Revista Geológica de Chile, Vol. 34, No. 2, p. 199-207, July 2007.
The first rodent from the Marino Formation (Miocene) at Divisadero Largo (Mendoza, Argentina) and its biochronological implications
El primer roedor de la Formación Marino (Mioceno) en Divisadero Largo (Mendoza, Argentina) y sus implicancias biocronológicas.
Esperanza Cerdeño
DepartamentodeGeologíay Paleontología, IANIGLA-CRICYT(CONICET), Avda. Ruiz Leal sln, 5500 Mendoza, Argentina espe@lab.cricyt.edu.ar
M. Guiomar Vucetich
Departamento Científico de Paleontología de Vertebrados, Facultad de Ciencias Naturales y Museo Universidad Nacional de La Plata, Paseo del Bosque sln, 1900 La Plata, Argentina. vucetich@fcnym.unlp.edu.ar
ABSTRACT
A new significant mammal fossil from the Marino Formation in the area of Divisadero Largo (Mendoza, Argentina) is described herein. The material consists of a mandibular fragment with the right series p4-m3 of the rodent Scleromys sp. It is compared with the Santacrucian species S. osbornianus Ameghino and S. angustus Ameghino, and with S. quadrangulatus Kramarz from the Pinturas Formation, all of them from the Santa Cruz Province. At the same time, the studied fossil presents a combination of characters that differ from those species: more transverse molars, with anterior wall slightly curved, lower degree of hypsodonty and smaller size. However, due to the great morphological variation within Scleromys, a specific determination is not provided based on a sole specimen. The presence of this genus, together with the previously studied mesotheriine, supports an early Miocene age for the middle member of the Marino Formation.
Key words: Rodents, Marino Formation, Early Miocene, Mendoza, Argentina.
RESUMEN
Se describe un nuevo resto de mamífero procedente de la Formación Marino en el área de Divisadero Largo (Mendoza, Argentina). Corresponde a un fragmento mandibular de roedor, con la serie p4-m3 derecha, identificado como Scleromys sp. Se lo compara con las especies santacrucenses, S. osbornianus Ameghino y S. angustus Ameghino de la Formación Santa Cruz y la S. quadrangulatus Kramarz de la Formación Pinturas, todas ellas de la Provincia de Santa Cruz. El ejemplar de Mendoza presenta una combinación de caracteres propia (molares más transversos, con la pared anterior ligeramente curva; menor desarrollo de la hipsodoncia; tamaño más pequeño) que lo diferencian de las especies anteriores. Sin embargo, debido a la gran variación morfológica observada en el género, se prefirió no hacer una determinación específica basada en un único ejemplar. La presencia de Scleromys sp., junto a la del mesoterino previamente descrito, apoya una edad miocena temprana para el miembro medio de la Formación Marino.
Palabras claves: Roedores, Formación Marino, Mioceno Temprano, Mendoza, Argentina.
INTRODUCTION
The Cenozoic outcrops of the Divisadero Largo area (Fig. 1), located 8 km west of the city of Mendoza (Mendoza Province, Argentina), provided a rich fauna in sediments of the Divisadero Largo Formation (Rusconi, 1946 a, b; Minoprio, 1947; Patterson, 1952; Simpson et al., 1962; López, 2002), which gave place to the definition of the Divisaderan land mammal age, assigned to Late Eocene-Early Oligocene (Pascual et al., 1965). The uniqueness of this mammal assemblage, however, is now under revision, since some data suggesta possible mixture of material from different stratigraphical origin (Cer-deño etal., 2005; López and Manassero, 2006).
Recently, the finding of a mesotheriid (Notoun-gulata) tooth series and several vertebral remains in the overlying Marino Formation has renewed the interest of this paleontological area, since they were the first mammal remains undoubtedly coming from this formation (Cerdeño et al., 2006; Cerdeño, in press). The Marino Formation (Biondi, 19361; Rolleri and Criado Roque, 1970) has been ascribed to the Miocene, and is also known in other zones near the city of Mendoza, such as the Cacheuta-Potrerillos area, from where Rusconi (1949) described the freshwater bivalve Corbicula elchaensis Rusconi. Sepulveda (1999) described some mi-crofloras from Salagasta area (northern Mendoza), which allowed him to assign a Late Oligocene-Early Miocene age to the Marino Formation. In addition, recent prospective studies in the Potre-rillos area provided some paleobotanical remains and a series of undetermined ichnites (Zavattieri et al., 2001 )2. Some ichnites have been also reported from fine sandy levels of Sierra de las Higueras, near Salagasta, in a unit correlated with the upper member of the Marino Formation (Ahumada, 2004). They were preliminarily assigned to an ungulate mammal, but a detailed study is still lacking.
