Timothy J Gaudin
email: timothy-gaudin@utc.edu

Alberto Boscaini

Bernardino Mamani Quispe

Rubén Andrade Flores

Marcos Fernández-Monescillo

Laurent Marivaux

Pierre-Olivier Antoine

Philippe Münch

François Pujos

Ruiz Leal

Running Head -New Nothrotheriid

Sloth Genus

From Bolivia

Recognition of a new nothrotheriid genus (Mammalia, Folivora) from the early late Miocene of Achiri (Bolivia) and the taxonomic status of the genus Xyophorus

Keywords: Xenarthra, Pilosa, sloth, auditory region, astragalus, skull, skeleton, late Miocene

New remains of a relatively plesiomorphic nothrotheriid sloth have been recovered from upper Miocene-aged deposits near the village of Achiri in the Altiplano of Bolivia. The new specimens appear allied to other middle and late Miocene remains from Argentina and Bolivia that have been assigned to the pseudo-genus "Xyophorus." "Xyophorus" has not previously been recognized as a distinct genus because of the paucity of material it encompasses. The new specimens, however, include a well-preserved squamosal with attached auditory region, and an isolated astragalus. These elements, which are described in detail, provide a sufficient number of distinctive characters to place the previous fossils assigned to "Xyophorus" into a new monotypic genus. Beyond exhibiting a suite of distinctive autapomorphies which justify its formal taxonomic designation, the new taxon shares several synapomorphies with more derived members of Nothrotheriidae, but also retains a number of plesiomorphies characteristic of basal megatherioid taxa, and shows a number of intermediate features. Although the new taxon is too incomplete to justify a full phylogenetic analysis, it appears to represent a basal member of Nothrotheriidae. Resolution of the taxonomic status of the genus Xyophorus awaits a better understanding of the taxonomy of early Miocene-aged basal megatherioids.

Introduction

Intensive, ongoing fieldwork in Neogene deposits from the Altiplano of Bolivia over the past two decades have yielded numerous remains of extinct sloths, including the large megatheriine Megatherium altiplanicum [START_REF] St-André | The smallest and most ancient representative of the genius Megatherium Cuvier, 1796 (Xenarthra, Tardigrada, Megatheriidae), from the Pliocene of the Bolivian Altiplano[END_REF], both scelidotheriine (cf. Proscelidodon; [START_REF] Pujos | The Miocene vertebrate faunas of Achiri, Bolivia[END_REF]) and mylodontine sloths (Simomylodon uccasamamensis; [START_REF] St-André | Nouveaux paresseux terrestres (Mammalia, Xenarthra, Mylodontidae) du Néogène de l'Altiplano bolivien[END_REF][START_REF] Boscaini | New well-preserved craniodental remains of Simomylodon uccasamamensis (Xenarthra, Mylodontidae) from the Pliocene of the Bolivian Altiplano: phylogenetic, chronostratigraphic and paleobiogeographic implications[END_REF][START_REF] Boscaini | Postcranial anatomy of the extinct sloth Simomylodon uccasamamensis (Xenarthra: Mylodontidae) from the Bolivian Altiplano and its evolutionary implications[END_REF] representing the Mylodontidae, with the latter the most abundant taxon, the nothrotheriid sloths Lakukullus anatisrostratus [START_REF] Pujos | Lakukullus anatisrostratus, gen. et sp. nov, a new massive nothrotheriid sloth (Xenarthra, Pilosa) from the middle Miocene of Bolivia[END_REF] and Aymaratherium jeani [START_REF] Pujos | A new nothrotheriid xenarthran from the early Pliocene of Pomata-Ayte (Bolivia): new insights into the caniniform-molariform transition in sloths[END_REF], and the peculiar megatherioid sloth Hiskatherium saintandrei [START_REF] Pujos | Hiskatherium saintandrei gen. et sp. nov.: an unusual sloth from the Santacrucian of Quebrada Honda (Bolivia) and an overview of middle Miocene, small megatherioids[END_REF]. Included among these remains are specimens reputedly pertaining to the enigmatic genus Xyophorus. The taxonomic status and phylogenetic relationships of this genus are controversial and have been widely discussed (e.g., [START_REF] De Iuliis | A new genus and species of nothrotheriid sloth (Xenarthra, Tardigrada, Nothrotheriidae) from the late Miocene (Huayquerian) of Peru[END_REF][START_REF] Pujos | Hiskatherium saintandrei gen. et sp. nov.: an unusual sloth from the Santacrucian of Quebrada Honda (Bolivia) and an overview of middle Miocene, small megatherioids[END_REF][START_REF] Brandoni | Xyophorus" sp. en el Mioceno medio de Chubut: implicancias sistemáticas, biogeográficas y biocronológicas del registro de un Nothrotheriinae en el Neógeno de la Argentina[END_REF][START_REF] Bargo | Early Miocene sloths (Xenarthra, Folivora) from the Río Santa Cruz valley (Southern Patagonia, Argentina). Ameghino, 1887 revisited[END_REF][START_REF] Brandoni | Chronological implications of the nothrotheriid "Xyophorus" (Mammalia, Xenarthra) from the Collón Curá Formation (Miocene of Patagonia, Argentina)[END_REF], in part because the fossil remains assigned to it are broadly distributed in time and space, spanning a geographic range from the Bolivian Altiplano in the north to the southern tip of Argentine Patagonia in the south, and a temporal range of nearly 12 Ma from the late early Miocene Santacrucian SALMA of Santa Cruz Province to the early Pliocene Montehermosan SALMA of La Rioja Province (Rodríguez [START_REF] Brizuela | Estratigrafía y mamíferos fósiles de la Formación Toro Negro (Plioceno), Departamento Vinchina, noroeste de la provincia de La Rioja, Argentina[END_REF][START_REF] Brandoni | Xyophorus" sp. en el Mioceno medio de Chubut: implicancias sistemáticas, biogeográficas y biocronológicas del registro de un Nothrotheriinae en el Neógeno de la Argentina[END_REF][START_REF] Brandoni | Chronological implications of the nothrotheriid "Xyophorus" (Mammalia, Xenarthra) from the Collón Curá Formation (Miocene of Patagonia, Argentina)[END_REF]). The taxon is also controversial because the remains assigned to it are so fragmentary and poorly preserved. [START_REF] Ameghino | Enumeración sistemática de las especies de mamíferos fósiles coleccionados por Carlos Ameghino en los terrenos eocenos de la Patagonia austral y depositados en el Museo de La Plata[END_REF] erected the genus based on partially preserved maxillary and mandibular specimens from the Santa Cruz Formation (Santacrucian SALMA, late early Miocene), which he placed in two species, Xyophorus rostratus [START_REF] Ameghino | Enumeración sistemática de las especies de mamíferos fósiles coleccionados por Carlos Ameghino en los terrenos eocenos de la Patagonia austral y depositados en el Museo de La Plata[END_REF] and X. simus [START_REF] Ameghino | Enumeración sistemática de las especies de mamíferos fósiles coleccionados por Carlos Ameghino en los terrenos eocenos de la Patagonia austral y depositados en el Museo de La Plata[END_REF], solely because of size differences. The type specimens of both species have subsequently been lost [START_REF] Bargo | Early Miocene sloths (Xenarthra, Folivora) from the Río Santa Cruz valley (Southern Patagonia, Argentina). Ameghino, 1887 revisited[END_REF]. Another four fragmentary specimens from Santa Cruz were assigned to four new species by [START_REF] Ameghino | Nuevos restos de mamíferos fósiles descubiertos por Carlos Ameghino en el Eoceno inferior de la Patagonia austral. Especies nuevas, adiciones y correcciones[END_REF][START_REF] Ameghino | Enumération synoptique des espèces de mammifères fossiles des formations éocènes de Patagonie[END_REF]; the former is a paper in which he also synonymizes Xyophorus with [START_REF] Mercerat | Datos sobre restos de mamíferos fósiles pertenecientes a los Bruta[END_REF] genus Eurysodon) based on size and minor morphological differences in the size and outline of the teeth: X. sulcatus [START_REF] Ameghino | Nuevos restos de mamíferos fósiles descubiertos por Carlos Ameghino en el Eoceno inferior de la Patagonia austral. Especies nuevas, adiciones y correcciones[END_REF], X. atlanticus [START_REF] Ameghino | Nuevos restos de mamíferos fósiles descubiertos por Carlos Ameghino en el Eoceno inferior de la Patagonia austral. Especies nuevas, adiciones y correcciones[END_REF], X. andinus [START_REF] Ameghino | Nuevos restos de mamíferos fósiles descubiertos por Carlos Ameghino en el Eoceno inferior de la Patagonia austral. Especies nuevas, adiciones y correcciones[END_REF] and X. crassissimus [START_REF] Ameghino | Enumération synoptique des espèces de mammifères fossiles des formations éocènes de Patagonie[END_REF]. [START_REF] Bargo | Early Miocene sloths (Xenarthra, Folivora) from the Río Santa Cruz valley (Southern Patagonia, Argentina). Ameghino, 1887 revisited[END_REF] recently assigned new mandibular and postcranial specimens to X. atlanticus, including a complete femur and astragalus. partial ulnae, and a partial radius and tibia. New species of Xyophorus from later in the late Miocene (Chasicoan-Huayquerian SALMAs) were erected by [START_REF] Scillato-Yané | Nuevo Nothrotheriinae (Edentata, Tardigrada) de edad Chasiquense (Mioceno tardío) del sur de la Prov. de Buenos Aires (Argentina). Su Importancia bioestratigrafíca, filogenética y paleóbiogeográfica[END_REF]X. bondesioi) and [START_REF] St-André | Deux nouveaux Édentés (Mammalia, Xenarthra) Trachycalyptoides achirense nov. gen. et nov. sp. (Glyptodontidae, Sclerocalyptinae) et Xyophorus villarroeli nov. sp. (Megatheriidae, Nothrotheriinae) du Huayquérien (Miocène supérieur) de l'Altiplano bolivien[END_REF]X. villarroeli), the former based on a partial mandible and the latter based on a partial right mandible (Figure 1) and an isolated astragalus. [START_REF] Frailey | The Miocene vertebrates of Quebrada Honda, Bolivia. II: Edentata[END_REF] described a partial anterior skull and mandible from slightly younger Laventan (Laventan SALMA) aged deposits at Quebrada Honda, in southern Bolivia, which he assigned to Hapalops angustipalatus, but these were subsequently placed in Xyophorus (X. cf. bondesioi) by Scillato-Yané and [START_REF] Carlini | Nuevos Xenarthra del Mioceno de Quebrada Honda, Bolivia[END_REF]. Additional partial mandibles were allocated to Xyophorus by Rodríguez [START_REF] Brizuela | Estratigrafía y mamíferos fósiles de la Formación Toro Negro (Plioceno), Departamento Vinchina, noroeste de la provincia de La Rioja, Argentina[END_REF]X. cf. bondesioi) from the early Pliocene (Montehermosan-Chapadmalalan SALMAs) of La Rioja Province (Argentina), and [START_REF] Croft | New data on Miocene neotropical provinciality from Cerdas, Bolivia[END_REF]X. cf. bondesioi) from the late early-middle Miocene (Friasian-Colloncuran SALMAs) of Cerdas (Bolivia), the latter study also allocating a manual and pedal ungual to the genus.

