Skull of Mylodon
|Lower to Upper Pleistocene|
|1.8? Millions of years to around 10,000 years|
|Owen , 1840|
Mylodon is a genus of the extinct family Mylodontidae that includes large, ground-living sloths . With a total length of 3 to 4 m, it is one of the best-known and largest representatives of the group. The genus is proven above all in the southern part of South America . The oldest finds probably date to the Lower Pleistocene . Most of the fossil remains , however, date from the Upper Pleistocene period . One of the most important sites of this phase is the Cueva del Milodón in southern Chile . Shortly after, about 10,000 years ago, Mylodon diedout. At this point in time, the sloth species appeared along with the first human colonizers in America. However, there is little evidence that it was increasingly hunted by humans.
From Mylodon not only remains of bones and teeth, but also various soft tissue such as skin and fur as well as food remains in the form of fossilized feces have come down to us. The Mylodon’s skull, which is greatly elongated and compared to other large Mylodonts, is clearly narrower and has a completely closed nasal arch as an outstanding characteristic . Other distinguishing features concern the structure of the front dentition.
The animals lived exclusively on the ground. A thick coat with long hair can be interpreted as an adaptation to a life under cold climatic conditions, such as those that prevailed in southern South America during the end of the last cold period . This also corresponds to a diet based predominantly on grasses in this region. The widespread distribution of Mylodon into the pampas region and some features on the skull show, however, that the animals had a much greater ecological range and could also cope with warmer temperature conditions and possibly a mixed vegetable diet. Some of the animals fell victim to larger predators .
The genus was scientifically introduced in 1840, usually only one species is recognized. The type material comes from the area of the pampas, where it was collected by Charles Darwin during his trip with the HMS Beagle . In the course of the research history, there were confusions and equations with other large ground sloths such as Glossotherium and Paramylodon , which could only be resolved in the 1920s. In addition, Mylodon was one of the first extinct sloths on which genetic studies were carried out to clarify the ancestral relationships.
Mylodon was a large representative of the Mylodontidae. Its total length was estimated to be around 3 to 4 m. Based on the size of the skull, a weight between 1 and 2 t is assumed with an approximate estimate of 1.65 t. This made Mylodon about the size of related forms such as Glossotherium or Paramylodon , but was significantly smaller than the giant Lestodon . In terms of physique, the animals largely corresponded to the other large ground-living sloths.
Skull and dentition features
Especially in the construction of the skull, Mylodon differed significantly from other related forms. After examining ten more or less well-preserved specimens, its length varied between 59.0 and 71.5 cm, which is significantly longer than Glossotherium or Lestodon . At the skull it was between 16.5 and 22.5 cm wide, in the front nasal area between 11.3 and 15.5 cm. The height of the posterior skull was 14.0 to 19.0 cm and the anterior 15.0 to 23.5 cm. As a result, the skull was elongated and narrow, in contrast to Glossotherium and Lestodon , which had a short and very broad skull. The extraordinary length of the skull of Mylodon was mainly achieved by stretching in the rostral section. Seen from above, the rostrum narrowed towards the front. This is where the most important difference to most of the other representatives of the Mylodontidae can be found: The nasal bone was long and narrow and curved downwards in the front area. At the front end, it connected to the middle jawbone , which was lengthened by an appendage and which in turn fused with the upper jaw. This resulted in a completely closed nasal arch in adult individuals, which is largely unknown in other sloths. In comparison, the skulls of Glossotherium and Lestodon , but also of Paramylodon , showed a widened nasal area when viewed from above, which was rather short and looked clearly cut off when viewed from the side, the middle jaw bone had no contact with the nasal bone. The roof of the skull was largely straight in Mylodon , only a slight indentation could occur above the orbit . On parietal significant Temporallinien put on, but no head crest formed. The zygomatic arch was slender, the front attachment began above the third and fourth molars. It did not form a solid end with the rear arch attachment. As is usual with sloths, the front arch base consisted of three appendages: one ascending, one horizontal, and one descending, the former of which was the longest. The rear arch formed a triangular plate. The occiput bent at an angle of 120 ° from the roof of the skull. The underside of the occiput was at about the level of the occlusal plane. When viewed from behind, the occiput appeared almost circular and not as depressed as in Glossotherium and Lestodon . On the underside of the skull, the palate was narrow and more or less triangularly oriented towards the front. Numerous small bone openings were characteristic here. The glenoid pit, in which the joint of the lower jaw engages, corresponded to that of other mylodonts with its weak form, but this provided free rotation overall.
The length of the lower jaw varied between 42.0 and 48.0 cm, measured on three fossil finds. It was elongated, more noticeable than in Glossotherium and Lestodon , since in Mylodon the area in front of the teeth in particular had stretched. The horizontal bone body increased continuously in height towards the rear, below the last molar it was about 10.5 to 12.7 cm. The symphysis at the front end for the jointing of the two halves of the lower jaw was about 12.4 cm long. Here the lower edge of the body of the lower jaw rose at an angle so that the front end of the symphysis was above the occlusal plane of the teeth. As with other sloths, the symphysis extended forward, it ended slightly rounded. According to the rostrum of the skull, the symphysis in Mylodon was narrow and not as wide as in Glossotherium and Lestodon . The mandible foramen opened shortly after the symphysis . The ascending branch started behind the last molar and formed an angle of 140 ° to the occlusal plane. The crown process rose up to 20 cm. In contrast, the articular process was lower, roughly at the level of the occlusal plane, resulting in a low cranial-mandibular connection. The angular process at the rear end of the lower jaw was clearly visible. Sometimes it tilted downwards and lay below the lower edge of the horizontal bone body. The upper side of the angular process does not reach the occlusal plane.
