A new Hypsodont Notoungulate (Hegetotheriidae, Pachyrukhinae) from the late Miocene of the Eastern Cordillera, Salta province, Northwest of Argentina

Late Miocene f luvial strata of the Palo Pintado Formation are broadly exposed to the northwest of the town of Angastaco, Salta province, Northwest of Argentina. These strata accumulated in the extensional Angastaco Basin. Recent field work at the Palo Pintado Formation (late Miocene), Valle Calchaquí, Salta province, Argentina has provided fossil remains that greatly increased the knowledge of the faunal assemblage of this site. A number of notoungulates and rodents were collected. A partial left jaw was collected at Quebrada Peñas Blancas along the west bank of the Río Calchaquí. Morphological and morphometric comparisons permit referral of this specimen to a new species of hegetotheriid notoungulate Paedotherium kakai sp. nov. It represents the first report of Paedotherium for the Eastern Cordillera and one of the few well-documented occurrences of this genus outside of middle-high latitudes Argentina. The widespread geographic range of Paedotherium, combined with its restricted temporal range, suggest it may be one of the most useful biostratigraphic indicator taxa for Neogene faunas. Paedotherium kakai would have been a mixed feeder that lived in gallery forests, feeding close to water bodies of a system river and lagoons, in food plains developed under humid and subtropical climate.


Introduction
Study of Neogene South American fossil mammals has traditionally focused on faunas from the middle-high latitudes in Argentina (Patterson and Pascual, 1972;Simpson, 1980;Cione and Tonni, 1995). The discovery of low latitude faunas elsewhere in Argentina (NW and NE of Argentina) and other countries of South America (e.g., Bolivia, Brazil, Chile, Colombia, Perú) have contributed to more complete understanding of the evolution of South American mammals. These low latitudes faunas demonstrate, for one, that latitudinal provinciality characterized South American mammal faunas for much of the Neogene, necessitating sampling of a wide geographic range for any given time slice to gain a fuller understanding of mammal evolution on the continent (Riggs and Patterson, 1939;Pascual et al., 1985;Flynn et al., 2002;Wyss et al., 2003;Reguero et al., 2007b;Reguero and Candela, 2011).
Fossil mammals previously collected from the lower levels within the Palo Pintado Formation near Calchaquí River indicate a late Miocene to early Pliocene age and palaeoenvironments characterized by relative humidity (Marshall et al., 1983;Anzótegui, 1998;Starck and Anzótegui, 2001).
Our research team has been working to uncover and study Miocene/Pliocene fossil mammal faunas throughout the northwestern area of Argentina (Jujuy and Salta provinces). In March 2010, one of the authors (D. Voglino) collected a small fossil mandible of a pachyrukhine (Notoungulata: Hegetotheriidae) from the upper levels of the Palo Pintado Formation at Quebrada Peñas Blancas in the area of Angastaco, Salta province (Fig. 1). From the same horizon more vertebrates were recovered, preliminary identifications of the unprepared specimens suggest the presence of at least three rodents, in addition to the small notoungulate specimen collected at Quebrada Peñas Blancas described below. The finding of a jaw with teeth in this unit has renewed the interest of this paleontological area, since represents the first hypsodont notoungulate remains from this formation.
Assignment of the Palo Pintado hegetotheriid to the subfamily Hegetotheriinae is precluded by the morphology of m3 and the small size; the Palo Pintado taxon has a trilobed m3, whereas in all known hegetotheriines this tooth is sharply bilobed.
Herein is described a new species of Paedotherium from the late Miocene Palo Pintado Formation, Salta province, Argentina. The new species provides insight into Pachyrukhinae diversity during the Neogene and allow valuable comparisons between low latitude late Miocene faunas and similarly aged middle-high latitude faunas. In addition, the presence of a hypsodont fossil ungulate species in a forested environment is discussed.

Materials and methods
The specimen of the new species described below is housed in the vertebrate paleontology collections of the Museum of San Carlos, Salta province, Argentina. Measurements of this and other specimens were made to the nearest 0.1 mm using digital calipers, unless otherwise noted. Comparative data were gathered from collections research at the Museo de La Plata (Argentina); additional data were gathered from published sources. Hypsodont dentition is characterized by highcrowned teeth, as opposed to brachydont dentition, which is low-crowned. Hypselodonty refers to ever-growing, rootless or open-rooted dentition. Hypsodonty index follow Reguero et al. (2007a), and were calculated by dividing the m1 height by the m1 anterobuccal-posteriorlingual length.
