Article

A new nothrotheriid xenarthran from the early Pliocene of Pomata-Ayte (Bolivia): new insights into the caniniform–molariform transition in sloths

Authors:
  • National Scientific and Technical Research Council (CONICET), Mendoza, Argentina
  • Museo Nacional de Historia Natural, La Paz, Bolivia
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Abstract

Tardigrade xenarthrans are today represented only by the two tree sloth genera Bradypus and Choloepus, which inhabit the Neotropical rainforests and are characterized by their slowness and suspensory locomotion. Sloths havebeen recognized in South America since the early Oligocene. This monophyletic group is represented by five clades traditionally recognized as families: Bradypodidae, Megalonychidae, Mylodontidae (†), Megatheriidae (†) and Nothrotheriidae (†). A new nothrotheriid ground sloth represented by a dentary and several postcranial elements, Aymaratherium jeani gen. nov., sp. nov., from the early Pliocene locality of Pomata-Ayte (Bolivia) is reported. This small- to medium-sized species is characterized especially by its dentition and several postcranial features. It exhibits several convergences with the ‘aquatic’ nothrotheriid sloth Thalassocnus and the giant megatheriid ground sloth Megatherium (M.) americanum, and is interpreted as a selective feeder, with good pronation and supination movements. The tricuspid caniniform teeth of Aymaratherium may represent a transitional stage between the caniniform anterior teeth of basal megatherioids and basal nothrotheriids (1/1C-4/3M as in Hapalops or Mionothropus) and the molariform anterior teeth of megatheriids (5/4M, e.g. Megatherium). To highlight the phylogenetic position of this new taxon among nothrotheriid sloths, we performed a cladistic assessment of the available dental and postcranial evidence. Our results, derived from a TNT treatment of a data matrix largely based on a published phylogenetic data set, indicate that Aymaratherium is either sister taxon to Mionothropus or sister to the clade Nothrotheriini within Nothrotheriinae. They further support the monophyly of both the Nothrotheriinae and the Nothrotheriini, as suggested previously by several authors.

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... Its members are mostly endemic to South America, although some of them participated in the Great American Biotic Exchange, occupying large territories in Central America, the Antilles, and North America both before and after the formation of the Isthmus of Panama (Woodburne 2010;Campbell et al. 2010). Although today the group has limited biological diversity, with the highest diversity found among cingulates, the fossil record of sloths is abundant, extending to the late Eocene (McKenna et al. 2006) and includes very diverse forms of terrestrial, semi-arboreal, and arboreal sloths with different body sizes, diets, and modes of locomotion (Pujos et al. 2016). Sloths (and xenarthrans in general) present unique dentitions among mammals, both in their micro and macrostructure. ...
... Many species possess an anterior tooth differentiated from the rest in a structure grossly similar to a canine (usually called caniniform), unlike the posterior teeth which are called molariforms and clearly have masticatory functions. Based on the discoveries of several Oligocene sloths in Chile, Bolivia, and Argentina (Hoffstetter 1956;McKenna et al. 2006;Pujos and De Iuliis 2007;Shockey and Anaya 2011), it is considered that the presence of caniniforms is a plesiomorphic state among sloths and several losses or even modifications to a molariform morphology have occurred (Hautier et al. 2016;Pujos et al. 2016;Delsuc et al. 2019). ...
... Sexual selection and/or niche segregation could also be related to the large body size that occurred in some sloth clades (Pant et al. 2014;Toledo et al. 2015). Likewise, they could also be related to the apparently high variability observed in sloths dentition in regard to the first tooth, which shows caniniform or molariform morphology in different clades, or its loss in many taxa (Hautier et al. 2016;Pujos et al. 2016;Delsuc et al. 2019). In fact, the ancestral reconstruction of first tooth morphology made by Delsuc et al. (2019) on a sloth phylogeny based on morphological data from Varela et al. (2019) and a molecular constraint based on their phylogenetic results, shows several cases of independent transitions from a caniniform to a molariform morphology, as well as cases of transitions from a molariform to a caniniform morphology or loss of the first tooth (especially in the mandible). ...
Article
Mylodontidae (Mammalia, Xenarthra) is a family of ground sloths widely distributed in the South American fossil record, with members also present in Central and North America. Within the Mylodontidae, Lestodon armatus is the largest species, with an estimated body mass of more than three tonnes. This work focuses on the enlarged lower caniniforms of L. armatus as possibly exaggerated sexually dimorphic structures. Lower caniniforms from the late Pleistocene of Argentina, Uruguay, and Bolivia were studied using specimens from seven palaeontological collections. The possible sexual dimorphism in the caniniforms and its implications regarding the existence of sexual selection was assessed through morphometric analyses. The results support the existence of sexual dimorphism in L. armatus. Sexual dimorphism in an exaggerated structure in a large mammal suggests the existence of sexual selection, via competition between males or female mate choice, resulting in the evolution of the dimorphic structure. In L. armatus, the enlarged caniniforms would correspond to males and could have functioned as armaments in intraspecific fights or ornaments for sexual display. Based on observations in extant mammals, a polygynous mating system is proposed as highly probable in L. armatus, although the existence or composition of social groups cannot be certainly determined.
... The subject of the present contribution, the late Mioceneearly Pliocene Pronothrotherium typicum Ameghino, 1907 (Huayquerian-Chapadmalalan SALMA;McDonald & De Iuliis, 2008), is also represented by extensive remains. This species has been used frequently in phylogenetic analyses (Gaudin, 1995;Muizon & McDonald, 1995;McDonald & Muizon, 2002;Muizon et al., 2003;Gaudin, 2004;De Iuliis et al., 2011;Pujos et al., 2016;Amson et al., 2017;Varela et al., 2019) where much of its anatomy has been coded into character matrices but, unfortunately, very little has ever been formally described. ...
... It is certainly possible that the dearth of published information on Pronothrotherium anatomy has hindered resolution of its phylogenetic relationships. Although recent cladistic studies have strongly supported the monophyly of the Pleistocene nothrotheriids Nothrotherium and Nothrotheriops (Muizon & McDonald, 1995;McDonald & Muizon, 2002;Muizon et al., 2003;Gaudin, 2004;De Iuliis et al., 2011;Pujos et al., 2016;Amson et al., 2017;Varela et al., 2019), the relationships of the other two well preserved nothrotheriids, Pronothrotherium and Mionothropus, to these two taxa have remained difficult to resolve. In some studies, one of the older representatives of the clade, the late Miocene Mionothropus, has been allied as the closest relative to the two Pleistocene taxa (Muizon & McDonald, 1995;Amson et al., 2017;Varela et al., 2019). ...
