow plodding travel on all fours but more recently with new discoveries and analyses, more movements and behaviors are believed possible, including tree-climbing, swimming, burrowing. (McDonald & DeIuliis 2008)
Small dwarf ground sloths in the West Indies may have been arboreal. (Steadman et al 2005)
Some sloths in the extinct family Nothrotheriidae might have been unspecialized swimmers, perhaps like capybara (Vizcaíno et al 1999)
Several South American ground sloth fossils were studied for the possible functioning of the limbs (Vizcaíno et al 2008)
Researchers using trackways and biomechanical calculations said Megatherium americanum and some mylodontid sloths could walk upright (were bipedal).
If ground sloths were bipedal, then arms were free for other tasks such as digging and/or defense.
Farina and Blanco (1996) suggested Megatherium arms and claws weren't used for digging, but instead were capable of fast, aggressive action perhaps in defense.
By contrast with Megatherium, two mylodontid sloths analyzed seemed to be very well suited for powerful digging and they may have produced burrows seen in Pleistocene sediments exposed along sea cliffs in Argentina. (Vizcaíno et al 2001)
Paramylodon harlani are often found with mammoths in North America, sharing a common habitat; not known how they avoided competition for plant resources. (McDonald & Pelikan 2005)
Researchers who examined a typical Late Pleistocene fauna from Virginia did not find much difference in the plant diets of all the large herbivores, including ground sloths; how these animals avoided competition was a puzzle.
(France et al 2007)
Niche compaction and collapse
may be reflected in these results, leading to eventual extinction or local extinction (extirpation) of many of the animals.
DIET & FEEDING* *How Do We Know This? Clues to fossil mammals' diets come from teeth,skull shape,
from fossil dung and gut contents, from
lab analysis of oxygen
in bone and teeth,
looking at diets of similar modern animals.
(Bargo et al 2006) (Coltrain et al 2004)(Fariña and Blanco1996) (France et al 2007) (Hansen 1978) (Martin 1975)(Poinar et al 1998) (Ruez 2005)
There are no modern analogues for ground sloths. (France et al 2007)
Ground sloths traditionally seen as non-ruminant herbivores with hindgut fermentation, subsisting on a wide variety of plants.
(France et al 2007)
A great deal is known about some species of fossil sloths' diets from deposits of dung found in caves in the southwestern U.S. (Poinar et al 1998)
Plant pollen in Shasta ground sloth (Nothrotheriops) dung from Rampart Cave, Arizona studied by Hansen (1978) contained 72 genera of plants; favored plants were Desert Globe Mallows (Sphaeralcea ambigua) and Mormon Tea (Ephedra nevadensis)
Dung from Gypsum Cave, Nevada of Shasta ground sloth analyzed for plant DNA revealed six families and two orders of plants, with plants in the caper family and the lilly family (probably yucca and agave) being common. (Poinar 1998)
Harlan's Ground Sloth (Paramylodon harlani) variously interpreted as a grazer, a browser, or a mixed feeder but looking at dentin isotopes Ruez ( 2005) said it was a mixed feeder with grass being one or the main component of the diet.
From tooth studies (Bargo 2001), anatomical studies of elbows (Fariña & Blanco 1996) conclude Megatherium from South America may have been omnivouous, consuming meat by either hunting or scavenging; other researchers have suggested diets with animal protein for other sloth species. (France et al 2007)
France et al (2007)
looked at carbon and nitrogen isotopes in sloth bone collagen and concluded that a Megalonyx from Virginia was primarily a herbivore, but that it might have have consumed a small amount of meat.
Other researchers examined isotopes from sloths at Rancho La Brea and said Paramylodon harlani looks like a strict herbivore. (Coltrain et al 2004)
Paramylodon and other mylodon sloths postulated to have dug with its powerful claws and arms for roots and tubers (Bargo et al 2006)
This digging behavior in search of food added grit to the diet which may have been one factor influencing evolution of their extremely high crowned teeth.
REPRODUCTION & DEVELOPMENT * *How do We Know This? Isotope studies of elements present fossil bones and tusks
about timing of reproductive stress, and timing of nursing.
Clues to stages of development come from tooth
replacement patterns and closure of sutures
in skull and limb bones.
A fossil sloth related to the Shasta ground sloth, found in a cave in Brazil, had a single foetus associated with the adult. (Auler & Smart 1999)
Shasta ground sloth juveniles found in many caves in southwestern U.S. (Hill & Gillette 1985)
A Megalonyx jeffersoni adult found in Iowa (Brenzel & Semken 2006) had two associated juveniles of different ages, suggesting extended parental care.
Ground sloths juveniles would have been vulnerable to the large cat predators (Smilodon, Homotherium, Panthera atrox) and perhaps Dire Wolves
Not known why ground sloths became extinct, but some researchers suggest humans had something to do with it. (Steadman et al 2007)
DISEASES AND PATHOLOGY*
*How do We Know This? Abnormalities in fossil bones may show
evidence of arthritis, cancer, nutritional stress, fractures and more.
A skeleton of Megatherium with pathologically fused teeth was once mistakenly thought to be a new species.
Some examples of trauma and infections have been noted.
Fusion of digits has been noted, but these are not necessarily arthritic changes and are thought to be evolutionary trends.
Important Web Resources (including where to view fossils in museums):
The Paleobiology Database: This site is a scientific organization run by paleontological researchers from around the world. It features taxonomic and distribution information for the entire fossil record.
George Page Museum at La Brea Tar Pits(affiliated with Los Angeles County Museum): Web site has virtual exhibits and information on an array of Pleistocene fossils. Page Museum exhibits exceptionally well-preserved fossils from the tar pits. A complete fossil skeleton of Paramylodon harlani is on permanent exhibit.
San Diego Natural History Museum's Fossil Field Guide: Web site with major Pleistocene mammals (and fossils from other time periods) from California described and illustrated. The museum also exhibits several Pleistocene fossils, casts, and original models of Pleistocene animals together with seven original murals of Pleistocene plants and animals from southern California.
A life-sized cast skeleton of Paramylodon is displayed under a mural by paleoartist William Stout of two species of ground sloths in California 20,000 years ago.
Fossil skeletons of giant ground sloths can also be seen at the Smithsonian's National Museum of Natural History in Washington D.C., at the Florida State Museum of Natural History in Gainesville, Florida and at the natural history museums in London, Paris, and Buenos Aires.