Mammalian biostratigraphy of Pleistocene fluvial deposits in northernUruguay, South America

Martin Ubilla

UBILLA, M. 2004. Mammalian biostratigraphy of Pleistocene fluvial deposits in northernUruguay, South America. Proceedings of the Geologists' Association, 115, 347-357. Thefossiliferous beds of the Sopas Formation, which outcrop in northern Uruguay, have yieldedfreshwater molluscs, plants and vertebrates, especially terrestrial and freshwater mammals andtrace fossils. Fluvial depositional environments predominate. The mammalian assemblage iscorrelated biostratigraphically with the Lujanian Stage (Late Pleistocene-Early Holocene) ofthe Pampean region of Argentina. Minimum radiocarbon ages (43000 years BP and 45000years BP) along with luminescence ages (43'5 ± 3·6 and 58·3 ± 7·4 ka) indicate a Late Pleistoceneage for this assemblage. Differences in comparison with the typical Lujanian mammal fauna ofthe fluvial Guerrero Member of the Lujan Formation in Buenos Aires province (Pampeanregion) could be a consequence not only of temporal differences but also of distinctbiogeographical and climatic phases. The Sopas Formation has mammals indicative of varioushabitats. Some of the extant species inhabit fluvial and lacustrine environments with riparianforest, whereas others indicate open woodland, savanna and grassland. There is discussion ofthe occurrence of tropical to temperate ('brasilic') mammals together with others related to aridconditions.

Key words: mammalian biostratigraphy, fluvial, Late Pleistocene, Uruguay, SopasFormation, Lujanian Stage

Paleontologia, Facultad de Ciencias, Universidad de la Republica, Igua 4225,11400 Montevideo, Uruguay (email: ubilla@jcien.edu.uy)

1. INTRODUCTION

It is widely accepted that mammalian biostratigraphyis a reliable way to correlate terrestrial and fluvialCenozoic deposits; despite the potential limiting factorof biotic fragmentation, caused by physical and ecologi­cal factors, mammals have proved helpful for temporalcontinental correlation and calibration of palaeoclimate(Cione & Tonni, 1995; Wood burne, 1996; Lindsay,1997; Walsh, 1998; Schreve, 2001). A terrestrial mam­malian biostratigraphical scheme has been establishedfor the Late Cenozoic in the Pampean area of Argentina(Cione & Tonni, 1999;Cione et al., 1999).Although it ispreliminary and, as yet, imperfect (Cione & Tonni,1999), it can be considered as a regional framework forthe mid-latitudes of South America.

In recent years, increasing information about thePleistocene mammal fossils of northern Uruguay haspaved the way for advances in biostratigraphy andstudies of palaeoenvironment and biogeography(Ubilla, 1996; Ubilla & Perea, 1999; Ubilla et al., 1999,2004). The fossiliferous Pleistocene beds that outcropin northern Uruguay (Fig. I) contain numerous verte­brate taxa, with mammals the dominant group. Themammalian assemblages include extinct taxa and ex­tant species of the neotropical region, some living atpresent in areas far from Uruguay.

Proceedings of the Geologists' Association, 115, 347-357.

This paper discusses the correlation of these mam­malian taxa with the Ensenadan, Bonaerian andLujanian Stages (Upper Pliocene to Pleistocene-earlyHolocene) in a regional context that includes thePampean area of Argentina. Consideration is alsogiven to palaeobiogeographical and palaeoclimaticprocesses.

2. GEOGRAPHICAL AREA AND GEOLOGICALSETTING

The study area in northern Uruguay is located between30° to 32° Sand 56° to 58° W (Fig. I). The landscapeis characterized by slightly undulating plains withriverine forest. The annual average temperatureranges from l7°C (Tacuaremb6 Department) to 19°C(Artigas Department), with an annual rainfall of1300 mm in a humid subtropical climate (Prohaska(1976) and author's compilation of annual data fromthe Servicio Meteorol6gico del Uruguay (1990-1999».There is a wide fluvial network with creeks, streamsand rivers, most draining into the Cuareim, Uruguayand Negro rivers (Fig, I), Northern Uruguay differsfrom the rest of the country due to its higher tempera­ture and humidity and its temperate and subtropicalfauna and flora.

0016-7878/04 $15.00 CO 2004 Geologists' Association

348 M. UBILLA

Fig. 1. The location of the study areas in northern Uruguay(delimited by the circle), southern Brazil and the Pampeanregion. A, Artigas; B, SaIto; C, Tacuaremb6 departments.

