Pliocene Pre-GABI herbivorous mammals from Espinar, Peruvian Andean Plateau

ABSTRACT Pliocene South American mammals prior to the Great American Biotic Interchange comprise the evolutionary pinnacle of a long-lasting process of continental isolation, in which large-scale orogeny and climate change were instrumental. These processes resulted in unique assemblages of extinct groups with peculiar anatomies. However, interpretation of the functional roles that these organisms would have played in the ecosystem is challenging because well-preserved fossils of this time interval in South America are scarce. Here, we describe a new Early Pliocene assemblage of native herbivorous megamammals documented by skeletons in anatomical connection from the Peruvian Andean plateau (city of Yauri in Espinar, Cusco Department). Fossil-bearing deposits pertain to Member C of the El Descanso Formation dated at 4.75 ± 0.5 Ma, when the plateau attained near-modern elevations and a Puna-type plant ecosystem. The mammal assemblage is dominated by xenarthrans, including three ground sloths (cf. Megatheriops rectidens, Simomylodon uccasamamensis, and Proscelidodon sp.) and Andinoglyptodon mollohuancai gen. et sp. nov., the first Pliocene glyptodontine known by a well-preserved cranium and dermal armor. Notoungulates are represented by the toxodontid Posnanskytherium viscachanense. Both selective and bulk feeders are identified, suggesting the existence of diverse plant resources in the Pliocene Puna ecosystem. These native mammals constituted the typical Pliocene herbivorous guild of the central Andean plateau and provide an opportunity to investigate the structure of mammalian communities just before the arrival of Holarctic immigrants.


INTRODUCTION
The modern Central Andean Plateau is characterized by physical traits such as extreme daily thermal oscillations, low relative humidity, high elevation, and reduced partial pressure of oxygen, which are believed to be responsible for the relatively low plant diversity in this ecosystem (Baied & Wheeler, 1993).Today, this altiplanic ecoregion (also known as Puna) supports an equally low diversity of herbivorous mammals whose biggest inhabitant, the camelid vicugna (Vicugna vicugna), does not exceed 50 kg (Acebes et al., 2018).Albeit some differences resulting from slightly wetter conditions (inferred by a higher number of fern taxa), these physical and floral traits typical of the modern Andean Plateau, were already in place by the Early Pliocene (Kar et al., 2016, Martínez et al., 2020).Herbivorous mammalian communities supported by this plant ecosystem in the Pliocene, however, differed drastically, not only in terms of taxonomic composition and diversity, but particularly in terms of body mass.Indeed, fossiliferous localities in the Pliocene, right before the main pulse of faunal arrival from North America during the Great American Biotic Interchange (GABI), document a rich mammalian diversity in the Andean Plateau with at least eight megamammal (i.e., > 40 kg) species (see Anaya & MacFadden, 1995).
In 2005, the first fossil mammal from the Peruvian Andean Plateau was discovered in the locality of Espinar, while excavations were being carried out for the construction of a soccer stadium in the city of Yauri (Tejada, Baez et al., 2009;Tejada, Gamarra et al., 2009).Since then, several well-preserved specimens have been recovered in the city documenting a rich pre-GABI fossil mammalian community.The paleontological record also includes finely preserved fishes, ostracods, diatoms, leaves, and fruits (Martínez et al., 2020;Tejada, Baez et al., 2009;Vélez et al., 2018).Here, we provide the first formal description of the Early Pliocene mammalian assemblage from Espinar, including one new glyptodontid species that reveals ecological differences between Pliocene and Pleistocene forms.Documenting the Espinar mammalian community during the Early Pliocene, shortly after the landscape would have reached nearmodern elevations, acquired modern environmental conditions, and before the arrival of immigrant herbivores during the GABI is important for several reasons.First, from an anatomical perspective, the information preserved in the Espinar fossil mammals allows assessment of character evolution associated with changes of feeding ecology in the Andes as well as a critical evaluation of the taxonomic significance of some morphological characters.Second, the Espinar fauna allows the evaluation of the regional impact of the rise of the Andes among coeval mammalian communities across the continent.Third, it represents crucial evidence toward understanding mammal community structures just before the arrival of Holarctic immigrants.

