Paramylodon

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Paramylodon
Skeletal reconstruction of paramylodon

Skeletal reconstruction of paramylodon

Temporal occurrence
Upper Pliocene to Upper Pleistocene
2.1? or 1.8 million years to 11,000 years
Locations
Systematics
Sub-articulated animals (Xenarthra)
Tooth arms (pilosa)
Sloths (folivora)
Mylodontoidea
Mylodontidae
Paramylodon
Scientific name
Paramylodon
Brown , 1903

Paramylodon is an extinct genus of the sloth and belongs to the also extinct family of the Mylodontidae . It was a large, ground-living sloth thatlivedin North America primarily in the Pleistocene from around 1.8 million years ago to around 11,000 years ago. The genus is documented through numerous fossil finds, their number increases visibly in the late phase of the appearance. They come predominantly from the south of today's USA , but in the western area they also spread to the south of Canada . The asphalt pits at Rancho La Brea near Los Angeles in California , where the genus is documented quite frequently, arean outstanding find.

Due to the sometimes very good fossil conservation of the animals, the body armor typical of the Mylodonts in the form of osteoderms has been handed down. In addition, a gender-specific difference in skull structure can be demonstrated. According to the finds, the representatives of Paramylodon lived in open landscapes, sometimes also in mountainous locations, and most likely ate a herbivorous diet or a mixed vegetable diet. Unique trace fossils also provide information about the locomotion of the animals, which took place on four feet. In addition, due to the design of the forelegs, a certain burrowing lifestyle cannot be ruled out.

Within the genus, only one species is recognized with Paramylodon harlani . The first fossil finds date from 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 at the beginning of the 20th century. Overall, the sloth representative can look back on an eventful research history. In the first 150 years, the finds that are now ascribed to Paramylodon and later the genus itself were repeatedly associated with other forms, first with Mylodon , but increasingly with Glossotherium since the 1950s . Especially with the latter genus, Paramylodon connects numerous characteristics that speak for a close relationship. Only since the 1990s have both genera been considered independent, Glossotherium being restricted to South America , while Paramylodon inhabited North America.

features

General

Paramylodon was a medium-sized representative of the Mylodontidae . A fully reconstructed skeleton, based on finds from Rancho La Brea in California , has a total length of 279 cm, of which the tail takes about 118 cm. At the shoulders it reaches a height of 112 cm, at the pelvis it measures 122 cm. The weight for these late members of the Upper Pleistocene is given as around 1.39  t , but earlier forms were definitely smaller. Overall, the Paramylodon was a robustly built animal. It was characterized by an elongated skull, a short neck, a short and compact body with a wide pelvis, and strong limbs and tail.

Skull and dentition features

Skull of Paramylodon

Paramylodon's skull was long and narrow. It reached a total length of 42.9 to 49.8 cm, a particularly large skull measured 54.0 cm. When viewed from above , it had a rather rectangular shape with an average width at the occiput of 18.8 cm, behind the eyes of 12.2 cm and at the snout of 14 cm. The snout, which widened continuously towards the front, was typical of numerous Mylodonts. Overall, however, the skull was clearly narrower than that of the comparably large Glossotherium , the latter exhibiting a dome-like bulge on the forehead line in a side view, which did not occur in Paramylodon . However, with the exception of the central area (the dome-like bulge in Glossotherium ) , the skull of Paramylodon was on average higher, measuring around 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 on the side and only opened to the front, which was about as high as it was wide, which was due to the overall narrower skull. The middle jaw bone was, typical of the sloth, only loosely connected to the upper jaw. On the frontal bone , the nasal bone protruded far back, so that the seam between the two skull bones was more V-shaped. In addition, the frontal bone represented the largest bone in the entire skull. Between the parietal bones there was a strong crest , which, however, appeared much narrower than in comparison to Glossotherium . In contrast to most sloths, the zygomatic arches were secondarily closed again. The front part of the arch, starting from the cheekbone and pointing backwards, had three processes, one of which was oriented upwards, one downwards and the middle horizontally. The posterior arch section attached to the temporal bone was finger-shaped and connected to the middle process of the anterior arch segment. On the underside of the skull, the palatine bone in Paramylodon protruded significantly further back than in Glossotherium , which was caused by the longer expansion of the bone behind the last molar. As with many Mylodonts, the two flanks of the wing bone were clearly inflated. In the case of Paramylodon , however, this was not quite as clear as in the case of Glossotherium , so that the bulging structures were much further apart due to the base phenoid of the sphenoid .

The lower jaw reached lengths from 31.5 to 43.6 cm. It was massive and broad. The horizontal bone body increased continuously from front to back in height, under the rearmost tooth its height was up to 10.5 cm. The sturdy symphysis was up to 11 cm long and - typical for Mylodonts - wide. She pulled forward, which is a hallmark of almost all sloths. This spoon-like extension of the symphysis symphysis did not protrude as clearly laterally in the Paramylodon as in the Glossotherium , so that the side edges were more straight and less curved than in the latter. The width of the symphysis in the anterior area was up to 15 cm. The articular process only protruded insignificantly from the occlusal plane, the crown process was significantly higher. Its leading edge ran in a straight line in Paramylodon , deviating from the curved shape in Glossotherium . As is common with sloths, the set of teeth consisted of 5 teeth per half of the upper jaw and 4 teeth per half of the lower jaw, so a total of 18 teeth were formed. The foremost teeth in each case had a canine-like ( canine - shaped ) shape, the others were molar-like ( molar-shaped ). The structure of the teeth is considered to be ancestral within the sloth. However, the upper caniniform teeth were often reduced in the later representatives of Paramylodon , so that the dentition consisted of only 16 teeth. A similar tooth reduction is not known with Glossotherium . With Mylodon, on the other hand, the foremost teeth in the upper dentition were no longer formed, but the lower canine-shaped teeth were similar to the rear molars. The caniniform teeth in Paramylodon had an oval cross-section and were curved backwards. However, they did not reach the size of Glossotherium or Lestodon . There was a short diastema to the back of the teeth . The molar-like molars had a flat shape with a slightly raised edge. In outline, they had a bilobed shape with a strong central constriction, except for the first molar, which was more rectangular in shape and formed the longest tooth in the upper jaw with an average length of 3.7 cm. The flap-like structure of the second upper molar was much more evident than that of Glossotherium . All teeth typically lacked tooth enamel , rather they consisted of a harder variant of the dentin ( orthodentine ), and there was also an outer layer of dental cement . The proportion of orthodentine in paramylodon reached 28%. The upper row of teeth was on average 14.4 cm long, of which the rear molars were 12.6 cm. Due to the forward widening of the snout, the rows of teeth diverged from one another.

