''Paramylodon'' is an extinct genus of

ground sloth Ground sloths are a diverse group of extinct sloths in the mammalian superorder Xenarthra. They varied widely in size with the largest, belonging to genera '' Lestodon'', ''Eremotherium'' and ''Megatherium'', being around the size of elephants. ...

of the family Mylodontidae endemic to

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

during the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

epochs, living from around ~4.9 Mya–12,000 years ago. Within the genus only two species are recognized: ''Paramylodon harlani,'' also known as Harlan's ground sloth known from

Early Pleistocene The Early Pleistocene is an unofficial epoch (geology), sub-epoch in the international geologic timescale in chronostratigraphy, representing the earliest division of the Pleistocene Epoch within the ongoing Quaternary Period. It is currently esti ...

to the

Late Pleistocene The Late Pleistocene is an unofficial Age (geology), age in the international geologic timescale in chronostratigraphy, also known as the Upper Pleistocene from a Stratigraphy, stratigraphic perspective. It is intended to be the fourth division ...

(

Irvingtonian The Irvingtonian North American Land Mammal Age on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), spanning from 1.8 million – 250,000 years Before Present, BP.

- Rancholabrean) and the earlier

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Blancan The Blancan North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 4,750,000 to 1,806,000 years BP, a period of .Richard Harlan, in whose honor the species was named. The genus ''Paramylodon'' was introduced by

Barnum Brown Barnum Brown (February 12, 1873 – February 5, 1963), commonly referred to as Mr. Bones, was an American paleontologist. He discovered the first documented remains of ''Tyrannosaurus'' during a career that made him one of the most famous fossil ...

in the early 20th century. Over 150 years after the description of the first species, the finds that are now attributed to ''Paramylodon'' were repeatedly placed in with other genera, first with '' Mylodon'', but since the 1950s increasingly with '' Glossotherium''. ''Paramylodon'' shares numerous features that suggest a close relationship with ''Glossotherium''. Only since the 1990s have both genera been considered distinct, with ''Glossotherium'' restricted to

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

, while ''Paramylodon'' inhabited North America. The species ''Paramylodon harlani'' is known from remains found across North America, with abundant remains known from the

La Brea tar pits La Brea Tar Pits comprise an active Paleontological site, paleontological research site in urban Los Angeles. Hancock Park was formed around a group of tar pits where natural Bitumen, asphalt (also called asphaltum, bitumen, or pitch; ''brea'' ...

in California. Like some other mylodontids, ''Paramylodon'' had

osteoderm Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, temnospondyls (extinct amph ...

s embedded within its skin. ''Paramylodon'' lived in open landscapes, sometimes also in mountainous locations, and were grazers or mixed feeders. Preserved footprints are known. The morphology of the forelimbs has led to suggestions that ''Paramylodon'' may have engaged in burrowing. Like other ground sloths, ''Paramylodon'' became extinct around 14-12,000 years ago as part of the Late Pleistocene megafauna extinctions of most large mammals across the Americas. ''Paramylodon'' overlapped in time with Paleoindians, the earliest human inhabitants of the Americas, who may have hunted ''Paramylodon''. Its extinction may be the result of climatic change, hunting, or a combination of both factors.

Taxonomy

''Paramylodon'' is an extinct genus of sloth from the extinct family Mylodontidae. Mylodontidae is grouped together with modern two-toed sloths of the family Choloepodidae and the extinct

Scelidotheriidae Scelidotheriidae is a family of extinct ground sloths within the order Pilosa, suborder Folivora and superfamily Mylodontoidea, related to the other extinct mylodontoid family, Mylodontidae, as well as to the living two-toed sloth family Choloepo ...

, in the superfamily Mylodontoidea, with the former family being their closest living relatives. ''Paramylodon'' is usually considered closely related to both '' Mylodon'' and '' Glossotherium''. In contrast, a study presented in 2019 by Luciano Varela and other involved scientists, which includes numerous fossil forms of the entire sloth suborder, partially challenged this. In this study ''Paramylodon'' and ''Glossotherium'' were found to be closely related, ''Mylodon'', on the other hand, forms the basis of the advanced mylodonts and ''Lestodon'' clades with some forms from northern South America. In the same year, a more-detailed phylogenetic analysis of the mylodonts was published by a research group led by Alberto Boscaini. According to the study, ''Paramylodon'', ''Glossotherium'', and ''Mylodon'' form a closer relationship within the Mylodontinae. This view also finds support from the aforementioned biochemical data, also presented in 2019.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 und Ross D. E. MacPhee: ''Palaeoproteomics resolves sloth relationships.'' Nature Ecology & Evolution 3, 2019, S. 1121–1130, doi:10.1038/s41559-019-0909-z Detailed morphological analyses published as early as 2009 by Robert K. McAfee also suggest that ''Paramylodon'' and ''Glossotherium'' are very closely related and share a close common ancestor. Below is a phylogenetic tree of the Mylodontidae, based on the work of Boscaini ''et al.''. 2019, showing the position of ''Paramylodon''.

History of research

Discoveries in North and South America

The research history of ''Paramylodon'' is complex and characterized by more than 150 years of confusion with '' Mylodon'' and '' Glossotherium''. The history begins with the first discoveries of Richard Harlan (1796-1843) at Big Bone Lick in Boone County in the U.S. state of

Kentucky Kentucky (, ), officially the Commonwealth of Kentucky, is a landlocked U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders Illinois, Indiana, and Ohio to the north, West Virginia to the ...

in 1831, which included a right mandible and a

clavicle The clavicle, collarbone, or keybone is a slender, S-shaped long bone approximately long that serves as a strut between the scapula, shoulder blade and the sternum (breastbone). There are two clavicles, one on each side of the body. The clavic ...

. Harlan recognized that they were remains of an extinct sloth and referred them to '' Megalonyx'', which was already known from North America at the time, and within the genus to the species ''Megalonyx laqueatus'' that he had established shortly before.Richard Harlan: ''Description of the jaws, teeth, and clavicle of the Megalonyx laqueatus.'' The Monthly American journal of geology and natural science 1, 1831, S. 74–76Richard Harlan: ''Description of the jaws, teeth, and clavicle of the Megalonyx laqueatus.'' Medical and physical researches, 1835, S. 334–336 The finds were originally preserved in New York, but are now lost. Between the years 1831 and 1836,

Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English Natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...

made his pioneering voyage on HMS ''Beagle'' to

and brought back from there a large number of fossils. These were then studied by

Richard Owen Sir Richard Owen (20 July 1804 – 18 December 1892) was an English biologist, comparative anatomy, comparative anatomist and paleontology, palaeontologist. Owen is generally considered to have been an outstanding naturalist with a remarkabl ...

, one of the most important explorers of the Victorian era, and the results published. In a first publication on mammalian remains in general in 1840, he introduced the genus ''Mylodon'' with the species ''Mylodon darwinii''. The genus and species were based on a mandible Darwin found in Punta Alta in the

Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...

Buenos Aires Buenos Aires, controlled by the government of the Autonomous City of Buenos Aires, is the Capital city, capital and largest city of Argentina. It is located on the southwest of the Río de la Plata. Buenos Aires is classified as an Alpha− glob ...

province. As a special characteristic, a total of four molar-like teeth per tooth row stood out. At the same time, Owen also noted similarities in tooth structure between Harlan's mandible and that of ''Mylodon darwinii''. Inferring this, he discarded the name coined by Harlan, ''Megalonyx laqueatus'', and created a new species, ''Mylodon harlani''.Richard Owen: ''Fossil Mammalia.'' In Charles Darwin (Hrsg.): ''Zoology of the Voyage of H.M.S Beagle, under the command of Captain Fitzroy, during the years 1832 to 1836. Part I. Fossil Mammals.'' London, 1840, S. 12–111 (S. 63–73) The genus name ''Paramylodon'' is composed of the

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

παρα (para meaning "beside" or "near"), ''μύλη'' (''myle'' " molar") and ''ὀδούς'' (''odoús'' "tooth"), thus translates as "molar tooth". Harlan commented two years later on the use of the name because, in his opinion, it did not describe any outstanding characteristic of the animal and could mean any extinct mammal because almost all of them had the posterior molars.Richard Harlan: ''Notice of two new mammals from Brunswick Cana, Georgia; with observations on the fossil quadrupeds of the United States.'' The American journal of science and arts 43, 1842, S. 141–144

