jurassic paleoclimates of argentina (volkheimer et al, 2008)

549Revista de la Asociacin Geolgica Argentina 63 (4): 549- 556 (2008)

JURASSIC PALEOCLIMATES IN ARGENTINA, A REVIEW

Wolfgang VOLKHEIMER1, Oliver W. M. RAUHUT2, Mirta E. QUATTROCCHIO3 and Marcelo A. MARTINEZ3

1 Instituto Argentino de Nivologa, Glaciologa y Ciencias Ambientales (IANIGLA), CCT - CONICET - Mendoza, Argentina. E-mail:[email protected] Bayerische Staatssammlung fr Palontologie und Geologie, Munich, Germany. E-mail: [email protected] INGEOSUR (Instituto Geolgico del Sur)-CONICET (Consejo Nacional de Investigaciones Cientficas y Tcnicas). UniversidadNacional del Sur, Departamento de Geologa, Baha Blanca, Argentina. E-mail: [email protected]; [email protected]

ABSTRACT

New paleoclimatic evidence from palynologic and vertebrate proxies provides more detailed data on climatic change duringthe Jurassic in Argentina. Comparison with paleomagnetic data shows that the Neuqun basin shifted from the highest pale-olatitudes (50S), by the end of the Triassic until the end of the Sinemurian. During the Pliensbachian-Toarcian it movednorthward, reaching the lowermost paleolatitudes (25S), and subsequently (Middle to Late Jurassic) the area moved again andattained eventually a position similar to its present-day position (30S). These movements are reflected in the Jurassic palyno-floras. The high frequency of the pollen genus Classopollis (Cheirolepidiacean gymnosperms) is of special paleoclimatic impor-tance in the Argentinian Jurassic, as it is indicative of seasonal aridity or semiarid conditions during large intervals of thisPeriod. During the shift of the South American continent to the northernmost position, the arrival of an important group ofAraucariaceae, represented by Callialasporites spp., in the Toarcian could indicate an amelioration related to more humid con-ditions. Jurassic dinocyst assemblages studied in the Neuqun basin have proved to be useful paleoclimatic and paleobiogeo-graphic proxies. Abundant remains of marine crocodiles in the Mid- and Late Jurassic of the Neuqun basin indicate warmwater temperatures for this basin, probably in excess of 20 C. The occurrence of abundant turtles and other ectothermicvertebrates in the Middle Jurassic Caadn Asfalto Formation of Chubut is in general accordance with the warm climate indi-cated for this unit by geologic evidence.

Keywords: Paleoclimate, Jurassic, Argentina, Vertebrate evidence, Palynology.

RESUMEN: Paleoclimas jursicos en la Argentina, una revisin. Nuevas evidencias de indicadores paleoclimticos (palinolgicos y de vertebrados) suministran datos detallados sobre los cambios climticos en el Jursico de la Argentina. La comparacin condatos paleomagnticos muestra como, desde fines del Trisico hasta el Sinemuriano Tardo, la cuenca Neuquina deriv ocu-pando las posiciones ms australes (50S). Durante el Pliensbaquiano-Toarciano se moviliz hacia el norte, alcanzando las msbajas latitudes (25S), para luego ocupar, durante el Jursico Medio y Tardo, posiciones similares a las actuales (30S). Estosmovimientos se reflejan en las palinofloras jursicas. La alta frecuencia de polen del gnero Classopollis (Cheirolepidiaceae, gim-nospermas) es de especial importancia paleoclimatolgica, ya que indica aridez estacional o condiciones semiridas durantelargos intervalos de este Perodo. Durante la deriva del continente sudamericano a su posicin ms septentrional en elToarciano, la llegada de un importante grupo de araucariceas, representado por Callialasporites spp., indicara el comienzo decondiciones climticas ms favorables para este grupo de conferas, relacionadas con condiciones ms hmedas. Tambin seensayan el significado climtico y la evaluacin paleobiogeogrfica de las asociaciones de quistes de dinoflagelados en la cuen-ca Neuquina. Frecuentes restos de cocodrilos marinos en el Jursico Medio y Tardo de la cuenca Neuquina indican tempera-turas de aguas clidas para esta cuenca, de probablemente ms de 20C. La presencia de abundantes tortugas y otros vertebra-dos exotrmicos en el Jursico Medio de la Formacin Caadn Asfalto de Chubut est de acuerdo con las condiciones declima clido generalizadas, indicadas para esta formacin por la evidencia geolgica.