S.Sal.Scar.Paleo.2012-045 was found in situ in the sandy facies of the bearing horizon and the adhering matrix is a fine sandstone which was deposited by large subaqueous dunes in fast-flowing confined channels (Fig. 2). Further, the preservation of this specimen indicates that it was no transported far, and there are no other potential source horizons in the immediate vicinity. Some taphonomic characteristic of the teeth show no abrasion by sedimentary transport, i.e., 1. the amount and distribution of cementum (dentinecementum boundary indicated in figure 3) is normal and suggests that little abrasion has occurred; 2. no rounding and no enamel loss at the edges of occlusal surfaces; 3. no removal of enamel along the salient angles. Histologic features of the crowns teeth also indicate that little, if any, dentine has been removed, other than by normal processes of occlusal attrition.

Geographic and geologic setting
The Eastern Cordillera area in Argentina is situated to the northwest between 22°05' and 27°00'S and 64°40' and 66°10' W; it represents the southern extremity of the same mountain range known from Bolivia. The narrow, NS trending Angastaco basin is located in the distal part of a once contiguous Paleogene Retroarc Foreland basin (Jordan and Alonso, 1987). The Angastaco basin is located in the southern part of the Eastern Cordillera. This basin includes >6 km of Eocene-Pliocene continental clastic strata of the Payogastilla Group (e.g., Díaz and Malizzia, 1983;Starck and Vergani, 1996). This basin is bounded to the west by the Precambrian-early Cambrian metasedimentary Puncoviscana Formation over the Cenozoic basin strata. To the east, the Cretaceous rocks of the Sierra de Los Colorados over the basin strata (e.g., Carrera and Muñoz, 2008).
The Palo Pintado Formation comprises fluvial systems deposits which are well exposed at Peñas Blancas outcrops. The lithofacies boundaries and the characteristics and geometries of the architectural elements are documented in the stratigraphic column ( Fig. 1). We interpret these facies as the deposits of laterally unstable shallow gravelly channels, perhaps in a braided fluvial system (e.g., Miall, 1996). The Palo Pintado Formation (ca. 10 to 5 Ma) consists of ~1,200 m of colored fine to mediumgrained sandstone beds with trough cross-stratified sandstones interpreted as f luvial channel sand bodies (e.g., Davis, 1983).
The geological context and the stratigraphic profile of the Palo Pintado Formation at the Quebrada Peñas Blancas area are fully detailed in figure 1. The new fossil material comes from about 410 m above the base of this unit, close to a left tributary of the Calchaquí River (Fig. 1). The age of this formation range between 10.29±0.11 Ma (Galli et al., 2008(Galli et al., , 2011 below the bearing horizon, and 5.27±0.28 Ma (Coutand et al., 2006) and 5.98±0.32 Ma (Bywater-Reyes et al., 2010) on the top of the unit. Magnetostratigraphic study in progress of the Palo Pintado Formation has dated the mammal-bearing horizon at ca. 8.8 Ma (personal communication, C. Galli, 2013). This formation is correlated with the Guanaco Formation in the La Viña area (Sierra de Los Colorados) that has been dated at 8.73±0.25 Ma by K-Ar on biotite from an ash (Del Papa et al., 1993) and by zircon U-Pb at 9.31±0.31 Ma (Hain et al., 2011).
The Palo Pintado Formation at Quebrada Peñas Blancas is gradational with the Angastaco Formation ( Fig. 1) and consists of ca. 1,000 m of green mud rocks and sandstones interbedded with green ripple-and planar-laminated mudstones interpreted as associated to a relatively fine grained, nonmarine setting with meandering stream (Galli et al., 2011). The f luvial system interpreted for this unit is intermediate between fluvial braided (with low sinuosity and simple channels) and meandering rivers (with high sinuosity and multiple channels) (Miall, 1996). The trough cross-stratified conglomerates and sandstones were deposited by large subaqueous dunes (or large 3D ripples) in fast-flowing confined channels.  (Cerdeño and Bond, 1998): P. minor from the late Miocene Chasicoan and Huayquerian stage/ ages, P. typicum and P. bonaerense from Huayquerian. Description: S.Sal.Scar.Paleo.2012-045 consists of a partial right mandible that has been partially prepared. Most of the horizontal ramus is preserved. The alveolar and inferior margins are roughly parallel, with the depth of the mandible averaging approximately 11.5 mm. All teeth are fully erupted and have undergone moderate wear, indicative of a fully mature animal.

