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Glossotherium is an extinct genus of mylodontid ground sloths of the subfamily Mylodontinae, which includes large ground-dwelling sloths. It represents one of the best known members of the family, along with Mylodon. Reconstructed animals were between 3 to 4 m long and possibly weighed up to 1700 kg. The majority of finds of Glossotherium date from the Middle and Upper Pleistocene, around 300,000 to 10,000 years ago, with a few dating older, possibly as far back as the Pliocene, about 3 million years ago. The range included large parts of South America, east of the Andes roughly from latitude 20 to 40 degrees south, leaving out the Amazon Basin in the north. In western South America, finds are also documented north of the equator. The animals largely inhabited the open landscapes of the Pampas and northern savanna regions.

Like other mylodonts, Glossotherium was adapted to a more or less grassy diet, as indicated by the broad snout and the design of the teeth. This view is confirmed by isotopic analysis. The anatomical Structure of the locomotor system suggests quadrupedal locomotion, but they were also capable of changing to a bipedal stance. The particularly strong construction of the forelimbs is remarkable, leading to the assumption that Glossotherium burrowed underground. Large fossil burrows with corresponding scratch marks support this assumption, possibly making it the largest known burrowing mammal ever. The structure of the auditory system shows that Glossotherium could perceive frequencies in infrasound and probably produce them with the help of its voluminous nasal cavity.

The research history of the genus is very complex. The first description was in 1840 by Richard Owen. However, he discarded the genus name just two years later. Subsequently, this led to persistent confusion and equation with Mylodon and other forms, which was not resolved until the 1920s. Especially during the 20th century, Glossotherium was considered identical to the North American Paramylodon. It was not until the 1990s that it became widely accepted that the two genera are independent.

Body size
Glossotherium was a medium-sized to large representative of the mylodonts. Reconstructed, the animals possessed an overall length of 3 to 4 m. A fully assembled skeleton was 335 cm long overall, of which about 90 cm was occupied by the tail. Weight was about 500 to 1000 kg for more gracile forms, and 1200 to 1700 kg for more robust ones. Like all ground-living sloths, Glossotherium had a massive body with short and powerful limbs. The head was elongate and the tail very long compared to modern arboreal sloths.

Cranial and dentition characteristics
The skull of Glossotherium was typically tubularly elongate and rectangular in elevational and lateral view. Its length, as measured on about a dozen specimens, varied from 39.6 to 46.8 cm, and its width averaged 16.0 cm at the anterior snout, 15.4 cm behind the eyes, and 18.3 cm at the posterior end. A striking feature was the very widened region of the anterior rostrum. The related Mylodon, on the other hand, possessed a differently structured skull, long narrow in shape and with a closed nasal arch created by the nasal bone curving forward and fused to the diastema at the anterior end. In appearance and size measurements, the skull of Glossotherium overlapped with that of the closely related Paramylodon. In general, however, the skull of Glossotherium was clearly more robust and broader than that of Paramylodon and possessed a dome-like bulge of the frontal line in lateral view, deviating from the latter. Also, in Glossotherium a more pronounced constriction occurred in the region of the lacrimal bone in plan view. The generally broader skull in Glossotherium also resulted in a wider nasal opening, with width exceeding height. In Paramylodon, the width and height of the nasal opening were about equal. In addition, the parasagittal ridges on the parietal bone were also more widely spaced than in Paramylodon. Consistent with most other sloths, the midaxillary bone was only loosely connected to the nasal bone in Glossotherium, whereas in Mylodon there was firm contact between the two bones due to the closed nasal arch. Again corresponding to most other sloths but deviating from Paramylodon, the zygomatic arch was not closed in Glossotherium. It consisted of an anterior and posterior arch segment. The posterior one, attached to the temporal bone, possessed a finger-like shape. The anterior one, beginning at the zygomatic bone, had three processes: one ascending, one descending, and one horizontally oriented. The occipital bone, when viewed from behind, showed a depressed outline with distinct narrowing above and below. This distinguished it from the more oval shape in Lestodon and the nearly circular one in Mylodon. Further peculiarities were developed in the region of the skull base. Here, the palatine bone in Glossotherium did not show quite as long as in Paramylodon, as it showed significant shortening behind the last molar. In return, the flanks of the wing bones were more inflated in Glossotherium and thus the distance between them was smaller. Similar formations are found in numerous other sloths, but their function is unclear. The mandible, referring to about half a dozen specimens found, measured between 30.1 and 37.0 cm in length, and was thus shorter on average than that of Paramylodon. The horizontal bone body had an extremely robust construction, increasing in height from anterior to posterior, reaching a maximum of 9.5 cm below the last tooth. The lower edge of the mandible was straight, as in other mylodonts. With respect to the articular process, it protruded less high compared to the crown process than in Paramylodon; consequently, the mandibular joint sat lower on average in Glossotherium. The entire crown process reached a height of 14.8 to 18.0 cm and was clearly above the masticatory plane, the anterior margin of the process was more convex in Glossotherium than in Paramylodon. A striking feature is represented by the symphysis, which was exceptionally broad, reflecting the broad snout. It ended posteriorly at the level of the first tooth, and also possessed in front a spoon or spatula-like process typical of sloths, which was also broad and rose at an angle of 45°, but did not reach the chewing plane of the teeth. As a characteristic difference from Paramylodon, the lateral margin of this process protruded much further in Glossotherium.