GEOGRAPHIC AND GEOLOGIC SETTING
The Andes are home to numerous intermontane basins containing thick continental successions, with the Descanso-Yauri basin in southern Peru being one of the best-known in the southern Peruvian region.It contains a 1.8 km-thick Mio-Pliocene sequence associated with the Descanso Formation.These exposures consist of three lithostratigraphic units, in ascending order: Member A, Member B, and Member C (Carlotto, 2013;Cerpa et al., 2002).Fossil remains discussed in this paper were collected from the latter unit.Member C is composed of finely laminated fossiliferous diatomaceous silt-and sandstones, interbedded with massive to cross-bedded sandstone and conglomerate beds, as well as local syndepositional ash beds.It represents a N-NE flowing fluvial system that gradually gave rise to lacustrine and palustrine environments (Cerpa et al., 2002), preserving abundant plant (pollen, leaves, wood), invertebrate (ostracods), and vertebrate (fish, mammal) remains.The age of Member C was recently assessed as latest Miocene to Early Pliocene (5.4-3.9Ma; Kar et al., 2016;Martínez et al., 2020).In this study, a 3 m-thick ash outcropping on the NW margin of the Yauri city, near the area of the first paleontological discoveries, was further analyzed for U/Pb radiometric dating (sample E19-14; 14°784′S, 71°418′W; Figs. 1, S1-S3).U/Pb analyses of zircons from this ash yielded an age of 4.75 ± 0.5 Ma (see Supplementary Data 1, Table S1).Mammal remains described herein are correlated with sedimentary levels from the Versalles area (Martínez et al., 2020;3907 m a.s.l.), but are considered at least 1 My older than those outcropping at Cerro Púcara (Garbage Pit locality, 3937 m a.s.l.; Vélez et al., 2018).
Diagnosis--Glyptodontine similar in size to Boreostemma acostae with relatively long and slender snout (shorter in Paraglyptodon uquiensis, Glyptodon, and Glyptotherium; wider in Glyptodon and Glyptotherium); nasal aperture trapezoid and wider-than-tall (trapezoid and taller-than-wide in other glyptodontines); ventral margin of the orbit closer to the alveolar plane than to the dorsal skull profile; semicircular spout-like predental symphysis; the height of the horizontal ramus increases posteriorly (more homogeneous height along the horizontal ramus in Glyptodon); long anteroposterior base of ascending ramus of the mandible (closer in shape to G. muñizi than to G. clavipes which is anteroposteriorly shorter); relatively slender femur; greater trochanter at the same level of the femoral head (similar to G. reticulatus); third femur trochanter positioned just distal to the midshaft; low level of the articular condyle of the mandibles relative to the tooth row (higher in Glyptodon and Glyptotherium).
Etymology--Andinoglyptodon in reference to the holotype being found in the high Andes; mollohuancai after Oscar Mollohuanca, former mayor of Espinar who fought against economic and political power to protect the environment in Espinar and was killed in unclear circumstances.
Comparative Description--The skull is relatively long and high at the frontoparietal region.From the posterior limit of the orbits, the rostrum descends anteriorly, thus the snout is low relative to other glyptodontines, such as MPP 4676 (referred to cf.P. chapalmalensis by Oliva et al., 2010), P. uquiensis (MACN-Pv 5377), Glyptodon and Glyptotherium, in which the snout is much higher.The attenuation of the rostrum is reinforced by the distinct sigmoidal shape of the alveolar plane in lateral view, which is elevated anterior to the zygomatic arch (Fig. 2B).In palatal view, right and left upper dental series (Mf1-Mf8) are roughly parallel to each other, only diverging slightly posteriorly (Fig. 2D).Molariforms are fully trilobate, except for Mf1 which is only weakly lobated on the labial margin.As in Glyptodon, there is an axial palatal ridge in the maxillae at the level of Mf1 (Fig. 2D).The palate bears several large foramina, particularly at the level of Mf2 and Mf3.The RMW (width at lingual Mf4/width at labial Mf1) in Andinoglyptodon is 0.742, comparable to that of Propalaehoplophorus australis (0.745) and slightly higher than those of Glyptodon species (0.631-0.714;Vizcaíno et al., 2011b).
In Andinoglyptodon the snout is not truncated, as opposed to that of Glyptodon, Glyptotherium, and possibly Paraglyptodon (Cuadrelli et al., 2018;Gillette & Ray, 1981;Oliva et al., 2010;Soibelzon et al., 2010).The narial opening bears a shape of an inverted trapezoid as in other glyptodontines (Figs.2A, 4A, C,  E) except for Boreostemma acostae (Zurita et al., 2013).In Andinoglyptodon the narial opening is wider-than-tall (Figs.2A, 4A), in contrast to the typical taller-than-wide opening of most glyptodontines.The dorsal margin of the narial opening is only slightly concave, differing from the deeply concave margin of Glyptodon.The internasal septum is either not ossified or retreated to the narial passage, and so it is not visible.The lacrimal foramen is surrounded by a rugous surface within the lacrimal bone, which is not particularly inflated as in Glyptodon.The zygomatic arch is massive and the descending process of the maxilla projects posteroventrally (Fig. 2B).The postorbital bar is incomplete.In contrast to the high position of the orbits of Paraglyptodon, Glyptodon, and Glyptotherium, in Andinoglyptodon the orbits are located relatively closer to the level of the toothrow.The orbital notch is narrow and U-shaped as in Boreostemma acostae (Zurita et al., 2013), whereas this notch bears more equivalent proportions (i.e., circular) in Paraglyptodon uquiensis (MACN-PV 5377), P. chapalmalensis (MPP 4676; Fig. 4D), Glyptodon (Fig. 4F), and Glyptotherium.The parietal and occipital regions are similar to those of Glyptodon, with the occipital plane tilted anteriorly.The occipital condyles are lateromedially expanded, a feature considered a synapomorphy of the Glyptodontinae (Fernicola, 2008; Fig. 2E).