... In some studies, one of the older representatives of the clade, the late Miocene Mionothropus, has been allied as the closest relative to the two Pleistocene taxa (Muizon & McDonald, 1995;Amson et al., 2017;Varela et al., 2019). By contrast, in perhaps the most detailed study of the matter and the one most focused on nothrotheriid relationships, De Iuliis et al. to one another as sister taxa (McDonald & Muizon, 2002;Muizon et al., 2003), or simply failed to unambiguously resolve their relationships (Perea, 1999;Gaudin, 2004;Pujos et al., 2016). ...
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Article
Pronothrotherium typicum is a late Miocene–early Pliocene (Huayquerian–hapadmalalan SALMA) nothrotheriid sloth known from the Catamarca Province of northwestern Argentina. Pronothrotherium is one of four nothrotheriid genera known from relatively complete skeletal material, but unlike the other three, the osteology of Pronothrotherium has not been formally described. The present study provides the first detailed description and illustration of the cranial anatomy of Pronothrotherium, based largely on a nearly complete, subadult skull of P. typicum from the collections of The Field Museum (Chicago, Illinois, USA), as well as a less well-preserved adult skull and isolated mandible from the same collections. A revised cranial diagnosis of P. typicum is provided in the text. The skull of this species shows a number of distinctive features, most notably a peculiar vomerine keel in the nasopharynx, terminating in a swollen knob, that is, as far we know, a unique morphology among mammals. Based on the results of the present study, there appears to be reason to recognize two contemporaneous species of Pronothrotherium, P. typicum and P. mirabilis, although the latter is less well supported. We do not accept the validity of a third described species, P. figueirasi, considering it instead to be synonymous with P. mirabilis. The present study does not resolve the uncertain phylogenetic relationships among the well-preserved nothrotheriine taxa Pronothrotherium, Mionothropus (late Miocene), and the two Pleistocene genera in Nothrotheriini, Nothrotherium and Nothrotheriops. However, we hope that the data provided will facilitate subsequent phylogenetic studies that may resolve these issues.
... Astragalus: N. cf. carcaranensis: MACN-Pv-14153; Nothrotherium maquinense (Lund, 1839): MCL-1020/217, 2819/62, 22.030/01; Aymaratherium jeani Pujos et al., 2016: MNHN-BOL-V-012983; Nothrotheriops shastensis (Sinclair, 1905) right astragalus HC-1875-R-4. ...
... The astragalus MACN-Pv-14153 was compared with that of N. maquinense (MCL-1020(MCL- /217, 2819.030/01), A. jeani MNHN-BOL-V-012983, and N. shastensis right astragalus HC-1875-R-4, using photographs or 3D-captures and photographs for the case of N. shastensis. The terminology adopted for description of astragalus follows Pujos et al. (2016). The femora were compared in anterior, posterior and distal views. ...
... Indeed, the similarities with Nothrotheriops in astragalar size may actually be of dubious significance since there is material assigned to Nothrotherium presenting a similar or even larger size than Nothrotheriops, at least from femoral measurements (Kraglievich 1926), and furthermore, the width of the sulcus tali is variable in Neogene nothrotheriids (Kraglievich 1928). The same can be said about similarities shared with Nothrotherium, such as the prominence of the odontoid tuberosity, given the variability found in Neogene nothrotheriids (Kraglievich 1928, Pujos et al. 2016. ...
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Article
The remains of Nothrotheriidae are very scarce in the fossil record of the Pleistocene of Argentina but the locality La Ribera (late Pleistocene) has an unexpected concentration of members of this family. In this work, an astragalus and a femur assignable to Nothrotheriidae from La Ribera are described as Nothropus cf. carcaranensis Bordas, 1942. Other material of this family of sloths from this locality, among which is the holotype of Nothropus carcaranensis, are reviewed with discussion about the intraspecific variation of the femora. The elements studied herein provide a better understanding of the diversity of the Nothrotheriidae in Argentina.
... Nothrotheriidae is characterised by caniniforms (when present) separated by a diastema from the molariform tooth row and quadrangular, rectangular or trapezoidal molariforms with longitudinal grooves on the lingual and labial surfaces (see De Iuliis et al. 2011;Brandoni et al. 2016;Pujos et al. 2016). The subfamily Nothrotheriinae includes several genera: Nothropus, Nothrotherium, Pronothrotherium, Nothrotheriops, Mionothropus, Lakukullus, Aymaratherium, Huilabradys, Chasicobradys and Mcdonaldocnus (Villarroel 1998;De Iuliis et al. 2011;Brandoni 2014;Pujos et al. 2014;Brandoni et al. 2016Brandoni et al. , 2016Varela et al. 2019;Gaudin et al. 2022). ...
... Nothrotheriinae is known from the Middle Miocene of Bolivia and Argentina to the Pleistocene of different regions of South America, Central America and North America (Brandoni 2014;De Iuliis et al. 2015;Brandoni and Vezzosi 2019). Miocene well-known representatives are Pronothrotherium, Huilabradys, Mionothropus, Aymaratherium, Lakukullus and Mcdonaldocnus, whose remains were found in Argentina, Colombia, Uruguay, Peru, Brazil and Bolivia (Rovereto 1914;Perea 1988;Villarroel 1998;Brandoni 2014;Pujos et al. 2014Pujos et al. , 2016Gaudin et al. 2022). ...
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Article
Among extinct sloths, Nothrotheriidae nothrotheriines are characterised by caniniforms (when present) separated by a diastema from the molariform tooth row and quadrangular, rectangular or trapezoidal molariforms with longitudinal grooves on the lingual and labial surfaces. The subfamily Nothrotheriinae is recorded from the Middle Miocene of Bolivia and Argentina to the Pleistocene of different regions of South America, Central America and North America. Neogene well-known representatives include Pronothrotherium, Huilabradys, Mionothropus, Aymaratherium, Lakukullus and the recently defined genus Mcdonaldocnus, whose remains were found from Argentina, Colombia, Uruguay, Peru, Brazil and Bolivia. Mcdonaldocnus includes materials from Argentina and Bolivia previously assigned as ‘Xyophorus’. New records from Late Miocene levels of the Cerro Azul Formation cropping out in the Chasicó creek locality, Buenos Aires Province, and in the localities of Telén and Loventué, La Pampa Province, allow us to describe cranio-dental remains of Nothrotheriinae Mcdonaldocnus bondesioi and Mcdonaldocnus sp., respectively. The identification of Mcdonaldocnus sp. in the Cerro Azul Formation at La Pampa Province constitutes the first record of Nothrotheriinae for this province.
... Hoffstetter 1968Hoffstetter et al., 1971aHoffstetter et al., , b, 1972 and colleagues (e.g. MacFadden et al., 1993;Saint-André, 1994;Anaya & MacFadden, 1995;de Muizon, 1999;Pujos et al., 2016). Pierre-Antoine Saint-André [Institut Français d'Études Andines (IFEA)], together with Federico Anaya [Museo Nacional de Historia Natural, La Paz, Bolivia (MNHN-Bol)], conducted several campaigns in the Bolivian Altiplano in the 1990s, collecting numerous well-preserved fossil vertebrate remains, most of them figured in Saint-André's PhD dissertation (Saint-André, 1994). ...