There is widespread exposure of the fossil-bearingdeposits of the Sopas Formation, which outcrop inriver, stream and creek sides and in sections up to 12 mhigh. Exposures of this sort have revealed variations inlithology, facies and fossil taphonomy. Panario &Gutierrez (1999) included this unit in the so-calledDolores-Sopas Formation, which forms part of the'Lower Terraces' within the Quaternary of Uruguay.Because of controversy regarding radiocarbon ages forthis unit (Ubilla, 2001), further consideration is re­quired. Bombin (1976) correlated these sediments withthe Late Pleistocene Touro Passo Formation of south­ern Brazil, whereas Marshall et al. (1984) identified theSopas Formation with the 'Lujanian Land MammalAge' (Fig. 2). However, these authors provided littlepalaeontological information about the Sopas Forma­tion. Recently, Rosa et al. (2000) established radio­carbon and thermoluminescence (TL) ages for theTouro Passo Formation ranging from 10 810 years BPto 30 000 years BP.

The sedimentological features of the Sopas Forma­tion include conglomerates with clay and calcareousmatrices, conglomerate-sandstones, siltstones andsandy siltstones. Carbonates occur very frequently as

ATLANTIC OCEAN

dust, concretions or duricrusts. There are greyish-greensilty and clayey lenses of both centimetre and metrescale. This unit, which has a predominantly brownishcolour, overlies Mesozoic beds (the Arapey andTacuaremb6 formations) unconformably (Fig. 3). De­spite the fact that fluvial contexts (channels, bars,floodplains) are the predominant depositional environ­ment, in some places palaeosol formation has maskedthe depositional evidence.

The Sopas Formation has a rich fossiliferous con­tent, with vertebrates, freshwater molluscs, plants andtrace fossils. Table I provides an updated list ofmammal taxa, which forms the basis for the ensuingdiscussion. The majority of fossils were found in thefluvial contexts of this formation. Nevertheless, theextinct rodent Microcavia criolloensis and the extinctlitoptern Neolicaphrium recens (Fig. 4F) werediscovered in a palaeosol that also has earthwormaestivation chambers (Verde et al., 2004) andTaenidium (ichnogenus) trace fossils (Verde et al.,1998).

Ubilla & Perea (1999) and Ubilla (1996) obtainedradiocarbon ages ranging from 45 000 to 43 000 yearsBP, based on several samples, which are considered tobe minimum ages for the Sopas Formation. They werebased on wood and freshwater mollusc shell samplesfrom three localities: Cuareim river, Canada Sarandiand Malo creek of Artigas, Saito and Tacuaremb6departments, respectively (Fig. I: A, B, C). Theseoutcrops were included in the basal member of theDolores-Sopas Formation by Panario & Gutierrez(1999). Ubilla (1996) obtained an age of 12 100 yearsBP, derived from wood picked from an outcrop of theupper member of the Sopas-Dolores Formation ofPanario & Gutierrez (1999), which is devoid of verte­brate remains, in the Cuareim river valley. The radio­carbon dating of the Sopas Formation was discussedby Ubilla (2001).

Luminescence ages of 43.5 ± 3.6 ka (LVD-647, mud­stone) and 58.3 ± 7.4 ka (LVD-646, mudstone inter­calated in conglomerates) were obtained from twofossiliferous localities, Sopas creek (Fig. I, area B; Fig.3) and Malo creek (Fig. I, area C; Fig. 3), respectively.These ages are averages of thermoluminescencemeasurements on quartz and optically stimulatedluminescence on feldspar.

Taphonomy of fluvial bodies of the Sopas Formation

To assess the time-scale and significance of transportprocesses, the taphonomy of vertebrate bones from theSopas Formation at Malo Creek (Figs IC, 3) wasstudied by Ubilla & Lorenzo (2001). This analysis wasperformed on three lithofacies - conglomerates, sand­stones and siltstones - and mainly concerned mammalsheavier than 5 kg. The majority of the mammal taxaincluded in Table I were found at this locality and canbe assumed to be representative of the Sopas Forma­tion. Cervids are the most important source of skeletal

400

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PAMPEAN

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MAMMALIAN BIOSTRATIGRAPHY OF S. AMERICAN DEPOSITS

PLEISTOCENE..l:..