The general shape of the mandible resembles that of Glyptodon and Glyptotherium, with a long tooth row (mf1-mf8) that occupies most of the horizontal ramus and a high anterodorsal ascending ramus, the latter in correspondence with the posterodorsal orientation of jugal-squamosal zygomatic portion.Although the articular condyle is markedly elevated relative to the tooth row (Fig. 2G, H), its position is proportionally lower than in Glyptodon and Glyptotherium.The posterior-most extremity of the ascending ramus is located slightly above the level of the tooth row, closer to the condition of Glyptodon than that of Glyptotherium (Gillette & Ray, 1981).It seems that the mandibular rami were not fully fused at the symphysis.The symphysis ends at the level of the anterior lobe of the mf4 and its distal tip turns downward, as in Glyptodon.The tip of the symphysis of Andinoglyptodon bears a slender semicircular spout (Fig. 2F) whereas this spout is wider and more quadrangular in Glyptodon and Glyptotherium (Gillette & Ray, 1981;Soibelzon et al., 2010).Two mental foramina are located at the level of mf1.As are the upper teeth, the lower teeth are trilobate, with an incipient lobation in the lingual margin of mf1-mf3.The maximum depth of the horizontal ramus occurs at the level of the m6 (Fig. 2G).Although the depth of the horizontal ramus is variable in Glyptodon and Glyptotherium, the maximum depth in these taxa generally occurs anterior to m6 within a ramus of nearly parallel ventral and alveolar margins (except in Glyptotherium arizonae).The mandible is unknown for Paraglyptodon.
Salas-Gismondi et al.-Pre-GABI Andean mammals from Peru (e2237079-5) radial sulci limit 6-10 peripheral figures within this single row.The sulci are U-shaped in transversal section.Foramina (hair follicles) are set at most intersections of radial and circular sulci.Within this rosette pattern, the shape of the osteoderms varies from quadrangular in anterior and lateral regions (Fig. 3B, C) to hexagonal in dorsal and posterior areas (Fig. 3D  of the carapace of Glyptodontidium, Paraglyptodon, Glyptodon, and some species of Glyptotherium were probably absent in Andinoglyptodon (Fig. 3F-H).Osteoderms of the distal row of the caudal rings show no evidence of conical bosses as well.In Glyptodon and Glyptotherium, these ring osteoderms usually exhibit a similar geometry to the marginal osteoderms of the caudal notch.
The femur is relatively slender (Fig. 3I, Table 2).The greater trochanter reaches the height of the femoral head.The third trochanter is located just distal to the midshaft, resembling Parapropalaehoplophorus (Croft et al., 2007).This trochanter is closer to the distal end in Glyptodon.The femur medial length of Propalaehoplophorus australis (198 mm; from Scott, 1903) represents 78% of that measurement in Andinoglyptodon (holotype: 255 mm).Because the femur of Andinoglyptodon is not much larger than that Propalaehoplophorus australis, the body mass of the Peruvian form is estimated as similar to or slightly higher than that of Propalaehoplophorus Geographic and Stratigraphic Provenance--Locality Huacracanto, Calle Apurimac, block 2, Huacracanto, city of Yauri, Espinar, Cusco; 14°47.808′S,71°24.645′W(locality 3 in Fig. 1).Collected in 2008 by Gregorio "Goyo" Agramonte and JG.Lower levels of the Member C of El Descanso Formation, Early Pliocene (4.75 ± 0.5 Ma).