... viscachanense, the pampatheriid Plaina sp. and two other armoured cingulates of uncertain affinities, a rodent and a giant carnivorous phorusrhacoid bird. The faunal composition is consistent with an earliest Pliocene age (Montehermosan; Hoffstetter et al., 1972;Marshall et al., 1983;Hoffstetter, 1986;Marshall & Sempéré, 1991;Saint-André, 1994;Pujos et al., 2016). ...
Article
Fossil remains of extinct terrestrial sloths have been discovered in numerous localities throughout the Americas, but knowledge of these animals remains poor in the tropical latitudes in comparison with the austral ones. Even where Pliocene mylodontine sloths are known from North and South America, well-preserved craniodental remains are extremely rare, hindering reliable assessment of their taxonomic assignment and phylogenetic affinities. Here, new craniodental remains of Simomylodon uccasamamensis, from the latest Miocene–Pliocene of the Bolivian Altiplano, are described and compared with those of other Neogene Mylodontinae from South and North America. The resulting morphological observations, combined with morphometric analyses, permit reliable differentiation among these moderate-sized Miocene–Pliocene mylodontids. Simomylodon uccasamamensis appears to be the smallest Pliocene mylodontine, and it is closely related phylogenetically to the late Miocene species Pleurolestodon acutidens. Simomylodon uccasamamensis is also an endemic taxon of the Andean highlands during the Pliocene, with a continuous chronological range extending throughout the Montehermosan, Chapdamalalan and (early) Marplatan South American Land Mammal Ages. This terrestrial sloth may have found its ideal ecological conditions in the Bolivian Altiplano, during a span of time falling between the important South American Late Miocene–Pliocene faunal turnover and the Great American Biotic Interchange around the Pliocene–Pleistocene transition.
... (2) modern domesticated animals such as cats ( Pool and Carrig, 1972), dogs ( Dingwall et al., 1970;Silver et al., 2001), and horses (Bertoni et al., 2012); (3) and extinct animals such as cave bears (de Torres et al., 2005), machairodontine felid carnivorans ( Salesa et al., 2014), and South-American endemic mammals like ground sloths (McDonald, 1989;Pujos et al., 2016), glyptodonts (Glyptodon sp.; Gillette and Ray, 1981), and notoungulates (Toxodon platensis;Guérin and Faure, 2013). ...
... These conditions drastically changed during the Pliocene, with a noticeable cooling (MAT 8-9 °C; Gregory-Wodzicki et al., 1998) associated with a substantial rise of that region, up to modern height at ca. 4000 m above sea level ( Garzione et al., 2008;Lamb, 2016). These drastic abiotic changes affected mammalian communities of the northern Altiplano as demonstrated by the major faunal turnover recorded around the Miocene-Pliocene transition: mesotheriines (primarily Plesiotypotherium) were dominant during the late Miocene whereas ground sloths became dominant during the Pliocene ( Marshall et al., 1983;Pujos et al., 2016). ...
... Hoffstetter 1968Hoffstetter et al., 1971aHoffstetter et al., , b, 1972 and colleagues (e.g. MacFadden et al., 1993;Saint-André, 1994;Anaya & MacFadden, 1995;de Muizon, 1999;Pujos et al., 2016). Pierre-Antoine Saint-André [Institut Français d'Études Andines (IFEA)], together with Federico Anaya [Museo Nacional de Historia Natural, La Paz, Bolivia (MNHN-Bol)], conducted several campaigns in the Bolivian Altiplano in the 1990s, collecting numerous well-preserved fossil vertebrate remains, most of them figured in Saint-André's PhD dissertation (Saint-André, 1994). ...
... viscachanense, the pampatheriid Plaina sp. and two other armoured cingulates of uncertain affinities, a rodent and a giant carnivorous phorusrhacoid bird. The faunal composition is consistent with an earliest Pliocene age (Montehermosan; Hoffstetter et al., 1972;Marshall et al., 1983;Hoffstetter, 1986;Marshall & Sempéré, 1991;Saint-André, 1994;Pujos et al., 2016). ...
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Article
Fossil remains of extinct terrestrial sloths have been discovered in numerous localities throughout the Americas, but knowledge of these animals remains poor in the tropical latitudes in comparison with the austral ones. Even where Pliocene mylodontine sloths are known from North and South America, well-preserved craniodental remains are extremely rare, hindering reliable assessment of their taxonomic assignment and phylogenetic affinities. Here, new craniodental remains of Simomylodon uccasamamensis, from the latest Miocene–Pliocene of the Bolivian Altiplano, are described and compared with those of other Neogene Mylodontinae from South and North America. The resulting morphological observations, combined with morphometric analyses, permit reliable differentiation among these moderate-sized Miocene–Pliocene mylodontids. Simomylodon uccasamamensis appears to be the smallest Pliocene mylodontine, and it is closely related phylogenetically to the late Miocene species Pleurolestodon acutidens. Simomylodon uccasamamensis is also an endemic taxon of the Andean highlands during the Pliocene, with a continuous chronological range extending throughout the Montehermosan, Chapdamalalan and (early) Marplatan South American Land Mammal Ages. This terrestrial sloth may have found its ideal ecological conditions in the Bolivian Altiplano, during a span of time falling between the important South American Late Miocene–Pliocene faunal turnover and the Great American Biotic Interchange around the Pliocene–Pleistocene transition.
... Intensive, ongoing fieldwork in Neogene deposits from the Altiplano of Bolivia over the past two decades has yielded numerous remains of extinct sloths, including the large megatheriine Megatherium altiplanicum (St-André and De Iuliis 2001), both scelidotheriine (cf. Proscelidodon; Pujos et al. 2012) and mylodontine sloths (Simomylodon uccasamamensis; St-André et al. 2010;Boscaini et al. 2019Boscaini et al. , 2021 representing the Mylodontidae, with the latter being the most abundant taxon, the nothrotheriid sloths Lakukullus anatisrostratus and Aymaratherium jeani (Pujos et al. 2016), and the peculiar megatherioid sloth Hiskatherium saintandrei (Pujos et al. 2011). Included among these remains are specimens reputedly pertaining to the enigmatic genus Xyophorus. ...
Article
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 recognised 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 not only 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.
... Finally, nothrotheres and megalonychids did not reach gigantic sizes (although some of them, such as Megalonyx, were very large), nor developed dermal armor or burrowing habits, but instead often used natural caves as dens (McDonald, 2003). In the case of megalonychids, their diversification in the Caribbean islands may have been related to achievement of small body sizes in insular environments (Pujos et al., 2016). Therefore, at present, dermal ossification cannot be reliably related to paleobiological aspects such as body size or burrowing habits. ...