Systematic
Paedotherium kakai is similar to P. typicum in morphology and is comparable in size. It was compared with the lectotype of P. typicum, MLP 12-1782, from the Montehermosan Stage/age of Buenos Aires Province, Argentina (Fig. 4).
The fourth premolar (p4) of Paedotherium kakai is similar to P. typicum in morphology but is less molariform, it has a very simple occlusal surface and exhibits a single deep labial sulcus separating the trigonid from the talonid. The sulcus is open and the lingual extremity has closely appressed sides. The talonid is wider than the trigonid in both p3 and p4 but is similar to it in length (mesiodistal). The lingual face of p4 is slightly convex. Enamel is absent from the distolingual and mesial surfaces, no cement covers the labial surface.
The molars are subequal in size, but gradually decrease from m1-m3. The first (m1) and second lower (m2) molars closely resemble p4 in size and morphology except that the lingual faces are slightly less convex and the labial sulci are deeper, extending more than halfway across the tooth. These attributes give the impression of a more slender tooth, but this is not ref lected in dental measurements (Table 1).
The morphology of m3 is the most notable aspect of the dentition of P. kakai. A labial sulcus separating the trigonid and the talonid, and a second shallow labial sulcus is present in approximately the middle of the talonid. The resulting 'trilobed' condition is common to all pachyrukhines but the shape and locations of the sulci is variable between the species. Comments: In overall morphology, Paedotherium kakai is similar to P. typicum. The most obvious difference between these two species is in morphology is the shape of the lobes and the locations of the labial sulci that conforms the tooth. Paedotherium kakai is a large pachyrukhine, but is significantly smaller than the Deseadan SALMA (late Oligocene) species Prosotherium garzoni and Medistylus dorsatus from Patagonia, the largest member of the subfamily; it is similar in size to P. typicum and P. bonaerense, overlapping in size some specimens described by Cerdeño and Bond (1998). Beyond of size features, the combined morphological difference cited in the diagnosis, clearly distinguish Paedotherium kakai from all previously described Neogene pachyrukhines.

Biostratigraphic and biogeographic significance of the genus Paedotherium
Pachyrukhines are extremely common in the Miocene and Pliocene of Argentina. From the late Miocene (Chasicoan/Huayquerian) to late Pliocene (Marplatan), only two genera are represented, Paedotherium and Tremacyllus.
Paedotherium is best known from the rich Chasicoan and Huayquerian (late Miocene) deposits along the Atlantic coast of Buenos Aires. The temporal record of Paedotherium ranges from late Miocene to late Pliocene of Argentina, Chile and Bolivia; it is one of the most abundant mammals in Montehermosan and Chapadmalalan faunas of the Buenos Aires (Monte Hermoso, Chapadmalal, Barranca de los Lobos, etc.) and La Pampa provinces (Zetti, 1972;Cerdeño and Bond, 1998).
The smaller species, Paedotherium minor, is recognized in the Arroyo Chasicó Formation (Vivero and Las Barrancas members) and the Epecuén Formation, which represent the late Miocene Chasicoan and Huayquerian ages, respectively (Cerdeño and Bond, 1998). Contreras and Baraldo (2011) mentioned Paedotherium minor from horizons assigned to the Chasicoan and Huayquerian ages in San Juan province. Brandoni et al. (2012) mentioned the presence of Paedotherium minor in the El Degolladito, La Rioja province, from sediments of Salicas Formation. This species also is recognized from sediments of Cerro Azul Formation assigned to the late Huayquerian for Verzi and Montalvo (2008) (but see Prevosti and Pardiñas, 2009 for a discussion about age of these bearing sediments).  One of the largest species, Paedotherium typicum, is abundantly recorded in the Monte Hermoso (Monterhermosan age; Tomassini et al., 2013) and the Chapadmalal (Chapadmalalan age) formations in Buenos Aires province. It is coeval with P. bonaerense, but P. typicum occurs more frequently in the Montehermosan (Tomassini et al., 2013), while P. bonaerense is more abundant in the Chapadmalalan age. Some scarce remains indicate the persistence of P. typicum in the younger Marplatan levels of the Barranca de Los Lobos and Vorohué formations. Also this species has been mentioned from late Miocene Río Quinto Formation of San Luis province (Prado et al., 1998). Paedotherium cf. typicum also is recorded in the Pliocene Alvear Formation (Candela et al., 2007). The size of P. typicum and P. bonaerense is very similar (Table 1).
