Journal of South American Earth Sciences 35 (2012) 1e9

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Biostratigraphy and correlation of the Monte Hermoso Formation (early Pliocene, Argentina): The evidence from caviomorph rodents Cecilia M. Deschamps a, *, María Guiomar Vucetich b, Diego H. Verzi c, A. Itatí Olivares c a

CIC, División Paleontología Vertebrados, Museo de La Plata, Paseo del Bosque s/n, B1900FWA La Plata, Argentina CONICET, División Paleontología Vertebrados, Museo de La Plata, Paseo del Bosque s/n, B1900FWA La Plata, Argentina c CONICET, División Zoología Vertebrados, Museo de La Plata, Paseo del Bosque s/n, B1900FWA La Plata, Argentina b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 June 2011 Accepted 26 October 2011

The Monte Hermoso Formation (Farola Monte Hermoso, Buenos Aires Province, early Pliocene) is very important because it is the type locality of biostratigraphic units of the South American mammal bearing Pliocene. These deposits were the base for the recognition of the Montehermosan and Lower Chapadmalalan stages/ages. The correlation of this unit, especially with the Chapadmalal Formation has been debated. Over the last twenty years, major improvements have been made in the knowledge of caviomorph rodents collected in the Monte Hermoso Formation. The results include information about systematics and morphological change within lineages of the families Echimyidae (incl. Myocastoridae), Ctenomyidae, Octodontidae, and Hydrochoeridae. This information allows refinement of biostratigraphic correlation with other localities of south-eastern Buenos Aires Province. First and last records, as well as evolutionary patterns of some lineages, suggest an important change between the faunas from the Monte Hermoso Formation (including Lithostratigraphic Units I and II) and the Chapadmalal Formation, rejecting the previously suggested similarity of the upper levels of the Monte Hermoso Formation (Unit II, Lower Chapadmalalan) with those of the Chapadmalal Formation (Upper Chapadmalalan). Studies of more lineages including new revisions are necessary to test this proposal. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Caviomorph rodents Biostratigraphy Pliocene Monte Hermoso Chapadmalal

1. Introduction The continental late Cenozoic is represented in central Argentina by a relatively homogeneous sedimentary sequence composed of fine volcanoclastic sediments and massive brownish silts, especially exposed in the southern coast of the Buenos Aires Province (Fig. 1). Fidalgo et al. (1975) made a revision and synthesis of the knowledge of these deposits and, more recently, Zárate (2005) published an update and new interpretations. Some exposures of these sediments, named generally “sedimentos pampeanos” (Fidalgo et al., 1975), are very important because they are the type localities of biostratigraphic units of the South American mammal bearing late MioceneePliocene. During the last years, both magnetostratigraphy and the application of absolute dating techniques supplied significant data for its chronologic calibration (Zárate, 2005). However, in most cases, fossil mammals are still the best tools for the correlation of different exposures.

* Corresponding author. Tel.: þ54 221 4257744x129; fax: þ54 221 4257527. E-mail addresses: [email protected] (C.M. Deschamps), vucetich@fcnym. unlp.edu.ar (M.G. Vucetich), [email protected] (D.H. Verzi), iolivares@ fcnym.unlp.edu.ar (A.I. Olivares). 0895-9811/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2011.10.006

Biostratigraphic units of the South American late Cenozoic are based on mammals. Reliability of these units lies on the authenticity of the stratigraphic position of the collected mammals, and precision of taxonomic diagnosis (Du et al., 1995). In this sense, comprehensive systematic and evolutionary revisions of the recorded species have to be made, because in many cases they have not been revised since the original mentions in the XIXth century. Among mammal faunas, rodents are very useful as biostratigraphic tools because of their wide geographic distribution, and high reproductive and rapid evolutionary rates (Vianey-Liaud et al., 2011). These characteristics, together with the large number of remains in palaeontological sites, turn this group into a basic tool for dating continental Cenozoic sediments worldwide (Fejfar and Heinrich, 1990; Woodbourne, 2004). For example, the biostratigraphy of the European Neogene continental deposits is based mostly in rodents (e.g. Mayhew and Stuart, 1986; Agustí, 1990; Cuenca-Bescós et al., 1992, 1997, 2010; Fejfar et al., 1997, 1999; Hernández Fernández et al., 2004; Minwer-Barakat et al., 2011). Despite there is much left to do, the improvement achieved through the study of caviomorph rodents over the last years allowed refining calibrations like that of the late Miocene of central Argentina (Verzi et al., 2008). Concerning the Pliocene, two key