The teeth typical of the sloth differ greatly from those of the other higher mammals and usually consist of five teeth at the top and four teeth at the bottom per jaw arch, making a total of 18 teeth. In the Mylodonts, the first tooth in each case was often canine-shaped ( caniniform ), while the rear teeth were more molar-like ( molar-shaped ). Within the sloth, this structure of the teeth can be described as original. Mylodon showed that the upper canine-like tooth of each row was completely regressed and only the molar-like four rear teeth were found here. In the lower row of teeth, the front canine- shaped tooth was transformed into a molar-shaped one . The dentition thus consisted of a total of 16 teeth. This is somewhat reminiscent of Paramylodon , in which the upper canine-shaped teeth were also missing, but the lower ones had retained their strikingly pointed shape. In contrast to this, Glossotherium and Lestodon had the original sloth bite. The flat, flap-like and largely indented structure of the molar-shaped teeth can be emphasized as a characteristic of the Mylodonts, which clearly differs from that of the Megatheriidae and Megalonychidae with their two transverse raised ridges per tooth. The shape of the teeth in Mylodon was designed more simply. They had a round to oval outline in the upper jaw row, and a diamond-shaped outline in the lower jaw row. The typically more complex bilobed design of the molar-like teeth of Glossotherium and Lestodon , caused by a central constriction, occurred in Mylodon only on the lower rearmost tooth. In general, the rows of teeth diverged towards each other, and the teeth were very high crowned ( hypsodontic ). The upper row of teeth extended over a length of 10.9 to 13.3 cm, the lower one was between 12.0 and 15.0 cm long.
Finds of the postcranial skeleton are far rarer in Mylodon than in the other large Mylodont sloths Glossotherium , Lestodon and Paramylodon . As a result, the body skeleton is less well documented. Only individual elements of the spine such as the atlas and various thoracic vertebrae have been described. The humerus was massive and extremely long at 46.0 to 48.0 cm. The joint head, the diameter of which was over 10 cm, stood out due to its hemispherical, but laterally somewhat flattened shape. A distinct deltopectoral ridge ran down the shaft, which acted as an anchor point for the shoulder muscles. As with many ground sloths, the lower end of the joint extended far and brought it here to a width of almost 26 cm. In part, this was caused by a massive internal epicondyle. The articular surfaces (capitulum and trochlea) were almost perpendicular to each other and did not form such an obtuse angle as in Glossotherium . The cubit was built gracefully. Their length was around 37 cm. The olecranon , i.e. the upper articular process, took up about 8.1 cm, which corresponds to about 22% of the total length and is significantly less than in comparison with Glossotherium and Lestodon . It was laterally narrowed, which is also found in Paramylodon . The spoke largely resembled that of Glossotherium and was compact and straight built with a length of about 30 cm. The head was oval in shape with a prominent lip. The pelvis was extremely expansive and 114 cm wide between the two iliac bones . The thigh bone measured between 55 and 59 cm in length. Typically for floor sloths, it was flat like a board. Its width decreased significantly on the shaft, the lowest value was reached just below the center point. Here the width was about 18 cm, the thickness about 7.5 cm. The joint ends, on the other hand, were significantly wider, around 30 cm at the knee end and around 26 cm at the foot end. The hemispherical joint head, a good 14 cm in diameter, towered over the Great Rolling Hill. The third rolling mound was only visible as a small elevation on the outer edge of the shaft below the large rolling mound. Compared to the thighbone, the tibia reached about 27 cm only about half the length, a characteristic of Mylodonts. This bone, too, was clearly flat with a thickness that was only half the value of the width at the shaft. The fibula is so far only fragmented. It was drawn in on the shaft and widened at the joint ends, with the upper joint end showing more pronounced curves than in Glossotherium .
The hand comprised a total of five rays (I to V), whereby the metacarpal bone was fused with the large polygonal bone on the first ray . This created the so-called Metacrapal Carpal Complex (MCC for short) , which is typical for many ground sloths . As a special feature of the wrist, the pea bone was clearly flat, its shape resembled that of Glossotherium , but differed from the corresponding bone of other Mylodonts with spherical, walnut-like or pyramidal shape. The fourth ray had formed the longest metacarpal bone, while that of the fifth was only slightly shorter. The respective bones measured there around 12.5 and 10.7 cm in length. As with Glossotherium and Paramylodon, only the three inner rays were probably clawed, but only of the second ray have all bone elements been documented. The metacarpal bone was 7.8 cm long and was built very gracefully. The first phalanx was extremely short and only about 2.5 cm long, the second was about 4.2 cm long and the third at least 11.5 cm. It was shaped like a tube and went forward into an appendage on which the claw rested. The first phalanges of the two outer rays were significantly reduced in length. There are only individual root bones of the foot, such as the talus .
Skin, fur and osteoderms
Mylodon is one of the few extinct mammals that has mummified skin remains. The most important location for such finds is the Cueva del Milodón in the Chilean province of Última Esperanza , where the first skin parts were unearthed at the end of the 19th century. Individual pieces are up to 150 cm long, but have shrunk due to the drying process. Its thickness is up to 1.5 cm in some places, but it is usually around 1 cm. The skin is densely covered with stiff, slightly wavy hair, with only the top hair being developed, while the undercoat is missing. This characteristic is similar to the two-toed sloth ( Choloepus ), but less so to the three-toed sloth ( Bradypus ), which have an undercoat. The length of the individual hair varies between 5 and sometimes over 20 cm with the shortest in the area of the back of the head, medium-long hair on the back and very long hair on the limbs. Their current color ranges from yellowish to reddish brown. The hair shafts are uniformly tubular, at the upper end they form blunt tips. As with today's sloths, the hair did not have a pith (medulla). In contrast to the hair of the two-toed sloth, they lack their characteristic longitudinal corrugation.
The mylodonts are the only representatives of the sloths to have bony plates embedded in their skin. Such formations, called osteoderms , are known today to a greater extent only in armadillos . In contrast to the outer armor of the armadillos, the bone platelets of the Mylodonts were rather loosely scattered. Hermann Burmeister published the first finds of individual osteoderms of Mylodon as early as the 1860s. The remains of skin found in the Última Esperanza caves give an impression of how they were embedded in the skin and distributed over the body. The bone platelets are all located in the lower section of the skin, while the hairs originate in the upper sections. The distribution turned out to be very inconsistent. Some areas with a dense array of osteoderms contain between 83 and 95 platelets per 10 cm². In the case of others, however, the number is very thin. However, even with a close arrangement, the osteoderms never unite to form a closed shell, but are always separated from one another by individual skin folds. In accordance with the armadillos' shells, the bone platelets form a single layer and do not appear stacked. Since all skin remains were found isolated from the body skeleton, it is sometimes difficult to assign the skin areas with a dense and thin arrangement of bone platelets to a specific part of the body. However, it can be assumed that the back was largely armored and the stomach was free. In the sections with dense osteoderm formation, these were larger than in the clear areas. Mylodon's osseous platelets were mostly of irregular oval shape with dimensions of 0.5 to 2.5 cm in length, 0.3 to 1.8 cm in width and 0.2 to 1.1 cm in thickness for weights of a maximum of 2 g. On the surface, they showed individual dimples. In cross-section, they consisted of numerous fiber bundles mixed with hard bone lamellae ( osteomas ). This made their structure much simpler than that of the armadillos, and they probably lacked the keratin layer known from the armadillos . In principle, the osteoderms of Mylodon were similar to those of other large Mylodonts.