In a biostratigraphic context the last appearance of Paedotherium is in the Paractenomys chapadmalensis Zone (Marplatan Age, Sanandresian subage, late Pliocene, Cione and Tonni,1995). The probable persistence of Paedotherium in the Ensenadan age of Buenos Aires Province involves scarce remains assigned to P. bonaerense. Nevertheless, the stratigraphic provenance of these remains is doubtful (Bond et al., 1995).
Except for the records of Paedotherium minor in the late Miocene (Chasicoan-Huayquerian) from the Guandacay Formation in Bolivia (Marshall and Sempere, 1991), middle Miocene from Cura Mallín Formation in Chile (Wyss et al., 2003), and from the late Miocene of the Catamarca, Argentina (Valle del Cajón) and Paedotherium kakai in the late Miocene from Palo Pintado Formation, Salta in Argentina, the genus Paedotherium had a strictly middle latitude Pampean distribution. This contrasts with most high-and middle-latitude faunas (i.e., those south of 23°S) in which Paedotherium account for the majority or entirety of hegetotheriid remains (e.g., Sinclair, 1909;Zetti, 1972;Cerdeño and Bond, 1998). Paedotherium kakai is the first unambiguous record of the genus in the Eastern Cordillera NW of Argentina. At Quebrada Peñas Blancas, the Palo Pintado Formation (i.e., fluvial association; Fig. 5) ranges in age between 10.29±0.11 Ma and 5.27±0.28 Ma (Galli et al., 2011;Carrapa et al., 2006). Additionally, interfingering of this Palo Pintado Fm. with the Guanaco Formation at La Viña area (Sierra de Los Colorados, a laterally equivalent facies) has been dated at 8.73±0.25 Ma by K/Ar on biotite from an ash (Del Papa et al., 1993) and by zircon U-Pb at 9.31±0.31 Ma (Hain et al., 2011).
The Quebrada Peñas Blancas specimen was collected from near the top of the middle part of the Palo Pintado Formation (i.e., fluvial association), suggesting an age slightly older than 6 Ma and younger than 10 Ma. Magnetostratigraphic study in progress of the Palo Pintado Formation has dated the mammal-bearing horizon at ca. 8.8 Ma (late Miocene, personal communication, C. Galli, 2013).

Hypsodonty of P. kakai and the paleoenvironment of the Valle Calchaquí in the late Miocene
The Pachyrukhinae are small terrestrial herbivorous hegetotheriids easily recognized by their specialized anterior dentition, 'rabbit-like' and evergrowing (rootless) incisors and cheek teeth. They were extremely abundant in the mid-Cenozoic of Argentina. Based on dental dimensions, their body mass would have ranged from about 1.8 to 2.2 kg (Castro, 2001;Elisamburu, 2011;Cassini et al., 2011); it is interesting to note that the only extant ungulate with similar size is the ruminant Asian tragulid mouse deer, the smallest artiodactyl known. Pachyrukhines are generally reconstructed as grassers and open habitat specialists that might have lived in burrows and resembled rabbits (leporids) or various South American rodents (caviids, chinchillids) in lifestyle (Sinclair, 1909;Kraglievich, 1926;Cifelli, 1985;Genise, 1989;Dozo, 1997;Cerdeño and Bond, 1998;Elissamburu, 2004).
