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Fossil land mammals from ten localities of southern Buenos Aires Province, Argentina …


Biology Articles » Paleobiology » Late Cenozoic mammal bio-chronostratigraphy in southwestern Buenos Aires Province, Argentina » Results

Results
- Late Cenozoic mammal bio-chronostratigraphy in southwestern Buenos Aires Province, Argentina

1 and 2. Grünbein. Two quarries near Grünbein village, approximately 5 km SE from Bahía Blanca city (38°46'S-62°11'W), were studied. In both quarries ?Pampean Sediments? assigned to the Saldungaray Formation crop out, with several intercalations of paleosoils, bioturbations, ?escorias? (scoria) and crotovines (figures 3.A-B). Profiles and the list of recovered taxa were described by Deschamps et al . (1998). 1. Cantera Seminario (38°44'08''S-62°12'19''W) is 11 m thick and was divided into five levels mostly because of the presence of calcrete. Fossils were found in the upper part of level 2 below the calcrete crust (figure 3.A). They are isolated scutes of Doellotatus cf. D. inornatus , D. cf. D. praecursor , Chorobates , and skull and mandible fragments of Paedotherium cf. P . minor , Tremacyllus cf. T. impressus , Dolicavia , Neocavia , Palaeocavia , Lagostomus ( Lagostomopsis ), Xenodontomys ellipticus , and Phtoramys cf. P. hidalguense (see Appendix 1). 2. Cantera Relleno Sanitario (38°46'24''S-62°09'25''W) has 8 m mean thickness divided into five levels on the basis of calcrete crusts and paleosoils. The materials were recovered from two contiguous levels (figure 3.B). The lower one is a paleosoil that yielded mandible fragments of cf. Borhyaenidium and Phtoramys cf. P . hidalguense , scutes of Macrochorobates , and metapods of Promacrauchenia . Level 2 is the calcrete level overlying the paleosoil and yielded scutes of Chasicotatus cf. C. peiranoi , Chorobates villosissimus , Macrochorobates , Macroeuphractus cf. M. morenoi , Aspidocalyptus , and Berthawyleria , mandible fragments of Paedotherium cf. P. minor , Tremacyllus cf. T . impressus , and part of the skull of Promacrauchenia .

The age of both localities is here reinterpreted on the basis of the known biochrons (see Appendix 2 and Comments) and especially, on new studies of the rodent Xenodontomys ellipticus (Verzi et al ., 2003, 2004c). A Late Huayquerian Age (Late Miocene) is proposed for these deposits.

3. Barrancas de Sarmiento. This locality is a cliff about 7 m high at Sarmiento Street in Bahía Blanca city (38°42'05''S-62°15'51''W). It is composed of brownish-reddish, sandy silts (?Pampean Sediments?) assigned to the Saldungaray Formation, with levels of carbonates, ephemeral fluvial deposits and paleosoils intercalated (González, 1984; Verzi and Deschamps, 1996). Fossils were found at a single level (figure 3.C), at approximately 1 m from the base: one scute of Eutatini, one isolated cheek tooth of Caviidae indeterminate, a fragment of skull and isolated teeth of Paedotherium bonaerense , isolated teeth of Lagostomus ( Lagostomopsis ), and several fragments of skull and palate of Xenodontomys ellipticus (Verzi and Deschamps, 1996).
Recent studies of the enamel microstructure of X. ellipticus (Verzi et al ., 2003; 2004c) suggested that the specimens of this locality would be coeval with those of Cantera Seminario (Grünbein) and older than those found in sediments of the Quequén Salado river, approximately 2° farther east. Consequently the age proposed for this locality is Late Huayquerian (see Appendix 2, Comments, and Verzi et al ., 2004a).

4. Las Obscuras. This site, on the right margin of the Sauce Grande river valley, was exposed during railway works, 2000 m from the bridge of national route N° 3 over the river (Deschamps et al ., 1989). The whole sequence is formed by ?Pampean Sediments? assigned to the Saldungaray-La Toma Formations, grayish sandy silts with compact levels of calcium carbonate (locally named as ?tosca?), paleosoils, crotovines up to 1,5 m diameter, that have not been lithostratigraphically defined. It is divided into three units, of which only unit 2 yielded fossil mammals (figure 3.D). Specimens found were a caudal tube and part of the carapace of Plohophorus cuneiformis , associated upper and lower cheek teeth of Pseudotypotherium , a mandible of Actenomys priscus , maxilar and mandible of Lagostomus ( Lagostomopsis ), a mandible of Orthomyctera cf. O. lacunosa , a fragment of palate of Dolicavia , and one astragalus of cf. Epitherium laternarium .
The record of Actenomys (see Comments), plus the absence of Xenodontomys , and on the basis of P. cuneiformis , the fossiliferous level of this locality was assigned to a Montehermosan Age (Early Pliocene) (see Appendix 2 and Comments).

