A review of the Quaternary Scelidotheriinae (Mammalia, Xenarthra, Tardigrada) from the Tarija-Padcaya basin, Bolivia

The Mylodontidae Scelidotheriinae (Mammalia, Xenarthra, Tardigrada) are a diversified clade of South American fossil ground sloths, with a wide geographic distribution, especially in high and middle latitudes. According to the last revision, the Quaternary diversity includes the genera Scelidotherium, Catonyx, and Valgipes. the clade Scelidotheriinae is well represented in the Pleistocene of the tarijaPadcaya basin, and the first mention of these ground sloths correspond to the middle of the XIX Century. Since then, several species (i.e., Scelidotherium tarijensis, Scelidodon tarijensis, Scelidotherium capellini) have been reported as inhabiting the tarija-Padcaya basin during the Pleistocene. Despite the abundance of fossil records of Scelidotheriinae in this area, no modern taxonomic revisions are available. In consequence, in this contribution a revision of the remains assigned to Scelidotheriinae from the tarija-Padcaya basin is accomplished, and some biostratigraphic and geographic implications are discussed. Our results show that one single species (Catonyx tarijensis) can be recognized in the studied area, whereas a supposed smaller one (Scelidotherium patrium) actually corresponds to juvenile specimens of C. tarijensis.


INTRODUCTION
the tarija-Padcaya basin is located in southern Bolivia, 140 km north of Argentina (see map in tonni et al. 2009). this locality is known for its high diversity of Pleistocene mammals since the XIX Century, being one of the most important in South America (see, among others, Gervais 1855, Ameghino 1902, Boule and Thévenin 1920, Tonni et al. 2009).
From a taxonomic perspective, the clade Scelidotheriinae (typified by Scelidotherium Owen 1839) is one of the most diversified among Mylodontidae. In this context, the Quaternary taxa are represented by three genera: Scelidotherium Owen 1839, Valgipes Gervais 1874, and Catonyx Ameghino 1891 (see Corona et al. 2013, Miño-Boilini 2016. Scelidotherines are recorded from the middle Miocene to the early holocene in Argentina, Bolivia, Brazil, Chile, Colombia, Ecuador, Peru, Paraguay, and Uruguay (Miño-Boilini and Carlini 2009). Despite their wide geographic distribution in South America, they are the only mylodontoids which did not migrate to Central and North America during the Great American Biotic Interchange (McDonald and Perea 2002).
From an anatomical viewpoint, scelidotherines are characterized by having a tubular and elongate skull (McDonald and Perea 2002), parallel dental series, laterally compressed molariforms (Pascual et al. 1966), and quadrangular and anteroposteriorly compressed femur, with concave facet of the astragalus for the cuboid (Pascual et al. 1966, McDonald andPerea 2002). this paper aims to: a) carry out a taxonomic revision of the Scelidotheriinae specimens from the tarija-Padcaya basin (Bolivia), including the descriptions of new specimens; b) discuss the chronostratigraphic and biogeographic implications.

PrEVIOUS StUDIES AND rEPOrtS OF SCELIDOthErIINAE IN BOLIVIA
The first mention of scelidotheres from Bolivia are those of Gervais (1855), Gervais and Ameghino (1880), Philippi (1893), andSefve (1915a, b). More precisely the first Bolivian remains were exhumed by Weddell from the tarija Valley (Weddell 1851), and were later mentioned and figured by Paul Gervais (1855: Plate 11 Figure  2, Plate 13 Figures 4-5). Sixty years later, Sefve (1915a) described and illustrated some specimens referred to Scelidotherium capellini from the same area.
Later, in an important field expedition to the tarija-Padcaya basin carried out between 1924-1927, riggs collected a large amount of remains that were assigned to Scelidotherium and are housed in the FMNh collections (Chicago, USA) (see Marshall 1978).
More recently, Werdelin (1991) mentioned specimens referred to Scelidodon tarijensis, which were collected by a Swedish expedition under the direction of Earland Nordenskiöld in 1901(Nordenskiöld 1902.
the age of the tarija palaeofauna is still problematic. recent studies, including magnetostratigraphy dating, suggested very different interpretations for those levels bearing fossil mammals. When compared to the biochronologic framework of the Argentinean Pampean region (see Cione et al. 2015), MacFadden et al. (2013) proposed they are exclusively Ensenadan in age (early to middle Pleistocene), but according to Coltorti et al. (2010) they are Lujanian (late Pleistocene), or even Ensenadan-Lujanian according to tonni et al. (2009).