As explained by Cerdeño et al. (2006), the only mention of a mammal specimen recovered from the Marino Formation, before the new records, corresponds to a specimen of the Proterotheriidae Licaphrium cf. floweri Ameghino from the Estratos de Marino in Cerro Cacheuta, probably Santa-crucian (late early Miocene) in age (Rolleri, 19503 in Yrigoyen, 1993). However, this specimen has never been described, localized, nor mentioned by later authors who have referred to the Cenozoic outcrops and faunas of Mendoza (Pascual and Odreman Rivas, 1973; Rolleri and Fernández Garrasino, 1979; Pascual and de la Fuente, 1993; Soria, 2001).
After the finding of the mesotheriid (Cerdeño et al., 2006), several field seasons have been developed. Although the thick sediments at Divisadero Largo are poor in fossils, we were successful in finding new fossil remains: some bony fragments too incomplete or insignificant for an accurate determination, an invertebrate trace (probable Skolithos; R. Melchor, personal communication, 2005), and, more importantly, a second significant specimen (Cerdeño and Vucetich, 2006), the fragmentary rodent mandible here described, which increases the Incipient knowledge of the Marino Formation mammal assemblage, and allows the refinement of the chronology of this formation. The material is stored at the Museo de Ciencias Naturales yAntropológicas'J.C.Moyano'(MCNAM) In Mendoza.
GEOLOGICAL AND CHRONOSTRATIGRAPHCAL SETTING
The Marino Formation, defined by Biondi (1936, nom. subst. Rolleri and Criado Roque 1970), is formed by three members: Conglomerados Violáceos at the bottom, Areniscas Entrecruzadas or Areniscas Inestratificadas in the middle of the section, and Estratos de Marino orSerie del Higueral In the upper section (Chiotti, 1946).
From a paleoenvironmental point of view, the lower and upper members correspond to fluvial and alluvial levels, deposited under arid and semi-arid conditions, whereas the middle member is assigned to aeolian deposits (Irigoyen, 1997).
The geological context and the stratigraphic profile of the Marino Formation in the Divisadero Largo area are fully detailed in Cerdeño et al. (2006). The new fossil material comes from about 45 m higher in the section than the mesotheriid remains, which come from the so-called level FM1 cited by those authors. The newfossiliferous point, FM3, is placed at 32° 52.684'S and 68° 56.453'W, at 1,081 m altitude, and presents the same characteristics as FM1, brown-grey sandstones of homogeneous texture, with massive stratification.
The age of the Marino Formation has been differently interpreted. Although firstly was thought to be Oligocene in age (Groeber, 1951), most authors have considered it as Miocene and correlated it with the Agua de la Piedra Formation, cropping out in Malargüe (south of Mendoza), and originally considered Late Oligocene-Early Miocene in age (Rolleri and Criado Roque, 1968; Gorroño et al., 1979; Yrigoyen, 1993) and more recently Middle Miocene (Combina et al., 1997; Combina and Nullo, 1999). Sepulveda (1999), based on palinologic data, also suggested a Late Oligocene-EarlyMioceneagefortheuppermember of the formation (Estratos de Marino). On the other hand, Irigoyen (1997) and Irigoyen et al. (2000, 2002) obtained an isotopic age of 12.03±0.45 Ma forthe uppermost part of the Marino Formation, and established a range of 15.7-12.2 Ma for the whole formation based on paleomagnetic correlations; this time span corresponds to the Middle Miocene, Colloncuran and Laventan ages (Flynn et al., 2002; Pascual et al., 2002). Moreover, the mesotheriid from the Marino Formation is very similar to a species from the Chucal Formation (Chile) (Cerdeño, in press), ascribed to the Santacrucian age, of the late Early Miocene (Croft et al., 2004). The rodent here described supplies new data concerning the age of the fossil-bearing levels.
SYSTEMATIC DESCRIPTIONS
| Order Rodentia Bowdich, 1821 Suborder Hystricognathi Tulberg, 1899 Superfamily Chinchilloidea Kraglievich, 1940 Family Dinomyidae Peters, 1873 Genus Scleromys Ameghino, 1887 |
Type species: Scleromys angustus Ameghino, 1887. Patagonia, Pinturas and Santa Cruz formations, late Early Miocene.