AGE >- u

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Lower Middle Late;:

0CIlW J:

Chronostratigraphic ages of SA Ensenadan Bonaerian Lujanian

~Tolypeutes pampaeus Mega~eriumamericanum Equus (A.) neogeus

Taxa

Dasypus aft. D. novemcinctus ---------------Pampatherium humboldt! ---------------cf. Wilfredomys oenax ---------------Coendou magnus ---------------Myocastor coypus ---------------Microcavia criolloensis ---------------Hydrochoerus hydrochaeris ---------------Lama (Vicugna) sp. ---------------Neuryurus rudisLundomys molitor •Lestodon cf. L. armatusNeolicaphrium recens~ntifer ultraMorenelaphus brachycerosGlyptodon clavipesPanochthus tuberculatusMegatheriumpmericanumGlossotherium robustumMacrauchenia patachonicaToxodon cf. T. platensisHippidion principaleHemiauchenia paradoxaMorenelaphus lujanensisParaceros fragilisTapirus terrestris •Equus (Amerhippus) neogeusNeochoerus ct. N. aesopiTayassu aft. T. pecari •Ozotoceros aft. O. bezoarticus •antra longicaudis •

Propraopus sp.Neothoracophorus sp.ct. Scelidotherium IFelis concolor •Panthera cf. P. onca •Reithrodon sp. I •Galea sp. •Catagonus Group •Cavia sp. I •Smilodon populatorDolichotis? sp. •Lycalopex gymnocercus •

349

Fig. 2. Mammalian taxa from the Sopas Formation and their distribution within Pampean chronostratigraphic units. Brokenlines indicate taxa found in the Sopas Formation but absent in the Pleistocene of the Pampean region. Arrows pointing leftindicate taxa also found in pre-Pleistocene contexts, while those pointing right indicate taxa still living.

elements. Ubilla & Lorenzo (2001) stated that, eventhough fragmentary bones outnumber complete ones(as would be expected in fluvial deposits), bones with­out abrasion dominate the sample considered. Mostfragmentary bones show fresh fractures and minimalevidence of weathering. In addition, some articulatedbones have been found. This pattern is suggestive ofshort-distance transport from the bone source areaand, at least for this locality, rapid emplacement,making it unlikely that reworking has occurred.

3. PAMPEAN MAMMALIANBIOSTRATIGRAPHY

A Pliocene to Recent biostratigraphical scheme basedon land mammals in the Pampean region of Argentinais provided by Cione & Tonni (1999). This provides asuitable framework for regional correlation in SouthAmerica, including mammalian assemblage biozones.These authors proposed three Pleistocene chronostrati­graphic units (Fig. 2): the Ensenadan, Bonaerian and

350 M. UBILLA

Mesozoic sediments

Luminescence43.5±3.6 kaLVD-647

3.0

Sopas Creek,Sa ito Department

Molluscs

Rhi/oconcreti ons

Gr - grave lSd - sandS - siltC - clay

Fossils

Vertebrates

n Trace foss ils~ (Ichnofossils)

Climbing ripplecross lamination

~ v;1 Arapcy Formation

Sed imentary structures

~ Convolute ~ Lenticular. ~ lamination stratification

d. d. Carbonate 8::::, Wm·yI'" 7> concretion :::z lamination

·..•: ... Tacuarcmbo Formation·..

Quaternary sediments

H~~ ~ l Mudstone

I: :::HSandstone

E:::~ Conglomerate• •I~: ~I Wackestone

E~ Mudstone with gravel clasts

Trough channelcross stratification

I I I I(' S Sd

I<if

i I I I(' S Sd

Ien-

Fig. 3. Stratigraphical profiles of fossiliferous localities in the Sopas Formation. Luminescence dating is indicated. LVD=Laboratorio de Vidros e Datacao, Faculdade de Teconologia de Sao Paulo, Universidade Estadual Paulista, Sao Paulo, Brazil.

Lujanian Stages, which are defined by the Tolypeutespampaeus, Megatherium americanum and Equus(Amerhippus) neogeus biozones, respectively. The geo­graphical area encompasses central-eastern and north­ern Argentina, including Buenos Aires Province andthe Mesopotamian region located between the Paranaand Uruguay rivers (Fig. I).