Body skeleton

Remnants of the body skeleton of Paramylodon

Above all, the extensive finds from Rancho La Brea allow a comprehensive reconstruction of the body skeleton. The spine was composed of 7 cervical, 16 thoracic 8 to 9 lumbar and sacrum and 21 tail vertebrae. The humerus was massive, the length was 46 cm. The head didn't stand out too clearly. A prominent bone ridge (deltopectoral ridge) was attached to the humerus shaft, but was less prominent in the upper part than in Glossotherium . The lower end of the joint protruded far from the side, an entepicondylar foramen , which occasionally occurred in some sloths, was not formed here. The ulna had a very extensive upper articular process, the olecranon . It was about 20 cm long, the entire bone reached 40 cm in length. The construction of the cubit was shorter and more robust than that of Glossotherium , the shaft was broad and narrowed at the top in front and behind. Likewise, the spoke was short and massive with a length of 29.6 cm. The longest bone represented the thigh bone with about 54.6 cm. Very short specimens from Rancho La Brea measured only 51 cm, very long 58 cm. The flat and wide design typical of the ground sloth was striking, so that the bone looked almost like a board. The head rose only a little from the surface and was more inwardly facing. The shaft was turned slightly inwards, 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 shin was significantly shorter than the thigh bone. This is a typical feature of the Mylodonts, in whose predominantly late representatives the lower section of the hind leg often only reached about half the length of the upper. In the case of Paramylodon , the tibia was 45% of the length of the femur. Its shaft, like that of the thigh bone, was flattened and also showed a slight twist. The upper end of the joint protruded laterally, the width here was about three quarters of the length of the entire bone. The fibula was not fused with the shin, it was 26.3 cm long.

Paramylodon skeleton with hands visible

Hands and feet showed a similar structure as in the other large Mylodonts Glossotherium and Lestodon , deviations exist in the details. The hand had a total of five rays (I to V), with only the three inner rays (I to III) having claws. The metacarpal bone of the first ray was fused with the large polygonal bone to form a unit, which is often proven in ground-dwelling sloths (so-called Metacarpal Carpal complex or MCC). The metacarpal bones of the third to fifth ray were massive and over 10 cm long, that of ray IV had the strongest structure. In addition, the first two phalanges were fused with one another on finger ray I, and there were three phalanges each on rays II and III, of which the first two had significantly reduced lengths. The respective end links of the three inner rays had extensive claw processes, which suggests correspondingly large claws. The length ranged from inside (I) to outside (III) from 7.5 cm to 15.4 cm up to 17.4 cm, the height varied from 2.9 to 5.7 cm. The clawless outer fingers had greatly reduced phalanxes in size. Paramylodon's foot had a total of four rays (II to V), the innermost ray was completely reduced. There were only claws on toes II and III, which were also the strongest. However, the metatarsal bones here had rather short lengths of 3.6 and 6.5 cm, respectively, at the outer rays they were each over 11.0 cm long and were very massive. As with the other two Mylodonts, the second ray only had two toe phalanxes, since the first and second phalanx were fused into a unit corresponding to the hand. In contrast to Glossotherium and Lestodon , the third ray in Paramylodon often consisted of only two members. The respective terminal phalanges with claws had an extremely strong structure, analogous to the hand, the claw extension alone measured around 8.5 cm on the second and 11.1 cm on the third ray and was 3.3 and 3.9 cm high respectively. The outer rays, in contrast, had greatly reduced end links.

Osteoderms

The Mylodonts are the only known line of sloths in whose representatives bone platelets, so-called osteoderms , were formed in the skin, analogous to today's armadillos . In contrast to these, however, they did not form solid bone armor in the Mylodonts, but were rather loosely scattered, as is shown by remains of skin from Mylodon . There are several hundred such osteoderms of Paramylodon from Rancho La Brea, among other things also as a dense layer on a plate from the 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 showed a rough surface with irregular depressions, while the underside was smooth and convex. In cross-section, they had a compact structure consisting of numerous fiber bundles mixed with hard bone lamellae ( osteomas ). In principle, the bone platelets of the Mylodonts were more simply structured than those of the armored articulated animals .

