Editing Megatherium (section)

== Description ==

=== Size ===
[[File:Megatherium size comparison.png|thumb|270x270px|Size of ''Megatherium americanum'' compared to a human]]
''M. americanum'' is one of the largest known ground sloths, with a total body length of around {{Convert|6|m|ft}}.<ref name=":92">{{Cite journal |last=Naish |first=Darren |date=November 2005 |title=Fossils explained 51: Sloths |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1365-2451.2005.00538.x |journal=Geology Today |language=en |volume=21 |issue=6 |pages=232–238 |doi=10.1111/j.1365-2451.2005.00538.x |issn=0266-6979}}</ref> [[Volumetric]] analysis suggests that a full grown ''M. americanum'' weighed around {{Convert|3700|-|4000|kg|lbs}}, comparable to an Asian elephant.<ref>{{Cite journal |last1=Brassey |first1=Charlotte A. |last2=Gardiner |first2=James D. |date=August 2015 |title=An advanced shape-fitting algorithm applied to quadrupedal mammals: improving volumetric mass estimates |journal=Royal Society Open Science |language=en |volume=2 |issue=8 |pages=150302 |doi=10.1098/rsos.150302 |issn=2054-5703 |pmc=4555864 |pmid=26361559|bibcode=2015RSOS....250302B }}</ref><ref name=":5">{{Cite journal |last=Casinos |first=Adrià |date=March 1996 |title=Bipedalism and quadrupedalism in Megatheriurn: an attempt at biomechanical reconstruction |url=https://www.idunn.no/doi/10.1111/j.1502-3931.1996.tb01842.x |journal=Lethaia |language=en |volume=29 |issue=1 |pages=87–96 |doi=10.1111/j.1502-3931.1996.tb01842.x |bibcode=1996Letha..29...87C |issn=0024-1164}}</ref><ref name="pmid24676170">{{cite journal | vauthors = Fariña RA, Czerwonogora A, di Giacomo M | title = Splendid oddness: revisiting the curious trophic relationships of South American Pleistocene mammals and their abundance | journal = Anais da Academia Brasileira de Ciências | volume = 86 | issue = 1 | pages = 311–31 | date = March 2014 | pmid = 24676170 | doi = 10.1590/0001-3765201420120010 | doi-access = free }}</ref> The Late Pleistocene Andean-Altiplano ''Pseudomegatherium'' species ''Megatherium celendinense'' was likely comparable in size. These species were only rivalled in size amongst ground sloths by the closely related ''[[Eremotherium]]'' and the distantly related ''[[Lestodon]]''. The Chilean ''Pseudomegatherium'' species ''M. sundti'' was much smaller, with an estimated body mass of only {{Convert|1253|kg|lbs}}, with the Peruvian ''Megatherium urbinai,'' Bolivian ''Megatherium tarijense'' and the Chilean ''Megatherium medinae'' (all also belonging to ''Pseudomegatherium'') also having a considerably smaller body size than ''M. americanum.''<ref name=":10">{{Cite journal |last=McDonald |first=H. Gregory |date=June 2023 |title=A Tale of Two Continents (and a Few Islands): Ecology and Distribution of Late Pleistocene Sloths |journal=Land |language=en |volume=12 |issue=6 |pages=1192 |doi=10.3390/land12061192 |doi-access=free |issn=2073-445X}}</ref> The Pliocene ''Megatherium'' (''Megatherium'') species ''M. altiplanicum'' has been estimated to weigh {{Convert|977-1465|kg|lbs}}.<ref name=":3">Saint-André P.-A. & de Iuliis G. 2001. — [https://sciencepress.mnhn.fr/en/periodiques/geodiversitas/23/4/le-plus-petit-et-le-plus-ancien-representant-du-genre-megatherium-cuvier-1796-xenarthra-tardigrada-megatheriidae-du-pliocene-de-l-altiplano-bolivien The smallest and most ancient representative of the genus ''Megatherium'' Cuvier, 1796 (Xenarthra, Tardigrada, Megatheriidae), from the Pliocene of the Bolivian Altiplano]. ''Geodiversitas'' 2001 (4): 625–645.</ref>