According to Cione & Tonni (1999) and Nabel et at.(2000), the Ensenadan spans the Late Pliocene to earlyMiddle Pleistocene, ranging from Chron C2n, Clr tothe earliest part of Cln. However, there are differentand controversial opinions about its lower age bound(MacFadden et al., 1983; Cione & Tonni, 1999).

The Bonaerian (Middle Pleistocene) and the Luja­nian (Late Pleistocene to early Holocene) span ChronCln. The boundary between the Ensenadan and theBonaerian is not well defined; suggestions include500 ka (Cione & Tonni, 1999) and 600 ka (Tonni et al.,1999). Even though the Bonaerian and the basal

Lujanian strata are similar lithostratigraphically, theyhave yielded different biostratigraphical information.The base of the Lujanian could be the base of the LaChumbiada Member of the Lujan Formation, which iscomposed of fluvial silts and clay. Nevertheless, theoccurrence of Equus (Amerhippus) neogeus in sedi­ments correlated with the Last Interglacial (OIS 5e)suggests that the base could be older (c. 120 ka:Pardifias et al., 1996). The La Chumbiada Member,which is similar to the 'brown zone' of the GuerreroMember of Prado & Alberdi (1999), is dated to c. 30 ka(Figini et al., 1995) and the overlying GuerreroMember of the Lujan Formation ranges from 21 kato 10 ka (Huarte et al., 1988; Figini et al., 1995; Tonniet al., 2003).

On the basis of the rich mammalian record from thefluvial beds of the Lujan Formation, an importantfaunal change between the La Chumbiada andGuerrero Members is recognized, presumably resulting

MAMMALIAN BIOSTRATIGRAPHY OF S. AMERICAN DEPOSITS 351

from climatic change (Prado & Alberdi, 1999; Tonniet al., 1999). Indeed, the assemblage from the LaChumbiada Member indicates warm and humid con­ditions, whereas that from the Guerrero Memberpoints to a colder and more arid environment(Bonadonna et al., 1995; Prado & Alberdi, 1999;Tonniet al., 1999).

There is agreement that the Guerrero Member of theLujan Formation dates from the Last Glacial cycle(Tonni et al., 1999) when the Andean glacial advanceoccurred (Clapperton, 1993), with cold and arid con­ditions in the Pampean area (Iriondo & Garcia, 1993;Iriondo, 1999). However, there is controversy aboutthe La Chumbiada Member. Tonni et al. (1999) sug­gested that it could be related to the OIS 3 interstadial(c. 60 ka), whereas Bonadonna et al. (1995) attributedit to a short warm period during the Last Glacial cycleat c. 30~25 ka. Prado & Alberdi (1999) correlated themammal assemblage of the La Chumbiada Memberwith the Last Interglacial (OIS 5e), disregarding theradiocarbon dates.

4. DISCUSSION

Biostratigraphy

Figure 2 lists the mammalian taxa from the Pleistoceneof northern Uruguay in relation to the chronostrati­graphic units of the Pampean area and also the age­range of each taxon. The pattern obtained hasimportant implications for South American mam­malian biostratigraphy and for the dating of Quatern­ary fluvial sequences.

The horse Equus (Amerhippus) neogeus (Fig. 4A),recorded from the Sopas Formation, is not only con­sidered an exclusive indicator of the Lujanian Stage,but is also the key diagnostic taxon of this chrono­stratigraphical unit of Buenos Aires province. Othermammalian taxa restricted to the Lujanian have alsobeen found, such as the extinct capybara Neochoerusaesopi, the tapir Tapirus terrestris, the deer Ozotocerosbezoarticus (Fig. 4G) and the white-lipped peccaryTayassu pecari.

The majority of the mammalian taxa consideredexclusive to the Tolypeutes pampaeus and Megatheriumamericanum biozones (Ensenadan and BonaerianStages) are absent in the Sopas Formation. Cione &Tonni (1999) described 47 exclusive taxa at genericand specific level for the Ensenadan and 16 for theBonaerian. Despite the fact that this is negative evi­dence, the absence of these taxa from the SopasFormation is unlikely to result from a sampling effectbut is, instead, probably due to chronological orbiogeographical factors.

Nevertheless, the glyptodont Neuryurus rudis, whichis an Ensenadan species, occurs in the Sopas Forma­tion. Furthermore, two extinct deer species Antiferultra and Morenelaphus brachyceros, which areBonaerian taxa, have now also been found in thislithostratigraphic unit, as have two taxa that span the

Lower to Middle Pleistocene boundary in the Pampeanarea, the ground sloth Lestodon armatus and thelitoptern Neolicaphrium recens (Table I).