The hypsodonty of Paedotherium kakai offers other possible paleoecological interpretation which is in agreement with the palaeoclimate and palaeonvironment inferred for the Palo Pintado Formation, characterized by habitats forested, with system of sinuous rivers, marsh and lagoons developed under a humid and subtropical climate, with short dry seasonality (Galli et al., 2011;Anzótegui and Horn, 2011) (Fig. 5). Study of megaflora of Palo Pintado indicate four paleocommunities: fresh water, marsh (e.g., Blechnum serrulatiformis, Acrostichum paleoaureum), riparian (e.g., Cedrella fissiliformis, Sapium haematospermoides, Nectandra saltensis, Ficus tressensii in the arboreal stratum, and the climbing Ranunculodendron anzoteguiae) and xerophytic forests areas (Anzótegui and Horn, 2011) grass were not the environments dominant inferred for Palo Pintado Formation. Note that the case of Paedotherium kakai is in contrast with the majority of the late Miocene extinct South American ungulates, for which no information on the paleoenvironment conditions of its bearing-units is little or not available. In this context, we assume that hypsodonty in P. kakai is not a feature associated with grasser feeding habits and strictly open habits. Then, if unquestionably hypsodont indicates high rates of tooth wear (Fortelius, 1985), we must explore which are the abrasive factors that explain the relatively high rate of tooth wear in P. kakai (Table 2). At least partially, hypsodonty is here interpreted as feature reelecting the evolutionary history of the group previously acquired in the Pachyrukhinae during late Oligocene in Patagonia, when extensive grasslands were certainly absent (Palasezzi and Barreda, 2012;Billet et al., 2009;Kay et al., 1999;Strömberg et al., 2011). On the other hand, hypsodonty in P. kakai is understood as feature associated to abrasive particles consumption during feeding. Janis (1988) concluded that hypsodonty alone could not be taken as evidence for grazing behavior in extinct ungulates and suggest that, although do not the only factor; the grit had an important role for hypsodonty. Likewise, other authors (e.g., Williams and Kay, 2001;Mendoza and Palmqvist, 2008;Damuth and Janis, 2011 and citations herein) indicated that soil ingestion in ungulates has a considerable explanatory power of hypsodonty, irrespective of the proportion of the grass in the diet.
In line with these ideas, soil consumption, including abrasive particles such as sand, dust, volcanic glass, etc., adhered to surface of food or accidentally ingested, could explain, at least partially, the high degree of hypsodonty in P. kakai. Examples of living ungulates indicate that hypsodonty and wear rates vary with levels of soil ingestion, but not entirely with proportion of grass in the diet (Damuth and Janis, 2011). Currently the pronghorn antelope (Antilocapra americana) is an examples of an hypsodont not grasser but mixed-feeding species, in which soil intake explains, at least as an important factor, the tooth wear (Damuth and Janis, 2011). On the other hand, among extant ungulates, these authors given examples of some populations of grazing or mixedfeeding species that live in rainforest regions (e.g., Bubalus mindorensis, buffalo, Syncerus caffer), where the grass is essentially absent. Ungar et al. (1995) support the presence of hypsodont species in forested habits obtaining source of wear on dust particles found in the canopies of tropical dry forests and rainforests.
Therefore, soil consumption, including probably sand near of border of rivers or lagoons, or consumption of dust or abrasive particles of soil adhered to food surfaces in floodplains or riparian forest could have acted as wear tooth in Paedotherium kakai. Compared with Paedotherium typicum and P. minor, the lesser degree of hypsodonty and the absence of cement of P. kakai would be indicative of a lesser amount of abrasive particle in the diet. P. kakai must have had a relatively lesser rate of abrasive particle consumption, indicative of a diet (food?) not highly abrasive, perhaps because it was being foraged close to water and so more free of grit. A high hypsodonty and the presence of cementum in Paedotherim typicum and P. minor evidence an increasing dietary abrasion as consequence of the grasslands became more widely available in the more open habits of Pampean region. These features common to these species suggest that their feeding habits were influenced by Cenozoic climate and its impact on paleoenvironment and habitat structure.
As discussed above, Paedotherium kakai probably was a mixed feeder compared with P. minor and P. typicum, consuming a variable diet obtained close to the ground. In the context of hypsodont species, this ungulate was probably a wide ranging species that lived in gallery forests, being able to eat close to water bodies of a system river and lagoons that occurred in the foodplains developed under humid and subtropical climate. Other hypsodont species, the giant rodent Eumegamys paranensis from the late Miocene of Argentina, adds to the examples of hypsodont but not grasser species (Candela et al., 2013) dwellers of forested habits, developed under warm and humid climate, where the grasses were not predominant.