=== Skull and jaws ===
[[File:Megatherium skull.png|thumb|270x270px|Skull and lower jaws of ''M. americanum'' (note: nasal septum is broken) Scale bar = 10 cm, ~ 4 inches]]
The head of ''Megatherium'' is relatively small compared to body size.<ref name=":5" /> The skull is roughly cylindrical in shape, with the cranial region of the skull being narrow. The [[Jugal bone|jugal]] bone of ''M. americanum'' has strongly developed ascending and descending [[Process (anatomy)|processes]].<ref name=":4">Bargo, M.S. 2001. [https://www.app.pan.pl/archive/published/app46/app46-173.pdf The ground sloth ''Megatherium americanum'': Skull shape, bite forces,  and diet]. – ''Acta Palaeontologica Polonica'' 46,2, 173–192.</ref> The skull of ''M. americanum'' has a relatively small [[cranial cavity]] (and thus brain) relative to skull size, with the skull having extensive [[Sinus (anatomy)|sinus]] spaces.<ref>{{Citation |last1=Boscaini |first1=Alberto |title=The Endocranial Cavities of Sloths (Xenarthra, Folivora): Insights from the Brain Endocast, Bony Labyrinth, and Cranial Sinuses |date=2023 |work=Paleoneurology of Amniotes |pages=737–760 |editor-last=Dozo |editor-first=María Teresa |url=https://link.springer.com/10.1007/978-3-031-13983-3_19 |access-date=2024-05-14 |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-031-13983-3_19 |isbn=978-3-031-13982-6 |last2=Iurino |first2=Dawid A. |last3=Sardella |first3=Raffaele |last4=Gaudin |first4=Timothy J. |last5=Pujos |first5=François |editor2-last=Paulina-Carabajal |editor2-first=Ariana |editor3-last=Macrini |editor3-first=Thomas E. |editor4-last=Walsh |editor4-first=Stig}}</ref> In many species of ''Megatherium'', the lower jaw is relatively deep, which served to accommodate the very long hypselodont (evergrowing) teeth,<ref name=":3" /> which are considerably proportionally longer than those of other ground sloths. Like other ground sloths, the number of teeth in the jaw is reduced to 5 and 4 teeth in each half of the upper and lower jaws, respectively, and the teeth lack enamel. The teeth of ''Megatherium americanum'' have sharp crests separated by v-shaped valleys, which interlock with the teeth on the opposing jaw.<ref name=":4" /> These teeth were self-sharpening, akin to rodent incisors.<ref name=":13">{{Cite journal |last1=Green |first1=Jeremy L. |last2=Kalthoff |first2=Daniela C. |date=2015-08-03 |title=Xenarthran dental microstructure and dental microwear analyses, with new data for Megatherium americanum (Megatheriidae) |url=https://academic.oup.com/jmammal/article-lookup/doi/10.1093/jmammal/gyv045 |journal=Journal of Mammalogy |language=en |volume=96 |issue=4 |pages=645–657 |doi=10.1093/jmammal/gyv045 |issn=0022-2372}}</ref> The skull of ''M. americanum'' has a relatively narrow snout/muzzle with a [[Ossification|ossified]] [[nasal septum]], and is suggested to have had a thick [[Prehensility|prehensile]] upper lip, similar to that of the living [[black rhinoceros]], which compensated for the lack of teeth at the front of the jaws.<ref name=":7">{{Cite journal |last1=Bargo |first1=M. Susana |last2=Toledo |first2=Néstor |last3=Vizcaíno |first3=Sergio F. |date=February 2006 |title=Muzzle of South American Pleistocene ground sloths (Xenarthra, Tardigrada) |url=https://onlinelibrary.wiley.com/doi/10.1002/jmor.10399 |journal=Journal of Morphology |language=en |volume=267 |issue=2 |pages=248–263 |doi=10.1002/jmor.10399 |issn=0362-2525 |pmid=16315216}}</ref> The morphology of the [[hyoid bones]] in ''Megatherium'' suggests that they were relatively rigid, this along with the short distance between the hyoid and the [[mandibular symphysis]] (the joint connecting the two halves of the lower jaw) suggests that the tongue had limited ability to protrude, and thus ''Megatherium'' did not have a long prehensile tongue, contrary to what was often historically suggested.<ref>{{cite journal |last1=Perez |first1=L. M. |last2=Toledo |first2=N. |last3=De Lullis |first3=G. |last4=Bargo |first4=M. S. |last5=Vizcaino |first5=S. F. |year=2010 |title=Morphology and Function of the Hyoid Apparatus of Xenarthran Fossils (Mammalia) |journal=Journal of Morphology |volume=271 |issue=9 |pages=1119–1133 |doi=10.1002/jmor.10859 |pmid=20730924 |s2cid=8106788}}</ref>  The skull and jaws of ''M. americanum'' show adaptation to powerful vertical biting.<ref name=":4" /> ''M. americanum'' and ''M. altiplanicum'' are distinguished from species of the subgenus ''Pseudomegatherium'' by the fusion of the [[maxilla]] and [[premaxilla]], while members of ''Pseudomegatherium'' are distinguished from those species by their flat [[occipital condyles]].<ref name="Pujos2006" />