It must be noted that the Sopas mammalian assem­blage includes one taxon, the Neotropical otter Lontralongicaudis, which in Pampean Argentina shows the'Lazarus' effect. As defined by Raup (1987), a'Lazarus-taxon' seems to suffer extinction but thenreappears later in the stratigraphic record. L. longi­caudis is recorded in the Ensenadan, disappeared in theBonaerian and eventually returned in the Lujanian toRecent. Although this pattern of occurrence may be anartefact of sampling, it could relate to biogeographicalvariations resulting from climatic and environmentalchange; the animal might have persisted in unidentifiedrefugia areas of South America.

A different pattern is shown by the sigmodontinerodent Lundomys molitor (Fig. 40), which lived in thePampean area from the Ensenadan to the Lujanianand then disappeared from this region. At present L.molitor inhabits a temperate area located in the northof Uruguay and southern Brazil (Voss & Carleton,1993).

The fossiliferous beds of northern Uruguay haveseveral taxa in common with the Pampean Mega­therium americanum and Equus (Amerhippus) neogeusbiozones. Typical representatives of the SouthAmerican 'megafauna' and common in Middle to LatePleistocene strata, they include the typical armouredxenarthrans Glyptodon clavipes (Fig. 4I) and Panoch­thus tubcrculatus and the ground sloths Megatheriumamericanum and Glossotherium robustum (Fig. 4J).Autochthonous South American ungulates are repre­sented by the Iitoptern Mucrauchenia patachonica andthe notoungulate Toxodon platensis(Fig. 4H), as wellas the horse Hippidion principale and the bizarrecamelid Hemiauchenia paradoxa.

Some taxa from the Sopas Formation are unin­formative biostratigraphically due to their unresolvedspecific level or prolonged occurrence in SouthAmerica, spanning the whole Pleistocene or longer.

Correlation of the fossiliferous fluvial beds of north­ern Uruguay with the Late Pleistocene Lujanian Stageof Buenos Aires Province is proposed. based on thefollowing:

l. the presence in the Sopas Formation of exclusivelyLujanian taxa, including its key diagnostic taxon;

2. the absence in this unit of the majority of theexclusively Ensenadan and Bonaerian mammaltaxa;

3. the presence of several mammal taxa that span theMiddle to Late Pleistocene (Bonaerian-Lujanian),thereby tending to discount an Ensenadan age.

Biogeography and palaeoclimate

Mammalian biostratigraphy provides an invaluabletool for evaluating potential causes of the observedvariations in taxa. Despite the biostratigraphical rea­sons for considering the mammalian assemblage of

352 M. UBILLA

Table 1. Updated list of mammals for the Sopas Formation of northern Uruguay.

Class MAMMALIAOrder XenarthraFamily Dasypodidae (armadillos)*Dasypus aff, D. novemcinctus Linnaeus, 1758Propraopus sp. Ameghino, 1881Pampatherium humboldti (Lund, 1839)Family Glyptodontidae (glyptodonts)Glyptodon clavipes Owen, 1839Neothoraeophorus sp. Ameghino, 1889Neuryurus rudis (Gervais, 1878)Panoehthus tuberculatus (Owen, 1845)'Chlamydotherium ' Lund, 1839Family Megatheriidae (ground sloths)Megatherium americanum Cuvier, 1796Subfamily Nothrotheriinae gen. et sp. indet.Family Mylodontidae (ground sloths)Glossotherium robustum (Owen, 1842)Lestodon cf. L. armatus Gervais, 1855cf. Scelidotherium Owen, 1839

Order CarnivoraFamily Felidaea Felis concolor Linnaeus, 1771 (mountain lion)a Panthera cf. P. onca (Linnaeus, 1758) (jaguar)Smilodon populator Lund, 1842 (sabre tooth)Family Canidae"Lycalopex gymnocereus (Fischer, 1814) (fox)Family Mustelidae"Lontra longicaudis (Olfers, 1818) (river otter)Family Ursidae"Subfamily Tremarctinae indet. (spectacled bears)

Order RodentiaFamily Cricetidae (field mice)"Reithrodon sp. Waterhouse, 1837"Cf, Wilfredomys oenax (Thomas, 1928)"Lundomys molitor (Winge, 1887) (marsh rat)Family ErethizontidaebCoendou magnus (Lund, 1839) (porcupine)Family Myocastoridae"Myocastor coypus (Molina, 1782) (coypu)Family Caviidae (guinea pigs)

