Scelidotheriinae (Mammalia, Pilosa) is a xenarthran mylodontid clade recorded in much of South America from the middle Miocene to the early Holocene. However, studies of their Neogene representatives are still scarce compared to their Quaternary representatives. The main goal of this contribution is to report new remains of Proscelidodon rothi (Ameghino, 1908) found in the basal levels of the El Polvorín Formation (Pliocene), cropping out near the city of Olavarría (Buenos Aires Province, Argentina). On the basis of the faunistic association, these deposits were assigned to the late Pliocene. The new materials include a skull, a mandible, and a calcaneal fragment (Xen-121), which enhance the characterization of this taxon, and allow more precise comparisons with other Neogene and Quaternary Scelidotheriinae. This new record extends the biochron and geographic distribution of the species, until now recorded in the latest Pliocene (Marplatan Stage/Age, Vorohuean Subage) of Olavarría.
Les Scelidotheriinae (Mammalia, Pilosa) représentent un clade de xénarthres mylodontidés enregistré dans une grande partie de l’Amérique du Sud depuis le Miocène moyen jusqu’à l’Holocène inférieur. Cependant, l’étude de leurs représentants dans le Néogène est encore rare par rapport aux représentants du Quaternaire. L’objectif principal de cette contribution est de fournir de nouveaux matériaux de Proscelidodon rothi (Ameghino, 1908) trouvés dans les niveaux basaux de la formation d’« El Polvorín » (Pliocène), qui affleurent près de la ville d’Olavarría (province de Buenos Aires, Argentine). Sur la base de l’association de leur faune, ces dépôts ont été assignés au Pliocène supérieur. Les nouveaux matériaux comprennent un crâne, une mandibule et un fragment calcanéen (Xen-121), ce qui a permis la caractérisation de ce taxon et d’effectuer des comparaisons plus précises avec d’autres Scelidotheriinae du Néogène et du Quaternaire. Ce nouvel enregistrement élargit la distribution biochronique et géographique de l’espèce, jusqu’à présent enregistrée dans le Pliocène supérieur (âge Marplatan, sous-âge Vorohuien) d’Olavarría.
Anatomy, Mylodontidae, Neogene, South America, Buenos AiresAnatomie, Mylodontidae, Néogène, Amérique du Sud, Buenos AirespresentedHandled by Lorenzon RookIntroduction
The South American endemic xenarthrans (Mammalia) include the clades Cingulata and Pilosa, the latter containing Vermilingua (anteaters) and Tardigrada (or Folivora or Phyllophaga, see Delsuc et al., 2001) (sloths). In turn, Tardigrada represents a well-diversified clade, especially in Neogene and Quaternary times (Vizcaíno and Bargo, 2014), in which five clades are recognized and traditionally considered as families: Megatheriidae, Nothrotheriidae, Megalonychidae, Mylodontidae, and Bradypodidae (see Gaudin, 2004). Among them, the family Mylodontidae includes the subfamilies Mylodontinae, Lestodontinae, and Scelidotheriinae (see Pitana et al., 2013 and Rinderknecht et al., 2010). Also, the subfamilies Urumacotheriinae, Nematheriinae, and Octomylodontinae are considered valid by some authors (see Rinderknecht et al., 2010), but their monophyly has never been demonstrated (Boscaini et al., 2018).
Scelidotheriinae are a group of extinct sloths restricted to South America, with a fossil record that ranges from the middle Miocene to the early Holocene of the current territories of Argentina, Bolivia, Brazil, Chile, Colombia, Ecuador, Peru, Paraguay, and Uruguay (Amson et al., 2016, McDonald and Perea, 2002 and Miño-Boilini et al., 2014a). During the Neogene (ca. 23.03–2.58 Ma) this subfamily achieved a considerable diversity, represented by four genera: NeonematheriumAmeghino, 1904, SibyllotheriumScillato-Yané and Carlini, 1998, Scelidotheridium Kraglievich, 1934, and ProscelidodonBordas, 1935 (see Brandoni et al., 2016). In this contribution, we follow Scillato-Yané (1977) in considering Nematherium as belonging to the subfamily Nematheriinae. According to Gaudin (2004)Nematherium is the sister taxon of the remaining mylodontids.
Despite their high Neogene diversification, Scelidotheriinae are mainly known from the Quaternary species Scelidotherium leptocephalumOwen, 1839, Catonyx cuvieri (Lund, 1839), Valgipes bucklandi (Lund, 1839), and Catonyx tarijensis (Gervais and Ameghino, 1880) (see Cartelle et al., 2009, Corona et al., 2013, McDonald and Perea, 2002 and Miño-Boilini, 2016, Lund, 1839, Miño-Boilini et al., 2019 and Owen, 1839), which are well characterized.
