Paramylodon — North American ground sloth of the Pliocene–Pleistocene

Height

Paramylodon has come down to us in abundant numbers mainly because of the finds from Rancho La Brea in California. The material of several dozen individuals recovered there served as the basis for numerous investigations, on which the following descriptive data are largely based. The sloth genus was a medium-sized representative of the Mylodontidae. A completely reconstructed skeleton from Rancho La Brea has a total length of 279 cm, of which the tail occupies about 118 cm. At the shoulders it reaches a height of 112 cm, and at the pelvis it measures 122 cm. The weight for this late member of the Upper Pleistocene is given as about 1.39 t, but earlier forms were quite smaller. Overall, Paramylodon was a robustly built animal. It was characterized by an elongated skull, a short neck, a short and compact body with a broad pelvis, and strong limbs and tail.

Skull and dentition characteristics

The skull of Paramylodon was long. It reached a total length of 42.9 to 49.8 cm, determined on about a dozen specimens. One particularly large skull measured 54.0 cm. In plan view, 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. Typical for many mylodonts was the continuously widening snout towards the front. The skull, however, was clearly narrower than that of the comparably sized Glossotherium, the latter showing a dome-like bulge at the frontal line in side view, which did not occur in Paramylodon. However, the skull of Paramylodon, with the exception of the middle area (the dome-like bulge in Glossotherium), was on average higher, measuring about 13.8 cm at the occiput and 13 cm at the snout. The nasal bone was laterally in contact with the maxilla. This resulted in a laterally closed and only forward open nasal cavity, which became about as high as it was wide, which was due to the overall narrower skull. The middle jawbone, typical of sloths, was only loosely connected to the upper jaw. At the frontal bone, the nasal bone protruded far back, so that the suture between the two skull bones was rather V-shaped. In addition, the frontal bone represented the largest bone of the entire skull. A strong parietal crest existed between the parietal bones, but it appeared much narrower than in Glossotherium. The zygomatic arches, in a departure from most sloths, were secondarily closed again. The anterior arch, originating at the zygomatic bone and pointing posteriorly, had three processes, one oriented upward, one downward, and the middle horizontal. The posterior arch section, attached to the temporal bone, had a finger-like shape and joined the middle process of the anterior arch section. On the underside of the skull, the palatine bone protruded much farther posteriorly in Paramylodon than in Glossotherium, caused by the longer extension of the bone behind the last molar tooth. As in many mylodonts, both flanks of the wing bone were distinctly inflated. In Paramylodon, however, this was not quite as evident as in Glossotherium, so that the distended structures were much farther apart through the basal phenoid of the sphenoid.

The mandible reached lengths of 31.5 to 43.6 cm after analysis of a good two dozen objects. It was massively built and broad. The horizontal bone body continuously increased in height from front to back, under the rearmost tooth its height was up to 10.5 cm. The robust symphysis grew up to 11 cm long and was - typical for mylodonts - broad. It extended forward, which is a characteristic of almost all sloths. This spoon-like extension of the symphysis did not project laterally as distinctly in Paramylodon as in Glossotherium, so that the lateral edges were rather straight and less distinctly curved than in the latter. The width of the symphysis in the anterior region was up to 15 cm. The articular process only marginally overhung the masticatory plane, the coronal process being much higher. Its anterior edge ran in a straight line in Paramylodon, deviating from the curved design in Glossotherium. The dentition consisted of 5 teeth per maxillary half and 4 teeth per mandibular half, as is common in sloths, so that a total of 18 teeth were formed. The front teeth had a caniniform shape, the others were molariform. The structure of the dentition is considered to be phylogenetically primitive within the sloths. However, in later representatives of Paramylodon, the upper caniniform teeth were often reduced, so that the dentition then consisted of only 16 teeth. A similar reduction of teeth is not known in Glossotherium. In Mylodon, on the other hand, the anterior teeth in the upper dentition were also no longer formed, while the lower caniniform teeth resembled the posterior molars. The caniniform teeth of Paramylodon had an oval cross-section and were curved backwards. However, they did not reach the size as in Glossotherium or even in Lestodon. There was a short diastema to the posterior row of teeth. The molar-like molars had a flat shape with a somewhat raised margin. In outline they possessed a two-lobed shape with strong median constriction, except for the first maxillary molar, which was more rectangular in shape and, with an average length of 3.7 cm, formed the longest tooth in the maxilla. In the second maxillary molar, the lobe-like structure was much more prominent than in Glossotherium. All teeth typically lacked enamel, but rather consisted of a harder variety of dentin (orthodentin), with an additional outer layer of dental cementum. The proportion of orthodentine reached 28% in Paramylodon. The upper row of teeth was 14.4 cm long on average, of which the posterior molars occupied 12.6 cm. Due to the forward widening of the snout, the tooth rows diverged from each other.