Distribution and important fossil finds
Overview and origins
Mylodon was mainly found in the southern area of South America . Fossil finds are from Argentina , Chile , Bolivia , Uruguay and Brazil . The populated regions thus encompass very southerly sites on the island of Tierra del Fuego as well as most of Patagonia as far north as the pampa region . The distribution area reached its southern limit at about 53 ° south latitude. The Tres Arroyos site on Tierra del Fuego and the region around the Cueva del Milodón in southwestern Patagonia are among the southernmost known records of a sloth representative in the Pleistocene . In the Pampas region, the northern border was found around the Chuí River in the southeastern Brazilian state of Rio Grande do Sul around the 30th southern parallel. Find points further north such as Ñuapua in Bolivia touch the 20th southern parallel . Finds reported from Paraguay are, however, considered to be rather uncertain.
The sloth genus may have first appeared in the Lower Pleistocene , but finds are rather rare. During this period, the possibly closely related form Archaeomylodon also appeared in the pampas , the foremost canine-like teeth of the upper row of teeth, although greatly reduced, but not yet completely reduced. One of the early finds from Mylodon located further north is a skull from the El Palmar Formation in the Argentine province of Entre Ríos , which dates back to the final warm period around 80,000 years ago. Also from the northern distribution areas, two partial skeletons are worth mentioning, one of which was unearthed on the Río Anisacate in the Argentine province of Córdoba and the other in Arroyo Quequén Salado near Oriente in the Argentine province of Buenos Aires . Mainly in the pampas there was an overlap of the occurrence of Mylodon with the two other large Mylodont sloth representatives Glossotherium and Lestodon in the Upper Pleistocene . However, an actual common occurrence is rarely proven. These include the important archaeological site of Paso Otero in the Province of Buenos Aires, the Arroyo de Vizcaíno locality in southern Uruguay and the Chuí River.
Significant finds from the Upper Pleistocene
As with most of the other large ground sloths, most of the Mylodon finds originate from the Upper Pleistocene, with an emphasis on the end of the last glacial period. It is also the phase in which Mylodon disappeared from the fossil record . From a global perspective, numerous larger animals died out in the transition from the Pleistocene to the Holocene , which is why this event is viewed as a Quaternary extinction wave. In South America this coincides with the first appearance of humans. Whether there is a causal connection between the two is often and controversially discussed. In addition to the potential hunting and possible landscape changes on the part of the early human hunter-gatherer groups , climatic changes can also have had an influence here. Numerous archaeological sites, especially in the Pampa region and in the Patagonian area, are between 13,500 and 10,000 years old. The majority of these testify to at least a coexistence of humans and ground sloths over a long period of time. Direct associations of human cultural products and fossil remains of Mylodon can be found in the Gruta del Indio at the eastern foot of the Andes , in Piedra Museo or in Las Buitreras , all Argentina, or in Tres Arroyos on Tierra del Fuego. Mylodon is often represented by individual osteoderms, bones or in the form of coprolites , while the human legacies are limited to stone artifacts and / or fireplaces. There is often no evidence as to whether humans also used more or less intensive raw material use of sloth bones. According to recent studies, numerous bone marks that were originally interpreted as being caused by human activity can be traced back to predatory damage. It is even more difficult to prove that humans have hunted the large ground sloths directly. Quebrada de Quereo , a site on an ancient coastline in northern Chile , is often regarded as evidence . From here come, among other things, skeletal remains of two individuals of Mylodon , distributed over a narrow area, but in two different stratigraphic units and at a spatial distance of 21 m from each other. One of the individuals was associated with around 70 stone objects, the anthropogenic origin of which is under discussion. There are no cut marks on the bones as indications of possible human manipulation. The age of the site is given as 11,600 to 10,900 years ago.
The Cueva del Milodón near Lake Sofía in the Chilean province of Última Esperanza is one of the most important sites and is best known for its skin remains. It belongs to a whole system of caves in the region, such as the Cueva del Medio or the Cueva Chica , which line the southern flank of the 556 m high Cerro Benitez like a string of pearls . Cueva del Milodón is a large cave 250 m long, 140 m wide and 30 m high at the entrance and 10 m at the rear. It was discovered in 1895 by the German captain Hermann Eberhard , who also found the first remains of skin. The great importance of these finds meant that the cave, initially known as "Cueva Eberhardt", was visited and explored by numerous scientists in the following period. As a result, a large number of finds accumulated over the course of time, among which Mylodon with bone fragments and numerous coprolites have a large share. Other finds include camels such as llama , horses such as Hippidion or South American ungulates such as Macrauchenia , and several predators are also represented, including the jaguar , smilodon as a member of the saber-toothed cats and the huge bear form Arctotherium . Some of the mammalian bones have marks that were originally associated with human activity, but according to current belief, they are more likely due to predatory bites. In addition to the faunistic remains, the cave also contained a myriad of botanical material. It also provided one of the most extensive data sequences from the Upper Pleistocene. Several radiocarbon dates , measured on the most diverse finds of Mylodon , go back over a period of around 16,700 to 10,200 years ago. The information above is one of the most recent that has been obtained directly from finds made by the sloth representative.
With around 11,480 to 11,250 years ago, similarly young data was also provided by the Baño Nuevo site in central-southern Chile. The osteoderms, from which these values originate, were stored in stratigraphically younger layers with an archaeological context. Cueva Lago Sofía , a group of several small caves not far north of the Cueva del Milodón, shows comparable age values. The small, dark rock chamber Cueva Lago Sofía 4 contained numerous bones, including four young Mylodon animals and more than 4200 bone platelets, the age of which is between 13,400 and 11,050 years. Even more recent are the dates of a Mylodon rib, which was associated with three dozen bone platelets on the neighboring Cueva Lago Sofía 1 and already belongs to the Lower Holocene at an age of 9700 years . The archaeological find area with stone artefacts, fireplaces and bones with cut marks, on the other hand, is older and can be classified in a period of 11,000 years, while other finds of Mylodon were deposited almost 2000 years earlier. Some Mylodon fossils come from layers that scatter even more clearly into the Holocene. In most cases, there is no direct age determination of the bones, so the context in which they were found is usually viewed as problematic. The reason for this is that numerous natural and anthropogenic processes have an impact in many cave and open-air sites that can lead to the displacement of bones or osteoderms, be it the burrowing activities of underground animals or various human activities. Like many other forms of the great ground sloth , Mylodon died in all probability in the transition from the Pleistocene to the Holocene.