In the same year, 1842, Owen presented a comprehensive description of a skeleton of a mylodont that came from the flood plains of the

Río de la Plata The Río de la Plata (; ), also called the River Plate or La Plata River in English, is the estuary formed by the confluence of the Uruguay River and the Paraná River at Punta Gorda, Colonia, Punta Gorda. It empties into the Atlantic Ocean and ...

north of

; he established for it the new species ''Mylodon robustus''.Richard Owen: ''Description of the skeleton of an extinct gigantic Sloth, Mylodon robustus, Owen, with observations on the osteology, natural affinities, and probable habitats of the Megatherioid quadrupeds in general.'' London, 1842, S. 1–176 At this point, then, the genus ''Mylodon'' consisted of three species, two of which occurred in South America and one in North America. Furthermore, it should prove problematic that Owen identified ''Mylodon darwinii'' as the type species of the genus, although, as he admitted, this was the second known and described species after ''Mylodon harlani''. Accordingly, ''Mylodon harlani'' would actually be entitled to the status of

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

. Subsequently, different type species were assigned to ''Mylodon'', for example, Johannes Theodor Reinhardt in 1879 considered it to be ''Mylodon robustus'',

Richard Lydekker Richard Lydekker (; 25 July 1849 – 16 April 1915) was a British naturalist, geologist and writer of numerous books on natural history. He was known for his contributions to zoology, paleontology, and biogeography. He worked extensively in cata ...

in 1887, on the other hand, considered it to be ''Mylodon harlani''.Richard Lydekker: ''Catalogue of the fossil Mammalia in the British Museum of Natural History. Part V.'' London, 1887, S. 1–345 (S. 106)Juan Carlos Fernicola, Sergio F. Vizcaíno und Gerardo De Iuliis: ''The fossil mammals collected by Charles Darwin in South America during his travels on board the HMS Beagle.'' Revista de la Asociación Geológica Argentina 64 (1), 2009, S. 147–159

''Paramylodon'' and the ''Mylodon''-''Glossotherium'' problem

Within the genus ''Paramylodon'', only one

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

, ''P. harlani'', is recognized. Another species, ''P. nebrascensis'', was described in 1903 by

on the basis of a partial skeleton from Hay Spring in

Nebraska Nebraska ( ) is a landlocked U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders South Dakota to the north; Iowa to the east and Missouri to the southeast, both across the Missouri River; Ka ...

, but was already synonymised with the

in the 1920s. Only ten years later,

Glover Morrill Allen Glover Morrill Allen (February 8, 1879 – February 14, 1942) was an American zoologist. Born in Walpole, New Hampshire, the son of Reverend Nathaniel Glover Allen and Harriet Ann (Schouler) Allen, he studied at Harvard University. While still a ...

created the species ''Mylodon garmani'' with the help of another partial skeleton from the Niobrara River in Nebraska,Glover M. Allen: ''A new Mylodon.'' Memoirs of the Museum of Comparative Zoology 40 (7), 1913, S. 319–346 but this is also considered a

synonym A synonym is a word, morpheme, or phrase that means precisely or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are a ...

of ''Paramylodon harlani''. The same applies to several species named by

Edward Drinker Cope Edward Drinker Cope (July 28, 1840 – April 12, 1897) was an American zoologist, paleontology, paleontologist, comparative anatomy, comparative anatomist, herpetology, herpetologist, and ichthyology, ichthyologist. Born to a wealthy Quaker fam ...

as early as the 1870s and 1890s, such as ''Mylodon sodalis'' and ''Mylodon sulcidens''. The original subdivision into two subspecies, ''P. h. harlani'' for a robust and ''P. h. tenuiceps'' for a gracile form, as suggested by Chester Stock in 1917, is no longer advocated today. However, the species "''Glossotherium''" ''chapadmalense'' is problematic. The species was originally identified in 1925 by Lucas Kraglievich from a 39 cm long, nearly undamaged skull with mandible from Middle Pliocene strata east of Miramar in the Argentina

Buenos Aires province Buenos Aires, officially the Buenos Aires Province, is the largest and most populous Provinces of Argentina, Argentine province. It takes its name from the city of Buenos Aires, the capital of the country, which used to be part of the province an ...

.Lucas Kraglievich: ''Cuatro nuevos gravigrados de la fauna Araucana "Chapadmalense".'' Anales del Museo Nacional de Historia Natural de Buenos Aires 33, 1925, S. 215–235 It shows similarities to ''Glossotherium robustum'', but also possesses individual divergences that may justify its own generic status; for example, the name "''Eumylodon''" (which Kraglievich already used for ''Eumylodon chapadmalense'' in 1925) has been suggested. The form could thus be the common ancestor of ''Glossotherium'' and ''Paramylodon''. However, whether this also applies to the North American finds from the Pliocene of Florida and Mexico, first listed under the same species name by Jesse S. Robertson in 1976, or these are closer to ''Paramylodon'' is currently unclear due to lack of comparative studies. Partially, the early mylodont remains are also listed as ''P. garbanii'', a species name given in 1986 to some Pliocene mandible and limb remains from Arroyo EI Tanque in the Mexican State of Guanajuato had been coined (under the scientific name ''Glossotherium garbanii'').Gary S. Morgan: ''Vertebrate fauna and geochronology of the Great American Biotic Interchange in North America.'' New Mexico Museum of Natural History & Science Bulletin 44, 2008, S. 93–140Marisol Montellano-Ballesteros und Oscar Carranza-Castaneda: ''Descripcion de un milodontido del Blancano temprano de la mesa central de Mexico.'' Instituto de Geología, Universidad Nacional Autónoma de México, Revista 6, 1986, S. 193–203 The species is not fully recognized, however, and other authors consider it a synonym of ''Glossotheridium/Glossotherium chapadmalense''. In 1903,

(1873-1963) introduced the generic name ''Paramylodon''. He used for this purpose a partial skeleton from Hay Spring in

that had been discovered in 1897 during an expedition of the

American Museum of Natural History The American Museum of Natural History (AMNH) is a natural history museum on the Upper West Side of Manhattan in New York City. Located in Theodore Roosevelt Park, across the street from Central Park, the museum complex comprises 21 interconn ...

. To the genus he assigned ''Paramylodon nebrascensis'' as a species. As defining differences from the North American ''Mylodon harlani'', which Brown considered the type species of ''Mylodon'', he gave the missing anterior caniniform teeth in the upper jaw. Thus, at that time, two distinct representatives of Mylodonts were recognized in the Pleistocene of North America. Later, Chester Stock (1892-1950), based on his studies of the Rancho La Brea find material, pointed out that the feature of missing upper front teeth is highly variably developed in ''Mylodon harlani''. Therefore, synonymized he replaced ''P. nebrascensis'' with ''Mylodon harlani'' in 1917. In 1928, however, Lucas Kraglievich restricted North American finds to ''Paramylodon'' and thus separated the genus from its South American representatives,Lucas Kraglievich: "''Mylodon darwini" Owen, es la especie genotipo de "Mylodon" Ow.'' Physis: Revista de la Sociedad Argentina de Ciencias Naturales 9, 1928, S. 169–185 an opinion that was echoed eight years later by Ángel Cabrera but it found little resonance among most researchers in the following period. Kraglievich, in the same move, also revised '' Glossotherium'' as an independent genus to be distinguished from ''Mylodon''. '' Glossotherium'' had also originally been placed by Owen in his 1840 paper on Darwin's discoveries on the basis of a skull fragment from Arroyo Sarandí in southwestern present-day

Uruguay Uruguay, officially the Oriental Republic of Uruguay, is a country in South America. It shares borders with Argentina to its west and southwest and Brazil to its north and northeast, while bordering the Río de la Plata to the south and the A ...

, but only two years later he united it with ''Mylodon''. Subsequently, after Kraglievich and Cabrera, ''Glossotherium'' evolved into a "wastebasket" taxon on the basis of naming priority, into which numerous closely related forms were placed.