"Cavia sp. Pallas, 1766"Galea sp. Meyen, 1831"Microcavia criolloensis Ubilla et al. 1999"Dolichotis? sp. Desmarest, 1820 (mara)Family Hydrochoeridae"Hydrochoerus hydrochaeris (Linnaeus, 1766) (capybara)Neochoerus cf. N. aesopi (Leidy, 1853)

Order LitopternaFamily MacraucheniidaeMacrauchenia patachonica Owen, 1838Family ProterotheriidaeNeolicaphrium reeens Frenguelli, 1921

Order NotoungulataFamily ToxodontidaeToxodon cf. T platensis Owen, 1837

Order ProboscideaFamily Gomphotheriidae (mastodonts)Genus et sp. indet.

Order PerissodactylaFamily Tapiridae"Tapirus terrestrls (Linnaeus, 1758) (brazilian tapir)Family Equidae (horses)"Equus ( Amerhippus) neogeus Lund, 1840Hippidion principale (Lund, 1845)

Order ArtiodactylaFamily Tayassuidae (peccaries)"Tayassu aff. T. pecari (Link, 1795)"Catagonus Group Menegaz & Ortiz-Jaureguizar, 1995Family Camelidae (camels)Hemiauchenia paradoxa Gervais & Ameghino, 1880"Lama (Vicugna ) sp. Menegaz et al. 1989Family Cervidae (deer)Antifer ultra (Ameghino, 1888)"Ozotoceros aff. 0. bezoarticus (Linnaeus, 1758)(pampas deer)Morenelaphus braehyeeros (Gervais & Ameghino, 1880)Morenelaphus lujanensis (Arneghino, 1888)Paraeeros fragilis (Ameghino, 1888)

Based on Ubilla (1996), Ubilla & Perea (1999), Ubilla et al. (1994,1999); Ubilla & Rinderknecht (2001), Vizcaino et al. (1995) and this paper."extant at generic or specific level;bextinct species of extant genus.

the Sopas unit as Lujanian, it is necessary to explainthe presence of one Ensenadan, two Ensenadan­Bonaerian and two Bonaerian taxa, which at first sightseem to contradict this interpretation. This may beexplained by one or more of a number of factors.

I. As noted above, this effect could be an artefact ofsampling; the Pampean biostratigraphical scheme ispreliminary.

2. There could be a geographical individual-taxonsurvival effect. Bond et al. (2001) reported Neolica-

phrium recens in the late Quaternary outside thePampean region. It was confined probably to a fewnorthern relict microhabitats and could, thus, havebeen part of a relict lineage, probably restrictedduring the Late Pleistocene to warmer and moreforested environments and found no further souththan the latitude of Uruguay and the northernArgentinian provinces of Cordoba and Corrientes.

3. There might be individual-taxon responses to geo­graphical barriers. The Parana-Uruguay river sys­tem acts as just such a barrier. It is a dynamic fluvial

MAMMALIAN BIOSTRATIGRAPHY OF S. AMERICAN DEPOSITS 353

A

c

20cm

I B

D

E

G H

Fig. 4. South American mammals with chronological, climatic and/or environmental significance in the Late Pleistocene ofnorthern Uruguay. A. Equus (Amerhippus) neogeus; B. Dasypus novemcinctus; C. Tapirus terrestris; D. Lundomys molitor; E.Lama sp.; F. Neolicaprhium recens; G. Ozotoceros bezoarticus; H, Toxodon platensis; I, Glyptodon clavipes; J, G/ossotheriumrobustum. Sources: A. H. I. J modified from Carlini & Tonni (2000); B. 0 and G modified from Gonzalez (200 I); C and Emodified from Hershkovitz (1972); F: modified from an unpublished drawing by C. Vildoso.

system that underwent striking fluctuations in thePliocene and Quaternary (lriondo & Santi. 2000)related to low and high sea-level variation (Violante

& Parker, 1999; Cavallotto et at., 1999). This pro­cess might have led to different palaeogeographicalscenarios, with emerging plains at low sea-level

354 M. UBILLA

times. This could have allowed the spread of somemammal species, such the two Bonaerian deer, fromPampean to northern latitudes, followed by localextinctions in the Pampean region.

