Ahytherium

''Ahytherium'' is an extinct genus of megalonychid sloth that lived during the Pleistocene of what is now Brazil. It contains a single species, ''A. aureum''.

Discovery and taxonomy

The almost-complete skeleton of ''Ahytherium'' alongside remains another extinct sloth species, ''Australonyx'', were discovered in Poço Azul, an underwater cave in Chapada Diamantina National Park in 2005. It was described by Castor Cartelle of Pontifícia Universidade Católica de Minas Gerais. The bones, which had a length of about when put together, belong to an animal which presumably was still growing.

Description

This animal is known from well-preserved and nearly complete fossils, including a skeleton of an immature specimen, which in life must have been about three meters long, one meter tall and weighing perhaps half a ton. Like all ground sloths, ''Ahytherium'' was equipped with a robust body and legs equipped with powerful claws. ''Ahytherium'' was similar to other ground sloths such as ''Megalonyx'' but possessed some features that clearly distinguished it: the head, for example, was extremely short and tall, and possessed dorsally swollen frontal bones. The zygomatic arches were wide, particularly the frontal processes, while the lacrimal bones were narrow, blade-shaped and directed anterolaterally. The dentition included canine teeth that were thin, curved and oval in cross-section. The humerus was equipped with a poorly developed deltopectoral crest, while the femur possessed a large trochanter located distally. The shape of the caudal vertebrae indicates that the tail was dorsoventrally flattened.

Classification

''Ahytherium'' is a genus of the extinct family Megalonychidae within the suborder Folivora and the superorder Xenarthra. The Megalonychidae represent a very diverse group. The closest relatives of the Megalonychidae are the Megatheriidae and the Nothrotheriidae. The former include the largest known representatives of the sloths, the latter consist of rather smaller members of the sloths. All three families together are relegated to the superfamily of Megatherioidea. Within the sloths, the Megalonychidae form a very old lineage, with the oldest known fossils dating to the Oligocene of Patagonia. Characteristic features are found in the caniniform and incisiform design of the anterior tooth in each case, as well as in the molar-like (molariform) designed posterior teeth. The latter are characterized by two transverse ridges (bilophodont) on the occlusal surface, which points to a rather leaf-eating diet in the Megalonychidae. Unlike the Megatheriidae and the Nothrotheriidae, the hind foot is plantigrade in shape and not twisted, thus retaining its original shape.H. Gregory McDonald und Gerardo de Iuliis: ''Fossil history of sloths.'' In: Sergio F. Vizcaíno und W. J. Loughry (eds.): ''The Biology of the Xenarthra.'' University Press of Florida, 2008, S. 39–55. The Megalonychidae were once widespread, with fossil remains found in both South America and Central America, as well as in North America as far north as the Arctic. In a classic view based on skeletal anatomy comparisons-based view, the Megalonychidae originally included the sloths of the West Indies as well as the extant two-toed sloths of the genus ''Choloepus'',Timothy J. Gaudrin: ''Phylogenetic relationships among sloths (Mammalia, Xenarthra, Tardigrada): the craniodental evidence.'' Zoological Journal of the Linnean Society 140, 2004, S. 255–305. which are now considered to belong to the superfamily Mylodontoidea.Frédéric Delsuc, Melanie Kuch, Gillian C. Gibb, Emil Karpinski, Dirk Hackenberger, Paul Szpak, Jorge G. Martínez, Jim I. Mead, H. Gregory McDonald, Ross D.E. MacPhee, Guillaume Billet, Lionel Hautier und Hendrik N. Poinar: ''Ancient mitogenomes reveal the evolutionary history and biogeography of sloths.'' Current Biology 29 (12), 2019, S. 2031–2042, doi:10.1016/j.cub.2019.05.043Samantha Presslee, Graham J. Slater, François Pujos, Analía M. Forasiepi, Roman Fischer, Kelly Molloy, Meaghan Mackie, Jesper V. Olsen, Alejandro Kramarz, Matías Taglioretti, Fernando Scaglia, Maximiliano Lezcano, José Luis Lanata, John Southon, Robert Feranec, Jonathan Bloch, Adam Hajduk, Fabiana M. Martin, Rodolfo Salas Gismondi, Marcelo Reguero, Christian de Muizon, Alex Greenwood, Brian T. Chait, Kirsty Penkman, Matthew Collins und Ross D. E. MacPhee: ''Palaeoproteomics resolves sloth relationships.'' Nature Ecology & Evolution 3, 2019, S. 1121–1130, doi:10.1038/s41559-019-0909-z Molecular genetic However, studies together with protein analyses failed to establish closer relationships between these individual groups.

Much of the fossil material of Megalonychidae is fragmentary and largely incomplete. The systematic Relationships of the individual representatives of the group could be worked out therefore so far only insufficiently. However, due to the richness of forms, different lines of development can be traced. One includes largely South American representatives such as ''Megistonyx'' or ''Ahytherium'' and '' Ortotherium'', respectively, another consists of the North American forms ''Megalonyx'' and ''Pliometanastes'' (here, since based on skeletal features, also Caribbean sloths such as ''Megalocnus'' or ''Neocnus''). Currently, however, it is not possible to determine direct ancestors for the North American representatives of Megalonychidae. Consequently, their relationship to the South American forms is rather unknown.

Below is a phylogenetic tree of the Megalonychidae, based on the work of Stinnesbeck and colleagues (2021).

Paleobiology

''Ahytherium'' must have been a large, rather slow-moving terrestrial sloth that could defend itself from carnivores with its powerful claws. The flattened shape of the tail suggests that ''Ahytherium'' might have been a good swimmer. The related ''Megistonyx,'' on the other hand, would seem to have adapted to high-altitude environments. Another swimming sloth, more specialized for marine life is ''Thalassocnus''.

See also

* ''Thalassocnus''

References

{{Taxonbar, from=Q4696453

Prehistoric sloths
Pleistocene xenarthrans
Prehistoric placental genera
Holocene extinctions
Pleistocene mammals of South America
Lujanian
Pleistocene Brazil
Fossils of Brazil
Fossil taxa described in 2008
Monotypic mammal genera