Distribution and important fossil finds

Paramylodon was endemic to North America and possibly also to Central America . The oldest finds that can be clearly assigned to the genus are known from the Lower Pleistocene . Older forms of mylodonts come from the Upper Pliocene of Mexico and the US state of Florida . Of the latter, the partial skeleton from the Haile 15A site is to be highlighted, a sediment- filled fissure in limestone in Alachua County , the age of which is estimated to be 2.1 to 1.8 million years. These early representatives are generally referred to as " Glossotherium " chapadmalensis , but their position within the genus Glossotherium is controversial. The fossil-rich El Gulfo local fauna from the mouth of the Colorado River in the Mexican state of Sonora are only a little more recent . They are already assigned to Paramylodon and date from 1.8 to 1.6 million years ago. Overall, fossil remains from the Lower and Middle Pleistocene are relatively rare and come from around 20 sites in North America. These are mainly distributed in the southern and central area of ​​today's USA and in northern Mexico, but also scatter in the west of the continent as far as the southern part of Alberta in Canada . They can be found in the lowlands as well as in mountainous areas, the highest point of discovery in Colorado is about 2900 m. One of the most important sites of discovery of that time is the Leisey Shell pit in Hillsborough County , Florida, from where several skulls and postcranial skeletal elements have been reported that are approximately 1.2 million years old. The locality Fairmead Landfill in Madera County in Colorado, which also produced several partial skeletons, already belongs to the transition to the Middle Pleistocene .

Rancho La Brea , reconstruction, a paramylodon skeleton lying in the foreground

Much more extensive is the lost material of Oberpleistozäns that more than 100 sites comes from, alone in California is paramylodon handed down by over 60 points Fund. The distribution of the genus is similar to that in the Lower Pleistocene, but it also occurs a little further east in the Midwest , for example in Iowa . It reached its northernmost point of discovery at that time with Sequim in Washington at 48.1 ° north latitude, in the south the genus has also been passed down from Mexico, where its southern limit of spread existed at Valsequillo at 19 ° north latitude, but some finds have since indicated this that Paramylodon may also have come down from Guatemala and El Salvador . Among other things, finds of a young animal and a full-grown individual could be recovered from the Stevenson Bridge in river deposits of Putah Creek in Yolo County of California, which belong to the beginning of the last glacial period . Two nearly complete skeletons have been reported from Shonto and Richville, Arizona, and are among the few known finds from the state. In general, paramylodon fossil remains on the Colorado Plateau in the southwest of today's USA and also in northwest Mexico are very rare, which may be related to the then drier climate in this area. The finds from the asphalt pits of Rancho La Brea in southern California are of outstanding importance worldwide . An extensive fossil fauna comes from here, the age of which ranges from 45,000 to 14,000 years ago. The first finds were discovered in the second half of the 19th century, but the much more important material can be traced back to targeted scientific investigations at the beginning of the 20th century, which include a total of over 100 documented sites. What is striking in the fauna spectrum is the unusual dominance of predators over herbivores. Most likely, the predators were attracted in large numbers to animals stuck in the bitumen 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 ( Eremotherium is only known from the eastern part of the USA). With over 70 individuals , however, the paramylodon is by far the most common representative; 30 skulls alone are to be highlighted among the finds. Another very extensive fossil complex is the Diamond Valley Lake Local Fauna in the Diamond Valley and the Domenigoni Valley in Riverside County, also in southern California. The material has been recovered during the construction of Diamond Valley Lake since the mid-1990s and currently comprises more than 100,000 objects of more than 100 taxa from over 2,600 different localities. In contrast to Rancho La Brea, the large herbivores dominate here, while the proportion of large predators is low. This means that it can be concluded that the fauna community is undisturbed. Paramylodon is occupied with around 280 individual finds, which makes up about 8% of the total mammal fauna. The ground sloth is the fifth most common mammal in the Diamond Valley Lake Local fauna, after the bison, the horses, the primeval trunk mammoth pacificus and the camel camelops . The other two sloths, Megalonyx and Nothrotheriops , also found in Rancho La Brea , on the other hand, play only a subordinate role with a combined 0.5% of the finds. The age of the Diamond Valley Lake Local Fauna is radiocarbon dating that of Rancho La Brea.

Like most other large, ground-living sloths, the Paramylodon also disappeared during the Quaternary Extinction Wave towards the end of the Pleistocene . In contrast to many other genera, however, paramylodon hardly has any radiometric data measured directly on fossil material . Among the most recent is a value from Rancho La Brea, which is 20,450 BP . However, clearly more recent finds are known, few of them came from archaeological sites that are related to the early settlement of the North American continent by humans. One of the rare evidence comes from El Fin del Mundo ("The End of the World") in Sonora. The station, discovered in 2007, could be dated to an age of 11,550 years BP using the radiocarbon method using charcoal. Due to the occurrence 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 in North America. In addition to two skeletons of proboscis , one of which clearly represents Cuvieronius and had obviously been dismantled by the hunters and collectors of the time , remains of paramylodon were also found . In addition, more than 130 osteoderms from the Aubrey Clovis site in north-central Texas are documented. The soil surrounding the finds was radiometrically dated to an age of 10,940 years BP. Stone artifacts also documented there , which comprise about 9800 pieces, can also be referred to the Clovis group due to a Clovis point. However, the remains of Paramylodon have no direct reference to the early settlers, as they were found with the exception of a single bone plate in a nearby watering hole. Due to the few common finds so far, it is unclear whether direct hunting led to the animals becoming extinct.