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Fig. 1. Location map showing the studied area. White circles, cities/villages; black squares, fossil localities mentioned in the text.

sites for the knowledge of the fossil mammal faunas are Farola Monte Hermoso and Chapadmalal area, known since the XIXth century (Fig. 1). Two faunas have been recognized in Farola Monte Hermoso, and the relationships between them and with that of the Chapadmalal Formation have been long debated. In this paper, we report an update of the systematics of several rodent taxa found in Farola Monte Hermoso which allowed assessment of previous correlations and a proposal of a new one. 2. Methodology The study of the rodents from Farola Monte Hermoso implied the revision of related taxa found in other localities, mainly in the Pampean region such as the area of Chapadmalal, Quequén Salado River, and Calera Avellaneda (Fig. 1). Part of this work was accomplished during revisions of the families Echimyidae (including Myocastoridae), Ctenomyidae, Octodontidae and Hydrochoeridae (see 7. Systematic list) made over the last 15 years (Deschamps et al., 2007, 2009; Olivares, 2009; Prado et al., 1998; Verzi, 1994, 1999, 2001, 2002, 2008; Verzi et al., 2002; Vucetich et al., 2005). These revisions involved exhaustive morphological analyses of type specimens and assigned materials, as well as new unpublished materials collected by the authors. Advances in the knowledge of caviomorph rodents over the last years, both in formal systematics and in their patterns of morphological change, allowed improving substantially the knowledge of the fossil record.

On the other hand, the exact stratigraphic provenance of fossils is essential in a biostratigraphic approach. Materials with dubious references were not taken into account, or are explained in the text. Institutional abbreviations: MACN, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Buenos Aires, Argentina; MD, Museo Municipal de Ciencias Naturales “Charles Darwin”, Coronel Rosales, Argentina; MLP, Facultad de Ciencias Naturales y Museo de La Plata, Universidad Nacional de La Plata, La Plata, Argentina; MMP, Museo de Ciencias Naturales de Mar del Plata “Lorenzo Scaglia”, Mar del Plata, Argentina. 3. Historical antecedents Farola Monte Hermoso is located on the coast of southern Buenos Aires province, 12 km west of Pehuén-Có Beach (Fig. 1). Coastal deposits form a marine cliff 12 m maximum height in which fossil-bearing Cenozoic sediments of the Monte Hermoso Formation are exposed. Stratigraphy and paleontology of this unit have been studied since the XIXth century when Charles Darwin visited this locality during his voyage on the HMS Beagle on October 19th 1832 (Darwin, 1846). The fauna was first described by Ameghino (e.g. 1887, 1888, 1889, 1898), who reported near a hundred taxa among amphibians (1 genus), reptilians (two families), birds (three families), marsupials (12 species), carnivores (two species), rodents (35 species), toxodonts (16 species), xenarthrans (16 species), “artiodactyls” and “perissodactyls” (six species). Then, Rovereto