Given the paucity of remains, it is perhaps not surprising that the diagnoses provided for the genus itself, and for its constituent species, have been less than compelling. St-André (1996, p. 96) offered the following diagnosis for the genus Xyophorus: "[cf1] déplacée vers la symphyse mandibulaire et séparée des autres dents par un diastème de longueur inférieure au diamètre antéropostérieur de [mf1]. Dents postérieures prismatiques." (our translation: cf1 displaced in front of the posterior edge of the mandibular symphysis [in dorsal view] and separated from the molariforms by a diastema of length less than the anteroposterior diameter of mf1. Posterior molariforms prismatic). The last of these features is characteristic of all megatherioid sloths [START_REF] Gaudin | Phylogenetic relationships among sloths (Mammalia, Xenarthra, Tardigrada): the craniodental evidence[END_REF]) in all but the very earliest stages of life, and is therefore not diagnostic. [START_REF] Pujos | Hiskatherium saintandrei gen. et sp. nov.: an unusual sloth from the Santacrucian of Quebrada Honda (Bolivia) and an overview of middle Miocene, small megatherioids[END_REF] also noted the strong resemblance between the astragali of Hapalops and Xyophorus. It is perhaps not surprising, then, that [START_REF] Scott | Mammalia of the Santa Cruz beds. Part I. Edentata[END_REF]followed by Lydekker 1894;[START_REF] Perea | Un singular grupo de perezosos, los "Nothrotheriinae" (Xenarthra)[END_REF], based on [START_REF] Ameghino | Enumeración sistemática de las especies de mamíferos fósiles coleccionados por Carlos Ameghino en los terrenos eocenos de la Patagonia austral y depositados en el Museo de La Plata[END_REF][START_REF] Ameghino | Nuevos restos de mamíferos fósiles descubiertos por Carlos Ameghino en el Eoceno inferior de la Patagonia austral. Especies nuevas, adiciones y correcciones[END_REF][START_REF] Ameghino | Enumération synoptique des espèces de mammifères fossiles des formations éocènes de Patagonie[END_REF] Santacrucian-aged specimens, considered Xyophorus a junior synonym of the much better known and highly variable genus Hapalops. [START_REF] Brandoni | Xyophorus" sp. en el Mioceno medio de Chubut: implicancias sistemáticas, biogeográficas y biocronológicas del registro de un Nothrotheriinae en el Neógeno de la Argentina[END_REF] and [START_REF] Brandoni | Chronological implications of the nothrotheriid "Xyophorus" (Mammalia, Xenarthra) from the Collón Curá Formation (Miocene of Patagonia, Argentina)[END_REF] suggested that the post-Santacrucian SALMA material placed in Xyophorus (in the species X. bondesioi and X. villarroeli) belongs in a separate genus from the Santacrucian species, but, given the sparse record of the group, they did not feel a genus could be adequately diagnosed, and hence they referred to this younger material as "Xyophorus" (following the "Open nomenclature" style of [START_REF] Bengston | Open Nomenclature[END_REF].

Although the published diagnoses do not establish this unambiguously, we agree with [START_REF] Brandoni | Xyophorus" sp. en el Mioceno medio de Chubut: implicancias sistemáticas, biogeográficas y biocronológicas del registro de un Nothrotheriinae en el Neógeno de la Argentina[END_REF] and [START_REF] Brandoni | Chronological implications of the nothrotheriid "Xyophorus" (Mammalia, Xenarthra) from the Collón Curá Formation (Miocene of Patagonia, Argentina)[END_REF] that the post-Santacrucian aged material assigned to X. bondesioi and X. villarroeli is marked by distinctive features that might be useful in distinguishing these species from other nothrotheriids. These include traits highlighted by [START_REF] St-André | Deux nouveaux Édentés (Mammalia, Xenarthra) Trachycalyptoides achirense nov. gen. et nov. sp. (Glyptodontidae, Sclerocalyptinae) et Xyophorus villarroeli nov. sp. (Megatheriidae, Nothrotheriinae) du Huayquérien (Miocène supérieur) de l'Altiplano bolivien[END_REF], De Iuliis et al. (2011), and[START_REF] Brandoni | Xyophorus" sp. en el Mioceno medio de Chubut: implicancias sistemáticas, biogeográficas y biocronológicas del registro de un Nothrotheriinae en el Neógeno de la Argentina[END_REF], i.e., cf1 displaced in front of the posterior edge of the mandibular symphysis [in dorsal view] and separated from the molariforms by a short diastema of length less than the anteroposterior diameter of mf1, postero-external opening of mandibular canal on lateral surface of the coronoid process, mf1 not mesiodistally compressed, and deep apicobasal grooves on the lingual and labial surfaces of mf1 and mf2. That said, the case for recognizing a distinct genus for this material would be bolstered greatly by the addition of new traits from new areas of the skeleton and not just the isolated and fragmentary material currently available.

The goal of the present report is to provide such characters and to reconsider the validity of post-Santacrucian SALMA Xyophorus species. We report here on an isolated squamosal and intact attached ear region, along with an isolated complete astragalus, recovered from deposits dating to the late Mayoan-early Chasicoan SALMAs (late Miocene, [START_REF] Fernández-Monescillo | Multiple skeletal and dental pathologies in a late Miocene mesotheriid (Mammalia, Notoungulata) from the Altiplano of Bolivia: paleoecological inferences[END_REF] near Achiri village, in the Bolivian Altiplano. This material clearly pertains to a nothrotheriid, and we reasonably consider it to be allied with the post-Santacrucian material assigned to "Xyophorus" by [START_REF] Brandoni | Xyophorus" sp. en el Mioceno medio de Chubut: implicancias sistemáticas, biogeográficas y biocronológicas del registro de un Nothrotheriinae en el Neógeno de la Argentina[END_REF] and [START_REF] Brandoni | Chronological implications of the nothrotheriid "Xyophorus" (Mammalia, Xenarthra) from the Collón Curá Formation (Miocene of Patagonia, Argentina)[END_REF]. These specimens display a distinctive array of traits that include plesiomorphic features shared with basal megatherioids, derived features shared with undoubted members of Nothrotheriidae (i.e., taxa included in [START_REF] Varela | Phylogeny, macroevolutionary trends and historical biogeography of sloths: insights from a Bayesian morphological clock analysis[END_REF], excluding Thalassocnus [following Amson et al. 2017], and any Santacrucian megatherioids), some transitional features, and a number of autapomorphies. These characteristics allow us to recognize the new specimens, and related material, as pertaining to a new extinct sloth genus. Moreover, they will allow us to better understand the phylogenetic affinities of this new taxon.

Materials & Methods

The basis of this report are two new specimens. The first, MNHN-Bol-V 012690, is a right squamosal and attached auditory region, including a complete auditory bulla and petrosal. The second specimen, MNHN-Bol-V 011887, is an isolated right astragalus. Both specimens were recovered from the Cerros Pisakeri and Virgen Pata, Mauri 6 Formation, Unit IV (Mayoan-early Chasicoan SALMAs; [START_REF] Evernden | Potassium-Argon ages of some Bolivian rocks[END_REF][START_REF] Fernández-Monescillo | Multiple skeletal and dental pathologies in a late Miocene mesotheriid (Mammalia, Notoungulata) from the Altiplano of Bolivia: paleoecological inferences[END_REF] 3-4 km southeast the village of Achiri (Pacajes Province, La Paz Department, Bolivia) roughly 50 km southwest of the capital city, La Paz (Figure 2).

The two new specimens are compared to other late Miocene and Pliocene nothrotheriids, in particular Pronothrotherium typicum, a species from the late Miocene-early Pliocene of southern South America (Huayquerian-Chapadmalalan SALMAs), represented by several nearly complete skulls, one with a well-preserved auditory region [START_REF] Patterson | The ear region in xenarthrans (= Edentata, Mammalia). Part II. Pilosa (sloths, anteaters), palaeanodonts, and a miscellany[END_REF][START_REF] Gaudin | Cranial osteology and taxonomy of Pronothrotherium (Xenarthra, Folivora, Nothrotheriidae) from the late Miocene-early Pliocene of Catamarca Province (Argentina)[END_REF], as well as a published description of an isolated astragalus [START_REF] Kraglievich | Descripción de los astrágalos de dos gravígrados terciarios de la subfamilia "Nothrotheriinae[END_REF]). We will also compare the skull material to Mionothropus cartellei, a species from the late Miocene (Huayquerian SALMA) of Peruvian Amazonia, represented by a single complete skull with a well-preserved auditory region [START_REF] De Iuliis | A new genus and species of nothrotheriid sloth (Xenarthra, Tardigrada, Nothrotheriidae) from the late Miocene (Huayquerian) of Peru[END_REF]. In addition, we will use the basal megatherioid sloth Hapalops, best known from the Patagonian Santa Cruz Formation (Santacrucian SALMA, late early Miocene; Scott 1904), as a reference taxon, in order to provide information on the plesiomorphic condition of megatherioid sloths.