Paleobiology

Height change and gender dimorphism

Like numerous other animal groups, the Paramylodon experienced a marked increase in body size in the course of its tribal history . The weight of the members of the Lower Pleistocene is given as about 915 kg, the late members of the Upper Pleistocene, on the other hand, reached a body weight of up to 1.39 t. The basis for the respective weight estimates are the thigh bones , whose lengths are 48.4 and 54.6 cm, respectively. The earliest forms from the Pliocene , whose position is widely discussed within the genus Paramylodon , had a total weight of around 310 kg with a femur length of 35.5 cm. Taking these early representatives into account, the weight of paramylodon increased by a factor of 4.5 over the course of a good 2.5 million years. It is particularly noticeable that especially in the end of the Pleistocene at the time of the last glacial period with its extremely pronounced climatic fluctuations, hardly any size variations occur, as studies on the numerous finds at Rancho La Brea from the time 45,000 to 10,000 years ago show. This is explained with a high flexibility of the species in relation to the environment and thus a high adaptability. However, the assumption ignores the fact that, according to Bergmann's rule , increasingly cooler conditions would lead to an increase in body size.

On the basis of the extensive fossil finds from the late Pleistocene, two morphotypes can be distinguished in Paramylodon , a graceful and a robust variant. The morphotypes are not reflected in the general size of the skull, but mainly affect their characteristics, such as the widths. Differences can also be seen, for example, on the occiput , which is vertical in the robust version, but diagonally backward in the more delicate version. As a result, the joint surfaces for attaching the cervical spine are more prominent in the latter than in the former. Further deviations can be found in the formation of the canine-shaped teeth, which, if present, end up pointed in robust individuals, but blunt in delicate ones. It is possible that the two morphotypes are not species or taxonomic variations in the sense of subspecies, as was originally assumed, since they often occur at the same site. Rather, they are more an expression of an intraspecific gender dimorphism . However, it is currently impossible to assign a morphotype to a specific gender. In the 30 known skull finds from Rancho La Brea, the ratio of robust to delicate is 3: 1, in the Americas Fall Reservoir in Idaho with three skulls it is 2: 1 and in Ingleside, Texas with three skulls it is 3: 0. It is noteworthy that the sex dimorphism is not reflected in the postcranial skeleton and so, as already noted with the skull, there is no size dimorphism in the form of significant differences in length in the extremity bones. In contrast, Eremotherium , which was also widespread in North America at the same time, but belongs to the Megatheriidae, is known to have a pronounced size difference between the sexes.

Locomotion

Generally a quadruped locomotion is assumed for the ground-living sloths. Due to the body's center of gravity, which was shifted far back, it was obviously also possible for them to change into a bipedal position, whereby they were able to support themselves with the strong - in contrast to today's tree sloths - very long tail. The rear foot of the Paramylodon is turned inwards, so that the main load when the foot touches down lies on the outer beam (V). This creates the pedolateral gait characteristic of numerous ground sloths , which required significant restructuring of the shape and positioning of the tarsal bones in relation to one another, especially in the ankle and heel bone . With Paramylodon the outer edge of the foot was slightly arched, so that it formed a more or less straight edge and the heel bone was almost in full length in contact with the ground. This is consistent with other Mylodonts, but differs greatly from the closely related Scelidotheriidae , which had a high arched foot with only the rear end of the calcaneus touching the ground. Another peculiarity is found in the hind limbs. Here the musculoskeletal system is characterized by an extremely short lower section. In paramylodon , the lower section reaches less than 50% of the upper. Such a blueprint, which almost all Mylodonts have, suggests a rather slow and clumsy movement. In comparison, the megatheria had significantly longer lower limb sections.

Paramylodon footprint (bottom) and reconstructed foot skeleton (top)

Trace fossils , which give clues to the locomotion of the ground sloth, are rarely preserved. For paramylodon , such step seals could be detected in the Nevada State Prison near Carson City . The traces were discovered in the second half of the 19th century during sandstone mining and initially, in 1882, interpreted as evidence of gigantic people. But already in the following year Othniel Charles Marsh recognized a connection with extinct ground sloths and looked for the cause of the tread marks among the Mylodonts, of which there are also bone fragments from the same site. Overall, in addition to the paramylodon, numerous traces of other mammals - such as mammoths , horses , moose and predators - and birds have been discovered near Carson City . The step seals are spread over an area of ​​around 8000 m², they are largely covered by the prison construction today, but are well documented by casts. A total of ten tracks could be observed from Paramylodon , consisting of 15 to 20 individual, mutually indented tracks. Each individual footprint is 47 to 51 cm long and around 20 cm wide, in outline they actually resemble human footprints, but they are much more dented on the sides. Further studies showed that the shape of the footprints matched the shape of the paramylodon 's foot very well and that the human-like shape is due to the sloth's outward-facing foot. The lateral distance between the tracks is about 60 cm, which roughly corresponds to the distance between the two hip joint sockets on the pelvis of Paramylodon , as well as the stride length of about 146 cm, which in turn corresponds to the known hind leg length of 95 cm. Strikingly, this means that almost exclusively hind footprints have survived, which was initially interpreted as the animals moving on bipeds, analogous to the corresponding trace fossils from Megatherium in South America. However, it could be determined that the individual footsteps of the hind feet cover those of the front feet. Since the front foot is much smaller than the back foot and touches down differently, it creates a much smaller footprint. In individual cases, footsteps have been handed down that testify that the forefoot is covered by the rear foot. For anatomical reasons, such as the bent legs when running, the position and orientation of the rear foot to the leg and the like, a permanent bipede of the large ground sloth is unlikely. The stride length of Paramylodon suggests an average speed of 1.8 to 2.2 m / s, which roughly corresponds to the determined speed of Megatherium .

Investigations on the shoulder blades of both young and adult individuals show a significant change in shape, which leads from a rather round shape in the former to a largely ovoid shape in the latter. The ontogenetic impressions are comparable to those of today's tree-dwelling sloths. The similarity of the shoulder blades of the young representatives of Paramylodon and the young of today's sloths suggests comparable behaviors. According to this, the pups of Paramylodon still had various climbing skills and possibly clung to their dams during transport.

