Editing Mylodon (section)

== Description ==
=== General ===
[[File:Mylodon size comparison.png|thumb|Size comparison of ''Mylodon darwini'' compared to a human]]
''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 tonnes is assumed, with an approximate estimate of 1.65 tonnes.<ref name="christiansenfarina2003">{{cite book |last1=Christiansen |first1=Per |last2=Fariña |first2=Richard A. |chapter=Mass estimation of two fossil ground sloths (Mammalia, Xenarthra, Mylodontidae) |pages=95–101 |chapter-url=https://www.researchgate.net/publication/292021908 |editor1-last=Farina |editor1-first=R. A. |title=Morphological Studies in Fossil and Extant Xenarthra (Mammalia) |series=Senckenbergiana biologica |volume=83 |issue=1 |date=2003 |publisher=E. Schweizerbartsche Verlagsbuchhandlung |isbn=978-3-510-61358-8 }}</ref> Thus, ''Mylodon'' had about the size of related forms such as ''[[Glossotherium]]'' or ''[[Paramylodon]]'', but was significantly smaller than the giant ''[[Lestodon]]''. In terms of physique, it largely corresponded with the other large ground-living sloths.<ref name="Bell2002">{{cite journal |last1=Bell |first1=C. M. |title=Did elephants hang from trees? – the giant sloths of South America |journal=Geology Today |date=March 2002 |volume=18 |issue=2 |pages=63–66 |doi=10.1046/j.1365-2451.2002.00334.x |bibcode=2002GeolT..18...63B |s2cid=130426084 }}</ref><ref name="forasiepietal2007">{{cite book |last1=Forasiepi |first1=Analía |last2=Martinelli |first2=Agustín |last3=Blanco |first3=Jorge Luis |title=Bestiario fósil: mamíferos del pleistoceno de la Argentina |trans-title=Fossil bestiary: Pleistocene mammals of Argentina |language=es |date=2007 |publisher=Albatros |isbn=978-950-24-1101-9 |oclc=230208342 |pages=60–61 }}</ref>

=== Skull and dentition features ===
Especially in the construction of the skull, ''Mylodon'' differed significantly from other related forms. Its length varied between 59 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.<ref name="brandonietal2010">{{cite journal |last1=Brandoni |first1=Diego |last2=Ferrero |first2=Brenda S. |last3=Brunetto |first3=Ernesto |title=Mylodon darwini Owen (Xenarthra, Mylodontinae) from the Late Pleistocene of Mesopotamia, Argentina, with remarks on individual variability, paleobiology, paleobiogeography, and paleoenvironment |journal=Journal of Vertebrate Paleontology |date=September 2010 |volume=30 |issue=5 |pages=1547–1558 |doi=10.1080/02724634.2010.501449 |bibcode=2010JVPal..30.1547B |s2cid=86181187 |hdl=11336/79988 |hdl-access=free }}</ref> The skull was thereby elongated and narrow, unlike ''Glossotherium'' and ''Lestodon'' that had a short and very broad skull. The extraordinary length of the skull of ''Mylodon'' was mainly due to elongations in the [[rostrum (anatomy)|rostrum]]. 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 nasal area, seen from above, which was rather short and looked clearly cut off when viewed from the side; the roof of the skull was largely straight in ''Mylodon'', only a slight indentation could occur above the orbit. On parietal, significant temporal lines were present, but no head crest formed. The zygomatic arch was slim, the anterior 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''.<ref name="brambilla&ibarra2018">{{cite journal |last1=Brambilla |first1=Luciano |last2=Ibarra |first2=Damian A. |title=The occipital region of late Pleistocene Mylodontidae of Argentina |journal=Boletín del Instituto de Fisiografía y Geología |date=15 November 2018 |volume=88 |pages=1–9 |url=https://www.fceia.unr.edu.ar/fisiografia/volumen88/brambilla_etal_bifg88.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.fceia.unr.edu.ar/fisiografia/volumen88/brambilla_etal_bifg88.pdf |archive-date=2022-10-09 |url-status=live |hdl=2133/14367 |hdl-access=free }}</ref> The palate was narrow and was more or less triangularly oriented towards the front of the skull. Numerous small bone openings were characteristic here. The glenoid pit, in which the joint of the lower jaw engages, corresponding to that of other mylodonts with its weak form, but this provided free rotation overall.<ref name="brandonietal2010" /><ref name="bargo&vizcaino2008">{{cite journal |last1=Bargo |first1=M. Susana |last2=Vizcaíno |first2=Sergio F. |title=Paleobiology of Pleistocene ground sloths (Xenarthra, Tardigrada): biomechanics, morphogeometry and ecomorphology applied to the masticatory apparatus |journal=Ameghiniana |date=2008 |volume=45 |issue=1 |pages=175–196 |url=https://www.ameghiniana.org.ar/index.php/ameghiniana/article/view/216 }}</ref><ref name="mcafee2007">{{cite thesis |last1=McAfee |first1=R.K. |year=2007 |title=Reassessing the Taxonomy and Affinities of the Myodontinae sloths, ''Glossotherium'' and ''Paramylodon'' (Mammalia: Xenarthra: Tardigrada) }}</ref>