Palabras clave: Paleoclima, Jursico, Argentina, Paleovertebrados, Palinologa.

INTRODUCTION

This brief essay mainly intents to be acritical revision of earlier contributionson Jurassic paleoclimates of southwes-tern Gondwana, especially through eva-luation of new data in vertebrate paleon-tology and continental and marine paly-

nology.From a palynological viewpoint, the uni-formity of Jurassic and earliest Creta-ceous floras appears to be more apparentthan real (Batten 1984). A modern re-view of the knowledge on distribution,diversity, and paleogeographic and paleo-environmental significance of Jurassic

floras and palynofloras of Argentina isgiven by Quattrocchio et al. (2007).The paleoclimatic situation of Argentinaduring the Jurassic Period was conditio-ned by its geographic position in mid-paleolatitudes of southwestern Gond-wana. Gondwana became part of the su-percontinent Pangaea in the Permian.

The disintegration of Pangaea/Gondwa-na during the Mesozoic is important forthe climatic evolution during the Jurassic,when drying occurred in Gondwana,which was interpreted as indicative ofthe breakdown of the Pangean Mega-monsoon (Parrish 1993).

PALEOCLIMATICINDICATORS

a) Sporomorphs

Large numbers of Classopollis pollen inMesozoic deposits are generally thoughtto reflect an abundance of representati-ves of Cheirolepidiacean gymnospermsof lowland vegetation, when the climatewas at least seasonally dry or semiarid(Batten 1975). Alvin (1982) suggests thatsome species of cheirolepids undoub-tedly were adapted to coastal environ-ments and may have formed extensivecoastal forests as the main componentsof a halophytic, salt-marsh community.Cheirolepidiaceae occupied a wide rangeof warm and semiarid habitats, and somegroups were probably associated withfreshwater environments (both lakes andrivers), or even inhabited freshwater stre-am banks or low-lying areas dissected bybraided streams that may have dried outseasonally or irregularly (Alvin 1983).Other Cheirolepidiaceae may have beencharacteristic for higher, better drainedsoils. Maximal abundances of Classopollishave been correlated with evidence ofwarmth and aridity (Vakhrameev 1981,1987).

b) Dinoflagellate cysts

The Liassic until Bajocian Jurassic dino-cyst assemblages of the Neuqun basin(Fig. 1) are characterized by low specificdiversity and predominance of acavateproximate cysts. Chorate/acavate ratiosincrease during the Callovian and theTithonian and specific diversity increasesduring the Jurassic. Chorate species aregetting common in the Callovian and in-crease during the Late Jurassic (Fig. 2).

The same tendency is registered offshoreeastern Canada, at 25-30 northern pale-olatitude, possibly reflecting warmingtrends in the Late Jurassic.

c) Vertebrates

Vertebrates, especially ectothermic te-rrestrial forms, have proved to be usefulas climate proxies in palaeoclimatic stu-dies of mainly Cenozoic environments(e.g. Markovick 1998, Bhme 2003), andthey might be helpful to interpret Me-sozoic climates as well (e.g. Vandermark etal. 2007), though additional caution isneeded when dealing with extinct groups(see Ostrom 1970). The record of Juras-sic vertebrates from Argentina is still ra-ther patchy, though a variety of dino-saurs, marine crocodiles, and a few othergroups have been reported.Although dinosaurs were sometimesused as climate proxies, allegedly indica-ting warm climates (e.g. Romer 1961, Col-bert 1964), they seem to be poorly suitedfor these purposes, due to uncertaintiesregarding their physiology (Ostrom1970). Indeed, recent finds of even sau-ropod dinosaurs at high latitudes (seeSmith and Pol 2007) indicate that dino-saurs cannot reliably be used to interpretpaleoclimates. The same applies to othergroups, such as the pterosaurs, and cau-tion is even needed when interpreting cli-mates on the basis of larger taxonomicunits that have living representatives, butare represented in the fossil localitiesunder study solely by lineages that lieoutside the radiation of the recent forms,such as crocodiles (Ostrom 1970, Marko-vick 1998). Keeping these restrictions inmind, some observations on possible cli-matic implications of the Jurassic verte-brates from Argentina will be offered.Marine crocodiles, mainly of the familyMetriorhynchidae, have been reportedfrom both the Middle and the Late Ju-rassic of Argentina, principally from ma-rine sediments of the Neuqun basin(Gasparini 1985, 1992, Gasparini et al.2005, Pol and Gasparini 2007). Althoughthe cautions noted above certainly apply