Diet

The mylodonts are generally considered to be sloths that are more adapted to grass food, the assumption is based on their high-crowned teeth and, unlike other lines of sloths, the flat chewing surfaces, which therefore resemble those of the ungulates that feed on this . The lack of enamel in the sloth's teeth makes comparisons difficult. The grass-eating diet was adopted very early on due to the special formation of the teeth. Analyzes of the Paramylodon's chewing apparatus showed that the food was mainly chewed in forward and backward as well as lateral chewing movements, which is also indicated by the corresponding grinding marks. This is not contradicted by the canine-shaped front teeth, which - when they are formed - are rather small. The mandibular joint in Paramylodon is broad and has an unspecialized surface, the associated glenoid pit on the skull appears flat, which is typical for herbivores with their rotating chewing movements. However, there is an additional, almost vertical joint facet on the inside, which anchors in a recess on the outer wall of the wing bone . As a result, excessive lateral chewing movements were rather limited. The arrangement of the masseter muscle meant that the paramylodon could only open its mouth by 22 °, which is significantly less than that of the two-toed sloths , which, however, have comparatively longer canine-shaped teeth. All in all, the structure of the chewing apparatus suggests a preference for mixed vegetable food ( mixed feeder ). The construction of the snout, which is not quite as wide as in Lestodon , for which a more grass-based diet is assumed ( grazer , analogous to today's white rhinoceros ), but significantly wider than in Megatherium , which is more leaves, speaks for such a way of feeding preferred ( browser , comparable to black rhinoceros ). The elongated symphysis of the lower jaw protrudes far beyond the nasal region. Since there is no ossification of the nasal septum as with Mylodon , strong cartilage development must be assumed here. It is also possible that the tongue also had a supporting function in the intake of food. Due to the position of the hyoid bone, which is far back in the skull and its robust structure with strong muscle attachment points, the geniohyoideus muscle, for example, was particularly strong and long, so that a very flexible tongue can be assumed.

Since paramylodon in contrast to mylodon no coprolites are known, the food remains cannot be determined directly. In addition, due to the lack of enamel, detailed isotope examinations are rarely possible. The implementation of such methods therefore requires excellent fossil conservation, in the case of Paramylodon it was achieved on the dentin of some teeth from the Upper Pleistocene site of Ingleside in Texas. The results, which were obtained with the help of the carbon isotopes , lie between the ranges of today's herbivores specializing in a hard ( grazer ) or soft ( browser ) vegetable diet and thus advocate a mixed diet, but possibly with a stronger tendency towards grasses. Thus the results so far agree well with the open landscape in which the paramylodon lived. But it is also believed that the sloth representative may have been digging for roots. This is supported by the strong front legs, which had a sturdy humerus that protruded widely at the lower end of the joint , a short spoke with a long olecranon for massive forearm muscles and somewhat flattened claws and were therefore very suitable for digging. In addition, there are differences in the structure of the teeth between early and late members of the genus. The forms from the Lower Pleistocene had even less high crowns and, accordingly, a lower lower jaw, while those from the Upper Pleistocene had significantly higher teeth and a more massive lower jaw. Possibly this reflects an increasing adaptation of paramylodon over time.

Others

The majority of the paramylodon finds include individual individuals, mass collections such as in Rancho La Brea represent accumulations over several millennia and thus have a rather random character. It can therefore be assumed that the sloth genus, like today's tree-living representatives, appeared solitary and at most formed mother-young animal groups. The digestive system was probably structured in a similar way to that of the recent species, which means that the paramylodon also had a rather slow metabolism with a long passage time for the food, which, however, was used to a large extent. In connection with the rather slow movement, this also speaks against larger seasonal migrations - in contrast to numerous other grass-eaters that inhabit open landscapes. Accordingly, the animals were relatively true to their location. Striking at numerous sites with Paramylodon is the frequent occurrence with the prairie mammoth and the bison . Both representatives of large herbivorous mammals showed a fundamentally different way of life with their herd formations and long migrations as well as deviating digestive systems. Therefore, Paramylodon probably used a different ecological niche to avoid direct competition with the other megaherbivores of the North American steppe landscapes .

Systematics

Internal systematics of the Mylodontidae according to Boscaini et al. 2019 (based on skeletal anatomical analyzes)
 Mylodontidae  


 Urumacotherium


   

 Pseudoprepotherium



   


 Paroctodontotherium


   

 Octodontotherium



   

 Brievabradys


   



 Lestodon


   

 Bolivartherium



   

 Thinobadistes


   

 Sphenotherus


   

 Lestobradys





   

 Pleurolestodon


   


 "Glossotherium" chapadmalense


   

 Simomylodon



   



 Kiyumylodon


   

 Mylodon



   

 Paramylodon



   

 Glossotherium









Template: Klade / Maintenance / Style
Internal systematics of the sloths according to Presslee et al. 2019 (based on protein analysis)
 Folivora  

 Megalocnoidea


   

 Megatherioidea


  Mylodontoidea  

 Scelidotheriidae


   

 Choloepodidae


  Mylodontidae  

 Lestodon


   

 Paramylodon


   

 Mylodon


   