5. Dique Paso Piedras. This locality is placed near the Paso Piedras dam, 2 km from route 51. The exposure is 30 m thick of ?Pampean Sediments? assigned to the Saldungaray Formation. A high terrace level formed by coarse conglomerates, partly carbonated, discontinuous along the Sauce Grande river valley, is known as La Delta Sequence (Zavala and Quattrocchio, 2001) (figure 4). Fossils were very scarce in both units. The Saldungaray Formation yielded a mandible fragment of Iguania indet., mandible fragments of Paedotherium bonaerense , aff. Pithanotomys and Caviidae indet. In La Delta Sequence, a scute of Plohophorini indet. and a mandible fragment of Microcavia were found.
The material of Pithanotomys of the Saldungaray Formation is not as derived as the materials from the Chapadmalal Formation in its type locality (3.3 Ma, Schultz et al ., 1998) suggesting a Montehermosan Age, in agreement with Paedotherium bonaerense . The record of a Plohophorini (a tribe known from the Hauyquerian to the Early Marplatan) in the overlying La Delta Sequence (see Appendix 2) and the unconformity between both units (figure 4), constrains its age. The Saldungaray Formation would be Late Huayquerian to Montehermosan (Late Miocene-Early Pliocene) and the La Delta Sequence would be Chapadmalalan to Marplatan.

6. Balneario Saldungaray. This site is exposed at the left margin of the Sauce Grande river at the riverside of Saldungaray village. The sequence begins with ?Pampean Sediments? at the water level, assigned to the Saldungaray Formation. This unit is overlain by coarse conglomerates and sands of the San José Sequence (figure 5.A). The profile ends with eolian sands of the Matadero Saldungaray Formation. The scarce mammal remains found in all these units do not constrain the age of the deposits but agree with the interpretation of Zavala and Quattrocchio (2001). A skull of Paedotherium bonaerense was found in the Saldungaray Formation, a complete pelvis of Glossotherium and another of Lama , in the lower section of the San José Sequence; a hind limb of Lagostomus , in the Upper Section of this sequence, and skulls of Ctenomys talarum and Lepus europaeus , in the Matadero Saldungaray Formation (see Appendix 2). These remains suggest a Montehermosan Age for the Saldungaray Formation (Early Pliocene), Pleistocene sensu lato for the San José Sequence, and Late Holocene-Historical Times for the Matadero Saldungaray Formation.