The anatomical terminology utilized in the present study follows that of [START_REF] Wible | On the cranial osteology of the yellow armadillo Euphractus sexcinctus (Dasypodidae, Xenarthra, Placentalia)[END_REF], [START_REF] Wible | Petrosal anatomy of the nine-banded armadillo, Dasypus novemcinctus Linnaeus, 1758 (Placentalia: Xenarthra: Dasypodidae)[END_REF], [START_REF] De Iuliis | A new genus and species of nothrotheriid sloth (Xenarthra, Tardigrada, Nothrotheriidae) from the late Miocene (Huayquerian) of Peru[END_REF][START_REF] Gaudin | On the osteology of the auditory region and orbital wall in the extinct West Indian sloth genus Neocnus Arredondo, 1961 (Placentalia, Xenarthra, Megalonychidae)[END_REF]. Measurements for all specimens were taken with a digital caliper to the nearest 0.1 mm. Aymaratherium, Mionothropus, Nothrotherium, and Nothrotheriops) or retains megatherioid symplesiomorphies, including: lack of a canal from occipital artery system perforating the paroccipital process; anterolaterally directed root of zygoma; lack of an odontoid process on astragalus; lack of broad dorsal exposure of fibular facet; broad astragalar neck; ectal facet not transversely oriented; small sustentacular facet; cuboid facet on plantar surface of astragalus; presence of a ridge connecting the head to the distal lateral trochlea in distal view; presence of a non-articular area on the plantar edge of the lateral trochlea, lateral to the head in distal view; distal contact between the fibular and ectal facets in lateral view; and, minimal lateral exposure of ectal facet in lateral view.

Museum

Can be distinguished from both basal megatherioids and other nothrotheriids by the following autapomorphies: elongated mandibular symphysis, with cf1 anterior to posterior edge of symphysis; short diastema, less than the maximum mesiodistal length of mf1; deep longitudinal grooves on the labial and lingual surfaces of mf1 and mf2; very rugose external surface of ectotympanic; contact between styliform process of ectotympanic and pterygoid; very high stapedial ratio; very small anteroventral process of tegmen tympani; subarcuate fossa strongly reduced; presence of medial prominence of petrosal; wide sulcus tali on ventral surface of astragalus; shallowly concave ectal facet. Bureau of Land Management, and the unquestioned dean of fossil sloth experts. He has been a tremendously important role model, mentor and friend to T.J.G, F.P., and A.B., and we are pleased to name this taxon in his honor, and to thank him for his unflagging enthusiasm for sloths and for his years of encouragement and collaboration with all who work on these fascinating animals.

Etymology-'McDonald

'Ocnus' comes from the Greek 'ocno,' meaning 'hesitating or inactive,' and is an ending frequently used in sloth genus names. [START_REF] Brandoni | Chronological implications of the nothrotheriid "Xyophorus" (Mammalia, Xenarthra) from the Collón Curá Formation (Miocene of Patagonia, Argentina)[END_REF], we recognize material assigned to "Xyophorus" bondesioi, "Xyophorus" villarroeli, and "Xyophorus" sp. as pertaining to a new nothrotheriid genus, Mcdonaldocnus, including: "Xyophorus" bondesioi MLP 32-V-20-1, partial left mandible (Scillato-Yané 1979); "Xyophorus" cf. bondesioi CRILAR-PZ 262, partial left mandible [START_REF] Brizuela | Estratigrafía y mamíferos fósiles de la Formación Toro Negro (Plioceno), Departamento Vinchina, noroeste de la provincia de La Rioja, Argentina[END_REF]; "Xyophorus" cf. bondesioi UATF-V-000871, partial right mandible, UATF-V-0008850, manual ungual phalanx, UATF-V-0008825, pedal ungual phalanx [START_REF] Croft | New data on Miocene neotropical provinciality from Cerdas, Bolivia[END_REF]; "Xyophorus" villarroeli MNHN.F. ACH 15, right astragalus, MNHN.F ACH 43, partial right mandible [START_REF] St-André | Deux nouveaux Édentés (Mammalia, Xenarthra) Trachycalyptoides achirense nov. gen. et nov. sp. (Glyptodontidae, Sclerocalyptinae) et Xyophorus villarroeli nov. sp. (Megatheriidae, Nothrotheriinae) du Huayquérien (Miocène supérieur) de l'Altiplano bolivien[END_REF]; "Xyophorus" sp. MPEF-PV 3264, partial left mandible [START_REF] Brandoni | Xyophorus" sp. en el Mioceno medio de Chubut: implicancias sistemáticas, biogeográficas y biocronológicas del registro de un Nothrotheriinae en el Neógeno de la Argentina[END_REF]; "Xyophorus" sp. LIEB PV 5129, right mandibular fragment [START_REF] Brandoni | Chronological implications of the nothrotheriid "Xyophorus" (Mammalia, Xenarthra) from the Collón Curá Formation (Miocene of Patagonia, Argentina)[END_REF]; as well as the material from the present study -MNHN-Bol-V 012690 right squamosal with auditory region, MNHN-Bol-V 011887, right astragalus.

Referred material-Largely following

Questionably Referred material-"Xyophorus" cf. bondesioi UF 26668, palatal portion of skull and partial mandible with complete dentition, UF 24200, partial left mandible with symphysis, which possesses some but not all of the diagnostic characters of the genus, though it falls within the appropriate age range (Laventan SALMA, middle Miocene; [START_REF] Frailey | The Miocene vertebrates of Quebrada Honda, Bolivia. II: Edentata[END_REF][START_REF] Brandoni | Chronological implications of the nothrotheriid "Xyophorus" (Mammalia, Xenarthra) from the Collón Curá Formation (Miocene of Patagonia, Argentina)[END_REF].

Distribution-late early-early middle Miocene Friasian SALMA to early Pliocene

Montehermosan SALMA, roughly 16/15-6/5 Ma; known from multiple localities in Argentina and Bolivia (see map in Brandoni 2014, Figure 3). †Mcdonaldocnus bondesioi [START_REF] Scillato-Yané | Nuevo Nothrotheriinae (Edentata, Tardigrada) de edad Chasiquense (Mioceno tardío) del sur de la Prov. de Buenos Aires (Argentina). Su Importancia bioestratigrafíca, filogenética y paleóbiogeográfica[END_REF], new combination Holotype-MLP 32-V-20-1, partial left dentary [START_REF] Scillato-Yané | Nuevo Nothrotheriinae (Edentata, Tardigrada) de edad Chasiquense (Mioceno tardío) del sur de la Prov. de Buenos Aires (Argentina). Su Importancia bioestratigrafíca, filogenética y paleóbiogeográfica[END_REF] Referred material-As for genus listed above.

Distribution-As for genus listed above.

Diagnosis-Because the newly constituted genus is monotypic, the diagnosis for this species, the type and only species, is the same as for the genus as a whole.

Remarks-In prior works, "Xyophorus" bondesioi and "Xyophorus" villarroeli were distinguished on the basis of size and the cross-sectional shape of the lower caniniform [START_REF] St-André | Deux nouveaux Édentés (Mammalia, Xenarthra) Trachycalyptoides achirense nov. gen. et nov. sp. (Glyptodontidae, Sclerocalyptinae) et Xyophorus villarroeli nov. sp. (Megatheriidae, Nothrotheriinae) du Huayquérien (Miocène supérieur) de l'Altiplano bolivien[END_REF][START_REF] Croft | New data on Miocene neotropical provinciality from Cerdas, Bolivia[END_REF][START_REF] Brandoni | Xyophorus" sp. en el Mioceno medio de Chubut: implicancias sistemáticas, biogeográficas y biocronológicas del registro de un Nothrotheriinae en el Neógeno de la Argentina[END_REF][START_REF] Brandoni | Chronological implications of the nothrotheriid "Xyophorus" (Mammalia, Xenarthra) from the Collón Curá Formation (Miocene of Patagonia, Argentina)[END_REF]), but we do not believe that either is sufficiently reliable as a basis for species definition (see, e.g., discussion in De Iuliis 2018 on use of size for diagnosing sloth taxa), so we choose instead to place all the material in the same species.

"Xyophorus" bondesioi has priority (see Article 23 of the ICZN 1999), and so all the material of the new genus is assigned to this species for the time being, recognizing that this makes for a very longlived species that may require subdivision pending discoveries of new, more anatomically extensive materials.

Description

Specimen MNHN-Bol-V 012690 is an isolated right squamosal, preserving much of the ear region, but missing the zygomatic process (Figure 3).

Ectotympanic-

The ectotympanic is complete and apparently tightly sutured to the skull. This bone is horseshoe-shaped, surrounding a dorsoventrally elongated porus acousticus. In both respects, Mcdonaldocnus resembles nothrotheriid sloths and differs from more basal megatherioids like Hapalops in which the ectotympanic is more circular and more loosely attached to the skull [START_REF] Patterson | The ear region in xenarthrans (= Edentata, Mammalia). Part II. Pilosa (sloths, anteaters), palaeanodonts, and a miscellany[END_REF][START_REF] Gaudin | The ear region of edentates and the phylogeny of the Tardigrada (Mammalia, Xenarthra)[END_REF][START_REF] Gaudin | Phylogenetic relationships among sloths (Mammalia, Xenarthra, Tardigrada): the craniodental evidence[END_REF]. As in most sloths the anterior crus is larger than the posterior crus (Figures 3A,C,4). The posterior crus is attached dorsally to the post-tympanic region of the squamosal laterally; posteromedially, it is connected to a bridge of bone linking the mastoid petrosal and the squamosal, as in other sloths (Figure 3A, C-D;[START_REF] Gaudin | The ear region of edentates and the phylogeny of the Tardigrada (Mammalia, Xenarthra)[END_REF]. The inner surface of the ectotympanic is visible at the tip of the posterior crus, so that the sulcus tympanicus and crista tympanica for the attachment of the tympanum are evident (Figure 3A, C-D). Immediately behind the posterior crus and the bridge of bone to which it attaches is the opening of the stylomastoid foramen (Figures 3C,4). The ventral portion of the ectotympanic is somewhat expanded transversely in ventral view, more so than in Hapalops but less than in early nothrotheriids such as Mionothropus and Pronothrotherium [START_REF] Patterson | The ear region in xenarthrans (= Edentata, Mammalia). Part II. Pilosa (sloths, anteaters), palaeanodonts, and a miscellany[END_REF][START_REF] De Iuliis | A new genus and species of nothrotheriid sloth (Xenarthra, Tardigrada, Nothrotheriidae) from the late Miocene (Huayquerian) of Peru[END_REF]. The surface of this portion of the ectotympanic is highly rugose (Figures 3A,C,4). Although this may be exaggerated by post-mortem erosion, we note that a rugose ectotympanic surface is characteristic of nothrotheriids and their relatives, including Pronothrotherium, Hapalops, and Analcimorphus [START_REF] Gaudin | The ear region of edentates and the phylogeny of the Tardigrada (Mammalia, Xenarthra)[END_REF][START_REF] Gaudin | Phylogenetic relationships among sloths (Mammalia, Xenarthra, Tardigrada): the craniodental evidence[END_REF]. The ventral portion of the ectotympanic is deep in lateral view, especially anteriorly, i.e., it is much broader than either the anterior or posterior crura. In this respect it resembles Pronothrotherium [START_REF] Patterson | The ear region in xenarthrans (= Edentata, Mammalia). Part II. Pilosa (sloths, anteaters), palaeanodonts, and a miscellany[END_REF][START_REF] Gaudin | Cranial osteology and taxonomy of Pronothrotherium (Xenarthra, Folivora, Nothrotheriidae) from the late Miocene-early Pliocene of Catamarca Province (Argentina)[END_REF]) and differs from Mionothropus, Hapalops, and Analcimorphus [START_REF] Scott | Mammalia of the Santa Cruz beds. Part I. Edentata[END_REF][START_REF] Patterson | The ear region in xenarthrans (= Edentata, Mammalia). Part II. Pilosa (sloths, anteaters), palaeanodonts, and a miscellany[END_REF]De Iuliis 2011), where the ectotympanic ring has more uniform proportions. This ventral segment of the ectotympanic appears to be attached medially to the tympanohyal and the entotympanic, with the opening for the Eustachian tube situated between the ectotympanic and the latter element (Figure 3C-D). An elongated styliform process of the ectotympanic forms the anterior margin of this opening, and forms a small distal contact with the pterygoid (Figure 3A). Although a sizable styliform process is present in Hapalops, Pronothrotherium, and Mionothropus [START_REF] Patterson | The ear region in xenarthrans (= Edentata, Mammalia). Part II. Pilosa (sloths, anteaters), palaeanodonts, and a miscellany[END_REF][START_REF] De Iuliis | A new genus and species of nothrotheriid sloth (Xenarthra, Tardigrada, Nothrotheriidae) from the late Miocene (Huayquerian) of Peru[END_REF]), the process does not contact the pterygoid in any of these taxa. The anterior crus is more massive and rugose than the posterior crus, and is exclusively attached to the squamosal (Figures 3A,4). The Glaserian fissure passes between the anterior crus of the ectotympanic and the squamosal (Figure 3A,C).