The dentition was greatly reduced in accordance with other sloths, consisting of five teeth in each half of the jaw in the upper jaw and four in each half of the lower jaw, for a total of 18 teeth. The foremost tooth in each jaw branch had a canine-like (caniniform) shape, the following ones were molar-like (molariform). This dentition structure is considered phylogenetically primitive within the sloths. Differing from Paramylodon and Mylodon, Glossotherium always had the caniniform teeth formed. In Paramylodon those of the upper jaw were partially reduced. Mylodon on the other hand had no caniniform teeth at all, in it the upper ones were formed back, but the lower ones were remodeled to molar-like. The caniniform teeth showed an oval or triangular cross-section, to the posterior dentition existed a short diastema. The molar-like molars of Glossotherium exhibited a flat occlusal surface with a somewhat raised margin. They possessed the two-lobed outline typical of mylodonts with a strong constriction in the middle. Only the anterior-most molar of the upper jaw was rather square in shape, which again is a difference from the rather long-rectangular first molariform tooth in Paramylodon. Differences in design between the two genera are also found in the second upper molar, as the two-lobed structure was less distinct in Glossotherium than in Paramylodon. As in all sloths, the teeth typically lacked enamel. Instead, they were composed of a softer interior and a harder exterior variety of dentine (vasodentin and orthodentin), with an additional layer of dental cement. The length of the upper row of teeth varied between 11.6 and 16.3 cm, but averaged 14.4 cm, of which the molar-like molars occupied about 11.2 cm. The forward widening snout conditioned the rows of teeth to diverge from each other.

Skeleton
The spine consisted of 7 cervical, 16 thoracic, 3 lumbar, 7 sacral, and at least 20 caudal vertebrae. However, the sacrum was formed as a synsacrum, which formed a unit with the lumbar vertebrae and the last thoracic vertebra, since the spinal processes of the corresponding vertebrae were connected to each other and the vertebral bodies were partially fused. The spine performed an S-shaped oscillation, with the lowest point at the fourth cervical vertebra and the highest point at the 15th thoracic vertebra. The spinous processes of the thoracic vertebrae protruded obliquely backwards, that of the eighth vertebra was up to 14 cm long. All thoracic vertebrae bore ribs, the first nine pairs of which were firmly attached to the sternum. The anteriormost caudal vertebrae were extremely massive and still resembled the remaining dorsal vertebrae with vertebral arches and transverse and spinous processes. There were also chevron bones on the underside, all in all the anterior tail section was designed to support extremely powerful musculature. Towards the rear, however, the size of the caudal vertebrae decreased greatly.

The postcranial skeleton has been preserved completely only in a few cases; the following data refer to a wide variety of finds, varying in number from single specimens to over a dozen. The humerus possessed a roughly columnar, short shape, but it widened massively toward the lower end of the joint, where it had a spatula-like appearance. Its length varied from 32 to 42 cm. Surrounding the shaft was a massive deltopectoral groin, a bony ridge that served as an attachment point for the humeral and shoulder muscles. Overall, the humerus was also extremely robustly built compared to that of the giant Megatherium. The same is true for ulna and spoke. The latter bone was 24 to 30 cm long, the former up to 36 cm. On the ulna, the upper joint end, the olecranon was conspicuously elongated. The femur was short as well as robust. It measured between 44 and 52 cm in length. It had a board-like flat but broad shape typical of ground-dwelling sloths, but the shaft tapered from top to bottom so that it was about twice as wide near the upper end of the joint as near the lower. Due to the high width of the shaft, the third rolling mound (third trochanter) was not clearly formed. The hemispherical condyle stood out prominently, but did not sit on a distinct neck. The laterally adjacent greater trochanter was nearly straight and then bent at a right angle to the shaft. The lower end of the joint sat somewhat asymmetrically, with the outer joint roll significantly larger than the inner. Compared to the femur, the tibia was extremely short, not even reaching half its length with measurements of 23 to 25 cm. The joint ends extended widely, and the upper (knee joint) possessed a width of three-quarters the length of the tibia. Similar to the femur, the shaft of the tibia was distinctly flattened. The almost triangular-prismatic fibula was not firmly fused to the tibia.

Osteoderms
Mylodonts are the only group of sloths in whose skin small bone platelets, so-called osteoderms were embedded. Such ossified skin formations are documented within the mammals today only in the armadillos. However, in mylodonts the bone platelets did not form a solid carapace, but were rather distributed randomly in the skin, as shown by finds of skin remains from Mylodon. From Glossotherium the osteoderms were discovered very early in the history of research, already in 1865 Hermann Burmeister Finds from the area of the Río Salado in Ecuador presented. These had a rhombic, trapezoidal or irregularly elliptical outline and were between 0.7 and 2.5 cm long. They possessed a roughened surface with irregular depressions 2 to 3 mm in diameter, while the underside was smooth and convex in design. The distribution of the bone platelets over the body of the animals is unknown, at the site of Toca dos Ossos in the Brazilian state of Bahia, however, the skeletal remains of one individual were associated with about 2500 osteoderms. Studies of these bony structures of "Glossotherium" chapadmalense, whose position within the genus is, however, rather uncertain, revealed that they were simpler in cross-section than those of xenarthrans. They had a compact structure and consisted of numerous fiber bundles mixed with hard bone lamellae (osteomas).