7. Bajo San José. This locality is placed near the bridge of route 51 over the Sauce Grande river. It is a terrace of conglomerates and coarse sand known as the San José Sequence exposed discontinuously along the valley, overlying the Saldungaray Formation and seldom covered by eolian units. The detailed sedimentological studies suggested that this deposit was formed by a braided fluvial system (Borromei, 1990). The San José Sequence yielded a rich vertebrate fauna (figure 5.B) listed in Deschamps and Borromei (1992) and also studied by Cione and López Arbarello (1995), Deschamps (1998), Deschamps et al . (2000), de la Fuente (1999), Pardiñas and Deschamps (1996), Tonni and Deschamps (2001) and Verzi et al . (2004b). The following taxa were collected in the Lower Section: Pimelodella aff. P . laticeps , Callichthys callichthys , Percichthys , Corydoras cf. C. paleatus , Hydromedusa tectifera , Rhea , Chloephaga sp. 1, Chloephaga sp. 2, Porphyrula , cf. Pseudoseisura-Pseudoseisuropsis , Motacillinae indet., Lestodelphys , Chaetophractus villosus , Zaedyus pichiy , Eutatus seguini , Tolypeutes n. sp., Propraopus , Glyptodon clavipes , Doedicurus , Panochthus tuberculatus , Sclerocalyptus cf. S. ornatus , Scelidotherium cf. S. leptocephalum , Glossotherium , Lestodon armatus , Megatherium americanum , ? Macraucheniopsis ensenadensis , Toxodon , Akodon cf. A. azarae , Akodon cf. A . iniscatus , Oxymycterus , Reithrodon auritus , Phyllotis , Lundomys , Ctenomys kraglievichi , Microcavia , Galea , Lagostomus , Neochoerus cf. N. tarijensis , Myocastor , Gomphotheriidae indet., Tayassu , Lama , Cervidae indet., Hippidion principale , Pseudalopex , and Herpailurus cf. H. yaguaroundi . In the Upper Section, only Scelidotherium , Myocastor columnaris and Lagostomus , were found.
Six detailed profiles were drawn, in which the collection level of each fossil could be identified. Then the taxa were joined in a schematic profile representative of the whole exposure (figure 5.B). The study of this fauna is in agreement with the geological interpretation, since the different abundance of materials among different sedimentological facies relies on taphonomy, the necessary energy for a quick burial, place of the facies within the depositional model, and preservation (Deschamps and Borromei, 1992).
The analysis of the vertebrate fauna found in Bajo San José required the revision of several taxa. Some taxa had their first record in this locality ( i.e . the fishes Pimelodella aff. P. laticeps , Callichtys callichthys and Percichthys ; the birds Porphyrula and Motacillinae; the Muridae Oxymycterus , Lundomys and Phyllotis , and the Tayassuidae Tayassu ), and others were new taxa ( Chloephaga sp. 1 and 2), consequently the analysis of their biochrons was difficult (see Comments). The Bonaerian Age is transitional between Ensenadan (Early Pleistocene) and Lujanian (Late Pleistocene) ages, and very few taxa are exclusive of this age. Accordingly, some authors have not recognized enough faunal differences to distinguish a separate age (see Marshall et al ., 1984; Cione and Tonni, 1999). Cione and Tonni (1999) confirmed the original Ameghino's ?Piso Bonaerense? and defined the Megatherium americanum Zone as the biostratigraphical base for this Age. The finding of the rodent Ctenomys kraglievichi (with short biochron) in Middle Pleistocene sediments of several localities of the Buenos Aires Province suggested their stratigraphic correlation calibrated through magnetostratigraphic data (Verzi et al ., 2004b). A Early Bonaerian Age (Middle Pleistocene) was proposed for the deposition of the Lower Section of the San José Sequence, correlated to the ISO 11, the most conspicuous warm pulse recorded for southern South America (Appendix 2, Comments, Deschamps, 2003, and Verzi et al ., 2004b).

8. Napostá Grande. This site is in the middle valley of the arroyo Napostá Grande, 29 km north from Bahía Blanca city by the road to Cabildo village known as ?La Carrindanga?. Quattrocchio et al . (1988) and modifications made by Zavala and Quattrocchio (2001), recognized the Agua Blanca Sequence and the Chacra La Blanqueada and Matadero Saldungaray formations. The exposure begins with fine sands to silts of the Middle Section of the Agua Blanca Sequence at the water level. This unit is overlain by overflow deposits of the Chacra La Blanqueada Formation and eolian sediments of the Matadero Saldungaray Formation. Many vertebrate remains were found in every unit (Quattrocchio et al ., 1988; Deschamps and Tonni, 1992) (figure 6.A). The palynological content and ostracods of this section were also studied (Quattrocchio et al ., 1988; Bertels and Martínez, 1990; Grill, 1995; Martínez, 2002). In the Middle Section of the Agua Blanca Sequence scarce remains of the following taxa were found: Rheidae indet., Chaetophractus villosus , Scelidotherium leptocephalum , Macrauchenia patachonica , Lama guanicoe , Lamini indet. and Equus ( Amerhippus ) neogaeus . But in the Upper Section fossils were more abundant and varied, especially in the lower levels with lamination: scales of Cyprinodontiformes indet., postcranial bones of Anura indet., Rhea , Nothura darwini , and Tinamidae indet., mandibles of Anas cf. A. platalea , and Dendrocygna , Anatidae indet., mandibles and/or postcranial bones of Lestodelphys halli , Thylamys cf. T. pusillus , Chaetophractus villosus , Zaedyus pichiy , Holochilus brasiliensis , Calomys cf. laucha - musculinus , Reithrodon auritus , Ctenomys , Cavia aperea , Lama guanicoe , Ozotoceros bezoarticus and Pseudalopex aff. P . gymnocercus . In the Chacra La Blanqueada Formation Ctenomys talarum and Lama guanicoe were found, and in the Matadero Saldungaray Formation, Lama guanicoe and Bos taurus .

These data suggest a Lujanian Age (Late Pleistocene-Early Holocene) for the Middle Section of the Agua Blanca Sequence (Appendix 2). The Upper Section of this unit is regarded as Platan in age (Late Holocene) because of the absence of Pleistocene species, the record of neospecies, and a radiocarbon dating of 1960±100 14 C years BP (Deschamps and Tonni, 1992). Although no remains of introduced fauna have been found in the Chacra La Blanqueada Formation at this site, they are common in others (Puesto La Florida, Rabassa, 1989; Rabassa et al ., 1991), suggesting that this unit may have been deposited from the Late Holocene up to the present. The Matadero Saldungaray Formation would have been deposited at least partially after the arrival of the Europeans (17th.century).

