Extinct Ground Sloth, Tardigrada April 2009

Fact Summary Taxonomy and Nomenclature Distribution and Habitat Physical Characteristics Behavior and Ecology Diet and Feeding Reproduction and Development Diseases and Pathology Web Resources Bibliography

Describer (Date): Paramylodon harlani - Owen 1840
                           Nothrotheriops shastensis - 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".
• Originally described as Mylodon harlani tenuiceps at Rancho la Brea (Stock 1917), McDonald (1996) said these fossils should be in a separate genus, Paramylodon.

• 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. (Gaudin 2004)
• All ground sloths are extinct today.

• Paramylodon: widely distributed across U.S., especially in western states (excluding the Colorado Plateau) (McDonald et al 2004)
• Nothrotheriops shastensis: 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.

• Paramylodon: 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)
• Megalonyx: Occupied 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)

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)

• 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

• 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.

• 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.

•  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)

• 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.

• 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.
• University of California's Paleontology Portal: Learn about famous fossil sites, exploring time and space, K-12 teachers' resources, careers
• 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.
• The Tarkio Valley Sloth Project: Web site with in-depth discussions of ground sloth fossil excavations and recent studies in Iowa.