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Research history

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Illustration of the first specimen of Megatherium americanum from 1796

The earliest specimen of Megatherium americanum was discovered in 1787 by Manuel de Torres a Dominican friar and naturalist, from a ravine on the banks of the Lujan River in what is now northern Argentina, which at the time was part of the Viceroyalty of the Río de la Plata. Torres described the bones as a ‘wonder and providence of the Lord’. On the orders of the then viceroy of la Plata, Nicolás Cristóbal del Campo, Marqués de Loreto, the specimen was moved to the capital Buenos Aires. There the skeleton was drawn for the first time by José Custodio Sáa y Faria in a horse-like posture, though what kind of animal it actually was unknown. Campo summoned a number of local indigenous leaders to ask if they had heard of the animal. The skeleton was then transferred by Campo to the Royal Cabinet of Natural History of Madrid (now the National Museum of Natural Sciences MNCN), which had been founded a decade prior in 7 crates, which had arrived and been unpacked by late 1788.[1]

At the direction of the cabinets main taxidermist Juan Bautista Bru, the specimen was then mounted in for public exhibition (which remains unaltered in the modern museum display). In 1796 a scientific description of the skeleton was published authored by Bru along with engineer Joseph Garriga, with engravings by Manuel Navarro. As the work was going through the process of publication in 1795, preliminary prints of the paper were obtained by French diplomat Philippe-Rose Roume who was in Madrid at the time, who sent them to the National Museum of Natural History (Muséum national d'histoire naturelle) in Paris, France, where they were seen by French anatomist and paleontologist Georges Cuvier.[1]

Cuvier, working solely from the prints from Madrid and not visiting the specimen personally,[1] and using comparative anatomy with "edenate" mammals (now recognised as members of the order Xenarthra) in the collection of the Paris museum,[2] correctly recognised that the remains represented those of a giant sloth, and an animal that was entirely extinct and not living. In early 1796, somewhat before the full publication of the work by Bru, Garriga and Navarro, Cuvier published a paper naming the species Megatherium americanum (literally "Great American beast"),[1] becoming the first fossil mammal to be identified with both a genus and species name. Which description had priority has been controversial in the past. Cuvier later wrote a fuller description in 1804, which was republished in his famous 1812 book Recherches sur les ossemens fossiles de quadrupèdes. Cuver identified Megatherium as a sloth primarily on the basis of its skull morphology and the dental formula and the shoulder, while regarding the anatomy of its limbs as more similar to armadillos and anteaters. Cuvier suggested that based on the proportions of its limbs (which are approximately equal to each other), that Megatherium did not jump or run, nor crawl like living sloths, with the presence of a cavicle and well developed crests on the humerus, suggesting to Cuvier that the animal probably used its forelimbs to grasp.[2] A later publication in 1823 by Cuvier suggested that giant carapaces found in the Pampas also belonged to Megatherium, but British paleontologist Richard Owen in 1839 demonstrated that these actually belonged to another extinct group of xenathrans called glyptodonts that were related to armadillos.[3]

Additional remains of Megatherium were collected by Charles Darwin during the Voyage of the Beagle in the 1830s, these remains were assigned by Richard Owen in 1840 to the species Megatherium cuvieri, which had been named by Desmarest in 1822. These remains are now assigned to M. americanum.[3]

Owen later wrote a monograph series from 1851 to 1860 thoroughly describing the anatomy of M. americanum.[4][5][6][7][8][9]

From the late 19th century onward additional species of Megatherium were described. In 1888 and 1921 the species Megatherium filholi (Late Pleistocene) and Megatherium gallardoi (Early-Middle Pleistocene) a were named based on remains found in the Pampas. The validity of these species is disputed, with some authors considering them to be synonyms of M. americanum.[10] In 1880 Paul Gervais and Florentino Ameghino described the species M. tarijense from remains of Pleistocene age found in Bolivia. In 1893 Rodolfo Amando Philippi erected the species M. sundti and M. medinae from remains found in the Pleistocene of Bolivia and Chile, respectively.[11][12] In 1921, Florentino's brother Carlos Ameghino and Lucas Kraglievich described the species Megatherium gallardoi based on remains found in the Pampas of Northern Argentina.[13] In 2001, the species M. altiplanicum was described based on remains found in the Pliocene of Bolivia.[14] In 2004, the species Megatherium urbinai was erected based on remains found in Pleistocene aged deposits Peru.[15] In 2006, the species Megatherium celendinense was erected for remains of Pleistocene age found in the Peruvian Andes.[16]