9. Puesto La Florida. This site is located at the Sauce Grande river valley between Bajo San José and Las Obscuras localities. The cliffs expose the lithostratigraphic units described for Arroyo Napostá Grande. Only some postcranial bones of Lama guanicoe were found in the Middle Section of the Agua Blanca Sequence. In the Upper Section, the following materials were found: a tarsus-metatarsus of Rhea americana , ulnae of cf. Anas , postcranium and scutes of Chaetophractus villosus , scutes of Zaedyus pichiy , large part of the skeleton of Cavia aperea , and postcranium of Ozotoceros bezoarticus . Several postcranial bones of Bos taurus were found in the Chacra La Blanqueada and Matadero Saldungaray Formations (figure 6.B).
These findings suggest the same ages as for the previous locality for the lithostratigraphic units.
The Upper Agua Blanca Sequence has a radiocarbon dating in the cliffs of the Sauce Grande river near the Bajo San José locality that yielded 5010±120 years 14 C BP, restricting this unit within the middle Holocene (Borromei, 1995). The Chacra La Blanqueada Formation was dated in 2830±90 years 14 C BP, Late Holocene (Borromei, 1995) and between 1570 ± 70 and 900±50 years 14 C BP (5 wood samples and 1 basal peat sample) at the type section of this unit (Rabassa et al ., 1991).

10. García del Río. This locality is placed upstream of the Napostá Grande locality in the arroyo Napostá Grande. The outcropping units are those described for Napostá Grande and Puesto La Florida localities. Only Lama guanicoe was recorded in all units (figure 6.C), which supplies no biostratigraphic information at this level. A radiocarbon dating of 2610±60 years 14 C BP (Quattrocchio et al ., 1998) assigns the Upper Section of the Agua Blanca Sequence to the Platan Age, Late Holocene.

Bio-and chronostratigraphy (figure 7)

Exposures of the ten localities were correlated in a chronostratigraphic chart. In this context, lithological units are considered unconformity bounded units representing the events occurred in the basin, with temporal and genetic meaning. This is the first time that a paleovertebrate study is based on these units. The genesis of the deposits determined through previous facies and sequence-stratigraphy analysis alerted about paleoenvironmental control on the findings that must be taken in mind when analyzing presence and absence of taxa.
The resulting biostratigraphic scheme allows chronostratigraphic correlation with other areas of the Pampean region, and refinement of the previous pattern, especially with those units based on micromammals.
Since no fossils were found in the Lower Section of the Agua Blanca Sequence, its age could not be assessed. But as the previous Bajo San José Sequence was assigned to the Early Bonaerian, it cannot be older than Middle-Late Bonaerian (late Middle Pleistocene).
The new biostratigraphic scheme here proposed is composed of six biozones for the studied area. From oldest to youngest these units are:

1. Xenodontomys ellipticus Zone

Definition. Total range of this taxon.
Age. Late Huayquerian (the end of the Late Miocene).
Reference section. The type area is Cantera Seminario in Grünbein. The stratotype is Unit 2 (figure 3.A). This is the single fossiliferous level of the profile, 0,5 m thick, between 2 m from the base and a level of calcrete crust.
Characteristic assemblage. Xenodontomys ellipticus , Phtoramys cf. P. hidalguense , Borhyaenidium , Aspidocalyptus , Berthawyleria .
Remarks. This biozone corresponds to the X. Ellipticus chron a Zone of Verzi et al . (2004a). It is also recognized in the other profile studied in the area of Grünbein (Cantera Relleno Sanitario), and in Barrancas de Sarmiento. The fossil fauna suggests a response to relatively arid conditions, open environments of grasslands and herbaceous or shrubby steppes with trees.

2. Actenomys priscus-Plohophorus cuneiformis Zone

Definition. Total range of these two taxa.
Age. Lower Montehermosan (Early Pliocene).
Reference section. The type area is Las Obscuras, with type profile in figure 3.D. The stratotype is the base of Unit 2, between 0.80 and 2.50 m from the base of the exposure.
Characteristic assemblage. Actenomys priscus and Plohophorus cuneiformis are frequently associated with Promacrauchenia and Pseudotypotherium which are not exclusive of this age.
Remarks. This biozone is chronostratigraphically correlative to the Saldungaray Formation outcropping at Balneario Saldungaray, and to part of the Montehermosan Stage based on the Trigodon gaudry Biozone, defined by Cione and Tonni (1999) in Farola Monte Hermoso. Although Plohophorus is not quite abundant, Actenomys priscus on the contrary is very abundant and well known. In addition, the lower boundary matches with the end of the record of the genus Xenodontomys .