Entotympanic-As in most sloths, the entotympanic in Mcdonaldocnus is composed of two vertical plates separated by an anteroposteriorly elongated sulcus for the internal carotid artery (Figure 3C). The lateral plate appears to be damaged posteriorly in the vicinity of the stylohyal fossa but the bone is otherwise nearly complete. In front of the stylohyal fossa, the ventral margin of the entotympanic lateral plate is thickened and rugose, and extends clearly ventral to the ectotympanic along its entire length (Figure 3C-D). The anteroventral process of the entotympanic appears to be missing but this is likely due to taphonomic damage. The medial portion of the entotympanic is deeper than that of Hapalops, similar in proportions to the basal nothrotheriids Pronothrotherium and Mionothropus [START_REF] Patterson | The ear region in xenarthrans (= Edentata, Mammalia). Part II. Pilosa (sloths, anteaters), palaeanodonts, and a miscellany[END_REF][START_REF] De Iuliis | A new genus and species of nothrotheriid sloth (Xenarthra, Tardigrada, Nothrotheriidae) from the late Miocene (Huayquerian) of Peru[END_REF]. The carotid sulcus is quite broad posteriorly but narrows dramatically roughly at the midpoint of the entotympanic, becoming a transversely narrow trough that curves medially and dorsally into the carotid foramen (Figure 3D). The entotympanic contacts the petrosal dorsally, the ectotympanic, mastoid, and tympanohyal laterally, and has a small anterior contact with the pterygoid (Figure 3C-D).

Stylohyal fossa-The stylohyal fossa is likely comprised of the same four elements that are typical for sloths: tympanohyal, mastoid portion of petrosal, entotympanic, and the paracondylar process of the exoccipital, although the latter is missing and the posterior portion of the entotympanic is damaged (Figure 3C-D). As preserved, the outline of the depression is oval as is in nothrotheriids, in contrast to the circular outline of Hapalops and other basal megatherioids [START_REF] Gaudin | The ear region of edentates and the phylogeny of the Tardigrada (Mammalia, Xenarthra)[END_REF](Gaudin : char. 54, 2004)). The bulk of the fossa is comprised by the circular head of the ventralmost portion of the tympanohyal.

Petrosal-The lateral surface of the petrosal is visible through the porus acusticus (Figure 4). As is typical for sloths, the promontorium of the petrosal is rounded posteriorly but carries an anterodorsal extension with a flat lateral surface. The fenestra vestibuli is the largest aperture observable on the promontorium (stapedial ratio = 1.93). It is an anteroposteriorly elongated, oval, perhaps more elongated than in any other known sloth [START_REF] Gaudin | On the osteology of the auditory region and orbital wall in the extinct West Indian sloth genus Neocnus Arredondo, 1961 (Placentalia, Xenarthra, Megalonychidae)[END_REF]: stapedial ratios for Neocnus, Choloepus, and Bradypus 1.5-1.6; [START_REF] Boscaini | Phylogenetic and functional implications of the ear region anatomy of Glossotherium robustum (Xenarthra, Mylodontidae) from the Late Pleistocene of Argentina[END_REF]: stapedial ratio for Glossotherium 1.35).

There is a broad, triangular crista interfenestralis separating the fenestra vestibuli from the aperture for the cochlear fossula (Figure 4; note the latter opening leads to the fenestra cochleae, and the latter term has been used for this opening; Wible 2010). Immediately lateral to the fenestra vestibuli is a deep and very narrow facial sulcus flanked laterally by a strong crista parotica. At its posterior end, the crista gives rise to the base of the tympanohyal. At its anterior end is a small expansion that represents the anteroventral process of the tegmen tympani (aptt). The small size of the aptt stands in stark contrast to the condition in basal megatherioids like Hapalops where the aptt forms a large concave plate that contacts the anterior crus of the entotympanic; in nothrotheriids, the aptt is even further enlarged [START_REF] Gaudin | The ear region of edentates and the phylogeny of the Tardigrada (Mammalia, Xenarthra)[END_REF][START_REF] Gaudin | Phylogenetic relationships among sloths (Mammalia, Xenarthra, Tardigrada): the craniodental evidence[END_REF]. Lateral to the crista parotica, there appears to be a deep epitympanic sinus as in typical for megatheroid sloths.

Medial surface-The medial surface of the petrosal in MNHN-Bol-V 012690 is well preserved (Figure 3B). Its most prominent feature is the large deep posteromedially directed internal acoustic meatus (iam) characteristic of sloths [START_REF] Gaudin | The ear region of edentates and the phylogeny of the Tardigrada (Mammalia, Xenarthra)[END_REF]. Recessed at the bottom of the canal, the openings of CN VII and VIII are visible. The cluster of dorsal openings (including the facial foramen and the superior vestibular area of the vestibulocochlear nerve; Wible 2010) is larger than the ventral cluster (including the remaining openings for branches of CN VIII, the inferior vestibular area, the spiral cribriform tract and the foramen singular; Wible 2010), the two clusters separated by a sharp-edged crest. The area posterodorsal to the iam shows only the barest impressions of a very shallow subarcuate fossa, an unusual condition for a xenarthran. Most sloths have a broad, fairly shallow but distinct subarcuate fossa (e.g., [START_REF] Stock | Cenozoic gravigrade edentates of western North America with special reference to the Pleistocene Megalonychinae and Mylodontidae of Rancho La Brea[END_REF][START_REF] Patterson | The ear region in xenarthrans (= Edentata, Mammalia). Part II. Pilosa (sloths, anteaters), palaeanodonts, and a miscellany[END_REF][START_REF] Gaudin | The basicranium and orbital region of the early Miocene Eucholoeops ingens Ameghino, (Xenarthra, Pilosa, Megalonychidae)[END_REF][START_REF] Boscaini | Phylogenetic and functional implications of the ear region anatomy of Glossotherium robustum (Xenarthra, Mylodontidae) from the Late Pleistocene of Argentina[END_REF][START_REF] Gaudin | Isolated petrosal of the extinct sloth Glossotherium tropicorum (Xenarthra, Folivora, Mylodontidae) from the island of Trinidad[END_REF]. The fossa is also well developed in cingulates (e.g., Wible 2010; Gaudin and Lyon 2017). The fossa is very deep in some anteaters, but is reduced in Myrmecophaga [START_REF] Guth | La Région Temporale des Édentés[END_REF][START_REF] Patterson | The ear region in xenarthrans (= Edentata, Mammalia). Part II. Pilosa (sloths, anteaters), palaeanodonts, and a miscellany[END_REF][START_REF] Gaudin | The phylogeny of the Myrmecophagidae (Mammalia, Xenarthra, Vermilingua) and the relationship of Eurotamandua to the Vermilingua[END_REF]. The latter taxon most closely approximates the condition in Mcdonaldocnus among xenarthrans, but the fossa is less well developed in Mcdonaldocnus. Above the iam, there is a strong crista petrosa that marks the boundary of the cerebellar surface of the petrosal. Anterior to the iam is a broad anteroposteriorly directed groove that may accommodate a portion of the inferior petrosal sinus (following [START_REF] Gaudin | Cranial osteology of the pampathere Holmesina floridanus (Xenarthra: Cingulata; Blancan NALMA), including a description of an isolated petrosal bone[END_REF]. Below this groove is an extremely large medially directed prominence of the petrosal. To our knowledge, this prominence has neither been described or identified previously in sloths.

Posterior surface of the petrosal-The lateral most feature of the petrosal is a prominent paraoccipital process (Figure 3C-D). This forms the lateral boundary to the ventral opening for the occipital artery. The artery travels within a partially closed canal inside the mastoid portion of the petrosal as in some megatheriids (i.e., Megatherium and Eremotherium), as opposed to the primitive condition observed in basal megatherioids where the artery is in an open groove (e.g., Hapalops, Analcimorphus, and Schismotherium), or the more derived condition in Nothrotherium and Planops where the artery is completely enclosed in a bony canal [START_REF] Gaudin | The ear region of edentates and the phylogeny of the Tardigrada (Mammalia, Xenarthra)[END_REF][START_REF] Gaudin | Phylogenetic relationships among sloths (Mammalia, Xenarthra, Tardigrada): the craniodental evidence[END_REF].