Comments--MUESP 10 corresponds to a subadult individual slightly smaller than, but with general skull and jaw morphology similar to, the holotype of M. rectidens (MACN 2818), from the Late Miocene of Huayquerías, Mendoza Province, Argentina.As in M. rectidens, MUESP 10 is characterized by having a relatively long and slender cranium with a high, dome-like (convex) skull roof, straight tubular rostrum, upper molariforms oriented anteroventrally, jugal rooted in the maxillae at the level of Mf2, and lower margin of the orbit leveled to the maxillary alveolar plane (Fig. 5A, Table 3).The palate between the molariforms is narrow, but probably not as narrow as that of the type of M. rectidens.Molariforms are quadrangular, exclusive of Mf1 which is trapezoid in shape (Fig. 6B).Some of the features recognized in the skull of MUESP 10 and Megatheriops rectidens are also observed in species of Pyramiodontherium (Cabrera, 1928;Carlini et al., 2002), but the latter shows differences in the shape of the rostrum.In Pyramiodontherium species the predental portion of the maxillae bears a lateral expansion toward its anterior end (Carlini et al., 2002), whereas in MUESP 10 and M. rectidens this is virtually parallel-sided.Maxillary tooth row is laterally bowed in Pyramiodontherium species, with the alveoli of Mf1 and Mf2 less quadrangular than in MUESP 10 and M. rectidens.The main discrepancy between the Peruvian Megatheriops and the holotype of M. rectidens is the orientation of the occipital condyle, being more downwardly inclined in the latter than in the former.This feature is related with the head posture and its variation might have ecological implications (Coutier et al., 2017).

MYLODONTIDAE Gill
Comments--Robust cranium with a high and convex skull roof and a lower, short, massive rostrum (Table 3).In front of the zygomatic process of the maxillae, the rostrum expands anteriorly.Consequently, the rostrum bears a wide V-shaped palate with straight, divergent tooth rows, as in other mylodontines.The upper and lower dental series show no diastema between the two most anterior teeth (Cf1/cf1 and Mf1/mf1).Upper tooth rows are straight in Espinar specimens, and not bowed medially as in Glossotherium chapadmalense (MMP 245-M; Boscaini et al., 2022).As is typical in Simomylodon, the premaxillae are short, either forming a rounded (Fig. 6C; MUESP 16) or wide V-shaped (MUESP 11) anterior margin.The horizontal ramus of the mandible is trapezoid along the tooth row (cf1, mf1-mf3), with the lowest height at cf1 and deepening posteriorly.The symphyseal spout raises and expands anteriorly.The lower tooth rows are oblique to the horizontal ramus in occlusal view.
The abundant cranial material of Simomylodon from Bolivia has shown enormous intraspecific variation, particularly in the relative size, robustness, and snout morphology, attributed in part to sexual dimorphism (Boscaini et al., 2018(Boscaini et al., , 2019)).The adult individual MUESP 16 from Espinar would correspond to the robust (i.e., male) morphotype, characterized by having large size and a wide snout with a rounded distal margin.The morphology of MUESP 11, a juvenile individual, lacks part of the premaxillae, precluding any confident morphotype determination.As identified, the gracile (i.e., female) and robust (i.e., male) morphotypes are not only based in relative robustness, but also in a distinct morphology of the premaxillae, a condition not strictly linked to body size or adultness.If skeletal differences between genders result from sexual selection in early stages of ontogeny as occur in some slow-growing mammals (Leigh, 1995), then an early ontogenetic differentiation within Simomylodon would reinforce disparities in niche occupation between morphotypes (Boscaini et al., 2019).Based on several bones of a specimen from Bolivia (MNHN-Bol-V-003375), the mean body mass of Simomylodon is estimated at 348.5 kg.SCELIDOTHERIINAE Ameghino, 1904PROSCELIDODON Bordas, 1935 Type Species--Proscelidodon patrius (Ameghino, 1888) Temporal Range--Late Miocene to Late Pliocene?Geographic Distribution--Argentina (Provinces of Mendoza, Catamarca, Buenos Aires, La Rioja, and Jujuy), Bolivia (Inchasi, Chuquisaca Department), and Peru (Espinar, Cusco Department).