Taxonomy and evolution

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Megatherium is divided into 2 subgenera, Megatherium and Pseudomegatherium, with at least 7 species typically being considered valid. Taxonomy according to Pujos (2006) and De Iuliis et al (2009):[16][17]

  • Subgenus Megatherium
    • †M. altiplanicum Saint-André & de Iuliis 2001
    • †M. americanum Cuvier 1796
  • Subgenus Pseudomegatherium Kraglievich 1931
    • †M. celendinense Pujos 2006
    • †M. medinae Philippi 1893
    • †M. sundti Philippi 1893
    • †M. tarijense Gervais & Ameghino, 1880
    • †M. urbinai Pujos & Salas 2004

Megatherium gallardoi Ameghino & Kraglievich, 1921 from the Pampas dating to the Pleistocene has been either regarded as a valid species of Megatherium (Megatherium) or a junior synomym of M. americanum. The species Megatherium filholi Moreno, 1888 also from the Pleistocene of the Pampas region, historically regarded to be a junior synonym of M. americanum representing juvenile individuals has been suggested to be valid by some recent authors.[18] Megatherium gaudryi Moreno (1888) from Argentina, of uncertain temporal provenance but possibly Pliocene in age, may also be valid.[10]

Mitochondrial DNA sequences obtained from M. americanum indicates that three-toed sloths (Bradypus) are their closest living relatives. Phylogeny of sloths after Delsuc et al. 2019.[19]

Folivora (sloths)

†Caribbean sloths

Mylodontoidea
Megatherioidea

Megatheriidae is suggested to have diverged from other sloth families during the Oligocene, around 30 million years ago.[19] The subfamily to which Megatherium belongs, Megatheriinae, first appeared in the Middle Miocene in Patagonia, at least 12 million years ago, represented by the genus Megathericulus.[20] The earliest known remains of the genus Megatherium are known from the Pliocene, found in Bolivia (M. altiplanicum) and the Pampas (indeterminate species), dating to at least 3.6 millon years ago.[10][14] M. altiplanicum is suggested to be more closely related to M. americanum than to species of Pseudomegatherium. Phylogeny of Megatheriinae after Pujos, 2006:[16]

Megatheriinae

Plesiomegatherium

Megathericulus

Megatheriops

Pyramiodontherium bergi

Pyramiodontherium brevirostrum

Eremotherium

Megatherium
Megatherium (Megatherium)

Megatherium americanum

Megatherium altiplanicum

Megatherium (Pseudomegatherium)

Megatherium sundti

Megatherium urbinai

Megatherium celendinense

Megatherium medinae

Megatherium tarijense

Megatherium americanum first appears in the fossil record during the second half of the Middle Pleistocene, from around 400,000 years ago.[21]

Description

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Size

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M. americanum is one of the largest known ground sloths. Volumetric analysis suggests that a full grown M. americanum weighed around 3,700–3,800 kilograms (8,200–8,400 lb), comparable to an elephant.[22][23] 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. The Chilean Pseudomegatherium species M. sundti was much smaller, with an estimated body mass of only 1,253 kilograms (2,762 lb), 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.[24] The Pliocene Megatherium (Megatherium) species M. altiplanicum has been estimated to weigh 1,465 kilograms (3,230 lb).[14]

Skull and jaws

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Skull and lower jaws of M. americanum. Scale bar = 10 cm, ~ 4 inches

The head of Megatherium is relatively small compared to body size.[23] The skull of M. americanum has a relatively narrow snout/muzzle with a ossified nasal septum, and is suggested to have had a muscular prehensile upper lip.[25] The skull is roughly cylindrical in shape, with the cranial region of the skull being narrrow. The jugal bone of M. americanum has a strongly developed ascending and descending processes.[26] In many species of Megatherium, the lower jaw is relatively deep, which served to accomodate the very long hypselodont (evergrowing) teeth,[14] which are considerably proportionally longer than those of other ground sloths. Like other ground sloths, the number of teeth in the jaw is reducted 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. The skull and jaws of M. americanum show adaptation to powerful vertical biting.[26] 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.[16]

Axial skeleton

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Like other xenathrans, 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, forming a synsacrum. The sacrum was composed of 5 vertebrae. The pubic symphysis is reduced. The tail is large in size.[23]

Limbs

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Foot of Megatherium showing enlarged and elongate calcaneum (right) and lateral digits

The bones of the forelimbs of M. americanum are relatively elongate and thin. The three fingers in the middle of the hand bore claws, while the cuneiform bones did not touch the ulna. The femur was massive and roughly rectangular in shape.[23] 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.[27] The foot was headily 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 reduction/loss of digit V and the 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.[28] M. urbinai differs from M. americanum based on various characters of the feet and hands.[15]