Mcdonaldocnus also lacks the canals which perforate the paroccipital process in Nothrotheriidae [START_REF] Gaudin | The ear region of edentates and the phylogeny of the Tardigrada (Mammalia, Xenarthra)[END_REF][START_REF] De Iuliis | A new genus and species of nothrotheriid sloth (Xenarthra, Tardigrada, Nothrotheriidae) from the late Miocene (Huayquerian) of Peru[END_REF][START_REF] Gaudin | Cranial osteology and taxonomy of Pronothrotherium (Xenarthra, Folivora, Nothrotheriidae) from the late Miocene-early Pliocene of Catamarca Province (Argentina)[END_REF], marked by openings on the lateral surface of the skull posterodorsal to the process.

Lateral surface of the squamosal-As noted previously, the lateral surface of the squamosal serves as the point of attachment for the ectotympanic. Just anterior to the attachment point between the anterior crus and squamosal is a well-developed postglenoid foramen (Fig. 3A). The postglenoid foramen is generally reduced or absent in pilosans [START_REF] Gaudin | The ear region of edentates and the phylogeny of the Tardigrada (Mammalia, Xenarthra)[END_REF]. It has been observed in other fossil sloths, mostly in taxa that are not one anothers' close relatives, e.g., Mionothropus (De Iuliis et al. 2011), Neocnus [START_REF] Gaudin | On the osteology of the auditory region and orbital wall in the extinct West Indian sloth genus Neocnus Arredondo, 1961 (Placentalia, Xenarthra, Megalonychidae)[END_REF], Parocnus (Fischer 1971: fig. 9) and in a few other sloth taxa identified by Gaudin (2004 -Megatherium, Nematherium, and Analcimorphus), but it is rarely this prominent. Ventral to the postglenoid foramen and anterior to the opening for the Eustachian tube is another large opening between the squamosal and ectotympanic. This opening emerges into a deep anteroventrally directed groove that has been identified as accommodating the tensor veli palatini muscle (gtvp; Figure 3A, C; Gaudin 1995). This groove is present in Hapalops and in Megatheriidae and Nothrotheriidae [START_REF] Gaudin | Phylogenetic relationships among sloths (Mammalia, Xenarthra, Tardigrada): the craniodental evidence[END_REF]). The foramen ovale opens medial to the end of the gtvp. It would appear that the foramen ovale is almost entirely surrounded by the squamosal bone with a small ventral contribution to its rim from the pterygoid (Figure 3C). This is typical for most megatherioid sloths [START_REF] Gaudin | Phylogenetic relationships among sloths (Mammalia, Xenarthra, Tardigrada): the craniodental evidence[END_REF], with the exception of Pronothrotherium, where the aperture lies entirely within the alisphenoid [START_REF] Gaudin | Cranial osteology and taxonomy of Pronothrotherium (Xenarthra, Folivora, Nothrotheriidae) from the late Miocene-early Pliocene of Catamarca Province (Argentina)[END_REF].

Zygomatic process-The zygomatic process of the squamosal is mostly preserved (Figure 3A, C-D). It is anterolaterally directed in ventral view with a flat lateral face and a sharp dorsal margin (Figure 3C). On its ventral surface, it carries a glenoid fossa with a well-demarcated medial edge.

The fossa appears to be wider transversely than it is elongated anteroposteriorly, and its medial limit is marked by a low, anteroposteriorly elongated entoglenoid process (Figure 3C). The fossa is deeply concave transversely and nearly flat anteroposteriorly. The post glenoid region is marked by numerous longitudinal ridges. This differs from the condition in basal megatherioids, Pronothrotherium, and megatheriids, where the post-glenoid region is characterized by rugose bone, but is similar to the condition in Mionothropus and Nothrotheriops [START_REF] Gaudin | The ear region of edentates and the phylogeny of the Tardigrada (Mammalia, Xenarthra)[END_REF][START_REF] Gaudin | Phylogenetic relationships among sloths (Mammalia, Xenarthra, Tardigrada): the craniodental evidence[END_REF][START_REF] De Iuliis | A new genus and species of nothrotheriid sloth (Xenarthra, Tardigrada, Nothrotheriidae) from the late Miocene (Huayquerian) of Peru[END_REF][START_REF] Gaudin | Cranial osteology and taxonomy of Pronothrotherium (Xenarthra, Folivora, Nothrotheriidae) from the late Miocene-early Pliocene of Catamarca Province (Argentina)[END_REF].

Astragalus-Two astragali of Mcdonaldocnus are available (Figure 5A-F). The astragalus was originally described by [START_REF] St-André | Deux nouveaux Édentés (Mammalia, Xenarthra) Trachycalyptoides achirense nov. gen. et nov. sp. (Glyptodontidae, Sclerocalyptinae) et Xyophorus villarroeli nov. sp. (Megatheriidae, Nothrotheriinae) du Huayquérien (Miocène supérieur) de l'Altiplano bolivien[END_REF], as part of the type specimen of "Xyophorus" villarroeli (MNHN.F.ACH 15, Figure 5A-C; St-André 1996). This specimen is well preserved, missing only a chunk of bone along its proximal edge, affecting the tibial trochlea and ectal facet (Figure 5B-C).

The new isolated right astragalus we recovered from Achiri (MNHN-Bol-V 011887; Figure 5D-F) is almost perfectly preserved, with the exception of a small amount of surface erosion around the margins of the sustentacular facet on the ventrolateral side of the bone, and a small break at the distal end of the lateral trochlea.

The dorsal surface of the bone in MNHN-Bol-V 011887 is dominated by the medial and lateral trochleae of the tibial facet (Figure 5E). The latter is relatively narrow transversely and elongated proximodistally, of uniform width, with a straight lateral edge. It is convex transversely, and strongly convex proximodistally, and its surface faces medially. The medial trochlea is confluent with the lateral trochlea at its medial end, but diverges distally at roughly a 70º angle. The medial trochlea is convex both transversely and proximodistally, and is also fairly uniform in width. The medial and lateral trochleae are similar in MNHN.F.ACH 15 (Figure 5B; St-André 1996), in X. atlanticus [START_REF] Bargo | Early Miocene sloths (Xenarthra, Folivora) from the Río Santa Cruz valley (Southern Patagonia, Argentina). Ameghino, 1887 revisited[END_REF], and in the basal megatherioid genus Hapalops (Figure 5H), except that in the latter three taxa the medial trochlea is less divergent from the lateral trochlea. The distal angle between the medial and lateral trochleae is roughly 40º in MNHN.F.ACH 15, 25º in X. atlanticus (Bargo et al. 2019, fig. 6), and it varies between 35º -57º in Hapalops (35º in H. Immediately distal to the junction of the medial and lateral trochleae in MNHN-Bol-V 011887 is a deep pit. A similar pit is described on the dorsal astragalar surface in Hapalops longiceps - [START_REF] Scott | Mammalia of the Santa Cruz beds. Part I. Edentata[END_REF] suggested its function is to accommodate the elongate anterior distal tibial process at the end of the tibia, serving as a stop to avoid hyper-dorsiflexion. The pit is also present in Pronothrotherium (FMNH P15523), albeit a bit more distally located along the medial edge of the lateral trochlea than in MNHN-Bol-V 011887, whereas it is more centrally located in X. atlanticus [START_REF] Bargo | Early Miocene sloths (Xenarthra, Folivora) from the Río Santa Cruz valley (Southern Patagonia, Argentina). Ameghino, 1887 revisited[END_REF] The plantar astragalar surface is largely occupied by two facets for the calcaneus, the lateral ectal facet and the medial sustentacular facet (Figure 5C,F). In Hapalops, the ectal facet is boomerang-shaped, i.e., it has a strong lateral indentation and a medial edge that is either medially convex (Figure 5I) or straight, with the distal and proximal ends extended laterally above and below the lateral indentation (Figure 5I). The facet is oriented mostly proximodistally, but tilted slightly laterally at its distal end. In MNHN-Bol-V 011887, the ectal facet has a similar orientation, but the lateral indentation is weaker than in Hapalops, and the facet only extends laterally at its proximal end, the distal end being rectangular in shape (Figure 5F). The preserved portion of the ectal facet in MNHN.F.ACH 15 (Figure 5C) is identical to that of MNHN-Bol-V 011887 (Figure 5F). Nearly the opposite condition prevails in X. atlanticus [START_REF] Bargo | Early Miocene sloths (Xenarthra, Folivora) from the Río Santa Cruz valley (Southern Patagonia, Argentina). Ameghino, 1887 revisited[END_REF], with the distal end extended laterally but the proximal end only weakly so. In Pronothrotherium (FMNH P15223), the ectal facet differs in both shape and orientation. In shape it is nearly rectangular, with slight indentations along its long edges, and it is oriented almost transversely, with only a slight distolateral tilt.

The sustentacular facet is roughly quadrangular in all the taxa examined here, and is much smaller than the ectal facet (though less so in Pronothrotherium), separated from that facet by a deep groove, the sulcus tali. The sulcus is fairly wide in Mcdonaldocnus (Figure 5C,F) and in X. atlanticus [START_REF] Bargo | Early Miocene sloths (Xenarthra, Folivora) from the Río Santa Cruz valley (Southern Patagonia, Argentina). Ameghino, 1887 revisited[END_REF]. It is somewhat narrower in Hapalops sp. (MLP 88-X-2-1.4), and is quite narrow in the other Hapalops specimens (FMNH P13123 and YPM-PU 15523) and in Pronothrotherium (FMNH P15223). The orientation of the sustentaculum varies among these taxa.

In Hapalops, it faces not only in the plantar direction, but also somewhat medially and proximally (Figure 5I). It has a similar orientation in X. atlanticus [START_REF] Bargo | Early Miocene sloths (Xenarthra, Folivora) from the Río Santa Cruz valley (Southern Patagonia, Argentina). Ameghino, 1887 revisited[END_REF]) and in MNHN.F.ACH 15 (Figure 5C). It is tilted proximally to a much lesser degree in MNHN-Bol-V 011887, and in Pronothrotherium it faces directly proximally.