for these extinct thalattosuchians, itmight be noted that recent amphibiouscrocodiles seem to prefer water tempera-tures in excess of 24 C (Markovick1998). Furthermore, longer spells oftemperatures below 18 C may lead toorgan malfunction in recent alligators,and feeding is already restricted at tempe-ratures below 22 C (Markovick 1998). Ifsimilar restrictions applied to metrior-hynchids, water temperature in the Neu-qun basin did probably not drop below20 C for longer periods of time in thelate Middle and Late Jurassic environ-ments in which these animals lived. Thelimits of a coldest month mean tempera-ture of 5.5 C and a mean annual tempe-rature of 14.2 C found by Markovick(1998) for recent amphibious crocodilesare probably not applicable to the fullymarine thalattosuchians. Temperatures inexcess of 20 C for the Neuqun basinare also in accordance with data fromoxygen isotopes, which indicate tempera-tures between 24 and 27 C (Gasparini etal. 2002).The Caadn Asfalto Formation hasyielded one of the most important verte-brate faunas from the Middle Jurassic ofGondwana (see Bonaparte 1979, Rauhutet al. 2001). Vertebrates recorded includea diverse dinosaur fauna (Bonaparte1979, 1986, Rauhut 2002, 2005) and a va-riety of other groups, such as anuran am-phibians (Bez and Nicoli 2008), turtles(Sterli 2008), and mammals (Rauhut et al.2002, Rougier et al. 2007a, b). Interes-tingly, no crocodile remains have beenconfidently identified from this forma-tion so far. However, this might be due toecological reasons other than climate,since geological evidence indicates a ra-ther warm climate for this formation (seeabove). It cannot be ruled out, though,that very high temperatures, in combina-tion with at least seasonal aridity, mighthave been a limiting factor for crocodi-lians in the Caadn Asfalto Formation(see Colbert et al. 1946). The abundantturtles found in the Caadn AsfaltoFormation (see Sterli 2008) are consis-tent with rather warm climates; in recent

550 W. VOLKHEIMER, O. W. M. RAUHUT, M. E . QUATTROCCHIO AND M. A . MARTINEZ

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turtles, even cold-adapted species do notoccur in areas with warm month meantemperatures below 17.5 C (Tarduno etal. 1998).

d) Paleolatitudes and paleomagneticdata

Comparison of paleoclimatic conclu-sions with data on paleolatitudes frompaleomagnetic studies shows a good co-rrespondence of paleoclimatic data andpaleomagnetic latitudes. A paleomagneticstudy in four ammonite-bearing marinesedimentary sections (Iglesia Llanos et al.2006) shows that during the Middle andLate Jurassic, the South American conti-nent eventually attained a position similarto its present-day position (Fig. 3). Butthat during the Hettangian-Sinemurianthe 50 S-latitude was crossing the Neu-qun basin, suggesting a cold climate inthis basin during the earliest Early Juras-sic (Spalletti et al. 1999, Gmez-Prez2003).Based on the palynofloras this shift couldbe correlated with the distribution of so-me Jurassic floras. The Sinemurian - Ear-ly Toarcian microflora from the Los Pa-tos Formation in the north of the basin(Volkheimer et al. 1978) and the Pliensba-chian microflora of the Sierra de Cha-caico Formation in the south (Volkhei-mer 1974, 1980) reflect the passage froma position of the Neuqun basin from50 S to 25 S. The lower member of theLos Patos Formation (lowermost Pliens-bachian - Sinemurian) exhibits a contentof Classopollis not higher than 60 % onaverage (at 50 S), while the Upper Mem-ber (Late Pliensbachian - Early Toarcian)and Sierra de Chacaico Formation showthe highest percentages of Classopollis (91and up to 99 %).During the shift of the South Americancontinent to the northernmost position,another important microfloral changecan be registered: The first appearance ofthe Callialasporites "complex" (Araucaria-ceae) is documented in the earliest LateToarcian associated with Araucariacitesaustralis and A. pergranulatus, for example

in arroyo Lapa, Neuqun (Volkheimer1971). The arrival of the new group ofAraucariaceae, represented by Callialas-porites spp., in the Toarcian (Volkheimerand Quattrocchio 1981) could indicatean amelioration related to more humidconditions. This conclusion could be ex-tended to Australia, where the same pat-tern of Classopollis and the Callialasporites"complex" is observed (Monteil 2006).