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(1914), Kraglievich (1934, 1946) and afterwards Bordas (1942), Parodi and Kraglievich (1948), Pascual and Bondesio (1961), Patterson and Kraglievich (1960), Reig (1978), Tonni (1974), among others, updated the knowledge of the fauna of Farola Monte Hermoso. Pascual et al. (1965a) based the Montehermosan South American Land-Mammal Age on faunas of this locality. Ameghino (1889, 1898) defined the “Piso Hermósico” for the silty unit of the lower and middle sectors of the exposure at Farola Monte Hermoso and considered it as a single unit. Vignati (1925, 1948) divided the “Piso Hermósico” into two units, the “Hermosense típico” (typical Montehermosan) below, and the “Piso Chapadmalalense” (Chapadmalalan) above, because he correlated the upper unit with the Chapadmalalan Stage of Miramar area. Since these papers, revisions vary between these two patterns (Fig. 2). Leanza (1948) recognized Formación I for the “Hermosense típico”, and formations II and III for the “Piso Chapadmalalense”. Bonaparte (1960) called the lower unit “Hermosense típico” and named the overlying levels as “Limolitas Claras” and “Limolitas Estratificadas” members. Fidalgo and Tonni (1982) defined the Lithostratigraphic Units I and II, and considered the latter more closely related to the Chapaladmalal Formation than to the underlying Unit I or “Montehermosense típico”. Zavala (1993) and Zavala and Navarro (1993) provided a detailed description of the deposits and a lithostratigraphic scheme focused on the facies analysis and paleoenvironmental reconstruction. The Monte Hermoso Formation (Fidalgo et al., 1975; Zavala, 1993) crops out at the abrasion platform and the lower part of the cliff. It is composed of clayey silts and silty sands, with architectural features of channel, lateral accretion and overflow deposits produced by high-sinuosity muddy rivers with high suspension load. The discontinuity observed between the “Hermosense típico” and “Piso Chapadmalense” (Lithostratigraphic Units I and II) would have been produced according to Zavala and Navarro (1993), by migration of the fluvial channel over the surrounding flood plain deposits; this interpretation gives a paleoenvironmental framework to assess the fossil record. Pascual et al. (1966), and Pascual and Odreman Rivas (1971, 1973) stated that the faunas from Monte Hermoso and Chapadmalal formations were not as different from each other as it had been supposed, and proposed to include the Chapadmalalan fauna into the Montehermosan Age. Simpson (1972) instead supported

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the independence of both faunas until a complete revision of Kraglievich’s 1934 paper was accomplished. Reig and Simpson (1972) supported that the fauna of Chapadmalal Formation is more closely related to that of the overlying Vorohué Formation than that of the Monte Hermoso Formation. Marshall et al. (1983) differentiated both ages. Tonni et al. (1992a) and Cione and Tonni (1995a,b,c; 1996; 2001; 2005) published revisions of the biostratigraphy and biochronology of Monte Hermoso Formation at Farola Monte Hermoso. The fauna from Lithostratigraphic Unit I was recognized as the Biozone of Trigodon gaudryi, biostratigraphic basis of the Montehermosan Stage, and the fauna from Lithostratigraphic Unit II, as the Biozone of Neocavia depressidens, biostratigraphic basis of the Lower Chapadmalalan Stage. These authors also proposed the correlation between these units and those of the marine cliffs of south-western Buenos Aires province, especially the Chapadmalal Formation, in which they recognized the Biozone of Paraglyptodon chapalmalensis, biostratigraphic basis of the Upper Chapadmalalan Stage. The last update reported by Cione and Tonni (2005) is as follows (with taxonomic and typographic corrections according to original papers or available revisions): 1- Biozone of T. gaudryi. Exclusive taxa: Notocynus, Parahyaenodon, Nopachthus, Myrmecophaga caroloameghinoi (Kraglievich, 1934), Alitoxodon, Trigodon, Diplasiotherium, Thylamys contrerasi (Mones, 1980), Palaeodaedicurus antiquus (Ameghino, 1887), Auliscomys formosus Reig, 1978. First record: Phtoramys homogenidens Ameghino, 1887; Actenomys priscus (Owen, 1840); Para1888); Eucelophorus myocastor diligens (Ameghino, chapalmalensis Ameghino, 1908; Proscelidodon patrius (Ameghino, 1888), Lestodon and Chapalmatherium. First record of sigmodontine rodents in South America. 2- Biozone of N. depressidens. Exclusive taxa: Necromys bonapartei (Reig, 1978), N. depressidens (Parodi and Kraglievich, 1948), and Sparassocynus bahiai (Mercerat, 1898). First record: Plaina, Palaeocavia. Taxa with last record: P. homogenidens Ameghino, 1887. 3- Biozone of P. chapalmalensis. Exclusive taxa: P. chapalmalensis (Ameghino, 1908), Pronothrotherium, Scelidotheridium, Plohophorus figuratus Ameghino, 1887; Glossotheridium; Dankomys simpsoni Reig, 1978; Graomys dorae Reig, 1978; Telicomys gigantissimus Kraglievich, 1926; Caviodon pozzii Kraglievich,

Fig. 2. Interpretation of the stratigraphy of the locality Farola Monte Hermoso according to different authors (modified from Zavala, 1993).