In contrast, knowledge of the Neogene taxa is still scarce because most of the species lack an adequate morphological characterization (paucity of remains) and several taxonomic aspects of the clade are in need of urgent revision under modern criteria (Miño-Boilini et al., 2011, Miño-Boilini et al., 2014a, Miño-Boilini et al., 2019, Ortega, 1967 and Taglioretti et al., 2014).
Among the more diversified genera is Proscelidodon, which according to the most recent revisions include P. patrius (Ameghino, 1888) from the Huayquerian Age/Stage to Chapadmalalan Age/Stage (late Miocene-late Pliocene, see Miño-Boilini et al., 2014b, Pujos et al., 2012 and Taglioretti et al., 2014); P. almagroi (Rovereto, 1914) and P. gracillimus (Rovereto, 1914) from the Huayquerian Age/Stage (late Miocene, see McDonald and Perea, 2002, Miño-Boilini et al., 2011 and Miño-Boilini et al., 2014b); and P. rothi (Ameghino, 1908) from the Chapadmalalan Age/Stage (late Pliocene, see McDonald and Perea, 2002, Miño-Boilini et al., 2014b and Taglioretti et al., 2014, see also type locality in this work).
Proscelidodon rothi was recognized by Ameghino (1908) based on a partial skull (MLP 3-762) lacking precise stratigraphic and geographic provenance; he noted only that the type material comes from “Barranca de Los Lobos”, Buenos Aires Province, Argentina. Some years later, Kraglievich (1923) improved the anatomical description of the type specimen, but without including any additional data about its provenance. Although P. rothi is commonly mentioned in the scientific literature (e.g. Miño-Boilini et al., 2011 and Pujos et al., 2012), this species was known only by its type material, and the postcranial anatomy remains unknown. Another skull (MMP 1001-M) was described and figured by McDonald (1987), but unfortunately the material is currently lost.
The recent discovery of a very complete and well-preserved specimen of P. rothi in the El Polvorín Formation (Pliocene) near Olavarría, Buenos Aires Province, allows us to describe and greatly improve the morphological characterization of this species. In addition, some comments on the geographic provenance and stratigraphic distribution of P. rothi are included in this report.
Material and methods
The material is housed at the Collection “Cementos Avellaneda”, Olavarría (Buenos Aires Province, Argentina), under the acronym Xen-121. Comparisons were carried out with several Mylodontidae Scelidotheriinae from the early and late Neogene of Argentina: Sibyllotherium guenguelianumScillato-Yané and Carlini, 1998 (MLP 90-XII-31-5, partial skull, partial dentary, holotype, Pedregoso Formation, middle-late Miocene, Santa Cruz Province); Neonematherium flabellatumAmeghino, 1904 (MACN A 11628, anterior portion of the skull, holotype, Río Mayo Formation, middle–late Miocene, Chubut Province); Proscelidodon patrius (MACN A 223 and MACN A 224, skull and mandible, respectively, holotype, Monte Hermoso Formation, late Miocene–early Pliocene, Buenos Aires Province); P. gracillimus (MACN PV 8470, skull and mandible deformed by lithostatic pressure, holotype, Las Huayquerías de San Carlos, Departamento de San Carlos, Huayquerías Formation, late Miocene, Mendoza Province); P. almagroi (MACN PV 2544, maxillae, holotype; Andalhuala, “Araucanian”, late Miocene–early Pliocene, Catamarca Province); P. rothi (MLP 3-762, skull, holotype; Barranca de Los Lobos, Chapadmalal Formation, late Pliocene Buenos Aires Province), and Scelidotheridium parodiiKraglievich, 1923 (MACN PV 5108, partial skull, holotype; Miramar, Chapadmalal Formation, late Pliocene, Buenos Aires Province).
Comparisons were also performed with the following Quaternary species of Scelidotheriinae from Argentina, Brazil, Peru, and Ecuador: Scelidotherium bravardi and Scelidotherium leptocephalum (see Miño-Boilini et al., 2014a); Catonyx cuvieri (see Cartelle et al., 2009 and Corona et al., 2013); C. tarijensis (see McDonald and Perea, 2002 and Miño-Boilini, 2016, Miño-Boilini et al., 2019); C. chiliensis (see Miño-Boilini, 2012, Miño-Boilini, 2016, Pujos, 2000 and Pujos and Salas, 2004), and Valgipes bucklandi (see Cartelle et al., 2009).