Some Sopas Formation mammals do not occur inthe Pleistocene of Buenos Aires province. This isparticularly apparent in the fluvial Guerrero Memberof the Lujan Formation, which has the typical Luja­nian mammals, and may be explained by palaeobioge­ography and/or palaeoclimate. Certainly, some ofthese taxa have a tropical and temperate distribution atpresent, such as the nine-banded armadillo Dasypusnovemcinctus (Fig. 4B) and the capybara Hydrochoerushydrochaeris. An extinct porcupine, Coendou magnus,belongs to the family Erethizontidae, which is nowfound in tropical to subtropical parts of SouthAmerica. Tapirus terrestris and Tayassu pecari also liveat present in tropical and temperate areas of theNeotropical region. The present southern limit ofTapirus terrestris, Lundomys molitor, Dasypus novem­cinctus and the family Erethizontidae equates roughlywith the 8°C and 15°C July isotherms and the 22°Cand 25°C January isotherms between 28° S to 34° Slatitude. Pampatherium humboldtii, an extinct cingu­late, was distributed in the east central region of SouthAmerica excluding the Pampean region. As shown byDe Iuliis et al. (2000), during the Quaternary the eastcentral area was perhaps a transitional zone betweenthe Argentinian plains and typically tropical zones,which could explain the pattern.

Fluvial contexts generally yield mammals that arecharacteristic of various environments; thus, theextant mammals from the Sopas Formation, such asLundomys molitor, Lontra longicaudis, Hydrochoerushydrochaeris and Tapirus terrestris, are related clearlyto fluvial and lacustrine environments with well­developed riparian forest. Others indicate open wood­lands, savannas and grasslands, such as Ozotocerosbezoarticus, Lyca/opex gymnocercus, Reithrodon, Galeaand Microcavia.

Evidently, the mammalian assemblage from theSopas Formation is not identical to that of theGuerrero Member of the Lujan Formation. Thehypothesis of overall homogeneity of the so-called'Lujanian biota', encompassing late Pleistocene sedi­ments of Buenos Aires province, Uruguay and south­ern Brazil (Bombin, 1975), is not supported by theavailable evidence. Although this hypothesis accom­modates some taxonomic differences between geo­graphical areas, it assumes similar climatic andenvironmental conditions. As mentioned above, theGuerrero Member is correlated with the Last GlacialMaximum and contains mammalian taxa adaptedto the cold and arid conditions of the Central andPatagonian areas (Tonni & Fidalgo, 1978). The mam­malian assemblage of the Sopas Formation seems to beolder than the typical Lujanian fauna and could rep­resent a warmer episode, perhaps correlated with theLa Chumbiada Member of the Lujan Formation or

even older. Taking into account this information andthe radiocarbon ages of 43 000 and 45000 years BP,which are considered to be minimum ages, a tentativeattribution to the Last Interglacial (OIS 5e) or to OIS3 has been proposed (Ubilla, 1996; Ubilla & Perea,1999). The radiocarbon ages, together with the lumi­nescence ages of 43.5 ka and 58.3 ka, suggest a cor­relation with OIS 3. However, these sediments innorthern Uruguay yield some mammal taxa that,today, live under arid conditions, such as the extantspecies of the genus Microcavia, Lama (Vicugna) sp.(Fig. 4E) and Dolichotis. They are distributed inthe Patagonian region, the Central to NorthernArgentinean region tMicrocavia and Dolichotis) and inthe Central Andean 'puna' (Lama vicugna), a cold, dryand often windy high-altitude grassland between3200 m and 4600 m altitude occurring between thetreeline and the snowline. These extant taxa areno longer found in the area surrounding northernUruguay. The terms 'disharmonious' or 'non­analogue' are applied to faunas possessing assemblagesof extant taxa that are now allopatric (Lundelius, 1983;Graham, 1985). The Sopas Formation has yielded adisharmonious fauna involving Dasypus novemcinctus,Lundomys molitor, Ozotoceros bezoarticus and Tapirusterrestris, which today do not overlap the ranges ofLama (Vicugna) sp. and Microcavia. The last genus isrepresented in sediments of northern Uruguay by anextinct species that might have had slightly differentclimatic or ecological requirements to the extant ones.In the Pleistocene of the Pampean region, there are noclear cases of non-analogue or disharmonious fauna.Cases have, however, been described in the Holoceneof this region (Tonni et aI., 1999). A likely explanationof disharmonious faunas assumes that some climaticregimes of the Pleistocene probably do not exist today(Lundelius, 1983).