=== Axial skeleton ===
Like other xenarthrans, the posterior trunk vertebrae of ''Megatherium americanum'' have additional xenarthrous processes that articulate with the other vertebrae. The [[ischium]] was connected to the [[caudal vertebrae]]. The caudal vertebrae were combined into a [[synsacrum]]. The [[sacrum]] was composed of 5 vertebrae. The [[pubic symphysis]] is reduced. The tail is large in size.<ref name=":5" />

=== Limbs ===
[[File:Megatherium cuvieri - right foot.JPG|thumb|Foot of ''Megatherium'' showing enlarged and elongate calcaneum (left) and lateral digits]]
The bones of the forelimbs of ''M. americanum'' are relatively slender. The three fingers in the middle of the hand bore claws, while the cuneiform hand bones did not touch the [[ulna]].<ref name=":5" /> The [[olecranon]] process of the ulna was relatively short.<ref name=":12" /> Like other xenarthrans, but unlike most other mammals, ''Megatherium'' possesses [[clavicle]]s (collarbones), which serves to support the forelimb. Like other sloths, the clavicle is merged with the [[acromion]] of the [[scapula]].<ref name=":11" /> The femur was massive and roughly rectangular in shape.<ref name=":5" /> As in most megatheriines, the [[tibia]] and [[fibula]] of ''Megatherium'' species are fused together at their proximal (closest to hip) end, while in ''M. americanum'' and ''M. tarijense'', they are also fused together at their distal (closest to foot) ends.<ref>{{Cite journal |last1=Bonini |first1=Ricardo A. |last2=Brandoni |first2=Diego |date=December 2015 |title=Pyramiodontherium Rovereto (Xenarthra, Tardigrada, Megatheriinae) from the Early Pliocene of San Fernando, Catamarca Province, Argentina |url=http://www.bioone.org/doi/10.5710/AMGH.16.06.2015.2902 |journal=Ameghiniana |language=en |volume=52 |issue=6 |pages=647–655 |doi=10.5710/AMGH.16.06.2015.2902 |hdl=11336/42299 |issn=0002-7014}}</ref> The foot was heavily modified from those of other mammals and earlier ground sloths, with a reduction in the number of digits on the inner part of the foot (digits I and II being lost), the increase in the size and robustness (thickness) of the [[metapodial]] elements of the outer digits, with the loss or reduction of the [[Phalanx bone|phalangeal]] bones. The [[calcaneum]] is wide and elongate posteriorly. The foot is suggested to have been inwardly rotated, historically the foot was suggested to be near vertical, though a recent study suggests that the angle was much shallower. The weight was primarily borne on the outer digits and the calcaneum.<ref>{{Cite journal |last1=Toledo |first1=Néstor |last2=De Iuliis |first2=Gerardo |last3=Vizcaíno |first3=Sergio F. |last4=Bargo |first4=M. Susana |date=December 2018 |title=The Concept of a Pedolateral Pes Revisited: The Giant Sloths Megatherium and Eremotherium (Xenarthra, Folivora, Megatheriinae) as a Case Study |url=http://link.springer.com/10.1007/s10914-017-9410-0 |journal=Journal of Mammalian Evolution |language=en |volume=25 |issue=4 |pages=525–537 |doi=10.1007/s10914-017-9410-0 |issn=1064-7554}}</ref> ''M. urbinai'' differs from ''M. americanum'' and other ''Megatherium'' species in the shape and position of the feet and hand bones, including the [[metacarpals]], [[metatarsals]], ectocuneiform, [[Hamate|hamate/unciform]], [[Navicular bone|navicular]] and [[Astragalus bone|astragalus/talus]].<ref name=":6" />