George Gaylord Simpson George Gaylord Simpson (June 16, 1902 – October 6, 1984) was an American paleontologist. Simpson was perhaps the most influential paleontologist of the twentieth century, and a major participant in the modern synthesis, contributing '' Tempo ...

expressed in his 1945 general taxonomy of mammals that if ''Paramylodon'' could not be clearly separated from ''Glossotherium'', ''Glossotherium'' would be preferable due to having priority over ''Paramylodon''.George Gaylord Simpson: ''The Principles of Classification and a Classification of Mammals.'' Bulletin of the American Museum of Natural History 85, 1945, S. 1–350 (S. 71) With the subsequent full inclusion of ''Paramylodon'' into the genus, accomplished by Robert Hoffstetter in 1952, ''Glossotherium'' was among the few sloth forms that occurred in South and North America, but it also possessed high variability as a result. Numerous researchers during the 20th century favored the view of the synonymy of the two sloth genera. In 1995, however, H. Gregory McDonald again separated the North American ''Paramylodon'' from the South American ''Glossotherium''. In doing so, he noted that there have been no studies to date that have demonstrated that the two genera are actually synonymous. In fact, the presence of ''Paramylodon'' in North America would rather distinguish it from ''Glossotherium'', which is, in turn, only known from South America. In the following time several skull studies could be presented, which clearly distinguished the two genera and plus ''Mylodon'' from each other.Diego Brandoni, Brenda S. Ferrero und Ernesto Brunetto: ''Mylodon darwini Owen (Xenarthra, Mylodontinae) from the Late Pleistocene of Mesopotamia, Argentina, with Remarks on Individual Variability, Paleobiology, Paleobiogeography, and Paleoenvironment.'' Journal of Vertebrate Paleontology 30 (5), 2010, S. 1547–1558 Some recent studies have questioned the attribution of the species ''P. garbanii'' to the genus.

Description

''Paramylodon'' measured about in length with a shoulder height of 1.8 meters and weighed as much as 1.5 metric tons. It is known from North America deposits, including in

Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...

and the United States and as far south as

Guatemala Guatemala, officially the Republic of Guatemala, is a country in Central America. It is bordered to the north and west by Mexico, to the northeast by Belize, to the east by Honduras, and to the southeast by El Salvador. It is hydrologically b ...

. ''Paramylodon'' exhibits the characteristic of having had dermal ossicles, small bones embedded in the skin, presumably adding a degree of protection to the animal. This characteristic is also shared by the South American '' Mylodon''.Otto Nordenskjöld (with the participation of other authors): Scientific results of the Swedish expedition to the Magellan lands 1895-1897, under the direction of Dr. Otto Nordenskjöld. Volume II: Zoology. Stockholm, 1899, pp. 1–170 (especially pp. 149–170)Wilhelm Branco: The application of X-rays in paleontology. Treatises of the Royal Prussian Academy of Sciences Berlin 1906, pp. 1–55 ''Paramylodon'' is abundant in numbers primarily because of the finds from Rancho La Brea in

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

. The material recovered there from several dozen individuals served as the basis of numerous investigations, on which the following descriptive information is largely based on.Chester Stock: ''Further observation on the skull structure of the mylodont sloths from Rancho La Brea.'' University of California Publications, Bulletin of the Department of Geology 10, 1917, S. 165–178Chester Stock: ''Structure of the pes in Mylodon harlani.'' University of California Publications. Bulletin of the Department of Geology 10 (16), 1917, S. 267–286Chester Stock: ''Cenozoic gravigrade edentates of western North America with special reference to the Pleistocene Megalonychinae and Mylodontidae of Rancho La Brea. Carnegie.'' Institute of Washington 331, 1925, S. 1–206 This genus was a medium-sized representative of the Mylodontidae. A completely reconstructed skeleton from Rancho La Brea shows a total length of 279 cm, of which the tail occupies about 118 cm. At the shoulders it reaches a height of 112 cm, and at the pelvis it measures 122 cm. ''Paramylodon'' weighs about 1.39

ton Ton is any of several units of measure of mass, volume or force. It has a long history and has acquired several meanings and uses. As a unit of mass, ''ton'' can mean: * the '' long ton'', which is * the ''tonne'', also called the ''metric ...

s, but earlier mylodontids were quite smaller.H. Gregory McDonald: ''Paleoecology of extinct Xenarthrans and the Great American Biotic Interchange.'' Bulletin of the Florida Museum of Natural History 45 (4), 2005, S. 313–333 Overall, ''Paramylodon'' represented a robustly built animal. It was characterized by an elongate skull, a short neck, a short and compact body with a broad pelvis, and strong limbs and tail.

Skull and dentition characteristics

Skull characteristics

The skull of ''Paramylodon'' was long and narrow. It reached a typical total length of . A particularly large skull measured 54.0 cm.T. D. A. Cockerell: ''A fossil ground-sloth in Colorado.'' University of Colorado Studies 6, 1909, S. 309–312 In plain view, it possessed a rather rectangular shape with an average width at the

occipital bone The occipital bone () is a neurocranium, cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone lies over the occipital lob ...

of 18.8 cm, behind the eyes of 12.2 cm, and at the snout of 14 cm. Typical for numerous mylodonts was the continuously widening snout towards the front. The skull, however, was shown to be altogether much narrower than in the comparably sized '' Glossotherium'', the latter showing a dome-like bulge at the frontal line in side view, which did not occur in ''Paramylodon''. However, the skull of ''Paramylodon'', with the exception of the middle region (the dome-like bulge in ''Glossotherium''), was on average higher, measuring about 13.8 cm at the occiput and 13 cm at the snout. The

nasal bone The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face and by their junction, form the bridge of the upper one third of the nose. Eac ...

was laterally in contact with the upper jaw. This created a nasal interior that was closed at the sides and open only at the front, becoming about as high as it was wide, which was due to the overall narrower skull. The diastema was, typical of sloths, only loosely connected to the upper jaw. At the

frontal bone In the human skull, the frontal bone or sincipital bone is an unpaired bone which consists of two portions.'' Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bo ...

, the nasal bone protruded far back, so that the suture between the two skull bones was rather V-shaped. In addition, the frontal bone represented the largest bone of the entire skull. A strong parietal crest existed between the parietal bones, but it appeared much narrower than compared to ''Glossotherium''. The

zygomatic arch In anatomy, the zygomatic arch (colloquially known as the cheek bone), is a part of the skull formed by the zygomatic process of temporal bone, zygomatic process of the temporal bone (a bone extending forward from the side of the skull, over the ...

es were secondarily closed again, deviating from most sloths. The anterior arch, originating at the

zygomatic bone In the human skull, the zygomatic bone (from ), also called cheekbone or malar bone, is a paired irregular bone, situated at the upper and lateral part of the face and forming part of the lateral wall and floor of the orbit, of the temporal fos ...

and pointing posteriorly, had three processes, one oriented upward, one downward, and the middle one horizontally. The posterior arch section, attached to the

temporal bone The temporal bone is a paired bone situated at the sides and base of the skull, lateral to the temporal lobe of the cerebral cortex. The temporal bones are overlaid by the sides of the head known as the temples where four of the cranial bone ...

, had a finger-like shape and joined the middle process of the anterior arch section. On the underside of the skull, the

palatine bone In anatomy, the palatine bones (; derived from the Latin ''palatum'') are two irregular bones of the facial skeleton in many animal species, located above the uvula in the throat. Together with the maxilla, they comprise the hard palate. Stru ...

protruded much further posteriorly in ''Paramylodon'' than in ''Glossotherium'', caused by the longer extension of the bone behind the last molar. As in many mylodonts, both flanks of the wing bones were distinctly inflated. In ''Paramylodon'', however, this was not quite as evident as in ''Glossotherium'', so the distended structures were much further apart through the basal phenoid of the sphenoids. Features that link ''Glossotherium'' and ''Paramylodon'' include, for example, the dentition structure with the anterior caniniform teeth and the tooth structure, such as of the second molar, or the position of the bony suture between the

and the maxilla near the posteriormost tooth. In contrast, ''Mylodon'' is more distinctly divergent, with its reduced dentition, more simply designed teeth, and the forward bone junction between the palatine and maxilla.