The mylodonts are often considered to be grass-eaters due to their dental structure with flat chewing surfaces on the molar-like teeth. This is also supported by the high ( hypsodontic ) tooth crowns and the wide mouth with numerous shapes. The ungulates are mostly used as analogous examples , in which shapes with high tooth crowns and broad-lipped mouths are usually grass-eating, such as various cattle , horses or the white rhinoceros . In contrast, those with low tooth crowns and narrow snouts such as the duiker or the black rhinoceros feed largely selectively on various leaves and other soft vegetable foods. In contrast to other large mylodont sloths such as Glossotherium , Paramylodon or Lestodon , the mouth of the Mylodon is relatively narrow. A special feature is the closed nasal arch, which is heavily roughened in its front area and thus offers muscle attachment points for a mobile upper lip. Something similar can be said about individual depressions in the vicinity of the infraorbital foramen , which also functioned as starting points for individual muscle strands of the nose and lip area. Possibly Mylodon was therefore more adapted to a mixed vegetable diet, which was ingested with the help of a movable upper lip. The loss of the front teeth in the upper row of teeth also leads to the assumption that, comparable to cattle, there was a horn-like structure on the middle jawbone that could be used to pluck the food.
The entire anterior cranial structure of Mylodon is relatively solid, combined with a partially ossified nasal septum, it can be assumed that relatively high chewing forces acted when the food was chopped up. In contrast to the sometimes huge representatives of the Megatheriidae , the joint between the lower jaw and the skull of the Mylodonts was relatively low, roughly at the chewing level of the teeth. The resulting decreasing lever arm of the masseter muscle experiences a certain compensation through the structure of the zygomatic arch , mainly the descending process, so that there should have been only minor differences to the Megatheria in terms of the biting force. The extended mandibular joint allows wide freedom of movement when chewing. Against this, however, is the zygomatic arch, which is not closed and therefore could only withstand the opposing forces of the masseter and wing muscles (musculus pterygoideus) to a limited extent. It can therefore be assumed that forwards and backwards directed chewing movements dominated in Mylodon . The flat tooth crowns lead to a comparatively small size of the total available chewing surface. In the case of Mylodon this is a good 1320 mm² corresponding to other Mylodonts of roughly the same size. The Indian rhinoceros, which is comparable in terms of its dimensions, has, on the other hand, double to four times the value with 2660 to 5190 mm². The situation is similar with the hippopotamus , the total surface area of which is between 3290 and 5410 mm². The small total occlusal surface of the teeth in Mylodon probably resulted in a rather low processing capacity for the food in the mouth. From this, either a high fermentation rate in the gastrointestinal tract and / or a very slow metabolism can be concluded. The latter is the case with today's sloths. This is due to the long passage time of the food of up to a week through the large, multi-chambered stomach . It can be assumed that this also applies to the extinct sloths. Possibly this made the stomach of the mylodonts a functional equivalent to the complex stomach of the ruminants , whereby a long throughput time of the food enabled efficient digestion, in which even more difficult to access nutrients could be provided, for example from foods with a higher fiber content. Such a digestive system could have compensated for the low processing volume in the mouth and thus ultimately also the small total sales area with Mylodon .
A direct analysis of the food resources used is possible, among other things, through the numerous dung residues in the form of coprolites , which are available from the Cueva del Milodón in the Chilean part of Patagonia , but also from other caves. The coprolites in Mylodon have a diameter of up to 18 cm. Investigations of the plant residues showed 80 to 95% sweet grass and 5 to 20% sour grass . Herbaceous plants , on the other hand, could only be detected in traces. According to this, Mylodon lived almost exclusively on grasses, at least in southwestern Patagonia. The food is reflected in the paleohabitat, as pollen analyzes show that the landscape at that time was tundra-like in character and was therefore almost free of trees with only a few low bushes. Occasional evidence of false beeches is interpreted as pollen carried by the wind.
In general, the large mylodonts are ground-dwelling animals. The lower section of the hind leg, which is very short compared to the upper part, is also found in Mylodon , whose shin bone, at 27 cm in length, is only half as long as the thighbone at 59 cm in length. In comparison, have megatheriidae significantly longer lower leg portions, about the almost equal Pyramiodontherium related to a 47 cm long shin leads to a 49 cm long femur. Possibly these differences in the hind leg structure result in a much more agile locomotion in the Megatheria in relation to the Mylodonts. Similar to other large ground sloths, the hand of Mylodon made contact with the ground with the outer side edge and thus sat up rotated. This is indicated by the long metacarpal bones of the external rays and the decreasing number of phalanges on them. The special hand position protected the long claws of the inner rays, which did not penetrate the ground while walking. A functionally similar but fundamentally different hand position can be found in the ankle duct of the distantly related present-day great anteater . The elbow joint was pointed slightly outwards when standing on four feet and the arms were thus angled slightly inwards, which is evident from the position of the olecranon . The hands came to rest slightly within the width of the elbow. Such an orientation of the arms can effectively support the large mass of Mylodon . As a result, the hands would also be in a line with the feet, which is also conveyed by the Paramylodon step seals . The laterally limited articular surface of the orberam head severely restricted the mobility of the arm. The same applies to the forearm, the straight spoke with the laterally elongated head of which did not allow any major rotational movements. These features can be interpreted as adaptations to a purely rural lifestyle. Finally, reference should be made to the muscle attachment points on the first cervical vertebra , which are more developed than, for example, in Paramylodon . Correspondingly, the occipital joint surfaces are also somewhat further apart. Both can be interpreted to mean that the more massive skull of Mylodon , caused by the lengthening of the snout region, required greater muscle support.