Article
Dermal ossifications (osteoderms, dermal ossicles, osteocutes) appear independently in various tetrapod lineages. In mammals, however, dermal ossifications are only present in some members of Xenarthra. This clade includes Cingulata (armadillos and their relatives), and Pilosa, including Vermilingua (anteaters) and Folivora (sloths). In extant xenarthrans, osteoderms are invariably present in cingulates whereas they are absent in pilosans. Among extinct sloths, however, a limited number of taxa possessed dermal ossifications. Records of mummified skins of ground sloths bearing osteoderms found in “Cueva del Milodón” (Southern Chile), with a late Pleistocene age, allowed us to analyze their micro‐ and macroscopic morphology. The main goal of this study is to closely examine a portion of a mylodontid skin portion using radiography. The arrangement, morphology and internal structure of the ossicles are analyzed and the results are discussed in the context of previous research. The results we obtained indicate that ossicles vary in shape and size, and the integument has four different patterns of arrangement of the ossicles: i.e., areas without ossicles, disorganized ossicles, rows, and mosaic areas. The latter has two variants, with clusters of ossicles forming rosettes or stars. Thin sections of the ossicles allowed us to recognize and describe anatomical features of the bone and its mode of growth. Finally, paleobiological and functional considerations of the dermal armor are discussed along with its phylogenetic and chronological implications.
... = upper and lower caniniform tooth, respectively; masl = meters above sea level; Mf/mf = upper and lower molariform tooth, respectively; MMCO = Middle Miocene Climatic Optimum; SALMA = South American Land Mammal Age. Anatomical abbreviations discf, ectf, fibf, hrm, Mf/mf, odontf, peomc, and symph (see figure captions for definitions) are modified fromPujos et al. (2016). ...
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Article
Xenarthra is an endemic South American lineage of mammals, probably the sister clade of the other placental mammals. The oldest records of Xenarthra are from the latest Paleocene, although its current diversity is much lower than that recorded in some intervals of the Cenozoic Era. A new Neogene Xenarthra (Pilosa and Cingulata) assemblage from two localities of the Argentine Eastern Puna (Calahoyo and Casira) is described. The newly recorded taxa—Cingulata, Dasypodidae, Eutatini: Stenotatus sp. indet. and Eutatini indet., Euphractini: Macrochorobates scalabrinii (Moreno and Mercerat, 1891), and Tardigrada, Mylodontinae: cf. Simomylodon sp. indet. and Simomylodon cf. S. uccasamamensis Saint-André et al., 2010—and those already published from Calahoyo—Cingulata: Macrochorobates chapadmalensis (Ameghino, 1908), Eosclerocalyptus sp. indet., and Tardigrada, Megatheriidae: Pyramiodontherium bergi (Moreno and Mercerat, 1891)—suggest a middle–late Miocene age for the fossil-bearing levels. In Calahoyo, the presence of Stenotatus sp. indet., in addition to some rodents currently under study in the lower levels, suggest a closer similarity with the palaeofauna of Cerdas (southern Bolivia), probably involving the last part of the Miocene Climatic Optimum. The Xenarthra recorded in the middle and upper levels of Calahoyo and Casira suggest a late Miocene–Pliocene age. A comparative analysis between Calahoyo and Casira highlights the absence of Cingulata in the latter and a high diversity in the former. This situation probably indicates different paleoenvironmental conditions. Finally, we present the first certain record of the genus Simomylodon Saint-André et al., 2010 in Argentina, which includes the oldest record of dermal ossicles for sloths in South America.
... Saint-André (1994) is also the first work in which the term Mylodontini is used extensively. Even though Saint-André (1994) does not formally establish the group, it has been subsequently adopted by some authors (e.g., Pujos et al., 2016;Saint-André et al., 2010). ...
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Article
The phylogeny of mylodontid sloths has recently been the subject of multiple studies. Contrasting hypotheses have been proposed, especially for the relationships among late Miocene–Pleistocene mylodontines and lestodontines. In this paper, a new and detailed phylogenetic analysis is conducted, after adding new characters and taxa previously unexplored from a phylogenetic point of view. New features derived from postcranial skeletal anatomy are added to previous studies based on craniodental evidence. In this way, the current reappraisal represents the first exhaustive phylogenetic study on the Mylodontidae that incorporates features coded for the entire skeleton. When available, multiple specimens of each bony element are observed for each operational taxonomic unit, in order to take into account intraspecific variation. The taxonomic sample of this study considers Mylodontinae at the specific level. However, many other Mylodontidae are considered, and their phylogenetic relation- ships tested. The taxonomic sample of this study is enriched with new taxa from central and northern South America, with the aim of compensating for the knowledge bias in favour of austral mylodontids, which have historically been more ex- tensively studied than those from tropical latitudes. Special emphasis is given to the phylogenetic relationships of Mylodontinae, and particularly to the mylodontine and lestodontine sloths, that are recovered in the present study as monophyletic clades, and together form a larger monophyletic group. According to the present results, the Mylodontini–Lestodontini split occurred at the middle–late Miocene transition, giving rise to independent adaptive radiations across South and North America.
... Among Nothrotheriidae, Nothrotheriinae include several genera (e.g. Nothropus Burmeister 1882; Nothrotherium, Pronothrotherium Ameghino 1907;Nothrotheriops Hoffstetter 1954;Mionothropus, De Iuliis et al. 2011;Lakukullus Pujos et al. 2014, Aymaratherium Pujos et al. 2016) (see Burmeister 1882;Ameghino 1887Ameghino , 1891Ameghino , 1907Lydekker 1889;Hoffstetter 1954;De Iuliis et al. 2011;Pujos et al. 2014Pujos et al. , 2016, which are known from the middle Miocene of Bolivia and Argentina to the Pleistocene of South America, Central America and North America (Brandoni 2014;Brandoni and McDonald 2015;De Iuliis et al. 2015). Miocene taxa of this group are included in the genera Pronothrotherium, Mionothropus, and Lakukullus, and are primarily recorded from Argentina, Uruguay, Peru, and Bolivia. ...
Article
The specimen described herein and assigned to ‘Xyophorus’ sp. (Mammalia, Xenarthra, Tardigrada) was collected in the locality Cerro Zeballos, northwestern Chubut Province, Argentina. The fossiliferous sediments bearing the specimen are correlated with Collón Curá Formation. The specimen has the features described for other members of ‘Xyophorus’ (e.g. shape and size of the molariforms, relationship between diastema length, m1 and m2 length) and has a Diastema Length/Tooth Row Length index (DL/TRL index) of ca. 14, between that of ‘X.’ villarroeli (12.07) from the Mauri Formation, Bolivia (ca. 10.3 Ma) and that of ‘X.’ bondesioi (16.45) from Arroyo Chasicó Formation, Argentina (ca. 10–8.7 Ma). The relationship between DL/TRL index and age of the bearing sediments, would suggest a Tortonian age (late Miocene) for the deposits of Collón Curá Formation at Cerro Zeballos, which results in a ‘younger age’ compared to the middle Miocene age traditionally accepted for the Collón Curá Formation bearing the Colloncuran fauna sensu stricto. Although no absolute ages for Cerro Zeballos are available yet, the geographic proximity of Cerro Zeballos to Cushamen River (with levels dated at ca. 11.2 Ma) supports the tentative Tortonian age indicated by the presence of ‘Xyophorus’ sp.
