Lestodon is an extinct genus of giant ground sloth native to South America during the Pleistocene epoch. Its fossil remains have primarily been found in the Pampas and adjacent regions. The largest member of the family Mylodontidae,
Research history and taxonomy
The genus
Lestodon and the species
Lestodon armatus was erected by
Paul Gervais in 1855, based on a fragments of the upper and lower jaws with teeth found in Late Pleistocene deposits what is currently Buenos Aires Province, Argentina.
[Gervais, P., 1855. Recherches sur les mammifères fossils de l′Amérique méridionale, Annales des Sciences Naturelles, Zoologie 3 (6) (1855), pp. 330–338.] The genus name, which means "thief tooth", is in reference to the large caniniform teeth at the front of the jaw.
In 1934, a second species
L. australis was erected by Lucas Kraglievich, but this is now regarded as a junior synonym of
L. armatus.
In 2004, two additional species
L. urumaquensis and
L. codorensis were described based on fossils found in Late Miocene/Early Pliocene aged deposits in Venezuela.
[Linares OJ. 2004. Nuevos restos del género Lestodon Gervais, 1855 (Xenarthra, Tardigrada, Mylodontidae), del Mioceno Tardío y Plioceno Temprano de Urumaco (Venezuela), con descripción de nuevas especies. New. Paleobiologia Neotropical. 2:1–14.] However other authors have doubted the placement of these taxa in
Lestodon, with later studies generally placing them in the separate genus
Bolivartherium.
Lestodon is a member of the family Mylodontidae, meaning that its closest living relatives are two-toed sloths. Phylogeny of sloths based on DNA after Delsuc et al. 2019.
Description
Size
Lestodon armatus is the largest known
Mylodontidae sloth, attaining a length of .
Volumetric estimates suggests a body mass of around ,
making it one of the largest known ground sloths, alongside the megatheres
Megatherium and
Eremotherium.
Skull
The skull of
Lestodon armatus has a very wide muzzle. The
Premaxilla are weakly attached to the
(often resulting in them being absent in recovered fossils). The
nasal septum was think and weak, and is rarely preserved. The nasal cartilage was apparently not ossified.
The roof of the muzzle is marginally concave along its midline. The maxilla extends more towards the front of the skull on its lower part. The nasofrontal suture is U-shaped, and opens forwards. The
foramen magnum is oval-shaped.
The teeth were quite
hypsodont (high-crowned), though not to the same degree as achieved in some other mylodontids.
[Bargo, M. S., G. De Iuliis, and S. F. Vizcaino. 2006. Hypsodonty in Pleistocene ground sloths. Acta Palaeontol. Pol. 51: 53.] At the front of the mouth there were two pairs of tusk-like "caniniform" teeth on the upper and lower jaws which are separated from the molariform teeth by a large diastema (gap).
The lower caniniform teeth appear to exhibit sexual dimorphism in regard to size.
The molariform teeth are largely similar to each other, aside from the last lower molar "which has two rounded lobes separated by a narrow constriction giving it a figure-8 shape".
There are 5 and 4 teeth in each half of the upper and lower jaw respectively, as is typical of mylodontids.
Axial skeleton
The first thoracic vertebra has a much higher neural spine than the seventh cervical vertebra.
Limbs
The
olecranon process of the
ulna of
Lestodom armatus is slightly short compared to body size, though large in absolute terms.
The
tibia and
fibula of
L. armatus are fused together at the end closest to the hip (proximal).
The tibia is relatively short compared to body size.
Like many other ground sloths, the foot is inwardly rotated, meaning that the weight was borne on the outer digits of the foot as well as on the
calcaneus.
Like other ground sloths, the hands had
ungual phalanges indicating well-developed claws.
Like many other ground sloths, the hind foot is inwardly rotated so that the body weight was borne on the fifth
metatarsal and
calcaneus. The first digit on the hindfoot has been lost, with the fifth and fourth digits additionally being reduced, while digits two and three retained claws. This condition is very similar to fellow mylodontids
Paramylodon,
Glossotherium and
Thinobadistes, though in the form of
Talus bone it more closely resembles members of the family
Megatheriidae.
[Clavijo, L. ; Tambusso, P.S. ; Fariña R.A. Anatomy of the foot of Lestodon armatus (Xenarthra, Folivora) and its relationships with other sloths. First Palaeontological Virtual Congress November 2018]
Distribution
Lestodon armatus is primarily known from the
Pampas and the
Chaco Basin-Paraná Basin, including what is now southern Brazil, Paraguay, Uruguay, and northern Argentina.
Ecology
Lestodon is generally regarded as having a grazing diet.
is thought to have been a bulk feeder that indiscriminately consumed large amounts of vegetation, using its probably square,
white rhinoceros-like lips to pluck grass and other low lying plants in combination with the tongue.
Isotopic analysis has suggested that
Lestodon consumed a varying proportion of C
3 and C
4 plants, depending on locality.
[Czerwonogora, A., Fariña, R.A. & Tonni, E.P. (2011): Diet and isotopes of Late Pleistocene ground sloths: first results for Lestodon and Glossotherium (Xenarthra, Tardigrada). – N. Jb. Geol. Paläont. Abh., 262: 257–266; Stuttgart.] Although the proportions of its forelimbs are similar to those of mylodontids like
Glossotherium that are suggested to have engaged in digging, the strength of its forelimb is much lower than those mylodontids, indicating that they were not substantially adapted to this task.
If
Lestodon engaged in digging at all, it may have engaged in it only in short intervals.
, like other ground sloths, was likely incapable of running from predators, instead relying on its claws to defend itself as living sloths do.
A bonebed of 13 Lestodon armatus individuals of different ages found together at the Playa del Barco site in Argentina suggests that the species engaged in gregarious behaviour, living at least some of the time in social groups.
Relationship with humans and extinction
Researchers working at the Arroyo del Vizcaíno site near Sauce, Uruguay suggested that
Lestodon was hunted by humans about 30,000 years ago. This was based on analysis of
Lestodon bones. Deep slash markings on some of them were suggested to be from the use of human stone tools.
[Richard A. Fariña et al., Arroyo del Vizcaíno, Uruguay: a fossil-rich 30-ka-old megafaunal locality with cut-marked bones, , Published 20 November 2013, ] However, there are no unambiguous stone tools at the site, and the supposed "cut marks" could easily have been generated by non-human activities, such as trampling.
The site is also considerably older than the earliest widely accepted dates for human presence in South America (which dates to around 16–14,500 years ago).
Lestodon became extinct at the end of the Late Pleistocene as part of the Late Pleistocene megafauna extinctions, along with the vast majority of large mammals native to South America, including all of those above .[G. Martínez, M. A. Gutiérrez, Paso Otero 5: A summary of the interdisciplinary lines of evidence for reconstructing early human occupation and paleoenvironment in the Pampean region, Argentina, in Peuplements et Préhistoire de l’Amérique, D. Vialou, Ed. (Muséum National d’ Histoire Naturelle. Departement de Prehistoire, U.M.R, Paris, 2011), pp. 271–284.]