[[File:Mylodon jaw.jpg|thumb|right|Lower jaw of ''Mylodon'']]
The lower jaw of ''Mylodon'' varied in length between 42 and 48 cm. It was elongated, more noticeable than in ''Glossotherium'' and ''Lestodon'', since in ''Mylodon'' the area in front of the teeth, in particular, is strongly elongated. 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 anterior 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, ''Mylodon'''s symphysis was narrow and not as wide as in ''Glossotherium'' and ''Lestodon''. The mandible foramen opened shortly behind 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&nbsp;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 tipped down and was below the lower edge of the horizontal bone body. The upper side of the angular process does not reach the occlusal plane.<ref name="brandonietal2010" /><ref name="bargo&vizcaino2008" /><ref name="mcafee2007" />

The dentition of ''Mylodon'' differs greatly from that of the other [[placental|placental mammals]] and usually consists of five teeth at the top and four teeth at the bottom per jaw arch, meaning a total of 18 teeth. In the mylodonts, the first tooth was often [[canine (tooth)|caniniform]] while the rear teeth were more [[molar (tooth)|molariform]]. Within the sloth, this structure of the teeth can be called original. A special feature of ''Mylodon'' was 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 anterior caniniform tooth was transformed into a molariform. 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 decayed teeth. The flat, flap-like and largely indented structure of the '' molariform '' 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 shapes of the teeth present in ''Mylodon'' were simpler. In the upper jaw row, they had a rather round to oval outline, in the lower jaw row a more [[diamond]]-shaped outline. The typically more complex bilobed design of the molar-like teeth of ''Glossotherium'' and ''Lestodon'', caused by a central constriction, only occurred on the lower rearmost tooth in ''Mylodon''. In general, the rows of teeth diverged to the front, and the teeth were very high crowned ([[hypsodont]]). The upper row of teeth ranges in length from 10.9 to 13.3&nbsp;cm, the lower row was between 12.0 and 15.0&nbsp;cm in length.<ref name="bargo&vizcaino2008" /><ref name="brandonietal2010" /><ref name="mcafee2007" /><ref name="bargoetal2006">{{cite journal |last1=Bargo |first1=M. Susan |last2=De Iuliis |first2=Gerardo |last3=Vizcaíno |first3=Sergio F. |title=Hypsodonty in Pleistocene ground sloths |journal=Acta Palaeontologica Polonica |date=2006 |volume=51 |issue=1 |pages=53–61 |url=http://webaccess.app.pan.pl/article/item/app51-053.html |citeseerx=10.1.1.728.7025 }}</ref>