The distal navicular facet is extended proximally onto the ventromedial surface of the astragalus (Figure 5D-F). In MNHN.F.ACH 15 (Figure 5A,C) and Hapalops sp. (MLP 88-X-2-1.4, Figure 5G, I), this extension is broadly confluent with the cuboid facet, separated by only a broad, shallow proximal indentation. The indentation between proximal navicular facet extension and cuboid facet is deeper in MNHN-Bol-V 011887, in X. atlanticus [START_REF] Bargo | Early Miocene sloths (Xenarthra, Folivora) from the Río Santa Cruz valley (Southern Patagonia, Argentina). Ameghino, 1887 revisited[END_REF], and in the other two Hapalops specimens (FMNH P13123 and YPM-PU 15523). The cuboid facet itself lies on the plantar surface of the astragalus in Mcdonaldocnus (Figure 5C, F), X. atlanticus [START_REF] Bargo | Early Miocene sloths (Xenarthra, Folivora) from the Río Santa Cruz valley (Southern Patagonia, Argentina). Ameghino, 1887 revisited[END_REF] and Hapalops (Figure 5I), as noted by [START_REF] Scott | Mammalia of the Santa Cruz beds. Part I. Edentata[END_REF] in the case of the latter. The facet is pointed proximally in MNHN.F.ACH 15, whereas it has a broad rounded proximal margin in MNHN-Bol-V 011887, X. atlanticus [START_REF] Bargo | Early Miocene sloths (Xenarthra, Folivora) from the Río Santa Cruz valley (Southern Patagonia, Argentina). Ameghino, 1887 revisited[END_REF], and Hapalops. In Pronothrotherium (FMNH P15223), the cuboid facet lies on the lateral side of the head, widely separated from the proximal extension of the navicular facet. Pronothrotherium also differs from the other taxa due to the presence of a large non-articular area occupying the proximomedial astragalar surface in ventral view, an area largely absent in the other forms.

The head of the astragalus is covered by a concave navicular facet in all the taxa examined for the present study (Figure 5D). A concave navicular facet is a synapomorphy of Pilosa [START_REF] Hoffstetter | Édentés Xénarthres[END_REF][START_REF] Gaudin | Morphology-based investigations of the phylogenetic relationships among extant and fossil xenarthrans[END_REF]. In MNHN.F.ACH 15 (Figure 5A), the facet is nearly circular (ratio of maximum dorsoplantar depth to maximum transverse width = 0.98), whereas in the others (Figure 5D, G) the facet is somewhat elongated transversely (ratio of depth/width: MNHN-Bol-V 011887 = 0.71; Hapalops sp. MLP 88-X-2-1.4 = 0.77; Hapalops longiceps YPM-PU 15523 = 0.72; X. atlanticus = 0.77 [based on Bargo et al. 2019, fig. 6]; Pronothrotherium typicum FMNH P15223 = 0.82). The head is connected by a strong, rounded ridge to the ventral edge of the lateral trochlea in MNHN-Bol-V 011887, as well as in X. atlanticus [START_REF] Bargo | Early Miocene sloths (Xenarthra, Folivora) from the Río Santa Cruz valley (Southern Patagonia, Argentina). Ameghino, 1887 revisited[END_REF]) and in Hapalops. The ridge is sharper but weaker in MNHN.F.ACH 15, and is absent entirely in Pronothrotherium. There is a sizable, non-articular area visible in distal view, lateral to the head abutting the distal edge of the lateral trochlea, in Mcdonaldocnus (Figure 5D), Hapalops (Figure 5G), and X. atlanticus [START_REF] Bargo | Early Miocene sloths (Xenarthra, Folivora) from the Río Santa Cruz valley (Southern Patagonia, Argentina). Ameghino, 1887 revisited[END_REF]. This is also missing in Pronothrotherium. The latter also differs from the others in that the lateral trochlea is much taller in distal view, and the odontoid process has a large distal exposure that the medial trochlea lacks in the other taxa.

The fibular facet has a similar shape in lateral view among all the taxa observed in the present study. It contacts the ectal facet in Mcdonaldocnus (Figure 5F), Hapalops (Figure 5I), and X. atlanticus [START_REF] Bargo | Early Miocene sloths (Xenarthra, Folivora) from the Río Santa Cruz valley (Southern Patagonia, Argentina). Ameghino, 1887 revisited[END_REF], whereas this contact is missing in Pronothrotherium. The latter taxon also has a broad exposure of the ectal facet and a globose cuboid facet in lateral view. These are missing in Mcdonaldocnus and Hapalops.

In medial view, MNHN-Bol-V 011887 strongly resembles Pronothrotherium (FMNH P15223) in that the lateral trochlea is much taller than the medial trochlea, and has a semicircular dorsal edge, whereas in Hapalops the lateral trochlea is lower and less strongly curved. In addition, the medial trochlea is proximodistally narrower, as in Pronothrotherium, and is bordered distally by a strong indentation. This indentation is missing in Hapalops, and the medial trochlea is more elongated proximodistally in medial view. The medial view shows that MNHN-Bol-V 011887 differs from both Pronothrotherium and Hapalops in the curvature of the ectal facet, with the concavity of the facet much less in the first taxon.

Discussion

Brandoni (2014) and [START_REF] Brandoni | Chronological implications of the nothrotheriid "Xyophorus" (Mammalia, Xenarthra) from the Collón Curá Formation (Miocene of Patagonia, Argentina)[END_REF] both suggested that post-Santacrucian sloths assigned to the nothrothere genus Xyophorus (i.e., X. bondesioi and X. villarroeli) likely comprised a distinct lineage from the material originally placed in this clade by [START_REF] Ameghino | Enumeración sistemática de las especies de mamíferos fósiles coleccionados por Carlos Ameghino en los terrenos eocenos de la Patagonia austral y depositados en el Museo de La Plata[END_REF][START_REF] Ameghino | Nuevos restos de mamíferos fósiles descubiertos por Carlos Ameghino en el Eoceno inferior de la Patagonia austral. Especies nuevas, adiciones y correcciones[END_REF][START_REF] Ameghino | Enumération synoptique des espèces de mammifères fossiles des formations éocènes de Patagonie[END_REF]). Ameghino's specimens were older, all deriving from the Santa Cruz Formation of Patagonia, which is late early Miocene in age (Santacrucian SALMA). However, [START_REF] Brandoni | Xyophorus" sp. en el Mioceno medio de Chubut: implicancias sistemáticas, biogeográficas y biocronológicas del registro de un Nothrotheriinae en el Neógeno de la Argentina[END_REF] and [START_REF] Brandoni | Chronological implications of the nothrotheriid "Xyophorus" (Mammalia, Xenarthra) from the Collón Curá Formation (Miocene of Patagonia, Argentina)[END_REF] hesitated to erect a new taxon for this material, because of the paucity of material available at the time and the consequent lack of a sufficient number of diagnostic features for such a clade, choosing instead to refer to these specimens with the generic epithet "Xyophorus," following the open nomenclature convention of [START_REF] Bengston | Open Nomenclature[END_REF], but rendering the understanding of this genus very difficult. Moreover, the majority of specimens assigned to "Xyophorus" were partial mandibles which, though distinct in several respects from the older Xyophorus material, indeed provide a very limited basis for erecting a new taxon (De Iuliis 2018).

The new specimens from Achiri described in the present report, and especially the skull fragment with ear region, significantly increase the information available for basal nothrotheriids.

The astragalus we recovered is similar to another astragalus from Achiri reported by St André (1996, part of the type of "Xyophorus" villarroeli), although it is better preserved. It bears a more medially divergent medial trochlea, more closely approaching the odontoid facet of derived nothrotheriids [START_REF] Stock | Cenozoic gravigrade edentates of western North America with special reference to the Pleistocene Megalonychinae and Mylodontidae of Rancho La Brea[END_REF][START_REF] Kraglievich | Descripción de los astrágalos de dos gravígrados terciarios de la subfamilia "Nothrotheriinae[END_REF][START_REF] Cartelle | Contribuição ao melhor conhecimento da pequena preguiça terrícola Nothrotherium maquinense (Lund) Lydekker, 1889[END_REF]; it possesses a dorsal pit alongside the lateral trochlea that receives the anterior distal tibial process, and a less proximally tilted sustentacular facet. However, the strong similarities between these two astragali (Figure 5A-F), confirm for the moment the presence of a single small-sized megatherioid species in the Bolivian locality of Achiri. However, the squamosal with its attached auditory region is even more indicative of the anatomy of Mcdonaldocnus, as it bears a significant number of diagnostic features of this genus. The ear region of mammals has long been recognized as one of the most complex regions of the skull [START_REF] Novacek | Patterns of diversity in the mammalian skull[END_REF], and one that is evolutionarily labile, such that it has proved particularly useful and sometimes fundamental as a source of taxonomically and phylogenetically informative characters for mammals in general [START_REF] Rougier | Major changes in the ear region and basicranium of early mammals[END_REF][START_REF] O'leary | The placental mammal ancestor and the post-KPg radiation of placentals[END_REF]) and xenarthrans in particular (e.g., Patterson et al. 1989[START_REF] Patterson | The ear region in xenarthrans (= Edentata, Mammalia). Part II. Pilosa (sloths, anteaters), palaeanodonts, and a miscellany[END_REF][START_REF] Gaudin | The ear region of edentates and the phylogeny of the Tardigrada (Mammalia, Xenarthra)[END_REF], as in the present case. The new ear region clearly shows that this new taxon Mcdonaldocnus, incorporating the remains of relatively primitive, small bodied nothrotheres from the late early-early middle Miocene (Friasian-Colloncuran SALMAs) and early Pliocene (Montehermosan-Chapadmalalan SALMAs) of Argentina and Bolivia, is a basal member of the clade Nothrotheriidae, but one lacking some of the traits of more derived members of the group. Nothrotheriid characters it shares from the auditory region include a dorsoventrally elongated ectotympanic with an elongated styliform process that is tightly sutured to skull, an ovate stylohyal fossa, and the presence of longitudinal ridges in postglenoid region (Figure 3). Additional nothrotheriid characters from the astragalus include a lateral trochlea of the astragalar tibial facet with a semicircular dorsal edge in medial view; a proximodistally narrow medial trochlea bordered distally by strong indentation in medial view; and a pit on the dorsal surface of the astragalus, medial to lateral trochlea, for the anterior distal tibial process (present only in the new specimen).