Dental characteristics

The lower jaw reached lengths of . It was massively built and broad. The horizontal bone body continuously increased in height from anterior to posterior, and below the posteriormost tooth its height was up to 10.5 cm.M. Susana Bargo, Gerardo de Iuliis und Sergio F. Vizcaíno: ''Hypsodonty in Pleistocene ground sloths.'' Acta Palaeontologica Polonica 51 (1), 2006, S. 53–61 The robust

symphysis A symphysis (, : symphyses) is a fibrocartilaginous fusion between two bones. It is a type of cartilaginous joint, specifically a secondary cartilaginous joint. # A symphysis is an amphiarthrosis, a slightly movable joint. # A growing together o ...

grew up to and was typical for mylodonts. It extended forward, which is a characteristic of almost all sloths. This spoon-like extension of the symphysis did not project laterally as distinctly in ''Paramylodon'' as in ''Glossotherium'', so that the lateral edges were rather straight and less distinctly curved than in the latter. The width of the symphysis in the anterior region was up to 15 cm. The articular process protruded only slightly above the masticatory plane, and the coronal process was much higher. Its anterior edge ran in a straight line in ''Paramylodon'', deviating from the curved design in ''Glossotherium''. The dentition consisted, as generally usual with the sloths, of 5 teeth per upper jaw half and 4 teeth per lower jaw half, thus altogether 18 teeth were formed. The front teeth had a canine-like (caniniform) shape, the others were molar-like (molariform). The dentition structure is considered phylogenetically primitive within the sloths. However, in later representatives of ''Paramylodon'' the upper caniniform teeth were often reduced, so that the dentition then consisted of only 16 teeth. A similar reduction of teeth is not known in ''Glossotherium''. In '' Mylodon'', on the other hand, the anteriormost teeth in the upper dentition were also no longer developed, but the lower caniniform teeth resembled the posterior molars. The caniniform teeth possessed an oval cross-section in ''Paramylodon'' and were curved backward. However, they did not reach the size as in ''Glossotherium'' or even in '' Lestodon''. A short diastema existed to the posterior row of teeth. The molar-like molars had a flat shape with a somewhat raised margin. They possessed a bilobed shape in outline with strong median constriction, except for the first maxillary molar, which was more rectangular in shape and formed the longest tooth in the maxilla with an average length of In the second maxillary molar, the lobe-like structure was much more prominent than in ''Glossotherium''. All teeth typically lacked enamel, instead consisting of a harder variant of

dentin Dentin ( ) (American English) or dentine ( or ) (British English) () is a calcified tissue (biology), tissue of the body and, along with tooth enamel, enamel, cementum, and pulp (tooth), pulp, is one of the four major components of teeth. It i ...

( orthodentin), with an additional outer layer of dental cement. The proportion of orthodentine reached 28% in ''Paramylodon.'' The upper row of teeth averaged 14.4 cm in length, of which the posterior molars occupied 12.6 cm. Due to the forward widening snout, the tooth rows diverged from each other.

Skeletal characteristics

The extensive find material from Rancho La Brea allows a comprehensive reconstruction of the body skeleton. The spine was composed of 7 cervical, 16 thoracic 8 to 9 lumbar, sacral, and 21 caudal vertebrae. The

humerus The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

was massive, the length was 46 cm, and the head did not stand out particularly clearly. A prominent bony ridge (deltopectoral groin) attached to the humeral shaft, but was less prominently developed in the upper part than in ''Glossotherium''. The lower end of the joint protruded widely laterally. An entepicondylar foramen, which occasionally occurred in some sloths, was not developed here. The

ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...

possessed a greatly expanded superior articular process, the

olecranon The olecranon (, ), is a large, thick, curved bony process on the proximal, posterior end of the ulna. It forms the protruding part of the elbow and is opposite to the cubital fossa or elbow pit (trochlear notch). The olecranon serves as a lever ...

. It grew to about 20 cm in length, the entire bone reaching 40 cm in length. The construction of the ulna appeared shorter and more robust than in ''Glossotherium'', the shaft was broad and narrowed above anteriorly and posteriorly. Likewise, the

radius In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...

was short and massive with a length of 29.6 cm. The longest bone was represented by the

femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...

at around 54.6 cm. Very short specimens from Rancho La Brea measured only 51 cm, very long 58 cm. The flat and broad design typical of ground sloths was striking, so that the bone appeared almost board-like. The head rose only slightly from the surface and had a more inward position. The shaft was slightly turned inward, and a third trochanter as a muscle attachment point, which appeared in '' Lestodon'', was not visible in ''Paramylodon''. With a length of 24.6 cm, the

tibia The tibia (; : tibiae or tibias), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two Leg bones, bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outsi ...

was significantly shorter than the femur. This is a typical feature of mylodonts, in whose predominantly late representatives the lower section of the hind leg often reached only about half the length of the upper. In the case of ''Paramylodon'', the tibia had 45% of the femur length. Its shaft was flattened like that of the femur and likewise exhibited a slight twist. The upper end of the joint was laterally projecting, the width here reaching about three-quarters of the length of the total bone. The

fibula The fibula (: fibulae or fibulas) or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. ...

was not fused to the tibia, reaching 26.3 cm in length.Barnum Brown: ''A new genus of ground sloth from the Pleistocene of Nebraska.'' Bulletin of the American Museum of Natural History 29, 1903, S. 569–583 Hands and feet showed a similar structure as in the other large mylodonts ''Glossotherium'' and ''Lestodon'', deviations are present in detail. The hand exhibited a total of five rays (I to V), with only the three inner rays (I to III) having developed claws. The

metacarpal In human anatomy, the metacarpal bones or metacarpus, also known as the "palm bones", are the appendicular bones that form the intermediate part of the hand between the phalanges (fingers) and the carpal bones ( wrist bones), which articulate ...

of the first ray was fused with the polygonal bone to form a single unit, which is frequently attested in ground-dwelling sloths (so-called ''metacarpal carpal complex'' or MCC). The metacarpal bones of the third to fifth rays were massive and over 10 cm long, with that of ray IV possessing the most robust construction. At the finger ray I the first two

phalanges The phalanges (: phalanx ) are digit (anatomy), digital bones in the hands and foot, feet of most vertebrates. In primates, the Thumb, thumbs and Hallux, big toes have two phalanges while the other Digit (anatomy), digits have three phalanges. ...

were additionally intergrown, at the rays II and III existed in each case three phalanges, of which the first two showed however clearly reduced lengths. The respective end members of the three inner rays had extended claw processes, which suggests correspondingly large claws. The length ranged from inner (I) to outer (III) from 7.5 cm to 15.4 cm to 17.4 cm, the height varied from 2.9 to 5.7 cm. The clawless outer fingers possessed phalanges greatly reduced in size. The foot of ''Paramylodon'' had a total of four rays (II to V), the innermost ray was completely reduced. Claws existed here only on toes II and III, which were also the most strongly developed. However, the metatarsals here had rather short lengths of 3.6 and 6.5 cm, respectively, at the outer rays they became over 11.0 cm long each and were very massive. As in the other two mylodonts, the second ray had only two phalanges, as the first and second phalanx were fused into one unit corresponding to the hand. Deviating from ''Glossotherium'' and ''Lestodon'', the third ray of ''Paramylodon'' also often consisted of only two limbs. The respective end phalanges with claws showed analogous to the hand an extremely strong construction, alone the claw process measured here about 8.5 cm at the second and 11.1 cm at the third ray and became in each case 3.3 and 3.9 cm high. The outer rays possessed opposite to this again strongly reduced terminal members.

Osteoderms

The mylodonts are the only known lineage of sloths whose representatives had bony platelets, so-called

s, formed in the skin, analogous to today's

armadillo Armadillos () are New World placental mammals in the order (biology), order Cingulata. They form part of the superorder Xenarthra, along with the anteaters and sloths. 21 extant species of armadillo have been described, some of which are dis ...

s. Unlike the latter, however, they did not form a solid bony armor in mylodonts, but were rather loosely scattered, as shown by finds of skin remains of ''Mylodon.'' Several hundred osteoderms of ''Paramylodon'' are known from Rancho La Brea,John C. Merriam: ''Recent discoveries of Quaternary mammals in Southern California.'' Science24, 1906, S. 248–250William J. Sinclair: ''Dermal Bones of Paramylodon from the Asphaltum Deposits of Rancho la Brea, near Los Angeles, California.'' Proceedings of the American Philosophical Society 49 (195), 1910, S. 191–195 in addition, among others, also as a dense layer on a slab from Anza-Borrego State Park in California and from Haile 15A, a fossil-rich

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

fissure in

Florida Florida ( ; ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders the Gulf of Mexico to the west, Alabama to the northwest, Georgia (U.S. state), Georgia to the north, the Atlantic ...