The chronological and biostratigraphic schemes used in this contribution follow Cione and Tonni, 1995 and Cione and Tonni, 2005 and Cione et al. (2015). Stratigraphic schemes follow De los Reyes et al. (2013) for the El Polvorín Formation. All the values included in the tables are expressed in millimeters, with an error range of 0.5 mm. The description and terminology partially follow Cartelle et al. (2009), Corona et al. (2013), McDonald and Perea (2002), Miño-Boilini, 2012 and Miño-Boilini, 2016, Miño-Boilini et al., 2011, Miño-Boilini et al., 2014a, Miño-Boilini et al., 2014b and Miño-Boilini et al., 2019. Linear measurements are available in Table 1.
Institutional abbreviations
BM(NH), Natural History Museum, London, UK; MACN A, Colección Ameghino, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Buenos Aires, Argentina; MACN PV, Colección Paleovertebrados, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Buenos Aires, Argentina; MNHN-Bol-V; Departamento de Paleontología de Vertebrados, Museo Nacional de Historia Natural de Bolivia, La Paz, Bolivia; MCL, Museu de Ciências Naturais da Pontifícia Universidade Católica de Minas Gerais, Belo Horizonte, Brazil; MMP, Museo Municipal de Ciencias Naturales “Lorenzo Scaglia,” Mar del Plata, Argentina; MLP, Museo de La Plata, La Plata, Argentina; ROM, Royal Ontario Museum, Canada; ZMUC, Zoological Museum, University of Copenhagen, Denmark.
HS, maximum height of snout; L, greatest proximal-distal length; LDS, length of dental series; LMC, length maxillary-condyle; MBH, maximum height of mandibular body; MWP, minimum postorbital width; OMf5L, distance between the posterior margin of occipital condyles and the distal margin of molariform 5; WB, external bicondylar width; WS, maximal width of snout.
Geographic and stratigraphic context
Xen-121 in this study was recovered near the city of Olavarría (Fig. 1), in the “Alicia” quarry (S37° 01.281’ W60° 19.974’), owned by Cementos Avellaneda S.A., from the Neogene El Polvorín Formation (Poiré et al., 2005). This geological unit is part of the Neogene sedimentary cover, which lies unconformably over limestones of the Neoproterozoic Sierras Bayas Group (Poiré and Gaucher, 2009) and La Providencia Group (Arrouy et al., 2015), in the “Tandilia System” mountain range (Nágera, 1940) of Buenos Aires province. The specimen of Proscelidodon rothi was collected from the lower section of the El Polvorín Fm. (Fig. 2), from the “Facies Calera” defined by De los Reyes et al. (2013), in a 1.70 m thick level composed of pink-brownish (dry 7.5YR7/4, humid 7.5YR4/4), well-sorted, silty-sandstone, which exhibits tangential cross-bedding, and some mud rip-up clasts and quartz, fine-grained pebbles (up to 2 cm in diameter). A diffuse lamination is observed to the top. All the levels show a slightly pedogenetic prismatic fabric and a strong vertical carbonate bioturbation, which cross this bed.
The following taxa were also recovered from the same stratigraphic level: Microtragulus reigi (Metatheria, Argyrolagidae), Paedotherium (Mammalia, Notoungulata), cf. Ringueletia (Mammalia, Dasypodidae), and Toxodontidae indet. This palaeofaunistic association suggests a Pliocene age (De Los Reyes et al., 2013) an interpretation reinforced by the recent discovery of the rodent Akodon (Abrothrix)magnus (Mammalia, Cricetidae), which has a biochron limited to the Vorohuean Age (late Pliocene) (Cione and Tonni, 2005 and Teta, 2013).
Systematic paleontology
Xenarthra Cope, 1889
Tardigrada Latham and Davies in Forster, 1795
Mylodontidae Gill, 1872
Scelidotheriinae Ameghino, 1904
ProscelidodonBordas, 1935
Proscelidodon rothi (Ameghino, 1908)
Figure 3 and Figure 4
Type species. Proscelidodon patrius (Ameghino, 1888) (see Mones, 1986)
Holotype. MLP 3-762, partial skull (Fig. 3A and B).
Type locality. Barranca de Los Lobos, Buenos Aires Province, Argentina (see Ameghino, 1908 and Kraglievich, 1923). The type specimen was collected by Santiago Roth (see Kraglievich, 1923) in the 19th century (see Kraglievich, 1923), the geology of the Chapadmalal cliffs had not been established; hence, the exact stratigraphic provenance of the type specimen cannot be determined. According to Cione and Tonni (2005, 2015) the biostratigraphic base of the upper Chapadmalalan Stage, the lower part of the Marplatan Stage (Barrancalobian Subage), Middle Marplatan (Vorohuean Subage), and Upper Marplatan (Sanandresian Subage) were defined for the area including Barranca de Los Lobos. In consequence, with the available evidence it is not possible to know the exact stratigraphic provenance of the type specimen.