 Glossotherium









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Paramylodon is an extinct genus from the also extinct family of the Mylodontidae . The Mylodontidae in turn form part of the suborder of the sloths (Folivora). In a classical system based on skeletal anatomical features , the Mylodontidae together with the Orophodontidae and the Scelidotheriidae represent the superfamily of the Mylodontoidea (sometimes the Scelidotheriidae and the Orophodontidae are also only listed as a subfamily within the Mylodontidae), which in turn is the second major family alongside the Megatherioidea and represents an important line of sloths. According to molecular genetic and protein-based studies, in addition to the Mylodontoidea and the Megatherioidea, the Megalocnoidea can also be identified as the third major line. With the two-toed sloths ( Choloepus ), one of the two species of sloth that still exists today can be assigned to the Mylodontoidea . The Mylodontidae form one of the most diverse groups within the sloth. Their characteristics include high-crowned teeth with, unlike the Megatherioidea, flat ( lobater ) chewing surfaces, which is interpreted as an adaptation to food with a higher concentration of grass. The rear teeth have a round or oval cross-section, the front teeth are shaped like a canine. The rear foot also shows a clearly laterally twisted shape. One of the earliest records of the Mylodonts is Paroctodontotherium , which is recorded in Salla-Luribay in Bolivia and dates back to the Oligocene .

The internal structure of the Mylodontidae is complex and varies depending on the processor. Most recognized are the late groups of the Mylodontinae with Mylodon as the type form and the Lestodontinae , whose character form is Lestodon (referred to as Mylodontini and Lestodontini on the tribal level). In some cases, numerous other subfamilies were established, such as the Nematheriinae for representatives from the Lower Miocene or the Octomylodontinae for all basal forms, which are not generally recognized. With the establishment of the Urumacotheriinae for late Miocene genera of northern South America, a further line within the Mylodonts could be worked out in 2004. In principle, a revision is requested for the entire family, since many of the higher taxonomic units have no formal diagnosis.

The subdivision of the terminal group of the Mylodonts into the Lestodontinae and Mylodontinae was generally confirmed in one of the most extensive studies to date on the tribal history of the sloths from 2004. As a result of this analysis, based on skull features and carried out by Timothy J. Gaudin , Paramylodon is close to Mylodon , but the relatively similar Glossotherium forms the sister taxon to the Lestodontinae. The results could be reproduced in some later phylogenetic work. A study presented in 2019 by Luciano Varela and other participating scientists, which includes numerous fossil forms of the entire subordination of the sloth, sees this more critically. Here Paramylodon and Glossotherium are closely related, Mylodon, however, forms the bases of the developed Mylodonts and Lestodon grouped with some forms from northern South America. In the same year, a higher-resolution phylogenetic analysis of the mylodonts was published by a working group led by Alberto Boscaini . This underpins the branching of the terminal representatives and combines Paramylodon , Glossotherium and Mylodon in one clade . As a fundamental difference between the Mylodontinae and Lestodontinae, the appearance of the canine-like front teeth can be used. In the latter, these are large and separated from the rear teeth by a long diastema; the former have only small canine-shaped teeth, which are sometimes also partially reduced, but are generally closer to the molar-like teeth. According to the study, Paramylodon , Glosstherium and Mylodon form a single community within the Mylodontinae. This view is also supported by the aforementioned biochemical data, also submitted in 2019. Detailed skull analyzes published by Robert K. McAfee in 2009 also suggest that Paramylodon and Glossotherium are very closely related and most likely share a common ancestor. The characteristics that connect the two genera include, for example, the structure of the dentition with the front canine-shaped teeth and the tooth structure, such as the second molar, or the position of the bone suture between the palatine bone and the upper jaw near the rearmost tooth. On the other hand, Mylodon with its reduced set of teeth, the more simply designed teeth and the advanced bone connection between the palatine bone and the upper jaw is more clearly different.

Skull of Mylodon garmani , a synonym form of Paramylodon harlani

Within the genus Paramylodon , only one species , P. harlani, is recognized. Another species, P. nebrascensis , was described by Barnum Brown in 1903 using a partial skeleton from Hay Spring in Nebraska , but was already combined with the nominate form in the 1920s . Just ten years later, Glover Morrill Allen created the species Mylodon garmani with the help of another partial skeleton from the Niobrara River in Nebraska , which is also regarded as synonymous with Paramylodon harlani . The same applies to several species named by Edward Drinker Cope in the 1870s and 1890s such as Mylodon sodalis and Mylodon sulcidens . The originally made subdivision into two subspecies, P. h. harlani for a robust and P. h. tenuiceps for a graceful shape, as suggested by Chester Stock in 1917, is no longer advocated today. However, the species " Glossotherium " chapadmalense is problematic . The species was originally established in 1925 by Lucas Kraglievich on the basis of a 39 cm long, almost undamaged skull with a lower jaw from layers of the Middle Pliocene east of Miramar in the Argentine province of Buenos Aires . It shows similarities to Glossotherium robustum , but also has individual deviations that may justify its own genus status , for example the name Eumylodon (which Kraglievich used as early as 1925 for Eumylodon chapadmalense ) was proposed . The shape could possibly 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, which were first used by Jesse S. Robertson in 1976 under the same species name, or whether these are closer to Paramylodon , is currently unclear due to a lack of comparative studies. Some of the early mylodont remains are also known as P. garbanii , a species name that was coined in 1986 for some pliocene mandibular and limb remains from Arroyo EI Tanque in the Mexican state of Guanajuato (under the scientific name Glossotherium garbanii ). The species is not fully recognized, however, other authors see it as a synonym for " Glossotherium " chapadmalense .

Research history

Discoveries in North and South America

Richard Harlan
The first finds of paramylodon published by Richard Harlan in 1831

The research history of Paramylodon is complex and marked by confusion and equation with Mylodon and Glossotherium for more than 150 years . But it begins with the first discoveries by Richard Harlan (1796-1843) at Big Bone Lick in Boone County , Kentucky, USA in 1831, which include a right lower jaw and a collarbone . Harlan recognized that it was the 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 which he had recently established . The finds were originally kept in New York, but are now lost.