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1927; Dolicavia minuscula Ameghino, 1908; Microcavia chapalmalensis Ameghino, 1908; Toxodon chapalmalensis Ameghino, 1908; and Platygonus marplatensis Reig, 1952. First record: Tayassuidae, Dolicavia, Ringueletia simpsoni (Bordas, 1933); Sparassocynus derivatus Reig and Simpson, 1972; Thylatheridium cristatum Reig, 1952; Microtragulus reigi Simpson, 1970. Last record: Argyrolagus; Lutreolina tracheia (Rovereto, 1914); Hyperdidelphys; Thylacosmilidae; Doellotatus; Macrochorobates; Chorobates; Macroeuphractus; Plaina; Plohophoroides; Trachycalyptus; Diheterocnus; Proscelidodon; Orthomyctera chapalmalense Ameghino, 1908; Cardiomys; Telicomys; Lagostomopsis; A. priscus; P. diligens (Ameghino, 1888); Brachytherium; Promacrauchenia chapalmalense Ameghino, 1908; Cyonasua; Thylatheridium pascuali Reig, 1958; Xotodon. 4. Update of systematics and stratigraphic range of the involved taxa Among the caviomorph rodents from Farola Monte Hermoso seven families are currently known to be represented: Echimyidae (Fig. 3), Ctenomyidae, Octodontidae (Fig. 4), Hydrochoeridae (Fig. 5), Caviidae, Chinchillidae, and Dinomyidae. In this paper, we focus on the first four because their knowledge has been improved by recent revisions. 4.1. Family Echimyidae The record of this family begins in the Oligocene, and after a moderate diversity reached in the late Miocene, it is represented in the Pampean Pliocene only by the genus Eumysops and another taxon related to Clyomys-Euryzygomatomys within Eumysopinae, and several Myocastorinae. The genus Eumysops, according to the revision of Olivares et al. (2006) and Olivares (2009), is recorded in Farola Monte Hermoso with two species, Eumysops laeviplicatus (Fig. 3A, B) and Eumysops formosus (Fig. 3C, D; Fig. 6). Both species have their first and last record in the Monte Hermoso Formation. Materials with precise stratigraphic provenance indicate that E. laeviplicatus is recorded in both levels (Lithostratigraphic Units I and II). E. formosus is recorded with certainty in the upper levels (Lithostratigraphic Unit II), but

Fig. 4. Occlusal surface of the lower dental series of Eucelophorus cabrerai (A), E. chapalmalensis (B), Phtoramys homogenidens (C), Pseudoplataeomys formosus (D), Pithanotomys columnaris (E) and P. chapalmalensis (F). A, MACN 7294, holotype (reversed); B, MMP 487-S; C, MACN-A 1643; D, MACN-A 1646, holotype; E, MLP 86-VI20-11; F, MACN 11761 (reversed).

remains with no precise stratigraphic provenance suggest not ruling out its record in the lower levels. E. laeviplicatus is also recorded in exposures of the Irene “Formation” of Cascada Grande, Quequén Salado River. In the Chapadmalal Formation, two other species are recorded, E. chapalmalensis (Fig. 3E, F) and Eumysops gracilis (Fig. 3G, H; Fig. 6). A third, undescribed, species of this genus is recorded in the Chapadmalal area, in the Vorohué and San Andrés formations (Olivares, 2009) (Fig. 6). The oldest remains of myocastorines (Patterson and Pascual, 1968; Olivares, 2009; Vucetich et al., in press) come from the “Mesopotamian” of Paraná, Entre Ríos (Fig. 1; late Miocene; Candela