Revised Diagnosis (revised from McDonald, 1987 and Miño-Boilini, 2012). It is the largest species of the genus (larger than P. patrius, P. almagroi, and P. gracillimus); robust skull with a sub-rectangular contour in dorsal view and lateral view, as in Catonyx spp.; the skull is narrow, whereas nasals and maxillaries are short, as in P. patrius, P. gracillimus, Si. guenguelianum, Catonyx spp., and V. bucklandi; the suture that separates the squamosal from the parietal and frontal bones is horizontal, and unlike P. patrius in which it is sub-horizontal; the Mf1 is anteroposteriorly elongated, with a lingual lobe at the middle of the tooth, the lingual lobe is well developed as in P. almagroi and Sce. parodii; mandible robust with mandibular spout wider and more sub-quadrangular than in P. patrius; the ventral edge of the mandibular body is convex as in Si. guenguelianum and C. tarijensis; the calcaneus is massive and robust, unlike that of P. patrius which is gracile.
Referred material. Xen-121 includes skull with complete right dental series, and left Mf4–Mf5, partial mandible, atlas, and partial left calcaneus (Fig. 4A–G).
Descriptions and comparisons
<italic>Skull</italic> (<xref rid="fig0020" ref-type="fig">Fig. 4</xref> A–C)
The skull Xen-121 is almost complete and well preserved; sutures can be seen between the nasal and maxilla, and between the squamosal and the parietal and frontal, suggesting it is an adult specimen.
The skull Xen-121 is narrow and elongated, with a length of 360 mm (from the posterior end of the occipital condyles to the anterior end of the premaxillae); nasals and maxillae are short, as typical in Si. guenguelianum (MLP 90-XII-31-5, holotype), P. patrius (MACN A 223, holotype), the holotype of P. rothi (MLP 3-762), P. gracillimus (MACN PV 8470, holotype), Catonyx spp. (see, Corona et al., 2013, Miño-Boilini, 2016 and Miño-Boilini et al., 2019), and V. bucklandi (Cartelle et al., 2009); this is different compared to Sc. bravardi and Sc. leptocephalum, in which the muzzle is relatively more elongated (see Miño-Boilini et al., 2014a).
In anterior view, the nasal opening is roughly circular in cross section, as in N. flabellatum (MACN A 11628), Si. guenguelianum (MLP 90-XII-31-5), P. patrius (MACN A 223), P. rothi (MLP 3-762), Catonyx spp. (Miño-Boilini, 2016 and Miño-Boilini et al., 2019). In contrast, in Sc. bravardi, Sc. leptocephalum, and V. bucklandi, the nasal opening has a sub-triangular contour (see, Miño-Boilini, 2012, Miño-Boilini, 2016, Miño-Boilini et al., 2014a and Taglioretti et al., 2014). The anterior process of the nasals extends beyond the anterior margin of the maxillae, and in lateral view it appears ventrally directed, as in P. patrius (MACN A 223) and P. rothi (MLP 3-762) (Taglioretti et al., 2014), and Catonyx spp.
In lateral view (Fig. 4 A), the skull is sub-rectangular, as in Si. guenguelianum (MLP 90-XII-31-5), P. patrius (MACN A 223), P. gracillimus (MACN PV 8470), P. rothi (MLP 3-672, Fig. 3 A), Sce. parodii (MACN PV 5108), Catonyx spp. (see Miño-Boilini, 2016), Scelidotherium spp. (Miño-Boilini et al., 2014a), and V. bucklandi (Cartelle et al., 2009). The dorsal margin of the skull is horizontal, with a slight depression at the level of the parietals, in lateral view. Posteriorly, the height of the posterior region of the skull is slightly sub-equal to the height of the rostrum, as in P. patrius (MACN A 223), P. rothi (MLP 3-762). The pterygoids are inflated at their base (Fig. 4 A and C). The pterygoid lamina is projected ventrally. The maxilla and the zygomatic process are preserved, but the dorsal and ventral processes of the zygomatic are lacking. The lacrimal foramen is just dorsal to the Mf3, as in N. flabellatum (MACN A 11628), Si. guenguelianum (MLP 90-XII-31-5), P. patrius (MACN A 223), P. rothi (MLP 3-672), P. gracillimus (MACN PV 8470), P. almagroi (MACN PV 2544), C. cuvieri (Cartelle et al., 2009, fig. 1B), C. tarijensis (Miño-Boilini, 2016, fig. 1C), C. chiliensis (Miño-Boilini et al., 2009, fig. 1B), V. bucklandi (Cartelle et al., 2009, fig. 1A), Sc. bravardi (Miño-Boilini et al., 2014a, fig. 2B), Sc. leptocephalum (Miño-Boilini, 2012, plate 17), and is slightly above the squamosal. In Sce. parodii the lacrimal foramen is unknown. The suture separating the squamosal from the parietal and frontal is horizontal in P. rothi, but in P. patrius it is sub-horizontal (see Taglioretti et al., 2014). In Xen-121 the premaxilla is preserved, and it projects anterodorsally. The contour of the palate is slightly convex as in N. flabellatum, P. rothi and C. tarijensis (Miño-Boilini, 2016, fig. 1C) in lateral view.