... Among Nothrotheriidae, Nothrotheriinae include several genera (e.g. Nothropus Burmeister 1882; Nothrotherium, Pronothrotherium Ameghino 1907;Nothrotheriops Hoffstetter 1954;Mionothropus, De Iuliis et al. 2011;Lakukullus Pujos et al. 2014, Aymaratherium Pujos et al. 2016) (see Burmeister 1882;Ameghino 1887Ameghino , 1891Ameghino , 1907Lydekker 1889;Hoffstetter 1954;De Iuliis et al. 2011;Pujos et al. 2014Pujos et al. , 2016, which are known from the middle Miocene of Bolivia and Argentina to the Pleistocene of South America, Central America and North America (Brandoni 2014;Brandoni and McDonald 2015;De Iuliis et al. 2015). Miocene taxa of this group are included in the genera Pronothrotherium, Mionothropus, and Lakukullus, and are primarily recorded from Argentina, Uruguay, Peru, and Bolivia. ...
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The specimen described herein and assigned to ‘Xyophorus’ sp. (Mammalia, Xenarthra, Tardigrada) was collected in the locality Cerro Zeballos, northwestern Chubut Province, Argentina. The fossiliferous sediments bearing the specimen are correlated with Collón Curá Formation. The specimen has the features described for other members of ‘Xyophorus’ (e.g. shape and size of the molariforms, relationship between diastema length, m1 and m2 length) and has a Diastema Length/Tooth Row Length index (DL/TRL index) of ca. 14, between that of ‘X.’ villarroeli (12.07) from the Mauri Formation, Bolivia (ca. 10.3 Ma) and that of ‘X.’ bondesioi (16.45) from Arroyo Chasicó Formation, Argentina (ca. 10–8.7 Ma). The relationship between DL/TRL index and age of the bearing sediments, would suggest a Tortonian age (late Miocene) for the deposits of Collón Curá Formation at Cerro Zeballos, which results in a ‘younger age’ compared to the middle Miocene age traditionally accepted for the Collón Curá Formation bearing the Colloncuran fauna sensu stricto. Although no absolute ages for Cerro Zeballos are available yet, the geographic proximity of Cerro Zeballos to Cushamen River (with levels dated at ca. 11.2 Ma) supports the tentative Tortonian age indicated by the presence of ‘Xyophorus’ sp.
... En Nothropus tarijensis (Figura 5B) se observa que alrededor del alvéolo del caniniforme se encuentra un tejido denominado "lámina dura", constituida por hueso compacto de aspecto diferente al hueso esponjoso del resto de la mandíbula; presenta un color homogéneo y más claro que el hueso circundante, lo que indica el proceso de cierre del alvéolo. Cabe destacar que la ubicación de los alvéolos es aproximadamente la misma en los tres ejemplares y que coincide con la localización de los caniniformes en las especies de Nothrotheriinae que presentan este diente (ver Brandoni, 2014;Pujos et al., 2014Pujos et al., , 2016. Las radiografías realizadas en los especímenes tipo confirman que Nothropus tarijensis y Nothropus carcaranensis presentan un alvéolo del caninifome, el cual se está cerrando (Figuras 7B, C), mientras que en Nothropus priscus (Figura 7A) se encuentra abierto. ...
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The Nothrotheriinae Ameghino (Xenarthra, Pilosa) are recorded in Argentina from the middle Miocene to the late Pleistocene. In the Quaternary of the current territories of Argentina and Bolivia it is possible to recognize the existence of the genus Nothropus Burmeister, which includes three Pleistocene species, all of them known only by their type materials: (i) Nothropus priscus Burmeister (Santa Fe Province, Argentina); (ii) Nothropus carcaranensis Bordas (Santa Fe Province, Argentina); (iii) Nothropus tarijensis (Burmeister) (Tarija Valley, Bolivia). This study shows that N. priscus is a juvenile specimen and must be considered as a species inquirenda. In turn, the mandible morphology of N. tarijensis is very different compared to that of N. priscus and N. carcaranesis. N. tarijensis shows a very robust dentary and a greater angle between the occlusal plane and the anterior margin of the ascending ramus, suggesting that this is a valid species. In summary, the evidence suggests that the diversity of Nothropus is limited to two species: N. carcaranensis in Argentina and N. tarijensis in Bolivia.
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The external surfaces of the bones of the skull of the yellow armadillo Euphractus sexinctus are described in detail based on six museum specimens (five from the Carnegie Museum of Natural History, the last from the Field Museum). Soft-tissue structures (e.g., nerves, arteries, veins, and muscles) are reconstructed onto the skulls based on a serially sectioned, 105-mm crown rump length yellow armadillo fetus (also from the Field Museum). One osteological specimen, a juvenile, retains sutures in the basicranium fused in the adults, which confirm the presence of a compound auditory bulla (with entotympanic and ectotympanic elements), a long anterior process of the malleus forming the lateral border of the Glaserian fissure, and a well-developed postglenoid process of the squamosal forming the anterolateral surface of the tubular external acoustic meatus (the postglenoid process has been reported as absent in extant xenarthrans). To place the cranial osteology of the yellow armadillo in a phylogenetic context, the morphology of 58 soft-tissue conduits (e.g., grooves, canals, and foramina) are compared among E. sexcinctus, the dasypodid Dasypus kappleri, the bradypodid Bradypus variegatus, the myrmecophagids Tamandua tetradactyla and T. mexicana, the dog Canis familaris, the Eocene palaeanodont Metacheiromys sp. and M. simpsoni, the Oligocene leptictid Leptictis dakotensis, and the Late Cretaceous stem placental Zalambdalestes lechei. Of the conduits considered, 11 distinguish Euphractus sexcinctus; 16 distinguish Dasypodidae; 12 distinguish Pilosa; 5 distinguish Bradypus variegatus; 4 distinguish Tamandua; 13 distinguish Xenarthra; and 4 distinguish Xenarthra + Metacheiromys. Noteworthy results within the E. sexcinctus sample are plasticity in number, size, and position both between and within individuals in some nervous and vascular foramina (e.g., the foramina on the palate for the major, accessory, and palatine nerves and vessels, the foramina in the squamosal, parietal, and frontal for the rami temporales of the stapedial artery).