. The bone platelets were round to oval, sometimes irregularly shaped, and 5 to 30 mm long. They exhibited a rough surface with irregular depressions, whereas the underside was smooth and convex in design. In cross-section, they possessed a compact structure consisting of numerous fiber bundles mixed with hard bone lamellae ( osteomae). In principle, the bone platelets of mylodonts were simpler in structure than those of other

xenarthra Xenarthra (; from Ancient Greek ξένος, xénos, "foreign, alien" + ἄρθρον, árthron, "joint") is a superorder and major clade of placental mammals native to the Americas. There are 31 living species: the anteaters, tree sloths, and ...

ns.

Distribution and important fossil finds

Sites and specimen ages (not complete): *El Golfo de Santa Clara Site, Sonora, Mexico ~4.9—1.8 Mya. *Tri-Britton Site, Hendry County, Florida ~2.1 Mya.—700,000 years ago. *Turin Pit Site (or Elliott Pit) Monona County, Iowa ~1.8—300,000 years ago. *Port Kennedy Cave Site,

Montgomery County, Pennsylvania Montgomery County, colloquially referred to as Montco, is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population of the county was 856,553, making it the third-most populous county in Pennsylvania after Philadel ...

~1.8 Mya.—300,000 years ago. *Fairmead Landfill Site,

Madera County, California Madera County (), officially the County of Madera, is a County (United States), county located at the geographic center of the U.S. state of California. It features a varied landscape, encompassing the eastern San Joaquin Valley and the central ...

~1.8 Mya.—300,000 years ago. *Rio de la Pasion Site, Petén,

~100,000—11,000 years ago. *Fanno Creek Site, Tualatin,

Oregon Oregon ( , ) is a U.S. state, state in the Pacific Northwest region of the United States. It is a part of the Western U.S., with the Columbia River delineating much of Oregon's northern boundary with Washington (state), Washington, while t ...

~15,000—10,000 years ago. *Medicine Hat Unit V Site,

Alberta Alberta is a Provinces and territories of Canada, province in Canada. It is a part of Western Canada and is one of the three Canadian Prairies, prairie provinces. Alberta is bordered by British Columbia to its west, Saskatchewan to its east, t ...

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

~1.8 Ma—11,000 years ago.

Overview and early occurrences

''Paramylodon'' was

endemic Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...

distributed in

and possibly also in

Central America Central America is a subregion of North America. Its political boundaries are defined as bordering Mexico to the north, Colombia to the southeast, the Caribbean to the east, and the Pacific Ocean to the southwest. Central America is usually ...

. The oldest finds clearly assignable to the genus are known from the Lower

Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

. Older forms of mylodonts are from the Upper

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...

H. Gregory McDonald: ''Fossil Xenarthra of Mexico: a review.'' In: Marisol Montellano Ballesteros und Joaquín Arroyo Cabrales (Hrsg.): ''Avances en los Estudios Paleomastozoológicos en México.'' Serie Arqueología, Instituto Nacional de Antropología e Historia, Córdoba, 2002, S. 227–248 and from the US state of

. Of the latter, noteworthy is the partial skeleton from site Haile 15A, a crevice filled with

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

s in the

in Alachua County, estimated to be 2.1 to 1.8 million years old. These early representatives are commonly referred to as "''Glossotherium"'' ''chapadmalensis'',Jesse S. Robertson: ''Latest Pliocene mammals from Haile XVA, Alachua County, Florida.'' Bulletin of the Florida Museum of Natural History. 20, 1976, S. 111–186 although the position within the genus ''Glossotherium'' is disputed.H. Gregory McDonald: ''Gravigrade xenarthrans from the early Pleistocene Leisey Shell Pit 1A, Hillsborough County, Florida.'' Bulletin of the Florida Museum of Natural History 37, 1995, S. 345–373 Only slightly younger are the finds of the fossil-rich El Gulfo local fauna from the estuary of the

Colorado River The Colorado River () is one of the principal rivers (along with the Rio Grande) in the Southwestern United States and in northern Mexico. The river, the List of longest rivers of the United States (by main stem), 5th longest in the United St ...

in the Mexican state of

Sonora Sonora (), officially Estado Libre y Soberano de Sonora (), is one of the 31 states which, along with Mexico City, comprise the Administrative divisions of Mexico, Federal Entities of Mexico. The state is divided into Municipalities of Sonora, 72 ...

. They are already placed in ''Paramylodon'' and date to 1.8 to 1.6 million years ago.Fred W. Croxen III, Christopher A. Shaw und David R. Sussman: ''Pleistocene Geology and Paleontology of the Colorado River Delta at Golfo de Santa Clara, Sonora, Mexico.'' In: Robert E. Reynolds (Hrsg.): ''Wild, scenic & rapid-a trip down the Colorado River trough. The 2007 Desert Symposium field guide and abstracts from proceedings.'' California State University, 2007, S. 84–89 Overall, Lower and Middle Pleistocene fossil remains are relatively rare and come from about 20 localities in North America. These are distributed primarily in the southern and central areas of what is now the United States and northern Mexico, but also scatter in the western part of the continent as far south as the province of

. They are found in both lowland and mountainous locations, with the highest finding point reaching about 2900 m elevation in

Colorado Colorado is a U.S. state, state in the Western United States. It is one of the Mountain states, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas ...

.H. Gregory McDonald und Steve Pelikan: ''Mammoths and mylodonts: Exotic species from two different continents in North American Pleistocene faunas.'' Quaternary International 142/143, 2006, S. 229–241H. Gregory McDonald, Robert G. Dundas und James C. Chatters: ''Taxonomy, paleoecology and taphonomy of ground sloths (Xenarthra) from the Fairmead Landfill locality (Pleistocene: Irvingtonian) of Madera County, California.'' Quaternary Research 79, 2013, S. 215–227 One of the most significant sites of the period is the Leisey Shell pit in Hillsborough County in Florida, where several skulls and postcranial skeletal elements have been reported to be about 1.2 million years old.

Late Pleistocene remains

The find material of the Upper Pleistocene is much more extensive, coming from more than 100 localities, in

alone ''Paramylodon'' is known from more than 60 localities. The distribution of the genus is similar to the Lower Pleistocene, but in addition it occurs somewhat further east in the

Midwest The Midwestern United States (also referred to as the Midwest, the Heartland or the American Midwest) is one of the four census regions defined by the United States Census Bureau. It occupies the northern central part of the United States. It ...

, such as in

Iowa Iowa ( ) is a U.S. state, state in the upper Midwestern United States, Midwestern region of the United States. It borders the Mississippi River to the east and the Missouri River and Big Sioux River to the west; Wisconsin to the northeast, Ill ...

.H. Gregory McDonald: ''Harlan's Ground Sloth (Paramylodon harlani) (Xenarthra: Mylodontidae) from the Late Pleistocene (Rancholabrean) of Iowa.'' The Journal of the Iowa Academy of Science 119 (1-4), 2012, S. 16–21 The northernmost find of the genus is at Sequim in Washington state at 48.1° north latitude; to the south, the genus is also known from across Mexico. Some finds now indicate that ''Paramylodon'' may also have lived in

and

El Salvador El Salvador, officially the Republic of El Salvador, is a country in Central America. It is bordered on the northeast by Honduras, on the northwest by Guatemala, and on the south by the Pacific Ocean. El Salvador's capital and largest city is S ...

. Among others, finds of a juvenile and an adult individual were recovered from ''Stevenson Bridge'' in stream deposits of Putah Creek in Yolo County of California, dating to the beginning of the Last Glacial Period.Robert G. Dundas und Laura M. Cunningham: ''Harlan's ground sloth (Glossotherium harlani) and a Columbian mammoth (Mammuthus columbi) from Stevenson Bridge, Yolo County, California.'' PaleoBios 15 (3), 1993, S. 47–62 Two nearly complete skeletons have been reported from Shonto and Richville in

Arizona Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...