In dorsal view (Fig. 4 B), the skull is sub-rectangular, as in the holotype of P. rothi (MLP 3-762), C. tarijensis, and C. chiliensis (Miño-Boilini, 2016); however, P. patrius, P. gracillimus, Scelidotherium spp., and V. bucklandi display a sub-triangular contour (see, Miño-Boilini, 2016 and Taglioretti et al., 2014). The anterior region of the skull is wider than the preorbital constriction. Xen-121 has well developed temporal crests, each extending from the postorbital process of the frontal toward the parietal, narrowing at the level of the frontal-parietal suture. This condition is similar to that observed in Catonyx spp. (see Miño-Boilini and Zurita, 2015). The preorbital constriction is slightly wider than the postorbital one. The lateral process of the nasal reaches the anterior margin of the maxilla as in Si. guenguelianum (MLP 90-XII-31-5), P. patrius (MACN A 223), P. rothi (MLP 3-762), P. gracillimus (MACN PV 8470), C. cuvieri (Cartelle et al., 2009, fig. 1D), C. chiliensis (Miño-Boilini et al., 2009, fig. 2C), C. tarijensis (Miño-Boilini, 2016, fig. 1B) and V. bucklandi (Cartelle et al., 2009, fig. 1C). However, in Scelidotherium spp. the lateral process extends more anteriorly than the anterior margin of the maxilla (see McDonald, 1987, Miño-Boilini, 2012 and Taglioretti et al., 2014). The frontonasal suture is V-shaped, as in N. flabellatum (MACN A 11628), Si. guenguelianum (MLP 90-XII-31-5), P. patrius (MACN A 223), C. cuvieri (MCL 4265), C. tarijensis (Miño-Boilini, 2016, fig. 1b), C. chiliensis (BM(NH) M 2819, holotype), Sc. bravardi (BM(NH) M 37626, holotype), and Sc. leptocephalum (MLP 3-402), but unlike P. gracillimus (MACN PV 8470) and V. bucklandi (MCL 4234) in which the nasal-frontal suture is almost straight.
In ventral view (Fig. 4C), the premaxillae are less developed, as in P. rothi (McDonald, 1987, Fig. 60C), C. tarijensis (Miño-Boilini, 2016, fig. 1d), C. cuvieri (ZMUC 831), C. chiliensis (Pujos and Salas, 2004, Fig. 3 B) whereas in Scelidotherium spp. the premaxillae are well developed (see McDonald, 1987, Miño-Boilini, 2016 and Miño-Boilini et al., 2014a). It has a palatal furrow parallel to the dental series, as in Sce. parodii (MACN 5108), P. patrius (MACN A 223) and most of Quaternary scelidotheriines species (e.g., Catonyx spp., V. bucklandi); but it is absent in N. flabellatum (MACN A 11628), Si. guenguelianum (MLP 90-XII-31-5), and Scelidotherium spp. The lateral ramus of the premaxilla is more developed than the medial ramus, as in the Quaternary scelidotheriines. The dental series are parallel to each other as in P. gracillimus, P. patrius, P. rothi (see, Taglioretti et al., 2014), and Sce. parodii (MACN 5108); but they diverge anteriorly in N. flabellatum (MACN A 11628) and Si. guenguelianum (MLP 90-XII-31-5), and posteriorly in V. bucklandi (MCL 4262). The maxillary-palatine suture is at the level of Mf4, as in P. patrius (MACN A 223), P. gracillimus (MACN PV 8470), P. rothi (see McDonald, 1987, fig. 60C; Taglioretti et al., 2014), P. almagroi (MACN PV 2544, holotype), and C. tarijensis (Miño-Boilini, 2016, fig. 1d), however, in S. leptocephalum the maxillary-palatine suture is at the level of Mf3 (Miño-Boilini et al., 2014, fig. 1C). The base of the jugal is at the level of Mf3 as in P. patrius (MACN A 223), P. rothi (MLP 3-762), P. gracillimus (MACN PV 8470), C. tarijensis (Miño-Boilini, 2016, fig. 1D), C. cuvieri (ZMUC 831), C. chiliensis (BM(NH) M 2819), S. bravardi (BM(NH) M 37626), S. leptocephalum (BM(NH) M 16579) and V. bucklandi (MCL 4262), Sce. parodii (MACN PV 5108, holotype), N. flabellatum (MACN A 11628), and Sibyllotherium guenguelianum (MLP 90-XII-31-5, holotype). In ventral view, the basicranium is difficult to observe, owing to complete fusion of the sutures (Fig. 4C).