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The anatomy of the skeletal elements of the hind limb of Thalassocnus is described. This genus of “ground sloth” comprises five species represented by Neogene specimens from the coast of Peru and Chile, mostly found in the Pisco Formation. The hind limb of the genus Thalassocnus as a whole is characterized by a small iliac wing, a gracile femur with well-formed femoral neck, teardrop shaped patella, long and slender tibia, triangular tuber calcis, and proximal development of the lateral process of the Mt V. The comparison of the species of Thalassocnus with each other suggests a progressive shift to a particular ecology from the earliest to the latest species of the genus, a conclusion in agreement with those of the studies of craniomandibular, dental, and forelimb gross morphology, and bone internal microstructure. The pedolateral stance, which involves the bearing of the weight on the lateral side of the foot, was practiced by the earliest species of Thalassocnus, as was the case for other Megatheria. This stance was apparently forsaken by the late species of the genus in favor of the acquisition of a secondary plantigrady. A plantigrade hind limb may have been more efficient for paddling and for bottom-walking. Additionally, the late species of Thalassocnus differ from the early ones in the morphology of the pelvis and the slight overall reduction of the hind limb. This suggests the decrease of the support function of the hind limb of these species when compared to that of the early species of Thalassocnus.
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The analysis of recently recovered ground sloth remains from Minas Gerais, Bahia, and Piauí (Brazil) results in a major reinterpretation of the Scelidotheriinae and Megalonychidae of tropical Brazil. Among the specimens collected from Lagoa Santa (Minas Gerais) by Lund during the first half of the 19th century are skeletal elements attributable to two scelidotheriine species, as Lund himself determined and Winge corroborated. This is in contrast to the interpretation of these remains as representing a scelidotheriine and the postcranial remains of a peculiar megalonychid, as most authors, following Hoffstetter, have believed. A new combination is proposed here for one of the species. Another scelidotheriine recently described from Piauí by Guérin and Faure is a synonym of this species.
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Our understanding of South American megatheriine ground sloths was traditionally based largely on abundant material from Brazil and Argentina, mainly because megatheriine remains from elsewhere in South America were scant and poorly preserved. In recent years, however, the recovery and description of remains from northwestern South America has led to the recognition of several new taxa and the validation of species originally based on sparse remains. Falling in the latter group is Megatherium (Pseudomegatherium) tarijense Gervais and Ameghino, 1880, which is based on a complete but eroded calcaneum from Pleistocene deposits of the Tarija Valley of southern Bolivia. Most authors of the past century viewed this species as poorly defined and probably synonymous with Megatherium (Megatherium) americanum. This uncertainty is attributable to both the poor nature of the remains of M. (P.) tarijense and the presence of M. (M.) americanum in the Bolivian Pleistocene. Well preserved and nearly complete remains of several individuals indicate that M. (P.) tarijense is indeed valid. This material includes abundant remains from the Tarija Basin (Bolivia) housed in the Field Museum of Natural History (USA) and the Museo Nacional de Paleontología y Arqueología de Tarija (Bolivia) and also from the Peruvian Andes (Yantac), housed in the Universidad Nacional de Ingeniería de Lima. M. (P.) tarijense differs from M. (M.) americanum mainly in its smaller size; shorter, less robust premaxillae; shallower mandibular ramus; reduced size of the humeral deltopectoral crest; less torsion of the femoral diaphysis; unreduced patellar trochlea; and relatively shorter, stockier calcaneum.
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New well-preserved remains of the megalonychid sloth Eucholoeops Ameghino, 1887 recovered under strict stratigraphic control from late Early Miocene Santa Cruz Formation (c. 19 to 14 Ma; Santacrucian Age), together with analysis of older collections, consideration of intraspecific variation in extinct and extant sloths, and assessment of the validity of the early literature on Santacrucian sloths, permit revision of the status of the numerous species erected for this genus. The current contribution deals with the systematics of E. ingens Ameghino, 1887, but its methodology provides a basis for revision of other Eucholoeops species, as well as other sloth genera recovered from the Santa Cruz Formation. The failure to make progress on the systematics of the Santacrucian taxa since their first description is shown to be due mainly to a combination of the poor quality of many of the specimens, which are often fragmented and incomplete and from older collections, as well as inadequate stratigraphic and geographic control of their recovery, an overly rigid reliance on the early literature that accompanied their descriptions, and lack of consideration for intraspecific variation. A neotype is designated for E. ingens, as the original specimen is no longer available. The species E. latirostris Ameghino, 1891, E. externus Ameghino, 1891, and E. curtus Ameghino, 1894 are considered as junior synonyms of E. ingens.
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A new genus and species of Mylodontidae (Mammalia: Xenarthra) from the late Miocene of southern Uruguay, with comments on the systematics of the Mylodontinae', Journal of Vertebrate Paleontology, 30: 3, 899 — 910 To link to this Article:terms-and-conditions-of-access.pdf This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
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Thalassocnus is a genus of “ground sloths” known from Neogene deposits, for the great majority of specimens, of the Pisco Formation (Peru). Five species are recognized, their description being currently restricted, for the most part, to the skull, mandible, and dentition. The bones of the forelimb are here described, and compared among the species of Thalassocnus and to other pilosans. The main characteristics of the forelimb of Thalassocnus relative to other sloths are the shortness of the humerus and radius, and the specialized digits. Moreover, the late species of the genus are character- ized by the development of the pronator ridge of the radius, stoutness of the ulna, widening of the proximal carpal row, and shortening of the metacarpals. Analogies with extant tetrapods are proposed in order to infer plausible aquatic functions of the forelimb of Thalassocnus. In addition to paddling, it is argued that the forelimb of Thalassocnus was involved in bottom-walking, a function similarly found in extant sirenians. However, the function of the forelimb of Thalassocnus differs drastically from that of the latter, since it was likely involved in an activity related to obtaining food such as uprooting seagrass rhizomes.
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The nothrotheriine sloth from riverbank deposits of the Río Acre region of Peru in western Amazonia was originally assigned to Nothropus priscus Burmeister, 1882. Although relatively complete, with essentially the pes unknown, its description was accompanied only by limited information on its cranial remains. The remains of this sloth, actually of late Miocene age, were extensively prepared. Subsequent analysis indicates that its original assignment is incorrect and that it belongs to a new genus and species, which is distinguished from other nothrotheriines by the following (among other) features: notably domed braincase; depressed, narrow snout; lack of parietal/alisphenoid contact; ulna with prominently projecting anconeal process; distal position of femoral greater trochanter; medial articular condyle of femur butts against patellar trochlea. Phylogenetic analysis places the new genus and species as sister group to the (Pronothrotherium (Nothrotheriops + Nothrotherium)) clade.