, among the few known finds from the state. In general, fossil remains of ''Paramylodon'' are very rare on the Colorado Plateau in the southwestern United States and additionally in northwestern Mexico, possibly related to the drier climate in this area at the time.H. Gregory McDonald, Larry D. Agenbroad und Carol Manganaro Haden: ''Late Pleistocene mylodont sloth Paramylodon harlani (Mammalia: Xenarthra) from Arizona.'' The Southwestern Naturalist 49 (2), 2014, S. 229–238 Of outstanding global importance, however, are the finds from the

tar pit Tar pits, sometimes referred to as asphalt pits, are large Bitumen, asphalt deposits. They form in the presence of petroleum, which is created when decayed organic matter is subjected to pressure underground. If this crude oil seeps upward via ...

s of Rancho La Brea in southern California. From here comes an extensive fossil fauna ranging in age from 45,000 to 14,000 years before present. The first finds were discovered as early as the second half of the 19th century, but the far more significant material is due to focused scientific investigations in the early 20th century, including a total of over 100 documented sites.Anthony R. Friscia, Blaire van Valkenburgh, Lillian Spencer und John Harris: ''Chronology and spatial distribution of large mammal bones in Pit 91, Rancho La Brea.'' Palaios 23(1), 2008, S. 35–42 A striking feature of the faunal spectrum is the unusual dominance of predators over herbivores. Most likely, the predators were attracted in greater numbers by animals stuck in the

asphalt Asphalt most often refers to: * Bitumen, also known as "liquid asphalt cement" or simply "asphalt", a viscous form of petroleum mainly used as a binder in asphalt concrete * Asphalt concrete, a mixture of bitumen with coarse and fine aggregates, u ...

and then fell victim to the natural traps themselves. Among the sloths, ''Paramylodon'', '' Megalonyx'', and '' Nothrotheriops'' are three of the four genera recorded in North America, with ''

Eremotherium ''Eremotherium'' (from Greek for "steppe" or "desert" "beast": ἔρημος "steppe or desert" and θηρίον "beast") is an extinct genus of giant ground sloth in the family Megatheriidae. ''Eremotherium'' lived in southern North America, Cen ...

'' being known only from the eastern part of the United States. However, ''Paramylodon'' represents by far the most abundant ground sloth at La Brea with over 70 individuals, and 30 skulls alone are notable among the finds.H. Gregory McDonald: ''Sexual dimorphism in the skull of Harlan's ground sloth.'' Contribution in Science 510, 2006, S. 1–9Leslie F. Marcus: ''A census of the abundant large Pleistocene mammals from Rancho La Brea.'' Contributions in Science 38, 1960, S. 1–11 Another very extensive fossil complex is present with the Diamond Valley Lake Local Fauna in Diamond Valley and Domenigoni Valley in Riverside County also in southern California. Material has been recovered during the construction of

Diamond Valley Lake Diamond Valley Lake is a man-made off-stream reservoir located near Hemet, California, United States. It is one of the largest reservoirs in Southern California and one of the newest. It has a capacity of . The lake nearly doubled the area's s ...

since the mid-1990s and currently includes more than 100,000 specimens from more than 100

taxa In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

, from more than 2600 different localities. In contrast to Rancho La Brea, large herbivores dominate here, while the a proportion of large predators is low. Thus, an undisturbed character of the faunal community can be inferred. ''Paramylodon'' is documented with about 280 individual finds, which represents about 8% of the total mammalian fauna. The ground sloth thus forms the fifth most abundant representative of mammals in the Diamond Valley Lake Local Fauna after bison, horses, the mastodon '' Mammut pacificus'', and the

camel A camel (from and () from Ancient Semitic: ''gāmāl'') is an even-toed ungulate in the genus ''Camelus'' that bears distinctive fatty deposits known as "humps" on its back. Camels have long been domesticated and, as livestock, they provid ...

Camelops ''Camelops'' is an extinct genus of camel that lived in North and Central America from the middle Pliocene (from around 4-3.2 million years ago) to the end of the Pleistocene (around 13-12,000 years ago). It is more closely related to living cam ...

''. In contrast, the two other sloths ''Megalonyx'' and ''Nothrotheriops'' that also occur in Rancho La Brea play are much less abundant, together accounting for 0.5% of the find record. The age of the Diamond Valley Lake Local Fauna corresponds to that of Rancho La Brea according to

radiocarbon dating Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for Chronological dating, determining the age of an object containing organic material by using the properties of carbon-14, radiocarbon, a radioactive Isotop ...

s.Kathleen Springer, Eric Scott, J. Christopher Sagebiel und Lyndon K. Murray: ''The Diamond Valley Lake Local Fauna: Late Pleistocene vertebrates from inland Southern California.'' Museum of Northern Arizona Bulletin 65, 2009, S. 217–236Kathleen Springer, Eric Scott, J. Christopher Sagebiel und Lyndon K. Murray: ''Late Pleistocene large mammal faunal dynamics from inland southern California: The Diamond Valley Lake local fauna.'' Quaternary International 217, 2010, S. 256–265 Paramylodon skull

Paleobiology

Body size change and sexual dimorphism

Like numerous other animal groups, ''Paramylodon'' underwent a marked increase in body size in the course of its

phylogeny A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or Taxon, taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, M ...

. The weight of members of the Lower

is given as about 915 kg, the late representatives from the Upper Pleistocene, however, probably reached up to 1.39 t body weight. Basis for the respective weight estimates are the

s, whose corresponding lengths are 48.4 and 54.6 cm, respectively. The earliest forms from the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58 It is particularly striking that especially in the late Pleistocene at the time of the Last Glacial Period with its extremely pronounced climatic fluctuations, there is hardly any variation in size, as studies of the numerous finds from Rancho La Brea dating from 45,000 to 10,000 years Before Present indicate. This is explained with a high flexibility of the genus in relation to the environment and thus a high adaptability. However, the assumption ignores the fact that increasingly cooler conditions should lead to an increase in body size according to the Bergmann's rule.Donald R. Prothero und Kristina R. Raymond: ''Stasis in the Late Pleistocene ground sloths (Paramylodon harlani) from Rancho La Brea Tar pits, California.'' New Mexico Museum of Natural History and Science Bulletin 53, 2011, S. 624–628 Based on the extensive fossil record from the late Pleistocene, two morphotypes can be distinguished in ''Paramylodon,'' a graceful variant and a robust variant. The morphotypes are not reflected in the general size of the skulls, but mainly concern their expression, for example in the width ratios. Also, differences can be found, for example, in the

, which is vertical in the robust variant, but oblique backward in the more graceful. Thus the joint surfaces for the attachment of the cervical spine are more prominently emphasized in the latter than in the former. Further deviations are found in the formation of the caniniform teeth which, if present, end pointedly in robust individuals, but bluntly in gracile ones. Possibly the two morphotypes are not species or

taxon In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

omic variations in the sense of subspecies, as it was originally assumed, because they often occur at one and the same locality. Rather, they are more an expression of intraspecific

sexual dimorphism Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

. However, it is currently impossible to assign a morphotype to a specific sex. In the 30 known skull finds from Rancho La Brea, the ratio of robust to gracile is 3:1; at the Americas Fall Reservoir in Idaho, with three skulls, the ratio is 2:1; and at Ingleside in

Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...

, also with three skulls, the ratio is 3:0. It is noteworthy here that sexual dimorphism is not reflected in the postcranial skeleton and thus, as noted earlier with the skull, no size dimorphism occurs in the form of significant length differences in limb bones. In contrast, ''

'', which was also common in North America at the same time but belongs to the

Megatheriidae Megatheriidae is a family of Extinction, extinct ground sloths that lived from approximately 23 Annum, mya—11,000 years ago. Megatheriids appeared during the Oligocene, Late Oligocene (Deseadan in the South American land mammal age, SALMA cl ...

