Paramylodon

''Paramylodon'' is an extinct genus of ground sloth of the family Mylodontidae endemic to North America during the Pliocene epochs, living from around ~4.9 Mya–11,000 years ago. It is also known as Harlan's ground sloth.

Due to the partly very good fossil preservation, the body armor typical for the mylodonts has mostly been preserved in the form of osteoderms. In addition, a sex-specific difference in cranial structure can be demonstrated. According to the finds, the representatives of ''Paramylodon'' lived in open landscapes, sometimes also in mountainous locations, and most likely fed on grass-eating or mixed plant diets. Unique trace fossils also provide information about the locomotion of the animals, which was quadrupedal. In addition, due to the design of the forelimbs, a certain burrowing way of life cannot be disputed.

Within the genus only two species are recognized: ''Paramylodon harlani'' and ''Paramylodon garbanii''. The first fossil findings date back to the beginning of the 1830s. They go back to Richard Harlan, in whose honor the species was named. The genus ''Paramylodon'' was introduced by Barnum Brown in the early 20th century. Altogether, this genus looks back on an eventful history of research. In their first 150 years, the finds that are now attributed to ''Paramylodon'', and later the genus itself, were repeatedly associated with other forms, first with ''Mylodon'', but since the 1950s increasingly with ''Glossotherium''. Especially with the latter genus ''Paramylodon'' shares numerous features that argue for a close relationship. Only since the 1990s have both genera been considered distinct, with ''Glossotherium'' restricted to South America, while ''Paramylodon'' inhabited North America.

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, in the superfamily Mylodontoidea, with the former family being their closest living relatives.

''Paramylodon'' is closely related to ''Mylodon'' and ''Glossotherium''. In some later phylogenetic work, the results could be reproduced. In contrast, a study presented in 2019 by Luciano Varela and other involved scientists, which includes numerous fossil forms of the entire sloth suborder, sees this more critically. Here, ''Paramylodon'' and ''Glossotherium'' are 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 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 and equation with ''Mylodon'' and ''Glossotherium''. However, it begins with the first discoveries of Richard Harlan (1796-1843) at Big Bone Lick in Boone County in the U.S. state of Kentucky in 1831, which included a right mandible and a clavicle. 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 made his pioneering voyage on HMS ''Beagle'' to South America and brought back from there a large number of fossils. These were then studied by Richard Owen, 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 Buenos Aires 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 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 north of Buenos Aires; 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. Subsequently, different type species were assigned to ''Mylodon'', for example, Johannes Theodor Reinhardt in 1879 considered it to be ''Mylodon robustus'', Richard Lydekker 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, ''P. harlani'', is recognized. Another species, ''P. nebrascensis'', was described in 1903 by Barnum Brown on the basis of a partial skeleton from Hay Spring in Nebraska, but was already united with the type species in the 1920s. Only ten years later, Glover Morrill Allen 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 of ''Paramylodon harlani''. The same applies to several species named by Edward Drinker Cope 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 graceful 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.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, Barnum Brown (1873-1963) introduced the generic name ''Paramylodon''. He used for this purpose a partial skeleton from Hay Spring in Nebraska that had been discovered in 1897 during an expedition of the American Museum of Natural History. 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, 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 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

Description

''Paramylodon'' measured about in length and weighed as much as It is known from North America deposits, including in Mexico and the United States and as far south as Guatemala, and often mistaken as ''Glossotherium''. Currently there is just one recognized species, ''P. harlani'', (Owen) 1840, which is commonly referred to as Harlan's ground sloth in honor of American paleontologist Dr. Richard Harlan, who first discovered and described a lower jaw in 1835. ''Paramylodon'' exhibits the interesting 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. 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 tons, 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 total length from . 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 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 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, 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 arches were secondarily closed again, deviating from most sloths. The anterior arch, originating at the zygomatic bone 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, had a finger-like shape and joined the middle process of the anterior arch section. On the underside of the skull, the palatine bone 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 palatine bone 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 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 ( 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 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 possessed a greatly expanded superior articular process, the olecranon. 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 was short and massive with a length of 29.6 cm. The longest bone was represented by the femur 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 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 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 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 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 osteoderms, formed in the skin, analogous to today's armadillos. 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 fissure in Florida. 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 xenarthrans.