Scleromys sp. Fig. 2, A-D
Material: MCNAM-PV 3655, right mandibular fragment with p4-m3 of a young adult individual. Geographic and stratigraphic provenance:
Divisadero Largo, Mendoza; Middle Member of the Marino Formation (Fig. 1).
Description: The p4 is somewhat longer than wide (Table 1), with the talonid slightly wider than the trigonid. The occlusal surface is clearly S-shaped because only hypo- and metaflexid are still open. The anterior wall is straight. The metalophid is very curved and there is a small anterofossettid at the antero-lingual angle.
The ml and m2 are similar in shape, but m2 is larger and more transverse. The mesoflexid is closed in ml, but still open in m2, and there is no anterofossettid in either of them.
The m3 is the smallest cheektooth (Table 1). It is little worn and thus the posterior lamina is isolated. A remnant of the metalophid is still distinguishable, but in this degree of wear it does not form an anterofossetid.
Mandible: The notch for the tendon of the M. masseter medialis pars anterior (Woods and Howland, 1979) is shallow, and the mental foramen is round and placed slightly ahead of p4. The alveolus of i1 extends backward to a point below the anterior portion of m3.
Comparisons: Scleromys sp. differs from the Santacrucian species, S. osbornianus Ameghino and S. angustus Ameghino, in its smaller size, particularly from the latter, the largest species of the genus (Table 1), and lesser degree of hypsodonty. Molars of both Santacrucian species are slightly longer than wide or subquadrangular in shape,with convex walls; the ml in particular is almost subcircularwhen highly worn. In Scleromys sp. the molars are more transverse, specially the m2, and the anterior walls are straighter. The notch for the tendon of the M. masseter medialis pars anterior and the mental foramen are less conspicuous and proportionallysmallerthan in S. osbornianus(these characters are not known in S. angustus). S. qua-drangulatus Kramarz from the Pinturas Formation (Early Miocene, Santa Cruz Province; Fleagle et al., 1995; Kramarz, 2001, 2006; Kramarz and Bellosi, 2005) differs from Scleromys sp. in its somewhat larger size and less transverse cheek teeth with almost straight anterior wall, but has a similar degree of hypsodonty, as attested by x-ray studies and comparisons between isolated and partially encased molars, as well as size and disposition of the notch for the tendon of the M. massetermedialis pars anterior and the mental foramen.
DISCUSSION AND CONCLUSIONS
The rodents of the genus Scleromys are here included in the Family Dinomyidae, which in turn is assigned to the Superfamily Chinchilloidea according to Horovitz et al. (2006). Scleromys is known from the different levels of the Pinturas Formation (Kramarz and Bellosi, 2005; Kramarz, 2006) and the Santacrucian levels of Patagonia. Two species of Scleromys were recognized for the Laventan (Middle Miocene, Colombia; Fields, 1957), but this generic assignment has been repeatedly challenged (Patterson and Wood, 1982; Walton, 1997; Kramarz, 2001).
The species of Scleromys are protohypsodont with a great amount of dental morphological change with ontogeny, basically due to flexilids closure, fossettelids disappearance, and change in molar (specially ml) size and outline. This makes an accurate determination difficult when dealing with only one specimen. Forthis reason, the new material Is not determined at specific level.
The Santacrucian species S. osbornianus Ameghino and S. angustus Ameghino are more derived than the one represented by the new material from Mendoza in their greater degree of hypsodonty. In contrast, this lesser hypsodonty is similar to that displayed by S. quadrangulatus Kramarz from the Pinturas Formation, suggesting they may represent a similar evolutionary stage, and that they could be similar in age. In fact, the above mentioned differences in dental morphology between Scleromys sp. and S. quadrangulatus (specially very transverse molars and moderately straight anterior wall) might be due to the different degree of wear displayed by the known specimens, and eventually both taxa might prove to pertain to the same species.