Comments--Bones of this skeleton are in anatomical connection, including the skull and jaws (Fig. 5D).This identification must be taken with caution because key anatomical structures (dentition, palatal surface, basicranium, dorsal shape of the skull, etc.) are still inaccessible (Fig. 5E).MUESP 20 (Table 3) is an adult individual of roughly same size of Proscelidodon patrius (holotype: MACN 8075).Proportions of the skull in the Espinar scelidotheriine closely correspond to those of the holotype of Pr. patrius.Comparisons with Pr. gracillimus are limited because the holotype (MACN 8470) is badly damaged.MNHN-Bol-V-003353 from the Pliocene of Inchasi (Bolivia) referred to Pr. patrius (Anaya & MacFadden, 1995) is more gracile than MUESP 20, and could correspond to a distinct genus (FP, pers. observ.).The convex skull roof in MUESP 20 in lateral view is a remarkable feature, since most specimens referred to Proscelidodon show a flat to sinuous dorsal profile in this region.The body mass is estimated at 316.0 kg.NOTOUNGULATA Roth, 1903TOXODONTIDAE Owen, 1845POSNANSKYTHERIUM Liendo Lazarte, 1943 Temporal Range--Pliocene.Geographic Distribution--Bolivia (Inchasi, Chuquisaca Department) and Peru (Espinar, Cusco Department).Comments--MUESP 3 is a relatively small individual almost indistinguishable from the holotype of Posnanskytherium viscachanense (MNHN VIZ 38), the latter collected at the Pliocene locality of Viscachani, south to La Paz.As typical in Posnanskytherium, in lateral view the skull roof is tall and flat (Fig. 5F), whereas the rostrum proportions vary among species.Posnanskytherium desaguaderoi (MHNH-Bol-V GB-004) from Bolivia bears a tall and relatively long rostrum.In Po. inchasense, the rostrum is comparatively short and robust.In MUESP 3 and MNHN VIZ 38, the rostrum is short as well, but not as massive and clearly lower than in the aforementioned species.The snout is particularly low at the narial opening because the dorsal profile of the rostrum uniformly descends anteriorly from the level of the postorbital processes.In dorsal view, the zygomatic arches are widely rounded in MUESP 3, not as the more paraxial sides of the zygomatics of Po. desaguaderoi.In ventral view, the proportions of the premaxillae and maxillae seem to be similar in all recognized species, but the dental formulae of MUESP 3 differ by the presence of a small upper canine, thus the dentition of this specimen is I2/3, C1/1, P4/3, M3/3 (Fig. 6D, Table 3).Instead, MUESP 15 bears no upper canine.Upper molars are very slender and have a distinctly elongated paracone.Lower molars of Espinar specimens (MUESP 3 and MUESP 8) bear a single lingual (meta-entoconid) fold as Po.viscachanense from Ayo Ayo (MNHN-AYO 191) and MNHN-Bol-V 003360 from Inchasi, the latter referred to Po. desaguaderoi (Anaya & MacFadden, 1995).This fold is virtually imperceptible in Po. inchasense (holotype, MNHN-Bol-V 350).Espinar specimens also differ from Po. inchasense in having a shallower horizontal ramus.The body mass is estimated at 157.5 kg.

Mammals from Espinar: A Glimpse of the Pliocene Andean Faunas
The central Andean plateau fauna from Espinar is remarkable by the amazing preservation of their fossil mammals, largely consisting of skeletons in anatomical connection and with no geometric deformation.As fossiliferous deposits are located within the urban area of the city of Yauri, searching and collecting procedures are limited.To date, we have identified five taxa of herbivorous mammals within two conspicuous native South American clades, Xenarthra and Notoungulata.Xenarthrans constitute the dominant component of the fauna, including a new glyptodontine, Andinoglyptodon mollohuancai gen.et sp.nov., and three taxa of ground sloths, the Megatheriidae cf.Megatheriops rectidens (Megatheriinae) and the Mylodontidae Simomylodon uccasamamensis (Mylodontinae) and Proscelidodon sp.(Scelidotheriinae).Notoungulates are represented by the Toxodontidae Posnanskytherium viscachanense.Small mammals and predators are currently unknown from the Pliocene of Espinar.Albeit rare, these faunal components are reported from coeval Andean localities that have been more extensively sampled (Anaya & MacFadden, 1995;Marshall & Sempere, 1991).
The vertebrate-bearing deposits of Espinar are dated at ∼4.8 Ma (Early Pliocene; Kar et al., 2016 and this contribution) and document taxa previously recovered from Late Miocene and Pliocene Andean localities in Bolivia and Argentina (Fig. 7).Among them, Simomylodon and Posnanskytherium were previously known only from the Bolivian-northernmost Argentina plateau and discovered in almost every locality encompassing the Pliocene interval (e.g., Pomata-Ayte, earliest Pliocene: Marshall et al., 1983;Inchasi late Early Pliocene [4.0-3.3Ma]: Anaya & MacFadden, 1995; and Ayo Ayo-Viscachani, Late Pliocene [∼2.9 Ma]: Marshall and Sempere, 1991; Casira, Late Miocene-Pliocene: Quiñones et al., 2019).The distribution of these animals is now northward into Peru, but still within the same Andean plateau domain.Proscelidodon attained a larger geographic distribution, including both Andean and non-Andean areas, but the species within the genus are poorly diagnosed (Anaya & MacFadden, 1995;Miño-Boilini et al., 2011).This is a crucial taxon because it might have been the sole scelidotheriine from the Late Miocene-Pliocene time frame (McDonald, 1987;Miño-Boilini et al., 2011;Pujos et al., 2012).The finely preserved skeleton from Espinar will be key to improve the characterization of Proscelidodon species and to provide a better understanding of the evolution of later relatives (i.e., Catonyx, Valgipes, and Scelidotherium).