For some of the Mylodonts of South America, such as Glossotherium , a partially burrowing way of life is being reconstructed, which results from the construction of the foreleg, among other things. An indicator for this is the upper articular process ( olecranon ) of the ulna . The longer the olecranon, the higher the leverage of the forearm, since more attachment surface is available for the forearm muscles. In Glossotherium , the olecranon takes up up to 35% of the total length of the cubit, the resulting ability to dig would be comparable to that of the spherical armadillos , which seldom build their own burrows, but are able to do so. The previous analyzes for Mylodon resulted in a much shorter olecranon, whose share in the total length of the cubit is only around 22%. The problem, however, is the fact that the values for Mylodon refer to a not fully grown specimen. Other clues can be derived from the construction of the hand. In Mylodon , for example, the metacarpal bones of the second and third ray are very delicate, in contrast to Glossotherium . A weakly pronounced central ray does not seem to support digging activity, as this is usually most strongly developed in underground mammals. However, the distal joint facet of the third metacarpal bone is flat, which means that the middle finger is generally stiff and stable. The same articulation surface on the second metacarpal is significantly more rounded and thus supports greater mobility of the finger when gripping. As a result, there were obviously functional differences between the individual rays of the hand. The rare signs of wear and tear on the last phalanx, which are isolated from the Cueva del Milodón several times, can serve as an additional indication of digging activities. The findings shown do not negate the occasional scratching underground, for example when looking for food or the like, for which the structure of the forearm with individual distinctive muscle marks on the forearm is obviously strong enough.
Mylodon occurred from Tierra del Fuego at the southern tip of South America via Patagonia to the Pampa region and thus possessed a distribution area that included the cold and cool temperate to partly subtropical climatic zones of South America. Above all, the numerous records in Patagonia and Tierra del Fuego from the end of the last glacial period , the reconstruction of the environment at that time as tundra-like open landscapes based on pollen analyzes, the resulting predominantly grassy diet and the tradition of fur remnants identify Mylodon as a cold-adapted animal. This is also supported by calculations of the thermal conductivity of the fur, which was very low and thus supported the thermal regulation . The value of around 1.9 is only about a fifth of what would be expected for an animal of around 1 t live weight. This is partly attributed to the low metabolic rate. If the latter also applies to Mylodon , this could have caused a thermoneutrality of around 28.5 ° C, which is much higher than that of the recent sloths. However, since the air temperatures at the end of the Pleistocene are likely to have often been lower, based on these calculations, a phase-wise increased metabolism can be assumed in order to keep the body temperature largely constant.
On the other hand, finds of Mylodon from areas significantly further north are well documented. The remnants of the El Palmar Formation in the Argentine province of Entre Ríos are assumed to have been in the final warm period, more precisely in the end of the heat isotope stage (OIS or MIS) 5 a good 80,000 years ago. The environment at that time consisted of alternating closed forests and more open grass areas in a landscape that was crossed by numerous waterways in the catchment area of today's Río Uruguay and which existed under warm and humid climatic conditions. Since Mylodon was still present here in the end of the Pleistocene under comparable climatic conditions, it can be assumed that the animals have a significantly greater ecological tolerance. It can be assumed that this also applies to diet. The more wooded landscapes in the more northerly refuges of the sloth walker provided less grass food, just as the already mentioned skull adaptations advocate a diet based more on a mixed vegetable diet.
On the basis of individual finds, certain differences in size can be determined in Mylodon . For example, fossil remains from more northerly sites indicate on average smaller individuals than those from more southern ones. In principle, this could reflect Bergmann's rule , but it is also possible that a previously undetected sexual dimorphism is expressed here. In general, the available fossil material is considered to be insufficiently extensive to allow a more precise sex determination of an individual. In other large ground sloths such as Paramylodon there are indications of a sexual dimorphism, as is the case with the recent tree sloths, which does not affect body size, but does affect body weight. On the other hand, Mylodon finds from very southern areas also testify to very small individuals, which is partly associated with their occurrence in mountainous locations.
Predators and Parasites
Especially in southern and southwestern Patagonia, numerous bone changes in finds of Mylodon can be proven to be caused by predators . This includes, above all, the remains from the Cueva del Milodón in southwestern Chile. Some caves in their immediate vicinity, such as Cueva Lago Sofía 4 and Cueva Chica, are interpreted as clumps of predatory animals. The same applies to the Cueva del Puma or the Cueva Fell in the Pali-Aike area of southern Chile. Some of the caves mainly contain smaller skeletal elements such as hand and foot bones or bone plates, which indicate that only part of the carcass was carried into the shelter. Whether this is the result of direct foraging or scavenging cannot be directly clarified in many cases. Other rock chambers in turn contained a larger proportion of young Mylodon animals . The largest predators occurring at that time were the puma and the jaguar as well as the extinct forms Smilodon as a representative of the saber-toothed cats and Arctotherium as a member of the bears . The latter two could, reconstructed, reach body weights of over 400 kg, with prey sizes between 1 and 2 t for the saber-toothed cat can be assumed, whereby Smilodon can be considered a great predator of Mylodon .
At various of the studied coprolites of Mylodon more eggs could of nematodes are documented. The eggs were ovaloid in shape with lengths of almost 50 µm in length and 29 µm in thickness. In addition, individual beetles could be detected.
Classic external system
Internal systematics of the Mylodontidae according to Boscaini et al. 2019 (based on skeletal anatomical analyzes)
Mylodon is an extinct genus from the also extinct family of the Mylodontidae . The Mylodontidae form part of the suborder of the sloths (Folivora) and form within this, together with the Orophodontidae and the Scelidotheriidae, the superfamily of the Mylodontoidea (sometimes the Scelidotheriidae and the Orophodontidae are only managed as a subfamily within the Mylodontidae). In a classic view, which largely takes into account the anatomical features of the skeleton , the Mylodontoidea represent the second large and important line of sloths alongside the Megatherioidea . The Megatherioidea include the Megatheriidae with the largest known representatives of the sloth, in addition also the Nothrotheriidae and the Megalonychidae . The two-toed sloths ( Choloepus ) are assigned to the latter as one of the two lines that still exist today. The Mylodontidae within the Mylodontoidea represent one of the most diverse groups of the sloths. Their most important defining features include the high-crowned teeth, which, unlike the Megatherioidea, have a flat ( lobate ) chewing surface. This tooth design is mostly seen as an adaptation to food that is more grassy. The rear teeth are often round or oval in cross-section; the foremost of a row of teeth is usually shaped like a canine. The rear foot is also clearly rotated so that the sole points inwards. The Mylodonts can be detected with Paroctodontotherium from Salla-Luribay in Bolivia as early as the Oligocene .