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The fossil xenarthrans include giant forms, the ground sloths (Tardigrada), characteristic of the mammal fauna of the Pleistocene of South America. Although most authors agree in considering them as herbivorous, these forms have not been studied in terms of detailed morpho-functional analyses of their masticatory apparatuses. The aim of this work is the study the masticatory apparatus of the large Pleistocene ground sloths Glossotherium robustum, Lestodon armatus, Mylodon darwini and Scelidotherium leptocephalum (Mylodontidae) applying biomecanichal and morphogeometrical methods, and to compare with the information obtained for Megatherium americanum (Megatheriidae). The results are integrated with recent ecomorphological analyses that include three variables (hypsodonty index, dental occlusal surface area and relative width and shape of the muzzle) providing useful information for the inference of dietary habits and to propose a niche partitioning among these species. Glossotherium robustum and Lestodon armatus, the wide-muzzled sloths, were mostly bulk-feeders (i.e. ingest great amounts of food with each bite; probably grass and herbaceous plants). Mylodon darwini and Scelidotherium leptocephalum, the narrow-muzzled sloths, were mixed or selective-feeders (i.e. select plants or plant parts; grass and/or tree and shrubs foliage). The tooth design of mylodontids indicates that teeth were used mainly for crushing and grinding turgid and fibrous items respectively. Megatherium americanum was probably the most selective feeder among these sloths, and selectively fed on particular plants (shrubs) or plant parts (leaves, twigs, fruits). Its dentition was designed mostly for cutting soft but tough items which might include flesh, leaving open the possibility of an omnivorous diet.
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Non-pathological densification (osteosclerosis) and swelling (pachyostosis) of bones are the main modifications affecting the skeleton of land vertebrates (tetrapods) that returned to water. However, a precise temporal calibration of the acquisition of such adaptations is still wanting. Here, we assess the timing of such acquisition using the aquatic sloth Thalassocnus, from the Neogene of the Pisco Formation, Peru. This genus is represented by five species occurring in successive vertebrate-bearing horizons of distinct ages. It yields the most detailed data about the gradual acquisition of aquatic adaptations among tetrapods, in displaying increasing osteosclerosis and pachyostosis through time. Such modifications, reflecting a shift in the habitat from terrestrial to aquatic, occurred over a short geological time span (ca 4 Myr). Otherwise, the bones of terrestrial pilosans (sloths and anteaters) are much more compact than the mean mammalian condition, which suggests that the osteosclerosis of Thalassocnus may represent an exaptation.
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The origins of the distinct sloth lineages are not well documented. The Deseadan SALMA site of Salla-Luribay presents four Tardigrada and constitutes one of the two oldest sloth assemblages. The peculiar glypto-sloth Pseudoglyptodon sallaensis is found together with two orophodontoid taxa and a Megalonychidae. The study of the P. sallaensis remains confirms that is effectively a sloth with various glyptodontoid convergences easily explicable by a grazing way of life. The orophodontids exhibit a common dental pattern with Octodontotherium from Patagonia. Except for the presence of Pseudoglyptodon, the Salla-Luribay Xenarthra assemblage closely resembles the La Flecha site from Santa Cruz province, Argentina. The Megalonychidae are also recorded, which increases the paleogeographic distribution of this clade in the late Oligocene. The phylogenetic status of Pseudoglyptodon remains doubtful and additional material is necessary to resolve it. Two species of Peltephilinae, a Glyptodontidae closely related to Eocoleophorus and Glyptatelus complete the xenarthran assemblage. All of the Bolivian specimens are considerably smaller than the Patagonian forms, a difference that may reflect distinct diets, latitudinal cline, and environments during the end of the Oligocene. The existence of Pseudoglyptodon, four Orophodontidae and two Megalonychidae at the end of the Oligocene (25.65 and 29.4 Ma) in South America is surprising and implies an early diversification of sloths and a Cingulata/Tardigrada split, probably before the Casamayoran, which would be in agreement with predictions based on molecular evidence.
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Middle Miocene remains of giant megatheriine ground sloths (Tardigrada: Megatherioidea) are scarce and generally located in southern South America. The discovery of a well-preserved edentulous dentary of Megathericulus sp. from the Middle Miocene (Laventan South American Land Mammal Age - SALMA; 13.5–11.8 Ma) of the Amazonian Peru increases our knowledge of this genus, which had previously been recognized in Argentina. A preliminary revision of the earliest Megatheriinae allowed clustering the four middle Miocene species within the genus Megathericulus Ameghino: M. patagonicus Ameghino, M. primaevus Cabrera, M. andinum (Kraglievich), and M. cabrerai (Kraglievich). This small-sized genus is mainly characterized by a lateral depression that borders m1, a posterior external opening of the mandibular canal anterior to the base of the ascending ramus that opens anteriorly or anterodorsally, the base of the symphysis located anteriorly to the m1, important anteroposterior compression of the teeth, elongation of the region of the maxilla anterior to the M1, humerus elongated and gracile, patellar trochlea of femur contiguous with medial and lateral articular facets for tibia, strongly developed odontoid tuberosity, and astragalus with prominent odontoid process. The genus Eomegatherium Kraglievich is therefore restricted to the Huayquerian SALMA of Argentina and represented by a single species, E. nanum Burmeister. Megatheriinae constitute the first clade of Tardigrada in which the caniniform tooth has been secondarily modified into a molariform tooth. Three molariform patterns can be observed during megatheriine evolution in relation to tooth compression and loph or lophid orientation. Middle Miocene Megatheriinae occur only in the westernmost part of South America. These giant ground sloths might have dispersed latitudinally from Colombia/Patagonian Argentina before colonizing eastern areas of Andean South America (Bolivia, Venezuela, north, and east of Argentina) during the late Miocene and early Pliocene.
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Prepoplanops boleadorensis, a new genus and species of Planopinae (Xenarthra, Tardigrada), is described herein. The new taxon is based on a nearly complete specimen recovered from the Cerro Boleadoras Formation (Miocene, Rio Zeballos Group), in northwestern Santa Cruz Province, Argentina. The shape and length of the predentary region of the skull and the length of the diastema of Prepoplanops boleadorensis differ from those present in the species of Planops. The posterolateral opening of the mandibular canal and the position of the posterior margin of the mandibular symphysis differ from those of species of Prepotherium. In addition, Prepoplanops boleadorensis differs from Planops martini in the size of the humeral tuberosities, the development of the deltoid crest, the position of the distal margin of the humeral trochlea, the shape and position of the olecranon, the development of the femoral epicondyles, and the shape of the medial margins of the patellar trochlea and medial condyle. On the other hand, it differs from Prepotherium potens in the shape of the medial margin of the medial condyle. The recognition of Prepoplanops boleadorensis increases the diversity of Planopinae for the Miocene of Patagonia, Argentina.