Fig. 3. Occlusal surface of the dental series of Echimyidae. A, B, Eumysops laeviplicatus; C, D, E. formosus; E, F, E. chapalmalensis; G, H, E. gracilis; I, Paramyocastor diligens. A, C, E, G, right upper series; B, D, F, H, I, right lower series. A, MACN-A 1623 (holotype); B, MMP 857-M, (reversed); C, MACN-A 7327 (holotype); D, MACN-Pv 7315; E, MMP 832-M; F, MACNPv 10963 (reversed); G, MLP 62-VII-27-75 (reversed); H, MMP 559 (reversed); I, MLP 46-V-13-104, holotype of Isomyopotamus albañiri.

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Fig. 5. Occlusal surface of the dental series of Phugatherium cataclisticum (A, B, E, G) and P. novum (C, D). A, MD-01-03, right p4-m2-m3; B, MLP 15-231, left M3; C, MMP 236-S, right p4-m3; D, FCS 92-V-15/1, right M3; E, F, P-14282; E, right p4-m3; F, left M3; G, MLP 57-VII-23-2, right M3. Arrows show diagnostic fissures in P. cataclisticum.

and Noriega, 2003). Currently they are represented by a single species Myocastor coypus. Verzi et al. (2002) made the revision of the myocastorids from the Pliocene of Buenos Aires coast, and concluded that the species Paramyocastor intactus, Isomyopotamus affinis, and I. albañiri represent different ontogenetic states of a single species P. diligens (Fig. 3I). As a result of this taxonomic arrangement, the species is present in both levels of the Monte Hermoso Formation as well as in the Chapadmalal Formation (Fig. 6), and levels 21 and 32 of the Corral Quemado Formation in Northwestern Argentina (Ameghino, 1904; Rovereto, 1914; Reig, 1950; Marshall and Patterson, 1981). Thus, P. diligens is not useful

to differentiate biochronologically the studied levels of the Monte Hermoso and Chapadmalal formations. 4.2. Family Ctenomyidae Two lineages that correspond to the genera Actenomys and Eucelophorus represent this family in the early Pliocene. Within the first one, A. priscus is the most derived representative of the lineage Xenodontomys-Actenomys (Verzi, 2008 and literature therein), and is recorded both in the Monte Hermoso and Chapadmalal formations (Fig. 6). Although a revision of the species of this genus has

Fig. 6. Stratigraphic chart with the record of the studied rodents from the Monte Hermoso and Chapadmalal formations. Dotted lines of records mean provenance data without specifying level of the Monte Hermoso Fm. Epochs according to International Commission of Stratigraphy; Stages/Ages according to Cione and Tonni (1995a); L.U., lithostratigraphic unit.