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 ~1.8 Mya.—300,000 years ago.
*Fairmead Landfill Site, Madera County, California ~1.8 Mya.—300,000 years ago.
*Rio de la Pasion Site, Petén, Guatemala ~100,000—11,000 years ago.
*Fanno Creek Site, Tualatin, Oregon~15,000—10,000 years ago.
*Medicine Hat Unit V Site, Alberta, Canada ~1.8 Ma—11,000 years ago.

Overview and early occurrences

''Paramylodon'' was endemic distributed in North America and possibly also in Central America. The oldest finds clearly assignable to the genus are known from the Lower Pleistocene. Older forms of mylodonts are from the Upper PlioceneH. 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 Florida. Of the latter, noteworthy is the partial skeleton from site Haile 15A, a crevice filled with sediments in the limestone 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 in the Mexican state of Sonora. 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 Alberta in Canada. They are found in both lowland and mountainous locations, with the highest finding point reaching about 2900 m elevation in Colorado.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 California 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, such as in Iowa.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 It reached its northernmost point of discovery at that time with Sequim in Washington at 48.1° north latitude; to the south, the genus has additionally survived from Mexico, where its southern limit of distribution was at Valsequillo at 19° north latitude, but some finds now indicate that ''Paramylodon'' may also have lived in Guatemala and El Salvador. 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, belonging 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, 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 pits 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 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'' being known only from the eastern part of the United States. However, ''Paramylodon'' represents by far the most abundant representative 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 since the mid-1990s and currently includes more than 100,000 specimens from more than 100 taxa, 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 primitive proboscidean '' Mammut pacificus'', and the camel ''Camelops''. In contrast, the two other sloths ''Megalonyx'' and ''Nothrotheriops'' that also occur in Rancho La Brea play only a minor role, 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 datings.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

Extinction

Like most other large, ground-dwelling sloths, ''Paramylodon'' disappeared toward the end of the Pleistocene in the wake of the Quaternary extinction event. Unlike many other genera, however, there is little radiometric data available from ''Paramylodon'' measured directly from fossil material. Data. Among the most recent is a value from Rancho La Brea, which is 20,450 BP.J. Tyler Faith und Todd A. Surovell: ''Synchronous extinction of North America’s Pleistocene mammals.'' PNAS 106 (49), 2009, S. 20641–20645 However, clearly more recent finds are known, few of which came to light from archaeological sites associated with early human settlement on the North American continent. One of the rare evidence comes from El Fin del Mundo ("The End of the World") in Sonora. Discovered in 2007, the station was dated to 13,390 years BP by the radiocarbon method using charcoal. Based on the presence of six Clovis Points, it belongs to a very early section of the Clovis Culture, which represents one of the earliest archaeological groups of the first settlers of North America. In addition to two skeletons of proboscideans, one of which clearly represents ''Cuvieronius'' and had apparently been dissected by the hunter-gatherers of the time, remains of ''Paramylodon'' also came to light.Joaquín Arroyo-Cabrales, Alejandro Lopez-Jimenez, Guadalupe Sanchez und Edmund P. Gaines: ''Faunal remains from El Fin del Mundo archaeological site, Sonora, Mexico.'' Journal of Vertebrate Paleontology 31 (suppl.), 2011, S. 64 In addition, more than 130 osteoderms 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 Group 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. It is unclear from the few common finds to date whether direct hunting led to their extinction.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

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

Notable discoveries

Recently a ''Paramylodon harlani'' specimen was identified from the Tarkio Valley site in Iowa association with ''Megalonyx jeffersoni''. This is the first occurrence of ''Paramylodon'' in Iowa, and probably the first time these species have been found in close association. These two species have since been uncovered at the Fairmead Landfill Site and Irvington Site in California.

On December 3, 2009, the Press-Enterprise newspaper reported a specimen skull to have been recovered in San Timoteo Canyon.

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. The weight of members of the Lower Pleistocene 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 femurs, whose corresponding lengths are 48.4 and 54.6 cm, respectively. The earliest forms from the Pliocene

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