The Peruvian fossil referred here as a putative subadult individual of Megatheriops rectidens is the only record besides the holotype (MACN 2818) from the Huayquerías of Mendoza in Argentina, a rich Andean vertebrate locality regarded as Late Miocene (∼5.8 Ma: Garrido et al., 2017;Fig. 7).Differences between the Peruvian and the Argentinian specimens are subtle.The Peruvian specimen might possess a wider palate between molariforms if the holotype of M. rectidens was not compressed by taphonomic processes.The preserved premaxillae reveal that the general configuration of those bones in Megatherium was already present in Megatheriops, including an elongate anterior process, a strong lateral process, and a nubbin-like medial process.Indeed, the premaxillae of the socalled Andean species of Megatherium (see De Iuliis et al., 2009), such as Megatherium tarijense and Megatherium celendinense, are essentially robust versions of those observed in the Peruvian Megatheriops.Since the premaxillary shape is suggested to be intimately linked to feeding habits (see below), changes in the morphology of the premaxillae within Andean megatheriines might reflect a shift in ecological niches for these animals in the high Andes from the Pliocene to the Pleistocene, yet a phylogenetic context is necessary to trace this evolution.

Andinoglyptodon and the Pliocene Glyptodontine Conundrum
The recognition of Andinoglyptodon as a glyptodontine stems basically on characters of the dorsal carapace, such as the presence of osteoderms with a single row of peripheral figures and 12 or less peripheral figures per osteoderm (Fernicola & Porpino, 2012;Zurita et al., 2013).Andinoglyptodon also bears a Ushaped osteoderm sulci as in Boreostemma, Glyptodontidium, Paraglyptodon, Glyptodon, and Glyptotherium.This feature might diagnose the glyptodontines (Carlini et al., 2008) but its distribution among tropical glyptodonts is poorly known (see Tejada-Lara et al., 2015).The rosette pattern of the osteoderms is characteristic of glyptodontines but is considered plesiomorphic by its presence in the archaic Propalaehoplophorinae (Fernicola & Porpino, 2012).We identified two cranio-dental characters that might diagnose glyptodontines and are observed in Andinoglyptodon: trilobate upper molariforms (incipient in Mf1; also present in Panochthus) and occipital condyles lateromedially expanded (see Carlini et al., 2008;Fernicola, 2008).Within glyptodontines, Andinoglyptodon possesses significant differences in the skull and jaws relative to Pliocene and Pleistocene forms known so far by cranial material.Pleistocene taxa, such as Glyptodon and Glyptotherium, are considerably bigger than Andinoglyptodon and exhibit a deep, massive, and more quadrangular rostrum (Fig. 4).This feature is emphasized in these genera by the presence of a truncated snout with a high narial opening.In contrast, in Andinoglyptodon the narial opening is wider-than-tall and the rostrum is lower, relatively long, and slender at the anterior end.The symphyseal spout of the mandible is semicircular and narrow.An elongated rostrum is also recognized in Boreostemma acostae from the Middle Miocene of the Villavieja Formation, Colombia (Zurita et al., 2013).
The Pliocene genus Paraglyptodon is problematic because its species (P.chapalmalensis and P. uquiensis after Oliva et al., 2010) were diagnosed on scattered remains of the dermal armor (Castellanos, 1953;Rovereto, 1914).The type series of Paraglyptodon uquiensis also includes a portion of a skull (MACN-Pv 5377) that preserves the rostrum, zygomatics, and partial dentition (Fig. 4C).Regarding P. chapalmalensis, wellpreserved cranial material (MMP 4676) recovered from the type locality was recently conferred to this species (Oliva et al., 2010; Fig. 4D).In a review of Paraglyptodon, Cruz and colleagues (2016) concluded that MACN-Pv 5377 cannot confidently be assigned to P. uquiensis because it was not found associated with the type osteoderms and shows no differences with Pleistocene glyptodontines.In the same work, the osteoderm type materials of P. chapalmalensis (MAVN-Pv 6162) and P. uquiensis were attributed to juvenile individuals of Pleistocene Glyptodon that might have rolled from upper levels, an asseveration supported by other authors only for P. chapalmalensis (Zurita et al., 2016).The present study revealed that both skulls (P.chapalmalensis: MACN-Pv 5377 and P. uquiensis: MMP 4676) are of comparable size and much larger (MACN-PV 5377, BZYW = ∼229 mm) than the skull of Andinoglyptodon (BZYW = ∼155 mm; Fig. 4).Most cranial features of Glyptodon species (which are absent in the Peruvian taxon) are recognized in these Argentinian fossils, such as the typical square, truncated, and high snout.If these skulls (i.e.,  are from the Late Pliocene, then Glyptodon species existed in that time frame.In contrast, lateral and anterior osteoderms of Andinoglyptodon are almost identical to those of Paraglyptodon and Glyptodon.It is true that osteoderm material of P. chapalmalensis bears features observed in juvenile stages of Glyptodon (see Zurita et al., 2016), but these features are also present in certain osteoderms of Andinoglyptodon (Fig. 3A-D), suggesting large intraspecific variation among glyptodontine taxa.