5. CONCLUSIONS

The fossiliferous beds of the Sopas Formation ofnorthern Uruguay, in the mid-latitudes of SouthAmerica, have yielded freshwater molluscs, plants,terrestrial and freshwater mammals and trace fossils.Using the Late Cenozoic continental chronostrati­graphical scheme from the Pampean region ofArgentina, which is based on mammalian biostratigra­phy, the mammalian assemblage of northern Uruguayis correlated with the Lujanian Stage (LatePleistocene-Early Holocene). The occurrence in theSopas Formation of exclusively Lujanian mammaltaxa, such Equus (A.) neogeus, Neochoerus aesopi,Tapirus terrestris, Ozotoceros bezoarticus and Tayassupeccary, supports this conclusion. Furthermore, mostexclusively Ensenadan (Late Pliocene--early MiddlePleistocene) and Bonaerian (Middle Pleistocene) mam­mals are absent from this unit. Despite the fact thatthe Lujanian Stage ranges in age from the latestPleistocene to Early Holocene, there is, to date,

M AMM A LI A N BIOS T R A TIGRAPH Y OF S . A MER I CA N DEPOSIT S 355

nothing to support an Early Holocene age for thisfauna. In fact , there is no evidence of Holocene extinctmegafauna from the Sopas Form ation . The minimumradiocarbon ages (43 000 years BP and 45 000 yearsBP) plus luminescence ages of 43.5 ka and 58.3 kaindicate a Late Pleistocene age for this mammalassociation .

The Sopas Formation has yielded mamm als adaptedto different environments; Lundomys molitor, Lontralongicaudis, Hydrochoerus hydrochaeris and Tapirusterrestr is live in fluvial and lacustrine environmentswith well-developed riparian forest. Open woodlands,savannas and grasslands are indicated by Ozotocerosbezoarticus, Lycalopex gymnocercus, Reithrodon, Galeaand Microcavia, amongst others.

The Sopas Formation contains some 'brasilic' taxa,which are cur rently distributed in tropic al and sub­tropical areas of South Amer ica . This differentiates itfrom the typical Lujanian fauna preserved in thefluvial deposits of the Gu errero Member of the LujanForma tion in the Pampe an region of Argentina. TheGuerrero Member includes taxa adapted to cold andar id cond itions (Patagonia and Central region ofArgentina) and is correlated to the Last Glacial Maxi­mum. This could imply not only a different age for theSopas Formation but different climatic conditionsas well. Mamm al assemblages from Late Pleistocenefluvial deposits in northern Uruguay might , thus, beolder than the typical Lujanian association and couldhave been associated with a warmer period which isalso indicated by the available luminescence ages.Nevertheless, these beds also contain some mammals

that are currently adapted to live in arid environmentsand do not overlap in their present distributional rangewith those of the 'brasilic' taxa. Although fluvialmixing may have affected the patt ern , a non-an aloguefaun a could have inhabited a large ecotone in the LatePleistocene of the mid-latitudes of South Americaunder a different climatic regime, as is predicted by the'd isharmonious' fauna hypothesis. Nevertheless, dueto remaining uncertainties (e.g. the ra te of depositionof sediments, the rate of environmental change, thepossibility of reworked materials), it is important toundertake more detailed fieldwork to test this hypoth­esis regarding the Late Pleistocene of northernUruguay.

ACKNOWLEDGEMENTS

The author is indebted to E. P. Tonni (La Plata), whorevised an early version of the manu script and pro­vided helpful suggestions on Pampean biostratigraphy ,and to D. Bridgland (Durham) and R. Westaway(Open University) who improved the English versionand provided useful comments. Thanks are due to D.Perea, S. Martinez, G. Piiieiro, M. Verde, N. Lorenzoand A. Rojas for their invaluable contributions. D.Schreve and an anonymou s referee also provided con­structive comments. C. Vildoso is thanked for thedra wing of Neolicaphrium recens. This paper is acontribution to the Comisi6n Sectorial de Investi­gaci6n Cientifica project: 'Cenozoico continental delUruguay' project (CSIC-M. Ubilla); and to IGCP-449'Global Correlation of Late Cenozoic fluvial deposits' .

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Manuscript received 12 June 2003; revised typescript accepted 16 July 2004