=== Postcrania ===
Postcranial skeletons are far rarer in ''Mylodon'' than in the other large mylodontid sloths. As a result, the 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 to 48&nbsp;cm. The joint head, the diameter of which was over 10&nbsp;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&nbsp;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&nbsp;cm. The olecranon, i.e. the upper articular process, took up about 8.1&nbsp;cm of it, 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&nbsp;cm. The head was oval in shape with a prominent lip. The pelvis was extremely expansive and 114&nbsp;cm wide between the two iliac bones. The thigh bone measured between 55 and 59&nbsp;cm in length. It was typical of ground sloths, being flat in shape. Its width decreased significantly on the shaft, the lowest value was reached just below the midpoint. Here the width was about 18&nbsp;cm, the thickness about 7.5&nbsp;cm. The joint ends, on the other hand, were markedly wider, around 30&nbsp;cm at the knee end and around 26&nbsp;cm at the foot end. The thighbone reached the shin with only about half of its 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''.<ref name="kraglievich1934">Lucas Kraglievich: Contribución al conocimiento de ''Mylodon darwini'' Owen y especies afines. Revista del Museo de La Plata 34, 1934, pp. 255–292</ref><ref name="mcafee2016">{{cite journal |last1=McAfee |first1=Robert K. |title=Description of New Postcranial Elements of Mylodon darwinii Owen 1839 (Mammalia: Pilosa: Mylodontinae), and Functional Morphology of the Forelimb |journal=Ameghiniana |date=August 2016 |volume=53 |issue=4 |pages=418–443 |doi=10.5710/AMGH.24.02.2016.2950 |s2cid=88450788 }}</ref><ref name="haroetal2017">{{cite journal |last1=Haro |first1=José A. |last2=Tauber |first2=Adan A. |last3=Krapovickas |first3=Jerónimo M. |title=Thoracic member (pectoral girdle and forelimb) bones of Mylodon darwinii Owen (Xenarthra, Mylodontidae) from the Late Pleistocene of Central Argentina and their phylogenetic implications |journal=PalZ |date=September 2017 |volume=91 |issue=3 |pages=439–457 |doi=10.1007/s12542-017-0350-z |bibcode=2017PalZ...91..439H |s2cid=90593541 }}</ref>

The hand comprised a total of five digits (I to V), whereby the metacarpal bone was fused with the large polygonal bone on the first digit. 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 a pyramidal shape. The fourth digit 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&nbsp;cm in length. As with ''Glossotherium'' and ''Paramylodon'', only the three inner digits were probably clawed, but only of the second digit have all bone elements been documented. The metacarpal bone was 7.8&nbsp;cm long and was built very gracefully. The first phalanx was extremely short and only about 2.5&nbsp;cm long, the second was about 4.2&nbsp;cm long and the third at least 11.5&nbsp;cm. It was tubular and went forward into an extension on which the claw rested. The first phalanges of the two outer digits were significantly reduced in length. Only individual root bones of the foot, such as the talus, are present.<ref name="mcafee2016" />

=== Integument ===
[[File:Mylodon darwini - skin, droppings and toenails.JPG|thumb|Toenails, dung and skin, Natural History Museum, London]]
[[File:Mylodon fur.jpg|thumb|Fur and skin at the Museum für Naturkunde, Berlin]]
''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 brought to light at the end of the 19th century.<ref name="moreno&woodward1899"/><ref name="nordenskjöld1899">Otto Nordenskjöld (with the participation of other authors): Scientific results of the Swedish expedition to the Magellan lands 1895–1897, under the direction of Dr. Otto Nordenskjöld. Volume II: Zoology. Stockholm, 1899, pp. 1–170 (especially pp. 149–170)</ref> Individual pieces have lengths of up to 150&nbsp;cm, but have shrunk through drying processes. Its thickness is up to 1.5&nbsp;cm in some places, but it is usually around 1&nbsp;cm. The skin is densely covered with stiff, slightly wavy hair, with only the top hair being developed, while the undercoat is missing. This feature is similar to the [[Choloepus|two-toed sloths]] but less so than the [[Bradypus|three-toed sloths]], which possess an undercoat. The length of the individual hairs vary between 5 and sometimes over 20&nbsp;cm with the shortest in the area of the back of the head, medium-length hair on the back and very long hair on the limbs. Their known 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 ribbing.<ref name="moreno&woodward1899">{{cite journal |first1=Francesco P. |last1=Moreno |first2=Arthur Smith |last2=Woodward |title=On a Portion of Mammalian Skin, named ''Neomylodon listai'', from a Cave near Consuelo Cove, Last Hope Inlet, Patagonia |journal=Proceedings of the Zoological Society |year=1899 |pages=144–156 }}</ref><ref name="nordenskjöld1899"/><ref name="lonnberg1900">{{cite journal |first1=Einar |last1=Lönnberg |title=On a remarkable piece of skin from Cueva Eberhardt, Last Hope Inlet, Patagonia |journal=Proceedings of the Zoological Society |volume=199 |year=1900 |pages=379–383 }}</ref><ref name="woodward1900">{{cite journal |last1=Woodward |first1=A. Smith |title=On some Remains of Grypotherium (Neomylodori) listai and associated Mammals from a Cavern near Consuelo Cove, Last Hope Inlet, Patagonia. |journal=Proceedings of the Zoological Society of London |date=21 August 2009 |volume=69 |issue=1 |pages=64–78 |doi=10.1111/j.1096-3642.1890.tb01704.x |url=https://www.biodiversitylibrary.org/part/72554 }}</ref><ref name="ridewood1901">{{cite journal |last1=Ridewood |first1=W. G. |title=Memoirs: On the Structure of the Hairs of Mylodon Listai and other South American Edentata |journal=Journal of Cell Science |date=1 May 1901 |volume=s2-44 |issue=175 |pages=393–411 |doi=10.1242/jcs.s2-44.175.393 |doi-access=free }}</ref>