However, Mcdonaldocnus lacks features of more derived nothrotheriids (Pronothrotherium, Aymaratherium, Mionothropus, Nothrotherium, and Nothrotheriops) as noted in the diagnosis above. These include a canal from the occipital artery system perforating the paroccipital process, an odontoid process on the astragalus, a broad dorsal exposure of the fibular facet on the astragalus, and a transverse orientation of the ectal facet of the astragalus. In addition, it retains numerous megatherioid symplesiomorphies, among them: an anterolaterally directed root of the zygoma; a broad astragalar neck; a small sustentacular facet; a cuboid facet on the plantar surface of astragalus; a ridge connecting the head to the distal lateral trochlea in distal view; a non-articular area on the plantar edge of the lateral trochlea, lateral to the head in distal view; a distal contact between the fibular and ectal facets in lateral view; and minimal lateral exposure of the ectal facet in lateral view.

Moreover, Mcdonaldocnus expresses some features that are transitional between basal megatherioids like Hapalops from the Santacrucian SALMA and more derived nothrotheriids like Pronothrotherium and Mionothropus, both from the Huayquerian SALMA, including: ectotympanic expanded transversely in ventral view (more than Hapalops, less than derived Nothrotheriidae); medial portion of the entotympanic is deeper than that of Hapalops, similar in proportions to the more derived nothrotheriids Pronothrotherium and Mionothropus; medial astragalar trochlea medially divergent (variable, more than Hapalops and Xyophorus atlanticus, less than derived Nothrotheriidae); ectal facet expands laterally at distal end only (extends laterally at both ends in Hapalops, not expanded at either end in Pronothrotherium); sustentacular facet less proximally oriented than in Hapalops and X. atlanticus, more so than in Pronothrotherium (variable); lateral trochlea elevated in medial view relative to medial trochlea (more than Hapalops and X. atlanticus, less than Pronothrotherium).

Although we have assigned all specimens included in "Xyophorus" by [START_REF] Brandoni | Xyophorus" sp. en el Mioceno medio de Chubut: implicancias sistemáticas, biogeográficas y biocronológicas del registro de un Nothrotheriinae en el Neógeno de la Argentina[END_REF] and [START_REF] Brandoni | Chronological implications of the nothrotheriid "Xyophorus" (Mammalia, Xenarthra) from the Collón Curá Formation (Miocene of Patagonia, Argentina)[END_REF] to our new genus, we have singled out one assignment as questionable. A partial skull and mandible reported by [START_REF] Frailey | The Miocene vertebrates of Quebrada Honda, Bolivia. II: Edentata[END_REF] from sediments in Quebrada Honda, Bolivia, assigned to the Laventan SALMA (middle Miocene, 13.1-12.4 Ma [see Gibert et al., 2020]), were originally placed in the species Hapalops angustipalatus, but were subsequently moved to "Xyophorus" cf. 'X' bondesioi by Scillato-Yané and [START_REF] Carlini | Nuevos Xenarthra del Mioceno de Quebrada Honda, Bolivia[END_REF]. There is a second mandible from the same locality (UF 242000) that we also questionably refer to the new genus. Although these specimens are of the right age to be included in Mcdonaldocnus -indeed, there is older material from Cerdas (Friasian -Colloncuran SALMAs; [START_REF] Croft | New data on Miocene neotropical provinciality from Cerdas, Bolivia[END_REF]) that we subsume in the new taxon -and the two mandibles do have a short diastema and elongated mandibular symphysis, as in other Mcdonaldocnus specimens, we note that these mandibles lack the most distinctive dental feature of Mcdonaldocnus, the deeply grooved first and second molariforms. The grooves are not entirely absent, they are just weakly developed, and hence more similar to the condition in basal megatherioids like Hapalops [START_REF] Scott | Mammalia of the Santa Cruz beds. Part I. Edentata[END_REF]. And though De Iuliis et al. (2011) caution against the use of apicobasal grooves on the lingual and labial surfaces of the molariforms as phylogenetic characters in nothrotheriids, given their variable expression in well-sampled taxa like the stem megatherioid Hapalops and the highly-derived nothrothere Nothrotheriops, we note that the grooves are particularly deep in the lower molariforms of other X. bondesioi and X. villarroeli (Figure 1), and so their poor development in the Quebrada Honda specimens merits some caution when assessing their taxonomic affinities. The partial skull recovered by [START_REF] Frailey | The Miocene vertebrates of Quebrada Honda, Bolivia. II: Edentata[END_REF] may eventually be helpful in more clearly determining the taxonomic provenance of the Quebrada Honda specimens, but only the palate is illustrated and described. Frailey (1988, p. 7) states that "a portion of the basicranium [is] preserved," but he provides no further information, and of course, the only other partial skull of Mcdonaldocnus, the one from Achiri, preserves neither the basicranium itself, nor any portion of the palate. Although on balance, given its age and geographic provenance, we feel there is reason enough to hypothesize a connection between the Quebrada Honda material and other specimens more confidently assigned to Mcdonalocnus, we also recognize that the morphological evidence for its inclusion is less compelling, and perhaps warrants some question pending the availability of more material.

It is important at the end to emphasize that the present study does not resolve the taxonomic status of the original material assigned to Xyophorus by [START_REF] Ameghino | Enumeración sistemática de las especies de mamíferos fósiles coleccionados por Carlos Ameghino en los terrenos eocenos de la Patagonia austral y depositados en el Museo de La Plata[END_REF][START_REF] Ameghino | Nuevos restos de mamíferos fósiles descubiertos por Carlos Ameghino en el Eoceno inferior de la Patagonia austral. Especies nuevas, adiciones y correcciones[END_REF][START_REF] Ameghino | Enumération synoptique des espèces de mammifères fossiles des formations éocènes de Patagonie[END_REF]. As noted in the Introduction to the present study, other authors (e.g., [START_REF] Lydekker | Contributions to a knowledge of the fossil vertebrates of Argentina[END_REF][START_REF] Scott | Mammalia of the Santa Cruz beds. Part I. Edentata[END_REF][START_REF] Perea | Un singular grupo de perezosos, los "Nothrotheriinae" (Xenarthra)[END_REF] have considered Xyophorus to be a junior synonym of Hapalops, based solely on Ameghino's specimens. [START_REF] Bargo | Early Miocene sloths (Xenarthra, Folivora) from the Río Santa Cruz valley (Southern Patagonia, Argentina). Ameghino, 1887 revisited[END_REF] provide an extensive discussion of the confusing taxonomy of this material. Although they suggest some of the original Xyophorus species (X. andinus, X. atlanticus, and X. crassissimus) might represent a distinct clade from other Santacrucian basal megatherioids based on dental features; and, moreover, that some of the new skeletal remains they assign to X. atlanticus exhibit autapomorphic traits, yet they do not attempt to formally designate any new genera or formally emend the diagnosis of any existing Xyophorus species. We consider that the ultimate fate of Xyophorus cannot be determined without an extensive revision of Hapalops and "allied genera" (e.g., Pseudohapalops, Amphihapalops, etc), like that recently completed for its fellow Santacrucian-aged megalonychid taxon Eucholoeops (Santacrucian SALMA; De Iuliis et al. 2014). Such an analysis could provide a clearer understanding of what should and should not be included in Hapalops. Until such systematic revision is completed, however, only those species erected by Ameghino and with a provenance from Santacrucian levels of Santa Cruz Province, must be included in the genus Xyophorus. In the absence of a comprehensive revision of Hapalops and its allies, which is well beyond the scope of the present study, we will refrain from making any definitive statement regarding the legitimacy of Xyophorus itself as a separate genus. That said, it is noteworthy that several of Ameghino's specimens do not satisfy the definition of Xyophorus as regards diastema length, possessing a diastema longer than the mesiodistal length of mf1 [START_REF] Brandoni | Xyophorus" sp. en el Mioceno medio de Chubut: implicancias sistemáticas, biogeográficas y biocronológicas del registro de un Nothrotheriinae en el Neógeno de la Argentina[END_REF], Table 1). Some also lack apicobasal grooves on the lower molariforms [START_REF] Bargo | Early Miocene sloths (Xenarthra, Folivora) from the Río Santa Cruz valley (Southern Patagonia, Argentina). Ameghino, 1887 revisited[END_REF].

Clearly, the taxonomy of Xyophorus requires further study before it can be resolved, though we are comfortable excluding this material from Mcdonaldocnus, following Brandoni (2014) and [START_REF] Brandoni | Chronological implications of the nothrotheriid "Xyophorus" (Mammalia, Xenarthra) from the Collón Curá Formation (Miocene of Patagonia, Argentina)[END_REF].

Finally, beyond an assignment to Nothrotheriidae, with the implication that it represents a fairly basal member of that clade, we have not yet attempted to better resolve the phylogenetic position of Mcdonaldocnus. At this point, there remains very little information on the skeletal anatomy of this taxon. The sum total of all the specimens assigned to date provide us with information on most of the mandible, but only the squamosal, auditory region and possibly the palatal portion of the skull, along with the astragalus and a few ungual phalanges. This ultimately represents a fairly small portion of its overall skeletal anatomy, and therefore we are not convinced that a formal phylogenetic analysis of Mcdonaldocnus is warranted as of yet. We are conscious that the specimens included in Mcdonaldocnus represent a lineage of 10 Ma and it is possible that new discoveries may justify the recognition of other species within this new genus, or even its division into multiple genera. We think that a greater diversity of skeletal material needs to be recovered and described in detail before a more definitive resolution of its affinities can realistically be achieved.