In occlusal view (Fig. 4C), the Mf1 is anteroposteriorly elongated, and the mesial lobe more developed than the distal one; besides, the lingual lobe is at the middle of the tooth, well developed as in P. rothi (MLP 3-762), P. almagroi (MACN PV 2544), and Sce. parodii (MACN PV 5108); P. patrius instead has no lingual lobe (Taglioretti et al., 2014). The Mf1 of Si. guenguelianum (MLP 90 XII-31-5) is reniform in section. Such a lingual lobe is also present in C. tarijensis, C. cuvieri, S. bravardi and V. bukclandi (see Miño-Boilini, 2016).
The Mf2 (Fig. 4C), is trilobate and is triangular in section as in N. flabellatum (MACN A 11628), Si. guenguelianum (MLP 90-XII-31-5), P. rothi (MLP 3-762), P. almagroi (MACN PV 2544), V. bucklandi (MCL 4262), and C. cuvieri (ZMUC 831). By contrast, in Sce. parodii (MACN PV 5108) and P. patrius (MACN A 223) the Mf2 is sub-triangular in section. Mf2 has an apicobasal sulcus on the lingual side that defines two lobes, the posterior being more developed than the anterior one, as in N. flabellatum, Si. guenguelianum, Sce. parodii, P. rothi, P. almagroi and P. patrius. The Mf2 of Xen-121 (Fig. 4C) is different from the Mf2 in the holotype of P. rothi (Fig. 3): it is more trilobate, probably reflecting intraspecific variation.
The Mf3 (Fig. 4C) is trilobate and has an apicobasal sulcus on the lingual side; it is triangular in section as in N. flabellatum (MACN A 11628), Si. guenguelianum (MLP 90-XII-31-5), P. rothi (MLP 3-762) and V. bucklandi (MCL 4262). In P. patrius (MACN A 223), P. almagroi (MACN PV 2544), and Sce. parodii (MACN PV 5108), it is sub-elliptical in section.
The Mf4 (Fig. 4C) also has an apicobasal sulcus on the lingual side and is sub-triangular in section as in P. rothi (MLP 3-672), N. flabellatum (MACN A 11628) and V. bucklandi (MCL 4262). The Mf4 in Sce. parodii (MACN PV 5108), P. patrius (MACN A 223) and P. almagroi (MACN PV 2544) is sub-elliptical in section.
The Mf5 (Fig. 4C) has an apicobasal sulcus on the labial side and another one on the lingual side as in P. rothi (MLP 3-762), P. patrius (MACN A 223), Catonyx spp. (Miño-Boilini, 2016), Scelidotherium spp. (Miño-Boilini et al., 2014a), Sce. parodii (MACN PV 5108) and V. bucklandi (MCL 4262).
<italic>Mandible</italic> (<xref rid="fig0020" ref-type="fig">Fig. 4</xref>D and E)
The mandible is incomplete; only the right mf1 and mf2 are preserved. The right mandibular condyle, part of the right coronoid process, the left coronoid process, left mandibular condyle, left angular process, and anterior portion of the mandible are missing.
In lateral view (Fig. 4D), the mandibular condyle is completely above the occlusal surface of the molariforms as in P. patrius, and P. gracillimus, and most of the Quaternary Scelidotheriinae (e.g.: Catonyx spp. V. bucklandi, and S. bravardi) (Miño-Boilini, 2012). Instead, in S. leptocephalum it is slightly above the occlusal plane (Miño-Boilini et al., 2014a). The ventral edge of the mandibular body is markedly convex, as in Si. guenguelianum (MLP 90-XII-31-5) and C. tarijensis (Miño-Boilini, 2016). In contrast, in P. patrius, P. gracillimus, Scelidotherium spp. C. cuvieri, C. chiliensis, and V. bucklandi, the ventral margin is straight or slightly convex (see Cartelle et al., 2009 and Miño-Boilini, 2016; Miño-Boilini, 2014a).
The postero-lateral opening of the mandibular canal is below the alveolar margin of the mf4, as in Si. guenguelianum (MLP 90-XII-31-5),P. patrius (MACN A 224), P. gracillimus (MACN PV 8470), C. tarijensisMiño-Boilini, 2016, fig. 1e), C. cuvieri (MCL 22685), C. chiliensis (ROM 4571), S. leptocephalum (BM(NH) M 16579) and V. bucklandi (MCL 22427).