, is known to have a pronounced size difference between the sexes.Donald R. Prothero und Kristina R. Raymond: ''Variation and sexual size dimorphism in Pleistocene ground sloths (Xenarthra).'' New Mexico Museum of Natural History and Science Bulletin 44, 2008, S. 331–333

Locomotion

In general, a quadrupedal locomotion is assumed for the ground-dwelling sloths. However, due to the body's center of gravity being shifted far to the rear, it was obviously also possible for them to change to a bipedal position, while being able to support themselves with the powerful - in contrast to today's tree sloths - very long tail. The hind foot of ''Paramylodon'' is turned inward, so that the main load when the foot is placed on the outer ray (V). This results in the pedolateral gait characteristic of numerous ground sloths, which required significant restructuring in the shape and bearing of the tarsal bones relative to each other, especially in the talus and

calcaneus In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel; : calcanei or calcanea) or heel bone is a bone of the Tarsus (skeleton), tarsus of the foot which constitutes the heel. In some other ...

. In ''Paramylodon'', the outer edge of the foot was little arched up, forming a more or less straight edge, and the calcaneus was in contact with the ground at nearly full length. This is consistent with other mylodonts, but differs greatly from the closely related

, which had a highly arched foot with only the posterior end of the calcaneus touching the ground.H. Gregory McDonald: ''Biomechanical inferences of locomotion in ground sloths: Integrating morphological and track data.'' New Mexico Museum of Natural History and Science Bulletin 42, 2007, S. 201–208H. Gregory McDonald: ''Evolution of the Pedolateral Foot in Ground Sloths: Patterns of Change in the Astragalus.'' Journal of Mammalian Evolution 19, 2012, S. 209–215 Another distinctive feature is found in the hind limbs. Here the locomotor system is characterized by an extremely short lower section. In ''Paramylodon'' the lower section reaches less than 50% of the upper. Such a construction plan, exhibited by nearly all mylodonts, suggests a rather slow and cumbersome locomotion. In comparison, the megatherians had significantly longer lower limb sections.

Trace fossil A trace fossil, also called an ichnofossil (; ), is a fossil record of biological activity by lifeforms, but not the preserved remains of the organism itself. Trace fossils contrast with body fossils, which are the fossilized remains of part ...

s that provide evidence of ground sloth locomotion are rarely preserved. For ''Paramylodon'', such definite stepping seals have been demonstrated at the Nevada State Prison near

Carson City Carson City, officially the Carson City Consolidated Municipality, is an independent city and the capital of the U.S. state of Nevada. As of the 2020 census, the population was 58,639, making it the 6th most populous city in the state. The m ...

. The tracks were discovered as early as the second half of the 19th century during

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

quarrying and initially, in 1882, were interpreted as evidence of giant humans. However, by the following year,

Othniel Charles Marsh Othniel Charles Marsh (October 29, 1831 – March 18, 1899) was an American professor of paleontology. A prolific fossil collector, Marsh was one of the preeminent paleontologists of the nineteenth century. Among his legacies are the discovery or ...

recognized a connection with extinct ground sloths and sought the originator of the stepping seals among the mylodonts, of which bone remains also exist from the same site.Othniel Charles Marsh: ''On the supposed human foot-prints recently found in Nevada.'' American Journal of Science 152, 1883, S. 139–140 In total, numerous tracks of other mammals - such as

mammoths A mammoth is any species of the extinct elephantid genus ''Mammuthus.'' They lived from the late Miocene epoch (from around 6.2 million years ago) into the Holocene until about 4,000 years ago, with mammoth species at various times inhabi ...

horses The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 milli ...

moose The moose (: 'moose'; used in North America) or elk (: 'elk' or 'elks'; used in Eurasia) (''Alces alces'') is the world's tallest, largest and heaviest extant species of deer and the only species in the genus ''Alces''. It is also the tal ...

, as well as predators - and additionally of birds have been discovered at Carson City in addition to ''Paramylodon''. The stepping seals are distributed over an area of about 8000 m², they are today largely covered by the prison construction, but well documented by casts. A total of ten tracks could be observed from ''Paramylodon'', consisting of 15 to 20 individual alternately impressed tracks. Each individual footprint is 47 to 51 cm long and about 20 cm wide, in outline they actually resemble human footprints, but they are much more laterally indented.Joseph LeConte: ''Carson footprints.'' Nature 1883, S. 101–102 Further studies showed that the shape of the prints corresponds very well with the shape of the foot of ''Paramylodon'' and that the outline reminiscent of humans is due to the outwardly turned foot of the sloth.Chester Stock: ''Origin of the supposed human footprints of Carson City, Nevada.'' Science 51, 1920, S. 514 The lateral distance between the tracks is about 60 cm, which roughly corresponds to the distance between the two hip sockets on the pelvis of ''Paramylodon'', as does the stride length of about 146 cm, which in turn corresponds to the known hind leg length of 95 cm. Strikingly, thereby almost exclusively hind footprints have survived, which was initially also interpreted with a bipedal locomotion of the animals, analogous to corresponding trace fossils of ''

Megatherium ''Megatherium'' ( ; from Greek () 'great' + () 'beast') is an extinct genus of ground sloths endemic to South America that lived from the Early Pliocene through the end of the Late Pleistocene. It is best known for the elephant-sized type spe ...

'' in South America. However, it was determined that the individual tread seals of the hind feet overlapped those of the forefeet. Because the anterior foot is much smaller than the posterior and sets on differently, it produces a much smaller impression. In individual cases, footprints have been preserved which show that the forefoot is covered by the hindfoot. For anatomical reasons, such as the bent legs when walking, the position and orientation of the hind foot to the leg, and similar, permanent bipedality of the large ground sloths is unlikely. The stride length of ''Paramylodon'' suggests an average speed of 1.8 to 2.2 m/s, which is similar to the determined speed of ''Megatherium.'' Studies of the scapulae of both young and adult individuals show a significant change in shape, from a more rounded form in the former to a largely ovoid form in the latter. The ontogenetic overprinting are comparable to those of present-day arboreal sloths. The similarity of the shoulder blades of the young representatives of ''Paramylodon'' and the young of present-day sloths suggests comparable behaviors. Accordingly, the young of ''Paramylodon'' still had various climbing abilities and possibly clung to their mothers during transport.

Diet

Feeding habits

Mylodonts are generally considered to be grazers (graminivores) based on their high-crowned teeth with flat chewing surfaces, similar to those of modern grazers. However, the lack of enamel in the teeth of sloths makes comparisons difficult. The graminivorous diet was inferred based on the special tooth formation, analyses of the masticatory apparatus of ''Paramylodon'' showed that food was predominantly crushed in forward, backward, and lateral chewing movements, which is also indicated by corresponding grinding marks. This is not contradicted by the caniniform front teeth, which - if formed - are rather small. The mandibular joint is broadly developed in ''Paramylodon'' and has an unspecialized surface, the associated glenoid fossa on the skull appears shallow, which is typical of herbivores with their rotary chewing movements. However, an additional, nearly vertical articular facet occurs on the inner side, anchored in a depression on the outer wall of the wing bone. This tended to limit overly strong lateral masticatory movements. The arrangement of the ''Musculus masseter'' caused ''Paramylodon'' to open its mouth only 22°, which is considerably less than in two-toed sloths, which, however, have comparatively longer caniniform teeth. All in all, the structure of the masticatory apparatus rather suggests a preference for mixed plant diets.Virginia L. Naples: ''The feeding mechanism in the Pleistocene ground sloth, Glossotherium.'' Contributions in Science Los Angeles County Museum of Natural History 415, 1989, S. 1–23M. Susana Bargo und Sergio F. Vizcaíno: ''Paleobiology of Pleistocene ground sloths (Xenarthra, Tardigrada): biomechanics, morphogeometry and ecomorphology applied to the masticatory apparatus.'' Ameghiniana 45 (1), 2008, S. 175–196

Feeding behaviour

The construction of the snout, which is not quite as broad as in '' Lestodon'', indicating a grass-based diet which is analogous to the living

white rhinoceros The white rhinoceros, also known as the white rhino or square-lipped rhinoceros (''Ceratotherium simum''), is the largest extant species of rhinoceros and the most Sociality, social of all rhino species, characterized by its wide mouth adapted f ...