Ecology

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Although some authors have suggested that Megatherium was an omnivore,[29] isotopic analysis has supported an entirely herbivorous diet for Megatherium.[30] Megatherium americanum is suggested to have been a browser that was a selective feeder on the foliage of trees and shrubs.[25][31] Megatherium is widely thought to have been able to adopt a bipedal posture to use its forelimbs to grasp vegetation, though whether it was capable of moving in this posture is uncertain.[32] Due to its very large body size, some authors have argued that Megatherium americanum was probably hairless like modern elephants for thermodynamic reasons.[33] Megatherium americanum inhabited temperate, arid-to semi arid open habitats.[34]

Based on fossil trackways and the anatomy of its inner ear, which is considerably different from living sloths and more similar to those of armadillos, species of Megatherium, while probably not capable of moving at considerable speed due to limitiations of their skeletal anatomy were likely significantly more agile and mobile than living sloths (likely capable of moving a few kilometers an hour, as opposed to around 0.5–0.6 kilometres per hour (0.31–0.37 mph) in living tree sloths). Species of Megatherium likely relied on their large adult body size to protect themselves against predators,[35] due to their inability to outrun them. Like other ground sloths, Megatherium likely used the claws on its hands for defense.[36]

  1. ^ a b c d Pimentel, Juan (2021), Pimentel, Juan; Thurner, Mark (eds.), "Megatherium", New World Objects of Knowledge, A Cabinet of Curiosities, University of London Press, pp. 231–236, ISBN 978-1-908857-82-8, retrieved 2024-04-23
  2. ^ a b Argot, Christine (2008), Sargis, Eric J.; Dagosto, Marian (eds.), "Changing Views in Paleontology: The Story of a Giant (Megatherium, Xenarthra)", Mammalian Evolutionary Morphology, Dordrecht: Springer Netherlands, pp. 37–50, doi:10.1007/978-1-4020-6997-0_3, ISBN 978-1-4020-6996-3, retrieved 2024-04-23
  3. ^ a b Fernicola, J. C., Vizcaino, S. F., & De Iuliis, G. (2009). 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), 147-159. Retrieved from https://revista.geologica.org.ar/raga/article/view/1339
  4. ^ Owen R (1851) On the Megatherium (Megatherium americanum, Blumenbach). Part I. Preliminary observations on the exogenous processes of vertebrae. Phil Trans R Soc Lond 141:719–764
  5. ^ Owen R (1855) On the Megatherium (Megatherium americanum, Cuvier and Blumenbach). Part II. Vertebrae of the trunk. Phil Trans R Soc Lond 145:359–388
  6. ^ Owen R (1856) On the Megatherium (Megatherium americanum, Cuvier and Blumenbach). Part III. The skull. Phil Trans R Soc Lond 146:571–589
  7. ^ Owen R (1858) On the Megatherium (Megatherium americanum, Cuvier and Blumenbach). Part IV. Bones of the anterior extremities. Phil Trans R Soc Lond 148:261–278
  8. ^ Owen R (1859) On the Megatherium (Megatherium americanum, Cuvier and Blumenbach). Part V. Bones of the posterior extremities. Phil Trans R Soc Lon149:809–829
  9. ^ Owen R (1861) Memoir on the Megatherium, or Giant Ground-sloth of America (Megatherium americanum, Cuvier). Williams and Norgate, London
  10. ^ a b c Chimento, Nicolás R.; Agnolin, Federico L.; Brandoni, Diego; Boh, Daniel; Magnussen, Mariano; De Cianni, Francisco; Isla, Federico (2021-04). "A new record of Megatherium (Folivora, Megatheriidae) in the late Pliocene of the Pampean region (Argentina)". Journal of South American Earth Sciences. 107: 102950. doi:10.1016/j.jsames.2020.102950. {{cite journal}}: Check date values in: |date= (help)
  11. ^ Philippi, R.A. 1893a. Vorläufige Nachricht über fossile Säugethierknochen von Ulloma, Bolivia. Zeitschrift der deutschen geologischen Gesellschaft 45: 87-96.