The internal structure of the Mylodontidae is complex and currently insufficiently worked out. Mostly recognized are the phylogenetically developed lines of the Mylodontinae with Mylodon as the type form and the Lestodontinae , whose character form is Lestodon (referred to on the tribal level as Mylodontini and Lestodontini). Paramylodon and Glossotherium are partly included in the latter . In addition to these two groups, numerous other subfamilies have been set up in the past. The Nematheriinae, which summarize the representatives from the Lower Miocene , or the Octomylodontinae for all basal forms should be mentioned. However, these are not generally recognized. Another line was worked out with the Urumacotheriinae in 2004, which include the late Miocene representatives of northern South America. In principle, some researchers are calling for a revision for the entire family, as many of the higher taxonomic units have no formal diagnosis.
The subdivision of the terminal representatives of the Mylodonts into the subfamilies of the Lestodontinae and Mylodontinae was confirmed in one of the most extensive studies to date on the tribal history of the sloths. The study was published in 2004 by Timothy J. Gaudin , is based on anatomical features of the skull and suggests a close relationship between Mylodon and Paramylodon , while Glossotherium is more closely related to Lestodon . The latter two, however, show strong anatomical similarities to Paramylodon . Although this relationship could be reproduced several times in the following years, other authors were sometimes more critical. An example of this is a study by Luciano Varela and colleagues from the year 2019, including numerous taxa from the entire suborder of the sloth, in which the mylodon occupies a basal position compared to all late mylodons. A higher-resolution phylogenetic analysis of the mylodonts by a team led by Alberto Boscaini from the same year underpins the two-part structure of the terminal representatives with the difference, however, to Gaudin's study from 2004, that mylodon , paramylodon and glossotherium form a closer unit and can be assigned to the mylodontinae while Lestodon is more closely tied to Thinobadistes . A fundamental difference between the Mylodontinae and Lestodontinae is the development of the canine-like front teeth. In the latter, they are large and separated from the rear teeth by a long diastema, in the former they are only small in size or are partially reduced and are closer to the molar-like teeth. A study published in 2009 by Robert K. McAfee dealt in detail with the cranial and tooth morphological differences between Glossotherium and Paramylodon and also included Mylodon . Accordingly, there are greater similarities between the former than the latter. The peculiarity of Mylodon is expressed in the reduced set of teeth, in which the upper canine-like teeth are missing, but the lower ones are transformed into molar-like ones. In addition, the genus stands out for its simpler molar teeth and an advanced bone suture between the palatine bone and the upper jaw at the level of the third molar-like tooth. Glossotherium and Paramylodon on the other hand have caniniform teeth, albeit in different forms, more complex molariform teeth and a palatine-maxillary connection at the level of the last molar.
Internal systematics of the sloths according to Presslee et al. 2019 (based on protein analysis)
Due to the good conservation of fossils with remnants of soft tissue , Mylodon was one of the few extinct sloths, along with Nothrotheriops , that were included in molecular genetic studies at a very early stage . The first analyzes took place at the turn of the 20th to the 21st century. In their consistent results, a closer relationship between the Mylodon and the two-toed sloth ( Choloepus ) emerged, which means that the latter can be assigned to the superordinate group of the Mylodontoidea. The three-toed sloths ( Bradypus ), on the other hand, were closer to Nothrotheriops from the group of Nothrotheriidae and thus belonged to the Megatherioidea. This turned out to be contrary to the classic, anatomy-based classification of the sloths, in which the two-toed sloths form a community with the Megalonychidae within the Megatherioidea, while the three-toed sloths represent the sister group to all other sloth lines. Collagen tests from 2015, which took into account other extinct forms in addition to today's sloths, were somewhat contradicting this . For the time being, there was a close relationship between the recent tree sloths. However, Mylodon was not part of this analysis. It can be shown as problematic in all of the work that too few fossil species were included and the data obtained remained so ambiguous. A separate study on the genetic relationship of mylodon to today's sloths, taking into account both mitochondrial and nuclear DNA, from 2018 confirmed the previously determined result of a close relationship to the two-toed sloth.
In 2019, two extensive studies on molecular genetics and protein structure were published, in which for the first time a significant number of representatives of almost all fossil families were integrated. The former took place under the leadership of Frédéric Delsuc , who had previously researched the genetic relationship of today's secondary articulated animals for years, the latter largely comes from Samantha Presslee . In both studies, the anatomically proven large groups of the mylodontoid and megatherioidea were largely confirmed. However, with the Megalocnoidea, a third major line could be added to them , which represents the sloths of the Caribbean islands . The recent sloths are distributed according to the previous studies with the three-toed sloths on the Megatherioidea and with the two-toed sloths on the Mylodontoidea. The picture deviating in this point from the anatomically justified classification scheme of the sloths had the consequence that on the one hand the three-toed sloths do not form an outer group despite some special characteristics and the two-toed sloths are not closely related to the Caribbean sloths. The latter two - originally combined in the Megalonychidae - were clearly separated. The Megalonychidae were therefore limited to the mainland large ground sloths, the two-toed sloths expelled and the Caribbean sloths assigned to their own large group. Within the Mylodontidae, Mylodon forms a closely related group with Paramylodon and Glossotherium . Lestodon, on the other hand, is in an external position to these three genera. The constellation can also be found in a similar way in some of the anatomy-based phylogenetic kinship models already mentioned.
Often only one type of mylodon is recognized:
- M. darwinii Owen , 1840
In addition, numerous other species have been described within the genus. The best known are the M. listai introduced by Florentino Ameghino in 1898 and the M. insigne, which goes back to Lucas Kraglievich in 1928 . Both forms are occasionally recognized as valid by individual authors, partly due to the smaller body size and / or individual deviating tooth features, while others see them as synonymous with M. darwinii . The same applies to M. zeballozi , which was scientifically named in 1880 by Henri Frédéric Paul Gervais and Ameghino. In principle, however, the scientists are calling for a revision of the genus. An examination of several skulls of Mylodon in 2010 comes to the preliminary conclusion that any size differences could possibly reflect a sexual dimorphism or ecological differences in the corresponding habitat or temporal differences. Apart from that, in 2018, Castrocopros hauthali, an Ichnotaxon for the numerous coprolites from the Cueva del Milodón in Chile was introduced.