In occlusal view (Fig. 4E), the mandibular spout is wide and sub-quadrangular, unlike P. patrius (MACN A 224) in which it is narrow and sub-rectangular. In occlusal view, the mf1 is shorter buccolingually than mesiodistally, as in Si. guenguelianum (MLP 90-XII-31-5), P. patrius (MACN A 224), Catonyx spp. (Miño-Boilini, 2016), Scelidotherium spp. Miño-Boilini et al., 2014a), V. bucklandi (MCL 22427). On the labial side it has a lobe as in Si. guenguelianum, C. tarijensis, C. chiliensis, and P. patrius (see Miño-Boilini et al., 2014b).
The alveoli of mf2 and mf3 are sub-quadrangular (Fig. 4E), with the maximum dimension oblique to the sagittal plane, as in Si. guenguelianum (MLP 90-XII-31-5), Catonyx spp. (Miño-Boilini, 2016), Scelidotherium spp. (Miño-Boilini et al., 2014a), V. bucklandi (MCL 22427), and P. patrius (Miño-Boilini et al., 2014b).
The alveolus of mf4 (Fig. 4E) is trilobate as in Si. guenguelianum, Catonyx spp., Scelidotherium spp., V. bucklandi, and P. patrius, and P. gracillimus, and wider mesiodistally than the rest of the molariforms (Corona et al., 2013 and Miño-Boilini et al., 2014b). The anterior lobe is more extended mesiodistally than the other two lobes, as in Si. guenguelianum (MLP 90-XII-31-5), Catonyx spp. (Miño-Boilini, 2016), Scelidotherium spp. (Miño-Boilini et al., 2014a), V. bucklandi (MCL 22427), P. patrius, and P. gracillimus (Taglioretti et al., 2014). The central lobe is slightly developed and in an oblique position with respect to the sagittal plane, as in Si. guenguelianum (MLP 90-XII-31-5), Catonyx spp. (Miño-Boilini, 2016), Scelidotherium spp. (Miño-Boilini et al., 2014a), V. bucklandi (MCL 22427), P. patrius (MACN A 224) and P. gracillimus (MACN PV 8470). The posterior lobe is more curved toward the lingual side, a characteristic present in Si. guenguelianum, Catonyx spp. and S. bravardi. In contrast, in S. leptocephalum and V. bucklandi, the lobe is straighter (Miño-Boilini, 2012 and Miño-Boilini et al., 2014a).
<italic>Calcaneum</italic> (<xref rid="fig0020" ref-type="fig">Fig. 4</xref>F and G)
The calcaneum of Xen-121 is more robust and massive than that of P. patrius (MACN PV 8000, see Miño-Boilini et al., 2011). The diaphysis is narrow as in P. patrius (MACN PV 8000), S. leptocephalum (MACN PV 5001), S. bravardi (MACN PV 1921), C. tarijensis (MACN PV 5110), and C. cuvieri (MCL 4265).
In anterior view (Fig. 4G), it has three facets: (1) the calcaneal ectal facet, (2) the calcaneal sustentacular facet, and (3) the facet for the cuboid. The ectal and sustentacular facets articulate with the astragalus and are separated by non-articular bone (sulcus calcanei) as in P. patrius (MACN PV 8000) and the Quaternary scelidotheriines [e.g.: C. tarijensis (MACN PV 5110), C. cuvieri (MCL 4265), V. bucklandi (MCL 4264), S. leptocephalum (Miño-Boilini et al., 2014a, fig. 3M), and S. bravardi (Miño-Boilini et al., 2014a, fig. 3N)]. However, the sustentacular facet is continuous with the facet for the articulation with the cuboid, as is typical of P. patrius and most Quaternary scelidotheriines (Miño-Boilini et al., 2011). On the contrary, there is no continuity between the sustentacular and cuboid facets in S. leptocephalum (Miño-Boilini et al., 2014b).
Discussion and conclusions
Xen-121 is assigned to P. rothi on the basis of the following morphological characteristics: 1) this is the largest species of the genus Proscelidodon; 2) the suture separating the squamosal from the parietal and frontal bones is horizontal; 3) the Mf1 is anteroposteriorly elongated, with a well-developed lingual lobe at the middle of the tooth; 4) the mandible is robust and the mandibular spout is wide and sub-quadrangular; 5) the ventral edge of the mandibular body is markedly convex; and 6) the calcaneum is massive and robust.