MATERIALS AND METHODS
the chronologic scale is based on Cione et al. (2015), whereas the stratigraphy follows Coltorti et al. (2007Coltorti et al. ( , 2010. recently, Miño-Boilini (2016) carried out a detailed anatomical description of the skull, mandible and other postcranial elements (e.g.: scapula, humerus, radius, ulna, femur, tibia, and astragalus) of C. tarijensis, including comparisons with other Quaternary Scelidotheriinae. Consequently, in this contribution only the diagnostic characters of the materials are considered in order to support the specific assignation.     (Figure 1a and c) is composed of an articulated partial skull and mandible, which precludes detailed descriptions. the narrow and elongate skull belongs to a large specimen, with an approximate length of 580 mm from the posterior end of the occipital condyles to the anterior end of the maxillaries.
In dorsal view, the parasagittal crests form an evident sagittal crest; the supraoccipital and occipital crests, as well as the external protuberances of the occipital, which are very developed, can also be observed. there is also a slight postorbital narrowing. the fronto-parietal suture is located behind the anterior part of the zygomatic process of the squamosal. In lateral view, the zygomatic process of the squamosal is very robust, whereas the lacrimal preserves a vestige of the suture with the frontal and the parietal. In anterior view, the contour formed by the nasals, maxillaries, and the palate is sub-circular. Finally, in posterior view, the foramen magnum is sub-circular. the left hemimandible is robust; in lateral view, the horizontal ramus is high and ventrally convex. the mandibular condyle is slightly above the occlusal surface of the dental series. chiliensis) (see McDonald 1987, Miño-Boilini 2012, Plate 2, 2016. this species has a lingual lobe with a slightly developed longitudinal groove; the mf2 and mf3 are sub-rectangular in cross section, with its maximum diameter oblique to the sagittal plane, as in all Scelidotheriinae, and that of mf4 is trilobated, as in the remaining scelidotheres (see McDonald 1987, Miño-Boilini 2012.
In distal view, the patellar trochlea of C. tarijensis is continuous with the lateral and medial condyles, as in C. cuvieri, C. chiliensis, Scelidotherium leptocephalum and S. bravardi. however, in V. bucklandi they are separated by non-articular bone (Miño-Boilini 2012). In C. tarijensis, the medial condyle is more massive than in C. cuvieri, C. chiliensis, Scelidotherium leptocephalum and S. bravardi.

MORPHOLOGY AND TAXONOMY
All the scelidothere remains of tarija-Padcaya basin are assigned to a single species, Catonyx tarijensis, on the basis of the following morphological characteristics: 1) skull with evident temporal ridges and sagittal crest, much better developed than in other Quaternary scelidotherines; 2) Mf1 mesiodistally extended with well-developed lingual lobe; 2) robust mandible, high horizontal ramus and ventral margin markedly convex; 3) mf1 with labial lobe having a slightly longitudinal groove; 4) robust and massive humerus with deltoid tuberosity more developed than in the remaining species; and 5) femur with short femoral neck.
In summary, Catonyx tarijensis is the only Scelidotheriinae recorded in the tarija-Padcaya basin. It is represented by several skulls, mandibles and postcranial elements. In turn, the hypothetical presence of the small Scelidotheriinae Scelidotherium patrium (see Boule and Thévenin 1920: 222) is rejected, according to the current evidence. Noteworthy, the materials assigned to this latter species (Scelidotherium patrium), currently housed in the MhNh tAr, were not figured (see Boule and Thévenin 1920). The preservation of these materials is bad, hindering a specific taxonomic assignation (C. Argot, pers. On the other hand it is important to remark that Scelidotherium patrium (currently Proscelidodon patrius) is recorded in Pliocene sediments of Argentina andBolivia (Anaya andMacFadden 1995, Miño-Boilini et al. 2011). In this scenario, the analyzed material (a left maxilla; MNPA-V s / n, ex MUt 452 (Figure 1f) shows evident sutures, which clearly indicate the early ontogenetic stage of the specimen. the taxonomic assignment to the species C. tarijensis is based on the particular morphology of the Mf1, which is mesiodistally extended, showing a very well developed lingual lobe. In turn, the hemimandibles (MNPA-V s / n, ex MUt 443 and 438) have a bulge in the body of the mandible; in addition, the lingual lobe of mf1 has a slightly marked longitudinal sulcus. this particular morphology is only observed in juvenile specimens of C. tarijensis.
the other species mentioned for the tarija Valley is Scelidotherium capellini (see Sefve 1915a andTakai et al. 1982), a taxon recognized by Gervais and Ameghino (1880) on the basis of a left dentary from the Pleistocene of the Buenos Aires province (Argentina) (see Miño-Boilini 2016, Figure 1g). In the type material of Scelidotherium capellini the ventral border of the mandible is convex, and the lingual lobe of mf1 shows a slightly marked longitudinal sulcus, as observed in C. tarijensis. this taxonomic interpretation is in agreement with the proposal of Hoffstetter (1963) and McDonald and Perea (2002) according to which this species is a junior synonym of C. tarijensis (see also Miño-Boilini 2016). Moreover, this interpretation is also in agreement with McDonald (1987) who supports that the only recorded species in the tarija Valley is C. tarijensis.

BIOGEOGRAPHY AND CHRONOSTRATIGRAPHY
According to Miño-Boilini (2012) the biogeographic history of the Scelidotheriinae was restricted with certainty to South America, so far this clade has not been found in Central or North America. Scelidotherines are recorded from the middle Miocene to the early holocene in Argentina, Bolivia, Brazil, Chile, Colombia, Ecuador, Peru, Paraguay, and Uruguay (Miño-Boilini and Carlini 2009).
taking into account that the age of the tarija-Padcaya basin is controversial (see tonni et al. 2009tonni et al. , MacFadden et al. 2013, with ages that range between the Ensenadan and Lujanian (MacFadden et al. 2013, Coltorti et al. 2007, 2010, tonni et al. 2009), the presence of C. tarijensis supplies no information to support any of the chronological hypotheses mentioned above.