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not been accomplished yet, the cheek teeth of the specimens from the Chapadmalal Formation are more elongated and narrow. This morphology is derived respect to that seen in the specimens from the Monte Hermoso Formation (Deschamps, 2003). The lineage of Eucelophorus is represented in Farola Monte Hermoso by the most primitive representative, Eucelophorus cabrerai (Fig. 4A; Fig. 6), with no other provenance data. In Chapadmalal area, the most derived E. chapalmalensis (Fig. 4B) is recorded both in the Chapadmalal Formation and the overlying Vorohué and San Andrés formations (Verzi, 2002, 2008, Fig. 6). 4.3. Family Octodontidae This family is represented by three genera in the Pliocene exposures of the coast of the Buenos Aires Province, Phtoramys, Pseudoplataeomys and Pithanotomys. Phtoramys includes three species (Ameghino, 1888; Rovereto, 1914; Pascual et al., 1965b). The type species, P. homogenidens (Fig. 4C) is the most modern and is last recorded in Farola Monte Hermoso (Ameghino, 1887, 1888; Verzi et al., 2008, Fig. 6). The genera Pseudoplataeomys and Pithanotomys are still in need of a deep revision. The type species of Pseudoplataeomys, Pseudoplataeomys formosus, is the most modern, and its record in Farola Monte Hermoso (Kraglievich, 1934) represents the last appearance of the genus (Fig. 4D). Ameghino (1887, 1888, 1889) described four species of Pithanotomys from Farola Monte Hermoso, Pithanotomys columnaris, P. similis, P. intermedius and P. macer, and two species from Chapadmalal area (Ameghino, 1908), P. chapalmalensis and P. ortorhynchus. According to a preliminary revision, P. columnaris (Fig. 4E) and P. macer are accepted here as valid species from the Monte Hermoso Formation; the first one is recorded in both Lithostratigraphic Units I and II, whereas P. macer lacks precise stratigraphic provenance. P. cf. columnaris is recorded as well in the “Irenense” of the Quequén Salado River. A more derived species, P. chapalmalensis (Fig. 4F), is recorded in the Chapadmalal, Vorohué and San Andrés formations (Fig. 6). 4.4. Family Hydrochoeridae Several genera and species have been described for the Pliocene. In the revision of the family, Mones (1991) considered Protohydrochoerus perturbidum, Phugatherium cataclisticum and Anchimysops villalobosi as valid species for Farola Monte Hermoso, and raised doubts about the validity of other taxa based on isolated teeth, or single specimens like Anchimysops ultra. Vucetich et al. (2005), in a revision based on characters of molars and mandibular morphology proposed that the materials assigned to P. cataclisticum and A. villalobosi belong to juvenile specimens, probably of Chapalmatherium perturbidum (¼Protohydrochoerus perturbidus, see Prado et al., 1998). By priority, the name of the species from Farola Monte Hermoso must be P. cataclisticum (Fig. 5A, B, E, G) (Vucetich and Deschamps, in press). For the area of Chapadmalal, Mones (1991) reports only Chapalmatherium novum. Prado et al. (1998), when studying two hydrochoerid skulls collected in Calera Avellaneda, Buenos Aires province (Fig. 1), revised the materials assigned to Chapalmatherium and concluded that most characters used to distinguish the other two species of the genus, Chapalmatherium irenense and Chapalmatherium saavedrai, described on materials found in the Quequén Salado River and Bolivia respectively (Fig. 1), correspond to intraspecific variation. These authors synonymyzed these species with C. novum and assigned the skulls from Calera Avellaneda to the latter. In addition, they synonymyzed the genus Protohydrochoerus, described for Farola Monte Hermoso, with Chapalmatherium. Part of the variation observed by Prado et al. (1998) also corresponds to