  12. ^ Philippi, R.A. 1893b. Noticias preliminares sobre huesos fósiles de Ulloma. Anales de la Universidad de Chile 82: 499-506.
  13. ^ C. Ameghino, L. Kraglievich Descripción del “Megatherium gallardoi” C. Amegh. descubierto en el Pampeano inferior de la ciudad de Buenos Aires Anales del Museo Nacional de Historia Natural de Buenos Aires, 31 (1921), pp. 134-156
  14. ^ a b c d SAINT-ANDRÉ P.-A. & DE IULIIS G. 2001. — 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.
  15. ^ a b Pujos, François; Salas, Rodolfo (2004-05). "A new species of Megatherium (Mammalia: Xenarthra: Megatheriidae) from the Pleistocene of Sacaco and Tres Ventanas, Peru". Palaeontology. 47 (3): 579–604. doi:10.1111/j.0031-0239.2004.00376.x. ISSN 0031-0239. {{cite journal}}: Check date values in: |date= (help)
  16. ^ a b c d Pujos, François (2006). "Megatherium celendinense sp. nov. from the Pleistocene of the Peruvian Andes and the phylogenetic relationships of Megatheriines". Palaeontology. 49 (2): 285–306. Bibcode:2006Palgy..49..285P. doi:10.1111/j.1475-4983.2006.00522.x. S2CID 84225654.
  17. ^ De Iuliis, Gerardo; Pujos, François; Tito, Giuseppe (2009-12-12). "Systematic and taxonomic revision of the Pleistocene ground sloth Megatherium (Pseudomegatherium) tarijense (Xenarthra: Megatheriidae)". Journal of Vertebrate Paleontology. 29 (4): 1244–1251. Bibcode:2009JVPal..29.1244D. doi:10.1671/039.029.0426. ISSN 0272-4634. S2CID 84272333.
  18. ^ Agnolin, Federico L.; Chimento, Nicolás R.; Brandoni, Diego; Boh, Daniel; Campo, Denise H.; Magnussen, Mariano; De Cianni, Francisco (2018-09-01). "New Pleistocene remains of Megatherium filholi Moreno, 1888 (Mammalia, Xenarthra) from the Pampean Region: Implications for the diversity of Megatheriinae of the Quaternary of South America". Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen. 289 (3): 339–348. doi:10.1127/njgpa/2018/0777. ISSN 0077-7749. S2CID 134660849.
  19. ^ a b Delsuc, Frédéric; Kuch, Melanie; Gibb, Gillian C.; Karpinski, Emil; Hackenberger, Dirk; Szpak, Paul; Martínez, Jorge G.; Mead, Jim I.; McDonald, H. Gregory; MacPhee, Ross D.E.; Billet, Guillaume; Hautier, Lionel; Poinar, Hendrik N. (2019-06). "Ancient Mitogenomes Reveal the Evolutionary History and Biogeography of Sloths". Current Biology. 29 (12): 2031–2042.e6. doi:10.1016/j.cub.2019.05.043. {{cite journal}}: Check date values in: |date= (help)
  20. ^ Brandoni, Diego; Carlini, Alfredo A; Anaya, Federico; Gans, Phil; Croft, Darin A (2018-09). "New Remains of Megathericulus patagonicus Ameghino, 1904 (Xenarthra, Tardigrada) from the Serravallian (Middle Miocene) of Bolivia; Chronological and Biogeographical Implications". Journal of Mammalian Evolution. 25 (3): 327–337. doi:10.1007/s10914-017-9384-y. ISSN 1064-7554. {{cite journal}}: Check date values in: |date= (help)
  21. ^ Prado, José Luis; Alberdi, María Teresa; Bellinzoni, Jonathan (2021-06). "Pleistocene Mammals from Pampean Region (Argentina). Biostratigraphic, Biogeographic, and Environmental Implications". Quaternary. 4 (2): 15. doi:10.3390/quat4020015. ISSN 2571-550X. {{cite journal}}: Check date values in: |date= (help)CS1 maint: unflagged free DOI (link)
  22. ^ Brassey, Charlotte A.; Gardiner, James D. (2015-08). "An advanced shape-fitting algorithm applied to quadrupedal mammals: improving volumetric mass estimates". Royal Society Open Science. 2 (8): 150302. doi:10.1098/rsos.150302. ISSN 2054-5703. PMC 4555864. PMID 26361559. {{cite journal}}: Check date values in: |date= (help)CS1 maint: PMC format (link)
  23. ^ a b c d Casinos, Adrià (1996-03). "Bipedalism and quadrupedalism in Megatheriurn: an attempt at biomechanical reconstruction". Lethaia. 29 (1): 87–96. doi:10.1111/j.1502-3931.1996.tb01842.x. ISSN 0024-1164. {{cite journal}}: Check date values in: |date= (help)