Comparisons between Andinoglyptodon and the Late Miocene species Glyptodontidium tuberifer show noticeable differences in the osteoderms of the posterodorsal region of the carapace.Posterodorsal osteoderms of Glyptodontidium bear a rosette pattern in which peripheral figures are comparatively larger than those of Andinoglyptodon.The central figure is convex in Glyptodontidium whereas it is concave in Andinoglyptodon.If present, conical bosses of the caudal notch osteoderms were poorly developed in Andinoglyptodon, in contrast to the prominent conical bosses of Glyptodontidium.
Among the rich Bolivian and Argentinian record of Neogene terrestrial mammals, we consider that only one is attributable to Andinoglyptodon.The material consists of a portion of a dorsal carapace and caudal elements from the locality of Inchasi and originally referred to Paraglyptodon (MNHN-Bol-V 003366; Anaya & MacFadden, 1995:fig. 4).Dorsal carapace osteoderms in this specimen are almost undistinguishable from the anterolateral osteoderms of Andinoglyptodon regarding the rosette pattern, surface texture, shape, size, and relative thickness (Fig. 8).Because these features are of only limited value for glyptodontine taxonomy, Andinoglyptodon-like osteoderms from Inchasi associated to cranial remains would be necessary for a conclusive identification.
Andinoglyptodon clearly differs on cranial grounds from Pliocene and Pleistocene relatives, but these are almost undistinguishable on the basis of the dermal armor, indicating that osteoderm evidence alone is insufficient to draw definitive taxonomic conclusions.Osteoderm differences between Pliocene and Pleistocene glyptodontines are subtle and generally range within the wide ontogenetic and individual variation shown by putative species.

Pliocene Environmental Conditions in the Central Andean Plateau: A Mammalian Perspective
At first glance, the Espinar mammal assemblage could be associated with grassland environments because it is composed by herbivorous forms with ever-growing (hypselodont) dentition (Damuth & Janis, 2011;Janis et al., 2000;Jardine et al., 2012).However, a closer look to the premaxillae and maxillae anatomy reveals a variety of morphologies that can provide a more complex environmental interpretation (Figs. 6, 9; Table 3).The shape of the premaxillae and maxillae reflects the shape of the muzzle.In herbivorous mammals, particularly in ungulates, the muzzle width has been linked to feeding strategies and diet, and by extension to plant resources (Janis & Ehrhardt, 1988;Janis et al., 2000).The predominance of xenarthrans in the Espinar fauna might be seen as a drawback for environmental inferences because their peculiar feeding anatomy lacks ecological analogs in modern ecosystems (Anaya & MacFadden, 1995;Bargo et al., 2006).For example, nitrogen isotopic analyses of amino acids on the Pleistocene ground sloth Mylodon darwini revealed that this species fed on items of animal origin in addition to plants (Tejada et al., 2021), a feeding behavior that would have been hard, if not impossible to infer, solely from morphology.In lack of geochemical data, however, muzzle reconstructions from craniodental morphology provide a widely used proxy to infer dietary preferences within the herbivorous guild of xenarthrans (Bargo et al., 2006).
Within the Espinar fauna, relatively wide muzzles are inferred for the toxodontid Posnanskytherium and the mylodontine Simomylodon based on their square-shaped and wide premaxillae (Boscaini et al., 2019).Posnanskytherium and Simomylodon would have exploited a wide range of resources close to the ground (e.g., grasses and small plants) because mammals with wide muzzles are generally reconstructed as grazers and bulk feeders (e.g., Toxodon [MacFadden & Shockey, 1997] and Glossotherium [Bargo et al., 2006]).Additionally, isotopic analyses indicate that Posnanskytherium from Inchasi fed on C3 plants in its diet, including woody plants and montane grasses (MacFadden et al., 1994).Robust and gracile morphs of Simomylodon might have exploited a wide range of resources even beyond the grazer guild (Boscaini et al., 2019).
Proscelidodon with a slender and long snout, and Megatheriops bearing slender premaxillae, were narrow-muzzled ground sloths with prehensile lips after the reconstruction criteria of Bargo and colleagues (2006) for Scelidotherium and Megatherium.This kind of anatomy is associated with selective feeders and browsers.Browsers make use of their prehensile lips to actively select parts of trees and shrubs, such as leaves and young shoots.Andinoglyptodon with its narrow and rounded premaxillae might have been a mixed or selective feeder.The RMW value obtained for Andinoglyptodon (0.742) places this taxon within the group proposed for putative selective feeders, which includes Early Miocene taxa and Glyptodon species (i.e., values above 0.6: Vizcaíno et al., 2011b).Thus, the Espinar assemblage depicts the existence of both selective and bulk feeders, as should be expected in a mixed, wooded grassland ecosystem (Fig. 9).We expect a comparable signal from Pliocene faunas described for the Bolivian Andes (Anaya & MacFadden, 1995;Marshall & Sempere, 1991).