The taxonomic history of Mylodon is complex. Over a long period of time there were confusions and equations with other mylodontic forms such as Glossotherium and Paramylodon . Part of this complexity can be traced back to the person who first described the genre himself, Richard Owen (1804-1892). From 1836 onwards, Owen, one of the most important researchers of the Victorian era, studied fossils that Charles Darwin had brought back from his groundbreaking trip to South America on the HMS Beagle . The collection also included a lower jaw from Punta Alta near Bahía Blanca in the south of the Argentine province of Buenos Aires (copy number NHM 16617). The almost complete piece was characterized by a row of four molar-like teeth. In an extensive document from 1840, Owen referred the lower jaw to the genus Mylodon , which he had newly created, and named the species M. darwinii (“ darwinii ” is the spelling used by Owen, but nowadays “ darwini ” is also often used ; according to the rules of zoological nomenclature , the former version is correct). He referred the generic name to the molar-like teeth (from the Greek μυλη ( myle ) for "molar" and ὀδούς ( odoús ) for "tooth", thus translated as "molar tooth"), with the specific epithet darwinii he honored Darwin as the finder of the Specimen copy . As a second species besides M. darwinii , Owen referred to M. harlani in his work . This form was based on a lower jaw and a collarbone, both of which came from Big Bone Lick in Boone County in the US state of Kentucky and which Richard Harlan had already described in 1831 as Megalonyx laqueatus . Owen, however, recognized similarities in the structure of the lower jaw with his M. darwinii and renamed Harlan's shape. Harlan used an essay two years later to comment on Owen's choice of name in relation to Mylodon , which he found not very descriptive. Accordingly, in his opinion, the name could be related to almost any extinct mammal form, because almost all of them had the back molars. In addition, the designation would be inappropriate, since based on the Latin version dens molaris for "molar" and the second part of the word don (from dens for "tooth"), a duplication arises that would translate as "molar tooth". Regardless of this, Owen's establishment of the genus Mylodon with two species meant that the extinct sloth representative was widespread in both South and North America.
From Glossotherium , Grypotherium, and Paramylodon
In his work from 1840, Owen established the genus Glossotherium in addition to Mylodon , but without identifying a specific species. The basis here was a rear skull fragment, which comes from the river bed of the Arroyo Sarandi in the Uruguayan Department of Soriano . In contrast to Mylodon , for which Owen assumed a relationship to the other then known large ground sloths such as Megatherium or Megalonyx , he placed Glossotherium in a row with the anteaters and pangolins and postulated an insectivorous way of life for the animals. Two years later, however, Owen rejected the name Glossotherium again. This happened in the course of the processing of an almost complete skeleton, which had been discovered the year before in the flood plains of the Río de la Plata north of Buenos Aires . The largely intact skull was characterized by a short and wide snout and a set of teeth consisting of a total of 18 teeth, the foremost tooth of which had a canine-like design. Due to the similarities in the tooth structure with the flat, molar-like teeth, Owen assigned the skeleton to the genus Mylodon and introduced the new species M. robustus . The skull fragment that he had originally placed with Glossotherium , he now connected with M. darwinii . As a result of this study, three species of the genus Mylodon already existed in the 1840s .
A skull and lower jaw found near Pergamino in the province of Buenos Aires served the Danish zoologist Johannes Theodor Reinhardt (1816–1882) in 1879 as the basis for a comprehensive description. Characteristic for the skull was the narrow snout and a closed nasal arch, which was created by the solid fusion of the nasal bone with the median jawbone. Furthermore, the dentition consisted of a total of 16 teeth, the upper front canine-like tooth was reduced, while in the lower jaw there were four molar-like teeth. Reinhardt noticed similarities to M. darwinii in the structure of the lower jaw , but in the skull design his find differed significantly from the broad-snouted M. robustus due to the narrow snout . However, according to Reinhardt, there were similarities in Owens Glossotherium skull fragment with corresponding skull sections in M. robustus . Due to the clear parallels between M. darwinii and his narrow- snouted skull find Reinhardt propagated the new genus Grypotherium with Grypotherium darwinii as the type species . Florentino Ameghino (1854–1911) took a different path around ten years later. In 1889 he confirmed the separation of both M. darwinii and M. robustus not only on the species, but also on the genus level. Deviating from Reinhardt, but in agreement with Owen, he saw the lower jaw of M. darwinii and the skull fragment of Glossotherium as belonging together. Since Glossotherium had priority over Mylodon (Owen mentioned the former over the latter in 1840) and Grypotherium in this scenario , Ameghino introduced the species Glossotherium darwinii . In contrast, he left the status of M. robustus untouched. Arthur Smith Woodward (1864–1944) in turn followed Reinhardt's argument. In an essay published in 1900, he presented finds of ground sloths from southern Patagonia and at the same time revised the collection of Charles Darwin. He equated the lower jaw of M. darwinii with Reinhardt's Grypotherium and subsequently restored Grypotherium darwinii . Smith Woodward assigned the skull fragment of Glossotherium to M. robustus analogously to Reinhardt .
In the period at the turn of the 19th to the 20th century, the number of fossil sloth finds in North America increased sharply. Among other things, an expedition of the American Museum of Natural History discovered a partial skeleton of a large mylodont sloth near Hay Spring in the US state of Nebraska in 1897 . Characteristic of the skull was the wide and open rostrum and a set of 16 teeth. The front canine-like teeth were missing in the upper row of teeth, but they were formed in the lower row. As a result, the design of the skull resembled that of Glossotherium , and the teeth, in turn, could be compared more with Mylodon . Barnum Brown (1873–1963) used the find in 1903 to establish the new genus Paramylodon with Paramylodon nebrascensis as the type species. In addition to the known finds of M. harlani from North America, the reduction of the upper canine-shaped teeth was shown to be decisive for Brown. However, the North American Mylodonts proved to be quite variable with regard to this characteristic, which was not least due to the immense increase in finds in the asphalt pits of Rancho La Brea in southern California and the resulting possible series investigations. Chester Stock (1892–1950), who had dealt with the fossil remains of Rancho La Brea for a long time, therefore reunited Paramylodon nebrascensis with M. harlani 14 years later .