, but significantly broader than in ''

'' and ''

'', which were both browsers, meaning that they ate the leaves of trees and bushes, which is comparable to that of the

black rhinoceros The black rhinoceros (''Diceros bicornis''), also called the black rhino or the hooked-lip rhinoceros, is a species of rhinoceros native to East Africa, East and Southern Africa, including Angola, Botswana, Eswatini, Kenya, Lesotho, Malawi, Moza ...

, which is also a browser.M. Susana Bargo, Néstor Toledo und Sergio F. Vizcaíno: ''Muzzle of South American Pleistocene Ground Sloths (Xenarthra, Tardigrada).'' Journal of Morphology 267, 2006, S. 248–263 The elongated symphysis of the mandible projects well beyond the nasal region. Since there is no ossification of the

nasal septum The nasal septum () separates the left and right airways of the Human nose, nasal cavity, dividing the two nostrils. It is Depression (kinesiology), depressed by the depressor septi nasi muscle. Structure The fleshy external end of the nasal s ...

as in '' Mylodon'', a vigorous cartilage development must be assumed here. In addition, the tongue may also have had a supporting function during feeding. Due to the position of the

hyoid bone The hyoid-bone (lingual-bone or tongue-bone) () is a horseshoe-shaped bone situated in the anterior midline of the neck between the chin and the thyroid-cartilage. At rest, it lies between the base of the mandible and the third cervical verte ...

, which was displaced far back in the skull, and its robust construction with strong muscle attachments, the geniohyoideus muscle, for example, was particularly strong and long, so that it could accommodate a very mobile tongue. Since no

coprolite A coprolite (also known as a coprolith) is fossilized feces. Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour (in this case, diet) rather than morphology. The name ...

s are known from ''Paramylodon'' in contrast to ''Mylodon'', the food remains cannot be determined directly. In addition, due to the lack of enamel, detailed isotopic examinations are rarely possible. Therefore, performing such methods requires excellent fossil preservation; in the case of ''Paramylodon,'' it was accomplished on the

dentary In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone ...

of several teeth from the Upper Pleistocene site of Ingleside, Texas. The results obtained from this using

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

isotopes fall between the ranges of present-day herbivores specializing in a hard grazing or soft browsing plant diet, thus advocating a mixed diet, but possibly with a stronger bias towards grasses. Thus, the results obtained so far are in good agreement with the open landscape in which ''Paramylodon'' lived. However, it is also hypothesized that the sloth representative may have been digging for roots. This is supported, for example, by the strong forelegs, which had a robust

widely projecting at the lower joint end, a short

with a long extended

for massive forearm musculature, and somewhat flattened claws, making them very well suited for digging. In addition, differences in dentition between early and late members of the genus are apparent. For example, Lower Pleistocene forms possessed even less high dental crowns and a correspondingly lower mandible, while those of the Upper Pleistocene had significantly higher teeth and a more massive mandible. It is possible that this reflects an increasingly strong adaptation of ''Paramylodon'' over time.

Social behaviour

The majority of the finds of ''Paramylodon'' are composed of single individuals, mass assemblages as for example in Rancho La Brea represent accumulations over several millennia. It can therefore be assumed that ''Paramylodon'', like present-day arboreal representatives, lived solitary and at most formed mother-young groups. The digestive system was probably similarly structured as in living sloths, so that also in ''Paramylodon'' a rather slow metabolism prevailed with long passage time of the food, which was highly digested. The suggested slow locomotion this suggest that they did not engage in larger seasonal migrations - in contrast to numerous other grass-eaters inhabiting open landscapes. According to this, the animals were relatively sedentary. A striking feature of numerous sites with ''Paramylodon'' is the frequent co-occurrence with the Columbian mammoth and the

bison A bison (: bison) is a large bovine in the genus ''Bison'' (from Greek, meaning 'wild ox') within the tribe Bovini. Two extant taxon, extant and numerous extinction, extinct species are recognised. Of the two surviving species, the American ...

. Both representatives of large herbivorous mammals, however, showed a different way of life with their herd formations and long migrations as well as deviating digestive systems. Therefore, ''Paramylodon'' probably used a different

ecological niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of Resource (biology), resources an ...

to avoid direct

competition Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, indi ...

with the other megaherbivores of the North American

steppe In physical geography, a steppe () is an ecoregion characterized by grassland plains without closed forests except near rivers and lakes. Steppe biomes may include: * the montane grasslands and shrublands biome * the tropical and subtropica ...

landscapes.

Extinction

Like most other ground sloths, ''Paramylodon'' became extinct at the end of Late Pleistocene as part of the Late Pleistocene megafauna extinctions. Unlike many other genera, however, there is little

radiometric Radiometry is a set of techniques for measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power in space, as opposed to photometric techniques, which ch ...

data available from ''Paramylodon'' measured directly from fossil material. One of the youngest known calibrated radiocarbon dates is from the locality of Gypsum Cave in Nevada, which dates to 14,603–14,213 years

Before Present Before Present (BP) or "years before present (YBP)" is a time scale used mainly in archaeology, geology, and other scientific disciplines to specify when events occurred relative to the origin of practical radiocarbon dating in the 1950s. Because ...

. The youngest date from Rancho La Brea is around 13,995–13,763 cal years Before Present, the youngest direct date for the species as of 2023. However, some more possible recent finds are known, few of which came to light from archaeological sites associated with early human settlement on the North American continent. More than 130

osteoderms Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of Extant taxon, extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, Temnospondyli, ...

have been documented from the Aubrey Clovis site in north-central Texas. The soil substrate surrounding the finds has been radiometrically dated to an age of 12,860 years BP. Stone artifacts also documented there, comprising about 9800 pieces, can also be referred to the

Clovis culture The Clovis culture is an archaeological culture from the Paleoindian period of North America, spanning around 13,050 to 12,750 years Before Present (BP). The type site is Blackwater Draw locality No. 1 near Clovis, New Mexico, where stone too ...

on the basis of a Clovis point. The remains of ''Paramylodon'', however, have no direct relation to the early settlers, having been found in a nearby waterhole with the exception of a single bone platelet.C. Reid Ferring: ''The Archaeology and Paleoecology of the Aubrey Clovis Site (41DN479) Denton County, Texas.'' Center for Environmental Archaeology, Department of Geography, University of North Texas, Denton, 2002, S. 1–276Donald K. Grayson und David J. Meltzer: ''Clovis Hunting and Large Mammal Extinction: A Critical Review of the Evidence.'' Journal of World Prehistory 16 (4), 2002, S. 313–359 Remains are also known contemporaneous to Clovis culture artifacts from the Kimmswick site in Missouri, though they do not bear cut marks. It is unclear from the few common finds to date whether direct hunting led to their extinction. That early colonizers of North America interacted with, followed, as well as possibly hunted large ground sloths is indicated by footprints from White Sands National Monument in

New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...

. Here, several hundred footprints of larger sloths are associated with those of humans on the shore of a former lake. In some cases they overlap, and in one case the human tracks lie within the track of a sloth. A striking feature of the intersecting sloth and human tracks is an abrupt change of direction in the former, suggesting a direct confrontation by the causers. However, no fossil remains are available from the site and the age of the tracks has so far only been indirectly dated (between 15,560 and 10,000 years before present). In addition, the sloth tracks have not been determined more precisely. They show strong size variations, which can be attributed either to animals of different ages or to different species. During the period in question, in addition to ''Paramylodon'', '' Nothrotheriops'', a smaller ground sloth from the

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 ...

group, and '' Megalonyx'', a large genus of the Megalonychidae, occurred in the region.

References

External links

Sloth World: an on-line bibliographyPage Museum: info on ground sloths from the La Brea tar pits
{{Taxonbar, from=Q1057386 Prehistoric sloths Prehistoric placental genera Pliocene xenarthrans Pleistocene xenarthrans Pliocene mammals of North America Pleistocene mammals of North America Blancan Irvingtonian Rancholabrean Pleistocene Canada Paleontology in Alberta Fossils of Canada Pleistocene California Geology of Los Angeles County, California Paleontology in California Pleistocene Mexico Fossils of Mexico Pleistocene Guatemala Fossils of Guatemala Fossil taxa described in 1903 Taxa named by Barnum Brown