Our preliminary body mass assessment indicates that most of the Pliocene mammals from Espinar largely surpassed 100 kg, with ground sloths ranging between about 316 kg (Proscelidodon sp.) and 1838kg (sub-adult cf.Megatheriops).Studies based on fossil leaves and pollen imply that these large herbivores inhabited the oldest known Puna ecosystem within the Central Andean plateau around 4.8 Ma, when paleoelevations might have reached near-modern values (∼3800 m a.s.l.; Kar et al., 2016;Martínez et al., 2020).Vegetation in this ecosystem was dominated by Poaceae (grasses) and Cyatheaceae (scaly tree ferns) and included taxa of shrubs and trees (such as Polylepis, Quercus, Myrsine, cf.Schinus, Podocarpus, Berberis, and Alnus) living either in the modern Puna or close to the treeline (Martínez et al., 2020).
The main difference between the Puna ecosystem in the Pliocene and in the present relates to a higher diversity and abundance of ferns and shrubs in the former, which is linked to slightly wetter conditions in the Pliocene (Martínez et al., Salas-Gismondi et al.-Pre-GABI Andean mammals from Peru (e2237079-13) 2020).Would the selective feeders documented in Espinar have fed upon ferns?Ferns are not the favorite food of herbivorous mammals in the present but there are reports of selective feeders (i.e., deer) that include ferns in their diet (Haeussler et al., 1990).Polylepis was also identified within the Pliocene Puna ecosystem and would have represented an important resource for selective feeders, such as Megatheriops and Proscelidodon since this tree constitutes relatively dense forests in the high Andes (Hansen et al., 1994).A recent discovery of a pristine Andean landscape in Cusco dominated by Polylepis suggests that prior to human occupation, tree-dominated forest-grassland mosaics prevailed (Sylvester et al., 2017).
An overall similar Puna ecosystem to the current one supported a community of at least five megamammals of different ecomorphotypes in the Early Pliocene of the Andean Plateau.Giant mammals, natives and immigrants, ruled and persisted throughout the Pleistocene in this ecoregion (Marshall et al., 1984;Lucas, 2008), suggesting that the low extant diversity of mammalian herbivores in the Andean plateau might be a consequence of the Pleistocene-Holocene extinction, rather than attributable solely to the modern-type of floral composition.
The Espinar mammal assemblage is the first known so far from the Pliocene of the Peruvian Andes.This initial study only includes the fauna living at about modern elevations from deposits of Member C, within the city of Yauri.However, fossils of vertebrates and plants with exquisite anatomical details were reported from earlier deposits in the region (Member B; Martínez et al., 2020;Tejada, Baez et al., 2009;Tejada, Gamarra et al., 2009).This pre-Pliocene record documents life during the Middle Miocene-Late Miocene interval, when the elevation of the plateau was only ∼1700m a.s.l.(Martínez et al., 2020).Future studies focusing on these older localities in Espinar will contribute to understanding the long-term evolution of South American native mammals and Andean ecosystems.
) of the carapace.A faint delimitation between central and peripheral figures characterizes the anterior and lateral osteoderms whereas deep sulci clearly define the central and peripheral figures of the posterodorsal osteoderms.Sutural margins separate osteoderms in the anterior and lateral parts of the carapace, whereas posterodorsal osteoderms are fused and peripheral figures are shared among neighboring osteoderms (Fig. 3D).In posterodorsal osteoderms, each figure bears a concavity (central depression), particularly noticeable in the central figure.A faint concavity in the central figure is also observed in dorsal osteoderms close to the anterior aperture (Fig. 3B).A similar concavity in the central figure was recognized in some specimens assigned to Paraglyptodon, Glyptodon muñizi, and Glyptodon clavipes from Argentina (Cruz et al., 2016), and an adult Pleistocene specimen from Cusco, here referred to G. clavipes (INC-CUS 1).In contrast, Glyptodontidium tuberifer (MLP 39-IV-25-10) bears posterior osteoderms with a convex central figure.Nuchal osteoderms are quadrilateral and flat.The osteoderms of the lateral and posterior borders (caudal notch) are not well preserved, but those preserved bear a flat to slightly convex surface with an enlarged central figure.Thus, the weak or prominent conical bosses of the lateral and posterior borders