Body skeleton

Especially the extensive find material from Rancho La Brea allows a comprehensive reconstruction of the body skeleton. The spine was composed of 7 cervical, 16 thoracic 8 to 9 lumbar, sacral, and 21 caudal vertebrae. The humerus was massive, the length was 46 cm and the head did not stand out particularly clearly. A prominent bony ridge (deltopectoral groin) attached to the humeral shaft, but was less prominently developed in the upper part than in Glossotherium. The lower end of the joint protruded widely laterally. A foramen entepicondylaris, occasionally present in some sloths, was not developed here. The ulna possessed a greatly expanded upper articular process, the olecranon. It grew to about 20 cm in length, the entire bone reaching 40 cm in length. The construction of the ulna appeared shorter and more robust than in Glossotherium, the shaft was broad and narrowed above in front and behind. Likewise, the radius was short and massive with a length of 29.6 cm. The longest bone was represented by the femur at about 54.6 cm. Very short specimens from Rancho La Brea measured only 51 cm, very long 58 cm. The flat and broad design typical of ground sloths was striking, so that the bone appeared almost board-like. The head rose only slightly from the surface and had a more inward position. The shaft was slightly turned inward, and a third trochanter as a muscle attachment point, which appeared in Lestodon, was not visible in Paramylodon. With a length of 24.6 cm, the tibia was significantly shorter than the femur. This is a typical feature of mylodonts, in whose predominantly late representatives the lower section of the hind leg often reached only about half the length of the upper. In the case of Paramylodon, the tibia had 45% of the femur length. Its shaft was flattened like that of the femur and likewise exhibited a slight twist. The upper end of the joint was laterally projecting, the width here reaching about three-fourths of the length of the total bone. The fibula was not fused to the tibia, it was 26.3 cm long.

Hands and feet showed a similar structure as in the other large mylodonts Glossotherium and Lestodon, deviations are present in detail. The hand had a total of five rays (I to V), with only the three inner rays (I to III) having developed claws. The metacarpal bone of the first ray was fused with the great polygonal bone to form a single unit, which is frequently documented in ground-dwelling sloths (so-called metacarpal carpal complex or MCC). The metacarpal bones of the third to fifth rays were massive and more than 10 cm long, the one of ray IV had the strongest structure. The first two phalanges of ray I were also fused together, while rays II and III each had three phalanges, the first two of which were considerably shorter. The respective end members of the three inner rays had extended claw processes, which suggests correspondingly large claws. The length ranged from inner (I) to outer (III) from 7.5 cm to 15.4 cm to 17.4 cm, the height varied from 2.9 to 5.7 cm. The clawless outer fingers possessed phalanges greatly reduced in size. The foot of Paramylodon had a total of four rays (II to V), the innermost ray was completely reduced. Claws existed here only on toes II and III, which were also the most strongly developed. However, the metatarsals here had rather short lengths of 3.6 and 6.5 cm, respectively, at the outer rays they became over 11.0 cm long each and were very massive. As in the other two mylodonts, the second ray had only two phalanges, as the first and second phalanx were fused into one unit corresponding to the hand. In contrast to Glossotherium and Lestodon, the third ray of Paramylodon often consisted of only two limbs. The respective terminal phalanges with claws had an extremely strong construction, analogous to the hand. The claw process alone measured about 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, on the other hand, had strongly reduced end members.

Osteoderm

The Mylodonts are the only known lineage of sloths whose representatives had bone platelets, so-called osteoderms, formed in the skin, analogous to today's armadillos. In contrast to the armadillos of today, the osteoderms of the Mylodonts did not form a solid bone shell, but were rather loosely scattered, as can be seen from the skin remains of Mylodon. Several hundred such osteoderms of Paramylodon are available from Rancho La Brea, and also as a dense layer on a plate from Anza-Borrego State Park in California and from Haile 15A, a fossil-rich limestone fissure in Florida. The bone platelets were round to oval, sometimes irregularly shaped, and 5 to 30 mm long. They showed a rough surface with irregular depressions, whereas the underside was smooth and convex in shape. In cross-section they had a compact structure consisting of numerous fibre bundles mixed with hard bone lamellae (osteomas). In principle, the bone platelets of the mylodonts were simpler in structure than those of the armoured collateral articulated animals.

Overview and early appearance

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 are from the Upper Pliocene of Mexico and the US state of Florida. Of the latter, the partial skeleton from the Haile 15A site, a sediment-filled crevice in limestone in Alachua County, estimated to be between 2.1 and 1.8 million years old, is noteworthy. These early representatives are commonly referred to as "Glossotherium" chapadmalensis, but their position within the genus Glossotherium is disputed. Only slightly younger are the finds of the fossil-rich El Gulfo local fauna from the Colorado River estuary in the Mexican state of Sonora. They are already classified as Paramylodon and date to 1.8 to 1.6 million years ago. Overall, fossil remains from the Lower and Middle Pleistocene are relatively rare and come from about 20 sites in North America. These are distributed primarily in the southern and central areas of what is now the United States and northern Mexico, but are scattered in the western part of the continent as far south as Alberta in Canada. They are found in the lowlands as well as in mountainous areas, the highest finding point reaching an altitude of about 2900 m in Colorado. One of the most important sites of that time is the Leisey Shell pit in Hillsborough County, Florida, where several skulls and postcranial skeletal elements have been reported to be about 1.2 million years old. Already in the transition to the Middle Pleistocene is the Fairmead Landfill locality in Madera County, Colorado, which also yielded several partial skeletons.