It is our hope that ongoing field work in Bolivia and Argentina by our group, and others, will ultimately provide such a resolution. ectotympanic; egp, entoglenoid process; ent(lp), entotympanic (lateral plate); eps, epitympanic sinuses; fG, fissura Glaseri; fo, foramen ovale; gEt, groove for the Eustachian tube; gf, glenoid fossa; gvn, groove for the vidian nerve; gtvp , groove for m. tensor veli palatini; iam, internal acoustic meatus; jf, jugular foramen; mpp, medial prominence of petrosal; oaf, occipital artery foramen; p, parietal; pcp, paracondylar process of exoccipital; pgf, postglenoid foramen; pop, paroccipital process of the petrosal (= mastoid process of squamosal of [START_REF] Patterson | The ear region in xenarthrans (= Edentata, Mammalia). Part II. Pilosa (sloths, anteaters), palaeanodonts, and a miscellany[END_REF]; prp, promontorium of the petrosal; pt(lp), pterygoid (lateral plate); saf, subarcuate fossa; sips, sulcus for inferior petrosal sinus; sq/f(?)s, suture between the squamosal and the frontal(?); shf, stylohyal fossa; stmf, stylomastoid foramen; zpsq, zygomatic process of the squamosal. Scale bar = 50mm. cp, crista parotica; ec, ectotympanic; ent(lp), entotympanic (lateral plate); fv, fenestra vestibuli (= fenestra ovalis); gEt, groove for the Eustachian tube; gtvp , groove for tensor veli palatini muscle; oaf, occipital artery foramen; pcp, paracondylar process of exoccipital (= paraoccipital process of [START_REF] Patterson | The ear region in xenarthrans (= Edentata, Mammalia). Part II. Pilosa (sloths, anteaters), palaeanodonts, and a miscellany[END_REF]; pgf, postglenoid foramen; pop, paroccipital process of the petrosal (= mastoid process of squamosal of [START_REF] Patterson | The ear region in xenarthrans (= Edentata, Mammalia). Part II. Pilosa (sloths, anteaters), palaeanodonts, and a miscellany[END_REF]; prp, promontorium of the petrosal; shf, stylohyal fossa; sq, squamosal; stmf, stylomastoid foramen. Scale bar = 10mm.

Figure Captions

  ' is in honor of Dr. H. Greg McDonald, recently retired from the U.S.

  elongatus FMNH P13123; 57º in Hapalops sp. MLP 88-X-2-1.4 [Figure 5H]; 45º in H. longiceps YPM-PU 15523), whereas in the nothrotheriid Pronothrotherium typicum (FMNH P15223; Kraglievich 1928), the medial trochlea is a fully developed odontoid process typical of many derived sloths (McDonald 2012), oriented at a 90º angle to the lateral trochlea. Pronothrotherium also differs from the other taxa in that the fibular facet is broadly exposed dorsally, and extends far laterally, making the astragalus much broader transversely in dorsal view.

  , in Hapalops longiceps(YPM-PU 15523; Scott 1904, plate 33, fig. 4) and H. elongatus (FMNH P13123), and is situated along the lateral edge of the medial trochlea in Hapalops sp. (MLP 88-X-2-1.4; Figure 5H). The depression is poorly developed in MNHN.F.ACH 15 (Figure 5B; St-André 1996). The astragalar neck, which connects the tibial facets to the distal articulation for the navicular on the head, is relatively broad in all of the taxa described here (Figure A, D, E), nearly as wide as the tibial facets themselves in Mcdonaldocnus, X. atlanticus and Hapalops. The neck is more constricted medially and laterally in Pronothrotherium (FMNH P15223; Kraglievich 1928).

Figure 1

 1 Figure 1 Caption:. Right dentary of the nothrotheriid sloth Mcdonaldocnus bondesioi (MNHN.F.ACH 43) from the late Miocene Bolivian locality of Achiri in lateral (A), medial (B), and occlusal (C) views. Abbreviations: cf1, lower caniniform; mf1-3, lower molariforms 1-3. Scale bar = 50mm.

Figure 1

 1 Figure 1 Alt text: Fragmentary right dentary, including caniniforms and molariforms, of the nothrotheriid sloth Mcdonaldocnus bondesioi (MNHN.F.ACH 43) shown in lateral, medial, and occlusal views with teeth labeled and a scale bar provided.

Figure 2

 2 Figure 2 Caption: Location map of the late Miocene fossil-bearing locality of Achiri (La Paz Department) and Cerros Pisakeri and Virgen Pata where the new remains of Mcdonaldocnus bondesioi were discovered; (B), general map of Bolivia.

Figure 2

 2 Figure 2 Alt Text: Location map of the late Miocene fossil-bearing locality of the village of Achiri (La Paz Department) and fossil localities of Cerros Pisakeri and Virgen Pata, where the new remains of Mcdonaldocnus bondesioi were discovered, shown next to a general map of Bolivia indicating the location of Achiri relative to the major cities in the country.

Figure 3

 3 Figure 3 Caption: Right posterolateral portion of the skull attributed to the nothrotheriid sloth Mcdonaldocnus bondesioi (MNHN-Bol-V 012690) from the Bolivian locality of Achiri in lateral (A), internal (B), lateroventral (C), and ventral (D) views. Abbreviations: bo, basioccipital; cf, carotid foramen; crp, crista petrosa; cs, cerebral surface; eam, external auditory meatus; ec,

Figure 3

 3 Figure 3 Alt Text: Right posterolateral portion of the skull attributed to the nothrotheriid sloth Mcdonaldocnus bondesioi (MNHN-Bol-V 012690) shown in lateral, internal, lateroventral, and ventral views, with structural details labeled, and a scale bar provided.

Figure 4

 4 Figure 4 Caption: Ear region in lateral view from the skull attributed to the nothrotheriid sloth Mcdonaldocnus bondesioi (MNHN-Bol-V 012690) from Bolivian locality of Achiri. Abbreviations:

Figure 4

 4 Figure 4 Alt Text: Ear region in close-up lateral view from the skull attributed to the nothrotheriid sloth Mcdonaldocnus bondesioi (MNHN-Bol-V 012690), with structural details labeled, and a scale bar provided.

Figure 5

 5 Figure 5 Caption: Photographs of two astragali of the nothrotheriid sloths Mcdonaldocnus bondesioi MNHN.F.ACH 15 (A-C) and MNHN-Bol-V 011887 (D-F) and the basal megathrioid sloth Hapalops sp. MLP 88-X-2-1.4 (G-I) in anterior (A, D, G), dorsal (B, E, H), and plantar (C, F, I) views. Abbreviations: ah, astragalar head; an, astragalar neck; cuf, cuboid facet; ef, ectal facet; ff, fibular facet; lt, lateral trochlea; mt, medial trochlea; nf, navicular facet; st, sulcus tali; suf, sustentacular facet. Scale bar = 50mm.

Figure 5

 5 Figure 5 Alt Text: Photographs of two astragali of the nothrotheriid sloth Mcdonaldocnus bondesioi (MNHN.F.ACH 15 and MNHN-Bol-V 011887) and the basal megathrioid sloth Hapalops sp.(MLP 88-X-2-1.4), each shown in anterior, dorsal, and plantar views, with structural details labeled, and a scale bar provided.

Fig. 1

 1 Fig. 1

Fig. 2

 2 Fig. 2

  Fig. 3

Other Abbreviations: aptt, anteroventral process of the tegmen typmani; Cf1/cf1, caniniform tooth; CN, cranial nerve; gtvp, groove for the muscle tensor veli palatini; iam, internal acoustic meatus; ICZN, International Code of Zoological Nomenclature; Mf1-4/mf1-3, molariform teeth; SALMA, South American Land Mammal Age.

  Acronyms: CRILAR-PZ, Colección de Paleozoología, Departamento de Geociencias, Small bodied member of Nothrotheriidae, similar to or smaller in size than Pronothrotherium. It lacks derived features of more derived nothrotheriids (i.e., Pronothrotherium,

	Diagnosis-
	Centro Regional de Investigaciones Científicas y Transferencia Tecnológica, Anillaco, Argentina;
	FMNH, The Field Museum, Chicago, IL, USA; LIEB PV, Laboratorio de Investigaciones en
	Evolución y Biodiversidad (-PV, Paleovertebrados), Esquel, Argentina; MLP, Museo de La Plata,
	La Plata, Argentina; MNHN-Bol, Museo Nacional de Historia Natural, La Paz, Estado
	Plurinacional de Bolivia; MNHN.F, Muséum national d'Histoire naturelle, Paris, France (ACH
	indicating Achiri); MPEF-PV, Colección de Paleontología de Vertebrados del Museo
	Paleontológico Egidio Feruglio, Trelew, Argentina; UATF, Universidad Autónoma Tomás Frías,
	Potosí, Estado Plurinacional de Bolivia; UF, Florida Museum of Natural History, University of
	Florida, Gainesville, FL, USA; YPM-PU, Princeton Collection, Peabody Museum, Yale University,
	New Haven, CT, USA.
	Systematic Paleontology
	XENARTHRA Cope, 1889
	PILOSA Flower, 1883
	FOLIVORA Delsuc et al., 2001
	NOTHROTHERIIDAE Gaudin, 1994
	Mcdonaldocnus new genus
	Type species: Xyophorus bondesioi Scillato-Yané 1979
	Figures 1, 3-5

Acknowledgments

Fieldwork in the Bolivian Altiplano has been carried out in the framework of an ongoing cooperation agreement (N°864/2014) between the MNHN-Bol (La Paz, Bolivia), the Institut des Sciences de l'Evolution de Montpellier (France) and the CCT-CONICET (Mendoza, Argentina). Fieldwork, lab work and dating were supported by the National Geographic Society (NGS EC-44712R-18 [AB] and NGS 9971-16 [FP]), the ECOS-SUD/FONCyT international program (2015-2017; FP and POA), and by the Investissements d'Avenir grant managed by the ANR (LabEx CEBA, ANR-10-LABX-25-01). For access to comparative material utilized in the present study, we thank Ken Angielczyk and Bill Simpson (The Field Museum, Chicago, IL); Jonathan Bloch and Richard Hulburt (Florida Museum of Natural History, University of Florida, Gainesville, FL), and W. Joyce and D. Brinkman (Peabody Museum at Yale University, New Haven, CT). T.J.G.'s work on this project was supported in part by a Faculty Research and Development Award from the University of Tennessee at Chattanooga, as well as funds from UTC's Department of Biology, Geology, and Environmental Science and the Bramblett gift fund.

Funding

Fieldwork, lab work and dating were supported by the National Geographic Society (NGS EC-44712R-18 [AB] and NGS 9971-16 [FP]), the ECOS-SUD/FONCyT international program (2015-2017; FP and POA), and by the Investissements d'Avenir grant managed by the ANR (LabEx CEBA, ANR-10-LABX-25-01). T.J.G.'s work on this project was supported in part by a Faculty Research and Development Award from the University of Tennessee at Chattanooga, as well as funds from UTC's Department of Biology, Geology, and Environmental Science and the Bramblett gift fund.

Data Availability Statement

The fossils described in the present study are housed in the Museo Nacional de Historia Natural, La Paz, Estado Plurinacional de Bolivia. Any requests for further information about the data presented in this study should be directed to the corresponding author.

Declaration of Interests Statement

The authors report there are no competing interests to declare.