ontogenetic variation. According to the proposal of Vucetich et al. (2005), the name of the species from the Chapadmalal Formation must be Phugatherium novum (Fig. 5C, D). The assignment of the materials from Calera Avellaneda to the species from Chapadmalal (P. novum) is correct, because the third upper molar (M3) lacks deep external fissures in prisms 4e6 and the fourth premolar (p4) lacks fifth internal fissure (h.5i.; Fig. 5), but the materials from the Quequén Salado River (“Chapalmatherium irenense”, P-14282, Chicago Natural History Museum, Fig. 5E, F and MLP 57-VII-23-02, Fig. 5G) are similar to those from Monte Hermoso (P. cataclisticum ¼ C. perturbidum) because of the M3 with very deep external fissures in prisms 4e6, and a deep h.5i. in p4 (Fig. 5G). The absence of such deep fissures in the M3 of the type specimen (Fig. 5F) could be due to its juvenile condition (M3 anteroposterior length ¼ 31 mm vs. ca. 60 mm of MLP 57-VII-23-2, estimated because of the lack of the first prism). Consequently, C. irenense is a junior synonym of P. cataclisticum and not of P. novum. 5. Correlations The information supplied by the updated systematics of the studied rodents (7. Systematic list), their first and last records, and also their evolutionary patterns, suggest that the biostratigraphic correlations so far proposed may be refined (Fig. 6). Among the echimyids, E. laeviplicatus and E. formosus have their first and last records in the Monte Hermoso Formation and Irene “Formation”; other three species of Eumysops are recorded in the Chapadmalal, Vorohué, and San Andrés formations. Myocastorines are represented by a single species, P. diligens, in the Monte Hermoso and Chapadmalal formations; consequently, this species does not support biochronological differences between the studied units and localities as previously proposed (e.g. Tonni et al., 1992b). Regarding the ctenomyids, although the genus Actenomys is recorded in both levels of the Monte Hermoso Formation and in the Chapadmalal Formation, its presence is significant because it allows constraining the maximum age of the bearing levels. This genus is presumably a derivative from the genus Xenodontomys; hence, its record suggests a younger age than that of the Cerro Azul Formation of La Pampa province, assigned to the Huayquerian. Through biochronology, an age of 5.7e5.8 Ma (late Miocene; Verzi et al., 2008) has been assigned to the most derived Xenodontomys species (X. elongatus). If this age is confirmed, the age of the exposures of the Monte Hermoso Formation should be younger, as originally proposed by Cione and Tonni (1995a, b, 1996). The octodontid P. homogenidens has its last record, but not the first one, in the Monte Hermoso Formation. The genus Pithanotomys is represented by the species P. columnaris and Pithanotomys macer in the Monte Hermoso Formation and by P. chapalmalensis in the Chapadmalal Formation. With regard to the family Hydrochoeridae, one species is recognized for the locality Farola Monte Hermoso, P. cataclisticum, and another one for Chapadmalal Formation, P. novum. Although the caviid rodents from the Monte Hermoso Formation have not been revised since their original descriptions, some comments can be made. The holotype of N. depressidens (MLP 46-V13-53) is currently lost. No other specimen referable to this species has been found in the palaeontological collections from the Monte Hermoso Formation. Paleocavia is not first recorded in the N. depressidens Biozone since it was also found in late Miocene sediments of the Cerro Azul Formation in Laguna Chillhué and Caleufú localities, La Pampa Province (Fig. 6; Montalvo et al., 1995). Besides caviomorphs, other mammal taxa used in the definition of these biozones provide additional information. The holotype and the only other three remains of the marsupial S. bahiai have no precise provenance within the Monte Hermoso Formation (Reig

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and Simpson, 1972). The dasypodid Plaina is not a valid taxon, being a synonym of a genus also recorded in older units (Gois and Scillato Yané, pers. communication). Consequently, among the taxa mentioned for the N. depressidens Biozone, only the sigmodontine N. bonapartei would be exclusive. The dasypodid R. simpsoni, mentioned as first recorded in the Paraglyptodon chapadmalensis Biozone, and the sigmodontine rodents, considered as first recorded in the T. gaudryi Biozone, have been recently reported from Huayquerian deposits of the Cerro Azul Formation (Urrutia et al., 2008; Verzi and Montalvo, 2008). The glyptodontid P. figuratus, considered as exclusive taxon of the P. chapalmalensis Biozone, is also recorded in both levels of the Monte Hermoso Formation (Zamorano and Scillato-Yané, 2011) and considered by Ameghino (1887) as the most common glyptodont of the Monte Hermoso Formation. The correlation of the “Irenense” (Kraglievich, 1934) of the Quequén Salado River has been long debated (see Marshall et al., 1983: 37; Goin et al., 1994). Rodent taxa here studied suggest levels of different age. Those bearing Actenomys, E. laeviplicatus, Pithanotomys cf. columnaris, and P. cataclisticum can be correlated to the Monte Hermoso Formation, instead of the Chapadmalal Formation as previously suggested. Those levels bearing Xenodontomys would be older (Verzi et al., 2008). The record of S. bahiai in the Irene Formation supports the correlation of the bearing levels with the Monte Hermoso Formation, since this species is not recorded in the Chapadmalal Formation, where S. derivatus was found (Reig and Simpson, 1972). This proposal also suggests a new interpretation for the age of the deposits of Calera Avellaneda bearing the skulls of P. novum (see Prado et al., 1998). The skulls were found together with the toxodontid T. gaudryi. Based on the association of P. novum and T. gaudryi, Tonni et al. (1992a) and Prado et al. (1998) assigned a Montehermosan age to the deposits of Calera Avellaneda. However, the biostratigraphy based on this revision of hydrochoerids suggests that the levels bearing P. novum of Calera Avellaneda should be correlated with the Chapadmalal Formation. Consequently, the biochron of T. gaudryi would be extended accordingly. 6. Conclusions Systematics and biostratigraphic revisions of different species and lineages of caviomorphs of the families Echimyidae, Ctenomyidae, Octodontidae and Hydrochoeridae allow a reassessment of the biostratigraphy of the Monte Hermoso Formation. These improvements in the knowledge of caviomorphs suggest that: 1) the species dealt here do not support previous hypotheses that proposed a stronger similarity between the upper levels of the Monte Hermoso Formation (Lithostratigraphic Unit II) and those of the Chapadmalal Formation; 2) on the contrary, the strongest similarities are found between the faunas of the lower and upper levels of the Monte Hermoso Formation, whereas the strongest differences are found between the faunas of the Monte Hermoso Formation as a whole and those of the Chapadmalal Formation (see Reig and Simpson, 1972: 520); 3) some species upon which the difference between Unit I and II of Farola Monte Hermoso was based, are not valid; 4) the interpretation of the N. depressidens Biozone as part of the Chapadmalalan Stage/Age is not supported (Fig. 6). Information on other taxa used in the definition of the N. depressidens Biozone is consistent with this proposal. These conclusions agree with Zavala’s (1993: 234) interpretation of the Monte Hermoso Formation, as sedimentary facies genetically related, without evidence of significant discontinuities. Further prospecting and systematic revisions are necessary to test the existence of two different biostratigraphic units in the Monte Hermoso Formation.