Lucas Kraglievich (1886–1932) in particular devoted himself in the 1920s to the increasing complexity of structuring and naming the large mylodontic forms of South and North America . With the help of the skull features, Kraglievich worked out that Mylodon , Glossotherium and Grypotherium clearly refer to two different forms that can be separated at the genus level. Owen's description of M. darwinii firmly linked the genus name Mylodon with the specific epithet . Kraglievich assigned genus and species, among other things, to the complete, narrow- snouted skull and lower jaw of Reinhardt's Grypotherium from Pergamino, as well as Owen's lower jaw from Punta Alta from the first description of the species as a type specimen. On the other hand, he referred Owen's broad-snouted M. robustus to the genus Glossotherium , which also included the skull fragment that Owen had once used to describe the genus. However, Kraglievich limited all North American finds to paramylodon . As a result, Mylodon remained only in South America, but here it was clearly differentiated from Glossotherium in the structure of the skull and teeth. This proposed structure was followed by individual authors such as Ángel Cabrera . In the course of the 20th century there was a controversial discussion about the synonymity of Glossotherium and Paramylodon , but this dissolved in the mid-1990s in favor of separating the two forms.
Discourse on the type species
The determination of the type species of Mylodon turned out to be somewhat problematic . Owen himself remained imprecise in his first description of Mylodon in 1840 in the definition of the type species. Although he based the genus on the lower jaw found by Darwin and thus on M. darwinii , he stated that this was the second species described after M. harlani . Two years later in his description of M. robustus , Owen identified M. darwinii as the primary species. In the course of research history, the nominate form of Mylodon changed depending on the processor. Joseph Leidy adopted Owen's point of view in 1855 and listed M. darwinii as the first species within the genus. For Reinhardt, M. robustus was the type form in 1879 , which can be ascribed to the fact that he had relocated the actual finds from Mylodon to the genus Grypotherium . Richard Lydekker , on the other hand, saw M. harlani as the nominated form in 1887 , which, by the way, was later supported by Barnum Brown. From today's point of view, the question of whether M. darwinii or M. harlani is to be regarded as a type form is not completely trivial, since both species are assigned to different genera and the latter species could well have priority by being mentioned first by Owen. However, in his 1928 revision, Kraglievich listed several reasons why M. darwinii is the actual nominate form. These are above all the already mentioned facts of the description of Mylodon on the basis of Darwin's lower jaw find and the listing by Owen as the first species. Kraglievich's approach was widely recognized in the subsequent period.
Domesticated sloths and modern myths
A special chapter in the research history of the genus Mylodon concerns the Cueva del Milodón in southwestern Patagonia . Shortly after its discovery by the German captain Hermann Eberhard in 1895 and the associated finding of a remnant of fur, various scientists visited the cave, for example the Swedish researcher Otto Nordenskjöld as part of his South America expedition in 1896. The piece of fur that was also recovered along with phalanxes was later examined by Einar Lönnberg . Shortly afterwards, an Argentine group of naturalists around Perito Moreno reached the region, including Rudolph Hauthal and Emil Racoviță . Moreno himself did not enter the cave, but his companions carried out individual investigations there. In addition, the expedition was able to secure Eberhard's original piece of fur (hanging as a curiosity in a tree). This later formed the basis for a description by Arthur Smith Woodward . Hauthal returned to Cueva del Milodón in 1899 and carried out extensive excavations. Numerous remains of the Mylodon came to light. Particular attention was paid to a collection of rubble in the back of the cave, which seemed to be arranged in a linear fashion. In addition, concentrations of grasses and coprolites were found . In a publication that appeared in the same year, Hauthal and his colleagues interpreted the stone structure as a kind of gate or pen in which the giant sloths were kept by the people of the time, and the grasses as a store of food for the animals. In addition, they presented another piece of fur that had an allegedly intentional cut made by humans and made the workers think of peeled skin. Santiago Roth , who examined the remains of Mylodon , established the species Grypotherium domesticum based on this in 1899 . The additional species refers to the assumed domestication of Mylodon by humans. Only a little later, Otto Nordenskjöld's cousin Erland , who carried out investigations in the Cueva del Milodón during the same period, contradicted the interpretation in parts , as he observed, among other things, a significantly wider spread of the manure residues. He also suspected that Mylodon was not kept in the cave by humans, rather he interpreted the bones as remains of prey from predators. From today's perspective, the idea of a tamed sloth could not be confirmed. The stones that formed the alleged pen are seen as a natural accumulation, created by the continuous breaking off of the cave ceiling over the past 12,000 years. In addition, the influence of early humans on the cave and its use is far less than initially assumed.
Closely related to the exploration of the Cueva del Milodón is the idea that Mylodon survived into historical times. Responsible for this is Florentino Ameghino , an Argentine naturalist who, at the end of the 19th century, classified the remnants of fur recovered by Moreno from the Cueva del Milodón as not very old due to its excellent preservation. Ameghino's reddish color of fur reminded him of a report he heard from Ramón Lista , an explorer and the governor of the Argentine province of Santa Cruz . According to this, Lista shot an animal at night in Santa Cruz at the end of the 1880s, but it escaped. The animal had Listas description, according to a red fur, but otherwise externally resembled a pangolin . Ameghino was convinced that the animal from Lista's story and the fur remnants belonged to the same species. Based on the bone platelets embedded in the skin, he identified this as a small representative of the Mylodonts. He also linked Lista's story with the traditions of indigenous inhabitants such as the Tehuelche about red-haired animals who dug burrows in the earth with their claws and allegedly had an impenetrable skin. For this animal and in honor of Lista, Ameghino created the species name Neomylodon listai in 1898 . At the same time, he encouraged the search for the animal in individual publications. Moreno doubted the existence of a modern ground sloth and attributed the good conservation of the fur residues to the special conditions of the Cueva del Milodón, just as he classified the reports of the Tehuelche about an animal called "Ellengassen" as fictional. However, Ameghino's remarks prompted Hesketh Vernon Hesketh-Prichard to embark on an expedition to Patagonia in 1900 to track down the supposedly existing Mylodon . Hesketh-Prichard published his travelogues in 1902 under the title Through the heart of Patagonia , but found that there were no traces of Mylodon on either side of the Andes . In the 1977 travel novel In Patagonia (German In Patagonia ), Bruce Chatwin processed the stories about Mylodon and the Cueva del Milodón.
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