Finds of the Upper Pleistocene

The find material of the Upper Pleistocene is considerably more extensive, originating from more than 100 sites; in California alone, Paramylodon is known from more than 60 sites. The distribution of the genus is similar to that of the Lower Pleistocene, but it also occurs somewhat further east in the Midwest, for example in Iowa. At that time it reached its northernmost locality in Sequim, Washington, at 48.1° northern latitude. In the south, the genus has also been found in Mexico, where its southern limit of distribution was at Valsequillo at 19° northern latitude, but some finds indicate that Paramylodon may also have been found in Guatemala and El Salvador. Among others, finds of a juvenile and an adult individual have been recovered from Stevenson Bridge in river deposits of Putah Creek in Yolo County, California, which belong to the beginning of the last cold period. Two nearly complete skeletons have been reported from Shonto and Richville, Arizona, which are among the few known finds from the state. In general, fossil remains of Paramylodon are very rare on the Colorado Plateau in the southwestern United States and additionally in northwestern Mexico, possibly related to the drier climate in this area at the time.

Of outstanding importance worldwide, however, are the finds from the asphalt pits of Rancho La Brea in southern California. From here comes an extensive fossil fauna, whose age ranges from 45,000 to 14,000 years before today. The first finds were discovered as early as the second half of the 19th century, but the far more significant material is due to targeted scientific investigations at the beginning of the 20th century, which include a total of over 100 documented sites. Striking in the faunal spectrum is the unusual dominance of predators over herbivores. Most likely, the predators were attracted in larger numbers by 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). However, Paramylodon is by far the most common representative with more than 70 individuals, among the finds 30 skulls alone are to be emphasized. Another very extensive fossil complex is the Diamond Valley Lake Local Fauna in Diamond Valley and Domenigoni Valley in RiversideCounty, also in southern California. The material was recovered during the construction of Diamond Valley Lake beginning in the mid-1990s and currently includes more than 100,000 artifacts from more than 100 taxa, from more than 2600 different localities. In contrast to Rancho La Brea, large herbivores dominate here, while the proportion of large prey predators is low. Thus, an undisturbed character of the faunal community can be inferred. Paramylodon is documented with about 280 individual finds, which represents about 8% of the total mammal fauna. The ground sloth thus forms the fifth most abundant representative of mammals in the Diamond Valley Lake Local Fauna after bison, horses, the primitive proboscidean mammoth pacificus, and the camel camelops. In contrast, the other two sloths, Megalonyx and Nothrotheriops, which also occur in Rancho La Brea, play only a minor role, accounting for a combined 0.5% of the find record. The age of the Diamond Valley Lake Local Fauna corresponds to that of Rancho La Brea according to radiocarbon dating.

Latest evidence

Like most other large, ground-dwelling sloths, Paramylodon disappeared towards the end of the Pleistocene during the Quaternary extinction wave. Unlike many other genera, however, there is little radiometric data from Paramylodon measured directly from fossil material. Among the most recent is a value from Rancho La Brea, which is 20,450 BP. However, clearly younger finds are still known, few of which came to light from archaeological sites associated with early human settlement on the North American continent. One of the rare records comes from El Fin del Mundo ("The End of the World") in Sonora. Discovered in 2007, the site could be dated to 11,550 years BP by radiocarbon dating using charcoal. Due to the presence of six Clovis points, it belongs to a very early section of the Clovis culture, which represents one of the earliest archaeological groups of the first colonizers of North America. In addition to two skeletons of proboscideans, one of which clearly represents Cuvieronius and had apparently been dismembered by the hunter-gatherers of the time, remains of Paramylodon also came to light. In addition, more than 130 osteoderms are attested from the Aubrey Clovis site in north-central Texas. The soil substrate surrounding the finds has been radiometrically dated to an age of 10,940 years BP. Stone artifacts also documented there, comprising about 9800 pieces, can also be referred to the Clovis Group on the basis of a Clovis point. The remains from Paramylodon, however, have no direct connection to the early settlers, as they were found in a nearby waterhole with the exception of a single bone platelet. It is unclear from the few common finds to date whether direct hunting led to the extinction of the animals.

That early colonizers of North America interacted with, followed, and possibly hunted large ground sloths is indicated by footprints from White Sands National Monument in New Mexico. Here, several hundred footprints of larger sloths are associated with those of humans on the shore of a former lake. In some cases they overlap; in one case the human tracks lie within the track of a sloth. A striking feature of the intersecting sloth and human tracks is an abrupt change of direction in the former, suggesting a direct confrontation by the causers. However, no fossil remains are available from the site and the age of the tracks has so far only been indirectly dated (between 15,560 and 10,000 years before present). In addition, the sloth tracks have not been determined more precisely. They show strong variations in size, which can be attributed either to animals of different ages or to different species. In addition to Paramylodon, Nothrotheriops, a smaller ground sloth from the group Nothrotheriidae, and Megalonyx, a large form of Megalonychidae, also occurred in the region during the period in question.