7

Acknowledgements We thank A. Dondas (MMP), A. Kramarz (MACN), T. Manera (MD), and M. Reguero (MLP) for access to material under their care. Valuable suggestions and comments from two anonymous reviewers helped to improve this manuscript. ANPCyT PICT 200538112 and 2007-01744, CONICET PIP 0270, and Subsidios automáticos UNLP N 568, partially funded this paper. Appendix. Systematic list Numbers in bold between parenthesis refer to the number of specimens studied. Order RODENTIA Bowdich, 1821 Suborder HYSTRICOMORPHA Brandt, 1855 Infraorder HYSTRICOGNATHI Brandt, 1855 Superfamily OCTODONTOIDEA Simpson, 1945 Family Echimyidae Gray, 1825 Subfamily “Eumysopinae” Rusconi, 1935 Genus Eumysops Ameghino, 1888 non Wilson, 1935 Eumysops laeviplicatus Ameghino, 1888 (13) Eumysops formosus (Ameghino, 1906) (4) Eumysops chapalmalensis (Ameghino, 1908) (186) Eumysops gracilis (Ameghino, 1908) (54) Eumysops sp. nov. (8) Subfamily Myocastorinae Ameghino, 1904 (see Patterson and Pascual, 1968) Genus Paramyocastor Ameghino, 1904 Paramyocastor diligens (Ameghino, 1888) (10) Family Ctenomyidae Lesson, 1842 Genus Actenomys Burmeister, 1888 Actenomys priscus Owen, 1840 (268) Genus Eucelophorus Ameghino, 1908 Eucelophorus cabrerai Kraglievich, 1927 (5) Eucelophorus chapalmalensis Ameghino, 1908 (30) Family Octodontidae Waterhouse, 1839 Genus Phtoramys Ameghino, 1887 Phtoramys homogenidens Ameghino, 1887 (1) Genus Pseudoplataeomys Kraglievich, 1934 Pseudoplataeomys formosus Kraglievich, 1934 (1) Genus Pithanotomys Ameghino, 1887 Pithanotomys columnaris Ameghino, 1887 (10) Pithanotomys macer Ameghino, 1888 (1) Pithanotomys chapalmalensis Ameghino, 1908 (11) Family Hydrochoeridae (Gray, 1825) Gill, 1872 Genus Phugatherium Ameghino, 1887 Phugatherium cataclisticum Ameghino, 1887 (9) ¼Chapalmatherium perturbidum, Chapalmatherium irenense Phugatherium novum (Ameghino, 1908) (11) ¼Chapalmatherium novum

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