Between the years 1831 and 1836 Charles Darwin made his seminal journey with the HMS Beagle to South America and brought back a large number of fossils from there. These were then examined by Richard Owen , one of the most important researchers 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 found by Darwin in Punta Alta in the Argentine province of Buenos Aires . A total of four molar-like teeth per row of teeth stood out as a special characteristic. At the same time, Owen noticed similarities in the tooth structure between Harlan's lower jaw and that of Mylodon darwinii . As a result, he discarded the term Megalonyx laqueatus, coined by Harlan, and created a new species with Mylodon harlani . The generic name Mylodon is made up of the Greek words μύλη ( myle " Molar ") and ὀδούς ( odoús "tooth"), so translated means so much like "molar tooth". Harlan commented on the use of the name two years later, since in his opinion it did not describe an outstanding characteristic of the animal and could mean any extinct mammal, because almost all of them had the back molars.

In the same year, 1842, Owen submitted a comprehensive description of a skeleton of a mylodont that came from the floodplains of the Río de la Plata north of Buenos Aires ; he established the new species Mylodon robustus for this . At that time, the genus Mylodon consisted of three species, two of which were found in South America and one in North America. In addition, it should turn out to be problematic that Owen identified Mylodon darwinii as a type species of the genus, although this, as he admitted, is the second known and described species after Mylodon harlani . Accordingly, Mylodon harlani would actually have the right to the status of nominated form . In the further course, Mylodon were assigned to different types, so Johannes Theodor Reinhardt saw them in Mylodon robustus in 1879 , Richard Lydekker in 1887 in Mylodon harlani .

Paramylodon and the Mylodon - Glossotherium problem

Barnum Brown
Skulls of Mylodon (above) and Glossotherium (below)

In 1903 Barnum Brown (1873-1963) introduced the generic name Paramylodon . He used a partial skeleton from Hay Spring in Nebraska , which was discovered in 1897 on an expedition to the American Museum of Natural History . He assigned a species to the genus with Paramylodon nebrascensis . As defining differences to the North American Mylodon harlani , which Brown regarded as a type of Mylodon , he indicated the missing front canine-shaped teeth in the upper jaw. As a result, two different representatives of the Mylodonts in the Pleistocene of North America were recognized at that time.

Later, Chester Stock (1892–1950) pointed out, based on his investigations on the finds from Rancho La Brea, that the feature of the missing upper front teeth in Mylodon harlani is very variable. Therefore in 1917 he synonymized P. nebrascensis with Mylodon harlani . In 1928, however , Lucas Kraglievich limited the North American finds to Paramylodon and thus separated the genus from the South American representatives, an opinion that Ángel Cabrera followed eight years later ; however, it found hardly any response from most researchers in the subsequent period. At the same time, Kraglievich also revised Glossotherium as an independent genus that can be distinguished from Mylodon . Glossotherium was originally also set up by Owen in his paper about Darwin's discoveries from 1840 on the basis of a fragment of a skull from Arroyo Sarandí in the southwest of present-day Uruguay , but only two years later he combined it with Mylodon .

In the period following Kraglievich and Cabrera, Glossotherium developed into a "trash can" taxon due to the priority of its name, in which numerous closely related forms were set. George Gaylord Simpson stated in his general taxonomy of mammals in 1945 that if Paramylodon could not be clearly separated from Glossotherium , Glossotherium would be preferable because of this naming priority . Due to the subsequent complete incorporation of Paramylodon into the genus, which Robert Hoffstetter completed in 1952, Glossotherium was one of the few sloth forms that occurred in South and North America, but it also had a high degree of variability. Numerous researchers throughout the 20th century favored the view that the two forms of sloth are congeneric . In 1995, however , H. Gregory McDonald separated the North American Paramylodon from the South American Glossotherium . He noted that there are no studies that have shown that both genera are actually identical. Rather, the isolation of Paramylodon in North America would rather speak for an independence of form. In the following time several skull studies could be presented which clearly differentiated the two genera and also Mylodon from one another.

literature

  • Robert K. McAfee: Reassessment of the cranial characters of Glossotherium and Paramylodon (Mammalia: Xenarthra: Mylodontidae). Zoological Journal of the Linnean Society, 155, 2009, pp. 885-903
  • H. Gregory McDonald and Steve Pelikan: Mammoths and mylodonts: Exotic species from two different continents in North American Pleistocene faunas. Quaternary International 142/143, 2006, pp. 229-241
  • Chester Stock: A mounted skeleton of Mylodon harlani. University of California Publications, Bulletin of the Department of Geology12 (6), 1920, pp. 425-430 ( [12] )
  • Chester 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, pp. 1-206