  24. ^ McDonald, H. Gregory (2023-06). "A Tale of Two Continents (and a Few Islands): Ecology and Distribution of Late Pleistocene Sloths". Land. 12 (6): 1192. doi:10.3390/land12061192. ISSN 2073-445X. {{cite journal}}: Check date values in: |date= (help)CS1 maint: unflagged free DOI (link)
  25. ^ a b Bargo, M. Susana; Toledo, Néstor; Vizcaíno, Sergio F. (2006-02). "Muzzle of South American Pleistocene ground sloths (Xenarthra, Tardigrada)". Journal of Morphology. 267 (2): 248–263. doi:10.1002/jmor.10399. ISSN 0362-2525. {{cite journal}}: Check date values in: |date= (help)
  26. ^ a b Bargo, M.S. 2001. The ground sloth Megatherium americanum: Skull shape, bite forces, and diet. - Acta Palaeontologica Polonica 46,2, 173-192.
  27. ^ Bonini, Ricardo A.; Brandoni, Diego (2015-12). "Pyramiodontherium Rovereto (Xenarthra, Tardigrada, Megatheriinae) from the Early Pliocene of San Fernando, Catamarca Province, Argentina". Ameghiniana. 52 (6): 647–655. doi:10.5710/AMGH.16.06.2015.2902. ISSN 0002-7014. {{cite journal}}: Check date values in: |date= (help)
  28. ^ Toledo, Néstor; De Iuliis, Gerardo; Vizcaíno, Sergio F.; Bargo, M. Susana (2018-12). "The Concept of a Pedolateral Pes Revisited: The Giant Sloths Megatherium and Eremotherium (Xenarthra, Folivora, Megatheriinae) as a Case Study". Journal of Mammalian Evolution. 25 (4): 525–537. doi:10.1007/s10914-017-9410-0. ISSN 1064-7554. {{cite journal}}: Check date values in: |date= (help)
  29. ^ "Megatherium , the stabber". Proceedings of the Royal Society of London. Series B: Biological Sciences. 263 (1377): 1725–1729. 1996-12-22. doi:10.1098/rspb.1996.0252. ISSN 0962-8452.
  30. ^ Bocherens, Hervé; Cotte, Martin; Bonini, Ricardo A.; Straccia, Pablo; Scian, Daniel; Soibelzon, Leopoldo; Prevosti, Francisco J. (2017-08). "Isotopic insight on paleodiet of extinct Pleistocene megafaunal Xenarthrans from Argentina". Gondwana Research. 48: 7–14. doi:10.1016/j.gr.2017.04.003. {{cite journal}}: Check date values in: |date= (help)
  31. ^ Bargo, M. S., & Vizcaíno, S. F. (2008). PALEOBIOLOGY OF PLEISTOCENE GROUND SLOTHS (XENARTHRA, TARDIGRADA): BIOMECHANICS, MORPHOGEOMETRY AND ECOMORPHOLOGY APPLIED TO THE MASTICATORY APPARATUS. Ameghiniana, 45(1), 175–196.
  32. ^ Milne, Nick; Toledo, Nestor; Vizcaíno, Sergio F. (2012-09). "Allometric and Group Differences in the Xenarthran Femur". Journal of Mammalian Evolution. 19 (3): 199–208. doi:10.1007/s10914-011-9171-0. ISSN 1064-7554. {{cite journal}}: Check date values in: |date= (help)
  33. ^ Fariña, R. A. (2002). Megatherium, the hairless: appearance of the great Quaternary sloths (Mammalia; Xenarthra). Ameghiniana, 39(2), 241-244.
  34. ^ Varela, Luciano; Tambusso, P. Sebastián; Patiño, Santiago J.; Di Giacomo, Mariana; Fariña, Richard A. (2018-12). "Potential Distribution of Fossil Xenarthrans in South America during the Late Pleistocene: co-Occurrence and Provincialism". Journal of Mammalian Evolution. 25 (4): 539–550. doi:10.1007/s10914-017-9406-9. ISSN 1064-7554. {{cite journal}}: Check date values in: |date= (help)
  35. ^ Billet, G.; Germain, D.; Ruf, I.; de Muizon, C.; Hautier, L. (2013-12). "The inner ear of Megatherium and the evolution of the vestibular system in sloths". Journal of Anatomy. 223 (6): 557–567. doi:10.1111/joa.12114. ISSN 0021-8782. PMC 3842198. PMID 24111879. {{cite journal}}: Check date values in: |date= (help)CS1 maint: PMC format (link)
  36. ^ McDonald, H. Gregory (2012-09). "Evolution of the Pedolateral Foot in Ground Sloths: Patterns of Change in the Astragalus". Journal of Mammalian Evolution. 19 (3): 209–215. doi:10.1007/s10914-011-9182-x. ISSN 1064-7554. {{cite journal}}: Check date values in: |date= (help)
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