3. Ctenomys kraglievichi Zone

Definition. Total range zone. Total extension of the rodent Ctenomys kraglievichi .
Age. Early Bonaerian (Middle Pleistocene).
Reference section. The type area is Bajo San José and the stratotype comprises the Lower Section of the San José Sequence 2 m from the base of the quarry (figure 5.B).
Characteristic assemblage. Other taxa exclusive from the Bonaerian Stage are Tolypeutes n. sp. (Scillato Yané, pers. com.) and Hippidion principale . Megatherium americanum , Glyptodon clavipes and Panochthus tuberculatus are all abundant taxa from the Bonaerian-Lujanian.
Remarks. This biozone was published by Verzi et al. (2004b) while this paper was under revision. This unit is also recognized in the Atlantic cliffs of Necochea and north of Mar del Plata city, Buenos Aires Province (Verzi et al ., 2004b). It corresponds chronostratigraphically to the basal part of the Bonaerian Stage, which is based on the Megatherium americanum Biozone (Cione and Tonni, 1999) of the Pampean region, without formal stratotype. The recorded taxa suggest a complex environment with open areas with scattered forests, temperate-warm, locally associated with water bodies such as the braided fluvial system suggested by facies analysis (Deschamps and Borromei, 1992; Deschamps, 2003; Verzi et al ., 2004b).

4. Equus (Amerhippus) neogaeus-Macrauchenia patachonica Zone

Definition. The association of the ranges of these two taxa.
Age. Lujanian (Late Pleistocene).
Reference section. The type area is Arroyo Napostá Grande (near Chacra Santo Domingo), the type section is shown in figure 6.A. The stratotype is the Middle Section of the Agua Blanca Sequence.
Characteristic assemblage. Equus ( A. ) neogaeus , Macrauchenia patachonica , Scelidotherium leptocephalum , Megatherium americanum , Glossotherium , Lama guanicoe .
Remarks. This biozone is also recognized in the Middle Section of the Agua Blanca Sequence cropping out at Puesto La Florida, and correlates chronostratigraphically with the Lujanian Stage, based on the Equus ( A .) neogeus Biozone (Cione and Tonni, 1999). The environment suggested is open areas with grasslands and steppes.

5. Ozotoceros bezoarticus Zone

Definition. Total range of this taxon in the area.
Age. Platan (Late Holocene), determined on absolute dating and the record of fragments of indian pottery.
Reference section. The type area is Arroyo Napostá Grande (Chacra Santo Domingo). The stratotype is the Upper Section of the Agua Blanca Sequence (figure 6.A).
Characteristic assemblage. Ozotoceros bezoarticus , Lama guanicoe , Lagostomus maximus , Cavia aperea , Ctenomys .
Remarks. Although O . bezoarticus is an extant species, it is restricted to protected areas of San Luis and Buenos Aires Provinces. This biozone is also recognized in the Upper Section of the Agua Blanca Sequence exposed at Puesto La Florida. It may be chronostratigraphically correlated with part of the Platan Stage, based on the Lagostomus maximus Biozone defined for Paso Otero area, Buenos Aires province (Cione and Tonni, 2001).

6. Bos taurus-Ovis aries Zone

Definition. The first record of these living taxa introduced by the Europeans. The upper boundary cannot be defined.
Age. Historical times-present.
Reference section. The type area is Arroyo Napostá Grande (Chacra Santo Domingo), the stratotype is composed by the eolian sediments at the top of the profile assigned to the Matadero Saldungaray Formation (figure 6.A). In other sites of the area (i.e. Puesto La Florida) this biozone is also recognized in the upper levels of the Chacra La Blanqueada Formation.
Characteristic assemblage. Bos taurus , Ovis aries , Lepus europaeus , Lagostomus maximus .
Remarks. This is a special biozone composed of living species, which is not considered in the Comité Argentino de Estratigrafía (1992), but it is regarded here because it limits the top of the previous biozone and helps recognizing this important period in archaeological studies. Although O. aries was not recorded in this site, the species is also considered in the name of the biozone because many bones of this conspicuous representative of the introduced fauna were observed in the surroundings together with Bos taurus , in the upper levels of the Chacra La Blanqueada Formation and the Matadero Saldungaray Formation.


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