Overview and early occurrences
Glossotherium was widespread in South America, but is largely known only from the Pleistocene. An exception is the controversial form Glossotherium chapadmalense (now known as Glossotheridium), which is from the Middle Pliocene (local stratigraphic Chapadmalalan) about 4 to 3 million years ago. Finds include those from the type locality at Miramar in the Province of Buenos Aires in Argentina and include a 39 cm long skull with mandible and a first cervical vertebra. Additional finds in the form of several mandible fragments were recovered from Inchasi about 50 km southeast of Potosí in the Bolivian departement Potosí, the site is located at an elevation of 3220 m above sea level.

The vast majority of the finds of Glossotherium date to the late Pleistocene. Older finds, still from the Middle Pleistocene, are very rare, including for example a right ankle bone from La Huaca in the northern Peruvian Piura region, which is about 304,000 years old. In the subsequent Late Pleistocene, Glossotherium is then widespread over much of South America. East of the Andes there are finds from a range between 40th and 20th degrees south latitude, and in western South America also from areas north of the equator. The Amazon basin is largely omitted. The main range includes central and northern Argentina, Uruguay, and southern Brazil. Other significant localities are documented from the coastal areas of southeastern and eastern Brazil in the states of Paraná, Minas Gerais, and Bahia, among others. Outstanding here is Toca dos Ossos in Bahia, where over a hundred specimens plus numerous osteoderms of several individuals came to light. In addition, the genus has been recorded in Venezuela, in Colombia, in the coastal lowlands, and (more rarely) in the Andean regions of Ecuador, Peru, and Chile, as well as in Bolivia. From Chile, a nearly complete skeleton from Lonquimay in the Araucanía Region forms the only voucher of the genus in the country to date. In contrast, extensive skull material comes from Ecuador and Peru, including not only adult but also several younger individuals. An equally singular find in the form of a skull bone was made from the island of Trinidad. The animals lived mainly in more open landscapes such as the pampas regions of central South America (Argentina, Uruguay, and Brazil), but also the more northern savanna landscapes. Mainly in the Pampas, the occurrence overlaps with those of the other major mylodonts of Pleistocene South America, Mylodon and Lestodon. At individual localities, such as in Arroyo del Vizcaíno in southern Uruguay or on the Chuí River in southeastern Brazil, all three genera of mylodonts coexisted together.

Extinction
The frequent record of Glossotherium at the end of the Pleistocene coincides with the disappearance of the genus during the Quaternary extinction wave. The latest finds of the genus also overlap with the arrival of early hunter-gatherer groups in South America. Whether the early human colonizers of the Americas directly influenced the disappearance of the ground sloth, however, is unclear; few definite interactions are known. Among the few sites with remains of hunter-gatherer groups associated with remains of Glossotherium is that of Arroyo Seco 2, a multiphase archaeological site in the Buenos Aires Province of Argentina, the age of which is dated by the radiocarbon method to between about 12,400 and 7,300 C14 years before present. In addition to numerous lithic artifacts, mainly containing fiber preparation debris used to make tools with sharp cutting edges, bone remains of large mammals were also found, including a few bones of Glossotherium. Direct age determinations on these finds indicate that the Glossotherium remains were emplaced during the earlier stage of sedimentary formation (circa 12,240 to 10,500 years BP ago). Another important site is Paso Otero which is also located in Buenos Aires province. In the archaeological area investigated since 1994, more than 80,000 bone fragments came to light, but some of them were badly shattered and so could hardly be determined. The assignable bones included two bone fragments of Glossotherium. In addition, about 80 stone tools made of quartzite were also discovered, among some fish tail points. The age of the site is 10,440 to 10,190 years BP, but the determination in this case was not made on the remains of Glossotherium but of Megatherium. Isolated evidence has also survived from other parts of South America, but its interpretation is uncertain. For example, from the rock of Santa Elina in the Brazilian state of Mato Grosso, a campsite with remains of a hearth was investigated in the 1990s, with jaw and vertebral fragments and hundreds of osteoderms of Glossotherium in the vicinity. The age of the hearth was dated to about 10,120 C14 years BP. One of the osteoderms was pierced, but this can also be attributed to biogenic influences. On the other hand, the direct relationship of the Glossotherium relics to the hearth is largely unresolved. In Taima-taima on the coastal zone of north-central Venezuela, isolated remains of Notiomastodon were unearthed alongside a skeleton of Notiomastodon, in the body of which was a projectile point of the El-Jobo type, but these may not be directly related to the hunting site. The age of the sediments in which the finds are embedded dates to about 13,390 to 12,580 years BP.