Individual evidence

  1. ^ A b H. Gregory McDonald: Paleoecology of extinct Xenarthrans and the Great American Biotic Interchange. Bulletin of the Florida Museum of Natural History 45 (4), 2005, pp. 313-333
  2. ↑ Chester Stock: A mounted skeleton of Mylodon harlani. University of California Publications, Bulletin of the Department of Geology12 (6), 1920, pp. 425-430 ( [1] )
  3. ^ TDA Cockerell: A fossil ground-sloth in Colorado. University of Colorado Studies 6, 1909, pp. 309-312
  4. ↑ a b c d e f 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, pp. 165-178
  5. ↑ a b c d e f g h i Chester 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, pp. 1-206
  6. ↑ a b c d e f g Robert K. McAfee: Reassessment of the cranial characters of Glossotherium and Paramylodon (Mammalia: Xenarthra: Mylodontidae). Zoological Journal of the Linnean Society, 155, 2009, pp. 885-903
  7. ↑ a b c M. Susana Bargo, Gerardo de Iuliis and Sergio F. Vizcaíno: Hypsodonty in Pleistocene ground sloths. Acta Palaeontologica Polonica 51 (1), 2006, pp. 53-61
  8. ^ Daniela C. Kalthoff: Microstructure of Dental Hard Tissues in Fossil and Recent Xenarthrans (Mammalia: Folivora and Cingulata). Journal of Morphology 272, 2011, pp. 641-661
  9. ^ A b Ascanio D. Rincón, H. Gregory McDonald, Andrés Solórzano, Mónica Núñez Flores and Damián Ruiz-Ramoni: A new enigmatic Late Miocene mylodontoid sloth from northern South America. Royal Society Open Science 2, 2015, p. 140256, doi: 10.1098 / rsos.140256
  10. ↑ a b c Barnum Brown: A new genus of ground sloth from the Pleistocene of Nebraska. Bulletin of the American Museum of Natural History 29, 1903, pp. 569-583
  11. ↑ Chester Stock: Structure of the pes in Mylodon harlani. University of California Publications. Bulletin of the Department of Geology 10 (16), 1917, pp. 267-286
  12. ^ FP Moreno and A. Smith Woodward: On a Portion of Mammalian Skin, named Neomylodon listai, from a Cave near Consuelo Cove, Last Hope Inlet, Patagonia. By Dr. FP Moreno, CMZS With a Description of the Specimen by A. Smith Woodward, FZS Proceedings of the Zoological Society 1899, pp. 144–156 ( [2] )
  13. ↑ John C. Merriam: Recent discoveries of Quaternary mammals in Southern California. Science24, 1906, pp. 248-250
  14. ^ William 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, pp. 191-195
  15. ↑ H. Gregory McDonald: An Overview of the Presence of Osteoderms in Sloths: Implications for Osteoderms as a Plesiomorphic Character of the Xenarthra. Journal of Mammalogy 25, 2018, pp. 485-493, doi: 10.1007 / s10914-017-9415-8
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  20. ↑ Fred W. Croxen III, Christopher A. Shaw and David R. Sussman: Pleistocene Geology and Paleontology of the Colorado River Delta at Golfo de Santa Clara, Sonora, Mexico. In: Robert E. Reynolds (Ed.): Wild, scenic & rapid-a trip down the Colorado River trough. The 2007 Desert Symposium field guide and abstracts from proceedings. California State University, 2007, pp. 84-89
  21. ↑ a b c H. Gregory McDonald and Steve Pelikan: Mammoths and mylodonts: Exotic species from two different continents in North American Pleistocene faunas. Quaternary International 142/143, 2006, pp. 229-241
  22. ^ A b H. Gregory McDonald, Robert G. Dundas and 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, pp. 215-227
  23. ↑ a b 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, pp. 16-21
  24. ↑ Robert G. Dundas and 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, pp. 47-62
  25. ↑ H. Gregory McDonald, Larry D. Agenbroad and Carol Manganaro Haden: Late Pleistocene mylodont sloth Paramylodon harlani (Mammalia: Xenarthra) from Arizona. The Southwestern Naturalist 49 (2), 2014, pp. 229-238
  26. ↑ Anthony R. Friscia, Blaire Van Valkenburgh, Lillian Spencer and John Harris: Chronology and spatial distribution of large mammal bones in Pit 91, Rancho La Brea. Palaios 23 (1), 2008, pp. 35-42
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  30. ↑ Kathleen Springer, Eric Scott, J. Christopher Sagebiel and Lyndon K. Murray: Late Pleistocene large mammal faunal dynamics from inland southern California: The Diamond Valley Lake local fauna. Quaternary International 217, 2010, pp. 256-265
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  33. ↑ Guadalupe Sanchez, Vance T. Holliday, Edmund P. Gaines, Joaquín Arroyo-Cabrales, Natalia Martínez-Tagüeña, Andrew Kowler, Todd Lange, Gregory WL Hodgins, Susan M. Mentzer and Ismael Sanchez-Morales: Human (Clovis) –gomphothere (Cuvieronius sp.) Association∼13,390 calibrated yBP in Sonora, Mexico. PNAS 111, 2014, pp. 10972-10977
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  37. ↑ Donald R. Prothero and Kristina R. Raymond: Variation and sexual size dimorphism in Pleistocene ground sloths (Xenarthra). New Mexico Museum of Natural History and Science Bulletin 44, 2008, pp. 331-333
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  51. ↑ a b Alberto Boscaini, François Pujos and Timothy J. Gaudin: A reappraisal of the phylogeny of Mylodontidae (Mammalia, Xenarthra) and the divergence of mylodontine and lestodontine sloths. Zoologica Scripta 48 (6), 2019, pp. 691-710, doi: 10.1111 / zsc.12376
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  73. ↑ Juan Carlos Fernicola, Sergio F. Vizcaíno and 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, pp. 147-159
  74. ↑ a b Lucas Kraglievich: "Mylodon darwini" Owen, es la especie genotipo de "Mylodon" Ow. Physis: Revista de la Sociedad Argentina de Ciencias Naturales 9, 1928, pp. 169-185
  75. ^ George Gaylord Simpson: The Principles of Classification and a Classification of Mammals. Bulletin of the American Museum of Natural History 85, 1945, pp. 1–350 (p. 71)
  76. ↑ Diego Brandoni, Brenda S. Ferrero and 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, pp. 1547-1558

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