Scleromys quadrangulatus comes from the lower and middle sequences of the Pinturas Formation, and it is replaced in the upper member by the Santacrucian species S. osbornianus (Kramarz and Bellosi, 2005; Kramarz, 2006). Lower and middle sequences bear the typical Pinturan fauna ('faune Astrapothericuléen'), considered by Ameghino (1906) older than the Santacrucian faunas, and informally recognized by some authors as Pinturan assemblage, mainly based on rodents (Kramarz and Bellosi, 2005). Contrarily, the upper member of the Pinturas Formation has been assigned to the Santacrucian based on its hys-tricognath rodent assemblage (Kramarz and Bellosi, 2005). These authors stated that the Pinturan rodents are more primitive than those from the Santacrucian, the lesser degree of hypsodonty being one of the most conspicuous differences, such as the one seen between S. quadrangulatus and S. osbornianus.
On the other hand, the previously known mesotheriid (Cerdeño, in press) is very similar if not identical to one species from the Chucal Formation (Chile), which has been assigned to the Santacrucian age (Croft et al., 2004), constrained between 17.4 and 18.79 or 21.7 Ma (based on different isotopic data).
Croft et al. (2004) stated that the time interval of the Chucal fauna overlaps or slightly pre-dates the Santacrucian age, but they did not consider the Pinturan fauna as a different mammal association. However, the radioisotopic data allow some equivalence with the Pinturas Formation and, in addition, other taxa establish differences (e.g., mesotheriid diversity, absence of interatheriids) with respect to the well known Santacrucian samples in Argentina and southern Chile, attributed by Croft et al. (2004) to ecological andlor biogeographic factors. In the uppermost part of the Sarmiento Formation at Gran Barranca (Chubut), there is a faunal assemblage similar to the Pinturan fauna, ranging between 18.28 and 18.78 Ma (Madden et al., 2005; Vucetich et al., 2005; Carlini et al., 2005). Therefore, the Pinturan assemblage could range from 18.78 to 16.5 Ma; theChucal Formation would be included in this temporal range.
The Marino Formation specimens support a preliminary biostratigraphic correlation between the middle member of thatformation and the Chucal fauna, on one hand, and the Pinturan fauna on the other, although the scanty material from Marino Formation precludes a finer analysis. Since the fossils of the Marino Formation indicate an Early Miocene age, this implies a longer temporal extension than that supposed by Irigoyen (1997) and Irigoyen et al. (2000, 2002).
A detailed comparison between the Chucal and Pinturan faunas is needed, but it is beyond the aim of this paper. Future findings in the Marino Formation, as well as a better understanding of mammals other than rodents form the Pinturas Formation and the description of mammals other than notoungulates from Chucal, will help to elucidate more precisely the biostratigraphic and biogeographic relationships among these three Early Miocene faunal assemblages. The geographic location of the Marino Formation, intermediate between Chucal and Patagonian areas, will eventually permit a better understanding of the biogeographic history of the Miocene South American faunas.
ACKNOWLEDGEMENTS
Special thanks are due to S. Devincenzi, E. Aránguez and R. Mons (Museo de Ciencias Naturales y Antropológicas 'J.C. Moyano', Mendoza), A. Acosta (Dirección de Recursos Naturales Renovables, Mendoza), and V. Silione, for their collaboration in the field studies and discovery of the specimens. B. González Riga (IANIGLA-CRICYT, Mendoza) helped with the geological data. A.G. Kramarz (Museo Argentino de Ciencias Naturales 'B.Rivadavia', Buenos Aires), gently permitted the study of unpublished material from the Pinturas Formation. The useful comments of the reviewers (D. Croft, A. Kramarz, and V. Ramos) have greatly improved the manuscript. R. Marín and G. Farias (MAGRAF-CRICYT, Mendoza) photographs. This research has been partially prepared the figures. A. Karam helped with the Rx supported by CONICET.
NOTAS
1Biondi, J.L. 1936. Exploraciones geológicas en las inmediaciones del cerro Cacheuta. Unpublished report, Yacimientos Petrolíferos Fiscales, Buenos Aires
2Zavattien, A. M.; Cerdeño, E.; Bottero, R.; Londero, S.2001. Estudio, prospecciony propuesta de rescate y protección de yacimientos paleontológicos en el área de Potrerillos y zonas aledañas, Mendoza. Unpublished report, Ministerio de Ambiente y Obras Públicas de Mendoza: 133p
3 Rolleri, E.0.1950. Informe Geológico de la Hoja 23c, Mendoza. Unpublished report, Dirección General de Yacimientos Petrolíferos Fiscales: 146 p.
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Manuscript received: August 30, 2006; accepted: May 2, 2007
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