During the middle Miocene-Pliocene, the Scelidotheriinae were highly diverse; in the current territory of Argentina the clade is represented by Si. guenguelianum, N. flabellatum, P. patrius, P. gracillimus, P. almagroi, P. rothi and Sce. parodii. However, during the Quaternary, the diversity of this clade decreased, and only two genera are recorded in Argentina: Scelidotherium (S. bravardi and S. leptocephalum), and Catonyx (C. tarijensis and C. chiliensis). The scelidotheriines were restricted to South America, with a wide latitudinal range from the middle Miocene to the Quaternary of Argentina, Ecuador, Peru, Colombia, Bolivia, Chile, Uruguay and Brazil (McDonald and Perea, 2002, Miño-Boilini, 2012, Miño-Boilini et al., 2011 and Miño-Boilini et al., 2019). To date, only two Scelidotheriinae taxa have not been recorded in Argentina: C. cuvieri is recorded in the late Pleistocene of Uruguay and Brazil, and V. bucklandi in the late Pleistocene of Brazil (Corona et al., 2013 and Miño-Boilini, 2012). In sum, the highest specific diversity of the clade is recorded in sediments from the late Neogene and Quaternary of South America, more precisely in Argentina.
Taglioretti et al. (2014) reported for the Chapadmalal Formation (late Pliocene) of Mar del Plata (Buenos Aires, Argentina) three Scelidotheriinae species included in the genera Proscelidodon (P. patrius, MMP 4849, and P. rothi (MLP 3-762, holotype) and Scelidotherium (Sce. parodii, MACN PV 5108, holotype). The Chapadmalal Formation has a wide temporal range (4.5–3.2 Ma, Zárate 2005) and the material referred to P. patrius (MMP 4849) was found in its lower levels (APSC, Alocapa 2, see Taglioretti et al., 2014). No stratigraphic data are available for the holotypes of Sce. parodii (MACN PV 5108), and P. rothi (MLP 3-762). Consequently, although these three species may have not been contemporaneous they demonstrate the diversification among Scelidotheriinae in the late Neogene of southern South America.
P. rothi has been reported by several authors (McDonald and Perea, 2002, Miño-Boilini et al., 2011, Miño-Boilini et al., 2014b and Pujos et al., 2012) as from the Chapadmalalan State/Age (Pliocene) in the area of Barranca de Los Lobos (General Pueyrredón County, Buenos Aires Province). However, the level from which the holotype of P. rothi (MLP 3-762) was recovered cannot be determined precisely given that the stratotypes of two stages/ages, the upper Chapadmalalan and Marplatan (Cione and Tonni, 2005 and Cione et al., 2015), were defined in the cliffs known as Barranca de Los Lobos. According to McDonald (1987), the specimen assigned to P. rothi (MMP 1001-M) was collected in the Barranca de Los Lobos Formation, near Barranca de Los Lobos (Buenos Aires Province, Argentina). However, Islas et al. (2015) stated that a large part of the materials reported as from the Barranca de Los Lobos Formation was found in fallen blocks, and therefore the fossil remains have no stratigraphic control. Hence, also the stratigraphic level of the lost specimen MMP 1001-M cannot be determined with certainty.
The new specimen (Xen-121) assigned to P. rothi represents the third record of this taxon and the first one with precise stratigraphic data. This new record widens the biochron and geographic distribution of the species, which is recorded in the latest Pliocene, Marplatan Stage/Age, Vorohuean subage, of Olavarría (Buenos Aires Province, Argentina). From a paleobiogeographic point of view, P. rothi is endemic to the Neogene of Buenos Aires Province.
The record of P. rothi in the El Polvorín Formation increases the diversity of ground sloths of this unit, which were previously represented by Megatheriinae indet. (Poiré et al., 2005), Nothotheriinae indet. and Mylodontinae indet. (Quiñones et al., 2017).
Outside Argentina, Anaya and MacFadden (1995) reported P. patrius (MNHN-Bol-V 003353) for the locality of Inchasi (Bolivia), in sediments that may be correlated with the Montehermosan and Chapadmalalan stages/ages. According to Miño-Boilini et al. (2011) the Mf1 of MNHN-Bol-V 003353 is more cylindrical in section than those of P. patrius, P. almagroi, and P. rothi, of Argentina. In addition, the lingual lobes of Mf2, Mf3, Mf4 and Mf5 are less marked than those of P. patrius and P. rothi. The Bolivian specimen may possibly represent a species different than those of Argentina (see also Pujos et al., 2012). Hence, the genus Proscelidodon is represented by P. patrius, P. almagroi, P. gracillimus,P. rothi, and a possibly new species from the Pliocene of Bolivia, making it so far the Neogene genus with the highest specific diversity.
Acknowledgments
We thank ‘Cementos Avellaneda’ for allowing the study of material under their care, Dr. Ulyses Pardiñas for helping with the determination of the cricetid, and Daniel Barasoain (CECOAL) for the revision texts in French. We thank the constructive suggestions from the two anonymous reviewers. This contribution was partially supported by grants PI Q 003/14 SGCyT-UNNE to ARMB.
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