The mylodonts are the only representatives of the sloths to have bony plates embedded in their skin. Such structures, 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.<ref name="burmeister1865">Hermann Burmeister: skin armor at Mylodon. Archives for anatomy, physiology and scientific medicine 1865, pp. 334–336</ref><ref name="burmeister1867">Hermann Burmeister: Fauna Argentina. Primera party. Mamiferos fósiles. Lista de los mamiferos fósiles del terreno diluviano. Anales del Museo Público de Buenos Aires 1, 1867, pp. 87–300 (p. 173)</ref> The remains of skin found in the caves of Última Esperanza 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&nbsp;cm<sup>2</sup>. For 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 residues were found isolated from the body skeletons, 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. The bone platelets of ''Mylodon'' were mostly of irregular oval shape with dimensions of 0.5 to 2.5&nbsp;cm in length, 0.3 to 1.8&nbsp;cm in width and 0.2 to 1.1&nbsp;cm in thickness, with weights of a maximum of 2&nbsp;g. On the surface, they showed individual dimples.<ref name="hill2005">{{cite journal |last1=Hill |first1=Robert V. |title=Comparative anatomy and histology of xenarthran osteoderms |journal=Journal of Morphology |date=December 2006 |volume=267 |issue=12 |pages=1441–1460 |doi=10.1002/jmor.10490 |pmid=17103396 |s2cid=22294139 }}</ref> In cross-section, they consisted of numerous bundles of fibers mixed with hard bone blades (osteoma). 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.<ref name="moreno&woodward1899"/><ref name="nordenskjöld1899"/><ref name="lopezmendozaetal2011">{{cite journal |last1=López-Mendoza |first1=Patricio |last2=Mena-Larraín |first2=Francisco |title=Extinct ground sloth dermal bones and their role in the taphonomic research of caves: the case of Baño Nuevo-1 (Andean Central Patagonia, Chile) |journal=Revista Mexicana de Ciencias Geológicas |date=December 2011 |volume=28 |issue=3 |pages=519–532 |url=http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1026-87742011000300013 }}</ref><ref name="branco1906">Wilhelm Branco: The application of X-rays in paleontology. Treatises of the Royal Prussian Academy of Sciences Berlin 1906, pp. 1–55</ref><ref name="hill2005"/><ref name="mcdonald2018">{{cite journal |last1=McDonald |first1=H. Gregory |title=An Overview of the Presence of Osteoderms in Sloths: Implications for Osteoderms as a Plesiomorphic Character of the Xenarthra |journal=Journal of Mammalian Evolution |date=December 2018 |volume=25 |issue=4 |pages=485–493 |doi=10.1007/s10914-017-9415-8 |s2cid=38600428 }}</ref>