Quaternary Scelidotheriinae (Mammalia, Xenarthra, Mylodontidae) are represented by three genera: Scelidotherium Owen, Valgipes Gervais, and Catonyx Ameghino. The genus Catonyx includes three species of ground sloths: C. cuvieri (Lund), C. tarijensis (Gervais and Ameghino), and C. chiliensis (Lydekker). Catonyx tarijensis and C. chiliensis were present during the Quaternary of Argentina. The aim of this contribution is to improve the knowledge of C. tarijensis of Argentina from a morphological, systematic, and biostratigraphic point of view. The presence of C. tarijensis is recorded in the Ensenadan-Lujanian South American Land Mammal Ages (SALMAs) (Early Pleistocene-Late Pleistocene) of Buenos Aires Province, in the Lujanian SALMA (Late Pleistocene) of Corrientes Province, and in the Pleistocene of Cordoba and Salta Provinces. Outside Argentina, this species is recorded in the Late Pleistocene of Bolivia and Uruguay.
Here we describe a new genus and species of giant ground sloth, Xibalbaonyx oviceps (Megalonychidae, Xenarthra), from the drowned cave system of the northeastern Yucatan Peninsula. The specimen is Late Pleistocene in age and was discovered in the Zapote sinkhole (cenote) near Puerto Morelos in the Mexican state of Quintana Roo. Xibalbaonyx oviceps differs significantly from all hitherto known Megalonychidae including those from the Greater Antilles and South America. The new taxon suggests a local Caribbean radiation of ground sloths during the Late Pleistocene, which is consistent with the dispersal of the group along a Mexican corridor.
Pyro clastic and epiclastic continental sediments bearing the "fauna Astrapothericulense" from the Pinturas Formation of Ameghino crop out mainly at several localities at the upper valley of the Pinturas River and its tributaries, northwestern Santa Cruz Province, Argentina. These continental sediments are referred to the Burdigalian Stage and constitute the basis for the recognition of the Pinturas Formation. The fauna recorded in the Pinturas Formation mainly consists of mammals, specially rodents, native ungulates, xenarthrans and primates. Here we describe the first association of Megatherioidea (Mammalia, Xenarthra, Tardigrada) from different localities of the Pinturas Formation with accurate geographic and stratigraphic provenance. The Megatherioidea from the Pinturas Formation presented herein are represented by (1) Schismotherium cf. binum; (2) Hapalops sp.; and (3) a Megatherioidea indet. In addition, the holotypes of Schismotherium binum (MACN A 11750), Hapalops curvus (MACN A 11140), and Pelecyodon arcuatus were collected from the "fauna Astrapothericulense" of Ameghino; unfortunately, it is not easy to determine if they were collected from the Pinturas Formation since they are part of Ameghino's Collection (MACN), but were collected a time when the Pinturas Formation had not yet been proposed. The presence of a species of Schismotherium and of Hapalops in the Pinturas Formation represent accurate early records for these genera, but not necessarily the earliest. The age of the Pinturas Formation could overlap, partially or totally, with the age of the Santa Cruz Formation at the Atlantic coast, where most Early Miocene Megatherioidea were collected.
A nearly complete skeleton, including most of the thoracic member bones of the sloth Mylodon darwinii, have been found in Upper Pleistocene strata from Anisacate River, Argentina. The thoracic member bones resemble their homologues in Glossotherium robustum, Paramylodon harlani, and Mylodonopsis ibseni in the following traits: (1) the olecranon is mediolaterally compressed; (2) the radius has an acute styloid process; (3) the radial diaphysis medial border is straight for two thirds of its length; (4) the radial shaft medial border forms an angle with the medial border of the styloid process. The radius presents a distinctive, mostly proximally facing articular circumference. The unfused epiphyses and feeble muscle attachment ridges indicate a sub-adult ontogenetic stage. Deviation of the olecranon and weak M. teres major origin and insertion, suggest a low fossorial specialization. The structure of the thoracic limb bones does not support climbing habits, because pronation-related features are reduced and the humeral head is not prominent. The structure of the radius suggests graviportal adaptations: the proximal head is mediolaterally expanded and the diaphysis straight. A phylogenetic analysis adding thoracic member characters recovers M. darwinii as part of a clade that includes Glossotherium robustum and Paramylodon harlani, but excludes Lestodon armatus. This contrasts with the results of previous analyses focusing on the head skeleton, highlighting the relevance of sampling postcranial characters in phylogenetic analyses of mylodontine sloths.