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The occasion of the Xenarthra Symposium during the ICVM 9 meeting allowed us to reflect on the considerable advances in the knowledge of sloths made by the “X-community” over the past two decades, particularly in such aspects as locomotion, mastication, diet, dental terminology, intraspecific variation, sexual dimorphism, and phylogenetic relationships. These advancements have largely been made possible by the application of cladistic methodology (including DNA analyses) and the discovery of peculiar forms such as Diabolotherium, Thalassocnus, and Pseudoglyptodon in traditionally neglected areas such as the Chilean Andes and the Peruvian Pacific desert coast. Modern tree sloths exhibit an upside-down posture and suspensory locomotion, but the habits of fossil sloths are considerably more diverse and include locomotory modes such as inferred bipedality, quadrupedality, arboreality or semiarboreality, climbing, and an aquatic or semi-aquatic lifestyle in saltwater. Modern tree sloths are generalist browsers, but fossil sloths had browsing, grazing, or mixed feeding dietary habits. Discovery of two important sloth faunas in Brazil (Jacobina) and southern North America (Daytona Beach and Rancho La Brea) have permitted evaluation of the ontogenetic variation in Eremotherium laurillardi and the existence of possible sexual dimorphism in this sloth and in Paramylodon harlani. A new dental terminology applicable to a majority of clades has been developed, facilitating comparisons among taxa. An analysis wherein functional traits were plotted onto a phylogeny of sloths was used to determine patterns of evolutionary change across the clade. These analyses suggest that megatherioid sloths were primitively semiarboreal or possessed climbing adaptations, a feature retained in some members of the family Megalonychidae. Pedolateral stance in the hindfoot is shown to be convergently acquired in Mylodontidae and Megatheria (Nothrotheriidae + Megatheriidae), this feature serving as a synapomorphy of the latter clade. Digging adaptations can only be securely ascribed to scelidotheriine and mylodontine sloths, and the latter are also the only group of grazing sloths, the remainder being general browsers.
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A cladistic investigation of the phylogenetic relationships among 21 extinct and extant genera of sloths (Mammalia, Xenarthra, Tardigrada) was performed on the basis of characteristics of the bony anatomy of the auditory region. This study was undertaken in order to evaluate specific hypotheses of relationship within the group. Questions of particular interest include the relationship among the three traditional family groupings of extinct ground sloths and the monophyletic or diphyletic origin of the two genera of extant tree sloths. Eighty-five discrete morphological characters were analyzed using the computer program PAUP. Characters were polarized via comparisons to the following successive outgroups, all members of the supraordinal grouping Edentata: the Vermilingua, or anteaters; the Cingulata, or armadillos and glyptodonts; the Palaeanodonta; and the Pholidota, or pangolins. Three most parsimonious trees result (for 21 ingroup taxa and 5 outgroup taxa; Length = 304 steps, CI = 0.405, RI = 0.712). The results of this analysis provide characters which support the monophyly of the Xenarthra as a whole, as well as its dichotomous division into the clades Cingulata and Pilosa. In addition, it strongly corroborates the monophyly of the Vermilingua and the Tardigrada. Within tardigrades, the living three-toed sloth Bradypus is suggested to represent the sister-taxon to all other sloths. The results support the monophyly of the three traditional ground sloth families Megatheriidae, Megalonychidae, and Mylodontidae (the latter two quite strongly), and suggest that the subfamily Nothrotheriinae may represent a paraphyletic stem group for the remaining sloths. A novel relationship between the families Megalonychidae and Mylodontidae is proposed which contrasts with previous hypotheses of a close relationship between megalonychids and megatheriids. Lastly, this analysis strongly supports the derivation of the extant two-toed sloth Choloepus from within the West Indian megalonychid ground sloths, contradicting the traditional systematic grouping of the extant tree sloths into a monophyletic family Bradypodidae and suggesting that the living tree sloths represent one of the most remarkable examples of convergent evolution known among mammals.
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The characters used by Engelmann (1985) to unite various Nothrotheriinae and Megatheriinae in the Family Megatheriidae are assessed for their suitability as synapomorphies for such a clade. The elongate, slender premaxillae were cited as a synapomorphy of these subfamilies. A tendency for the posterior external opening of the mandibular canal to be located medially, together with the presence of an odontoid tibial articular process of the astragalus, were cited to unite Nothrotherium with megatheres. However, these characters are unsuited to resolving the relationships among Nothrotheriinae, Megatheriinae, and Planopsinae. The Y–shaped premaxillae of some Nothrotheriinae sensu lato may be a synapomorphy of the Plio-Pleistocene and some Santacrucian nothrotheres. These premaxillae are morphologically distinct from the V–shaped, triangular type seen in Eremotherium, which is probably the plesiomorphic condition in sloths, and the robust, quadrangular, and derived premaxillae of Megatherium. The posterior external opening of the mandibular canal may be located more medially in Nothrotheriops and some species of Hapalops, but not in Nothrotherium, Nothropus, and Pronothrotherium. The presence of an odontoid tibial articular process is not restricted to Nothrotheriinae sensu stricto and Megatheriinae. The greater separation of the astragalar ectal and tibial facets (probably a derived condition) occurs only in some mylodonts, and thus is not useful in distinguishing the significance of the presence of the process. Other evidence (size; form of the astragalus, auditory region, and tibial process for the long digital flexors) suggest a closer relationship between Megatheriinae and Planopsinae than some other combination of these subfamilies. The astragalar odontoid tibial process is usually considered to be independently derived in megatheriids and mylodontids, based on the interpretation of other morphological evidence. The alternate hypothesis, that the process is a synapomorphy of these families, is explored. Alternate polarities to those usually accepted for the dentition and dentary are suggested on the basis of Pseudoglyptodon.
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Specimens of Nothrotheriops shastensis (25 juvenile mandibles or fragments, and 22 maxillae and fragments) from San Josecito Cave, Mexico, Shelter Cave, Conkling Cavern, Rampart Cave, and the tar seeps at Rancho La Brea were examined to determine (1) the patterns of tooth growth, eruption, and wear, and (2) the pattern of anterior facial growth. Although they lack enamel, sloth teeth have a hard outer layer of dentine structurally distinct from the softer central dentine. These two dentine types were present in all specimens, including the smallest juvenile (length of mandibular tooth row = 20.3 mm). Some tooth wear was present in all specimens. The alveoli for the ever-growing teeth extend to the ventral border of the mandibular ramus in adult sloths but not in juveniles. Both maxillary and mandibular tooth roots in juvenile sloths are tilted and curved, but in adults are more vertical and straight. Changes in tooth size and orientation during growth result in size and shape changes in the occlusal surfaces. “Cusp-like” structures form exclusively through dental wear.Based upon comparison of feeding habits and social behavior in Recent tree sloths, it is suggested that N. shastensis was primarily solitary, except when females had dependent young. Young sloths may have had somewhat higher metabolic rates than adults; nevertheless, they probably needed additional protection from cold temperatures. Caves may have been refugia for many generations of females with small to medium sized offspring. Shelter-seeking behavior explains the large concentrations of undisturbed juvenile remains exclusively at these locations.