During the Late Jurassic (and Early Cre-taceous) a broad arid belt, whose mostconspicuous feature was the Botucatdesert, (studied by Bigarella and Sala-muni 1964), extended from approxima-tely 15 to 30 of southern paleolatitude.Evaporites of considerable thickness anddistribution along the Pacific margincomplete the picture of this warmaridregion. Only in southern Patagonia pre-

Jurassic paleoclimates in Argentina, a review.

Figure 1: Location map showing theposition of the localities and regionsmentioned (Ro Limay region of nor-thern Patagonia, Neuqun basin, Sierrade Chacai Co, Mendoza region,Caadn Asfalto basin).

Figure 2: Number of species in various encystment modes and total specific diversity in theJurassic and earliest Cretaceous of the Neuqun basin (adapted from Quattrocchio 1986). Formethodology see Quattrocchio (1984).

vailed moist conditions, as indicated byarborescent ferns and products ofancient weathering (Volkheimer 1969,1970a, b).The Jurassic magmatic paroxysm was theimmediate predecessor of a large scaleGondwanaland disintegration (Ulianaand Biddle 1988). Those basins close tothe western margin of South Americacontinued to accumulate marine depositsduring the Neocomian. Large areas ofthe southern South American interiorbecame depositional sites for non-marineclastic material.The Late Jurassic to Neocomian sequen-ce records active crustal stretching inareas along the present margins of theSouth Atlantic. By 130 Ma the first ocea-nic floor was formed in the SouthAtlantic (Rabinowitz and LaBrecque1979, Gerrard and Smith 1982).

SUMMARY OF JURASSICPALEOCLIMATES

a) Early Jurassic paleoclimates

Neuqun basin: The Sinemurian - EarlyToarcian palynoflora from the Los PatosFormation in the north of the basin andthe Pliensbachian microflora of the Sie-rra de Chacaico Formation in the southof the same basin reflect the passagefrom a position of the Neuqun basin at50 S to 25 S, taking into account thecheirolepidiacean pollen content, whichrises from 41.5 % in the lower memberto 91 % of Classopollis in the upper mem-ber (Volkheimer et al. 1978), indicatingincreasing warmth and aridity during thedeposition of the Los Patos Formation.The relative abundance of the thermo-philic Classopollis (reaching 25 % as themaximum of the total spectrum of paly-nomorphs) in the Late Toarcian NestaresFormation of the Ro Limay region ofnorthern Patagonia probably reflects thelocal environments. The presence ofwarm (-temperate) and humid conditions(at least temporarily) is also compatiblewith the composition of associated paly-nomorph assemblages consisting mainly

of hygrophile elements: bryophyte/pteri-dophyte spores, associated with compa-ratively smaller amounts of conifer pol-len (Zavattieri et al. 2008).A fundamental feature of the climaticevolution of the Jurassic in the southernAndean region is the gradually increasingaridity during this Period. A cool climateis inferred for the Neuqun basin for theearliest Early Jurassic, based on the gene-ral absence of carbonate deposits and ahigh siliciclastic input into the basin (Spa-lletti et al. 1999, Gmez-Prez 2003). TheMiddle and Late Liassic of the andeanand pre-andean regions of Neuqun andMendoza were characterized by moist cli-matic conditions, as shown by the fre-quency of coal and well preserved fossilfloras.

b) Middle Jurassic paleoclimates

Central Patagonia: Caadn Asfalto basin: Incentral Extra-Andean Patagonia the con-tinental volcano-sedimentary sequenceof mainly siliciclastic lacustrine strata of

the Caadn Asfalto Formation is ofspecial value for paleoclimatic considera-tions. The abundant mega- and microflo-ral evidence (Volkheimer et al. 2001) sug-gest warm climatic conditions during theMiddle Jurassic of this region. The Chei-rolepidiaceae dominated the spectrum,associated with Araucariaceae. Warm andrelatively humid climatic conditions areindicated by high percentages of thethermophilic Cheirolepidiaceae, associa-ted with Araucariaceae, which need rela-tively humid conditions.Lacustrine carbonates (marls and algal li-mestones) are frequent in this formation.They are good climatic indicators, "be-cause they are restricted to generally dryclimates where the ground waters are notunduly acid" (Fairbridge 1967). However,thin coal seams found in parts of the Ca-adn Asfalto sequence also indicateshort periods of more humid conditionsat least in the basal parts of the formation.The conchostracans from this formationare occurring in banded shales. As anexample, fifteen or more successive sea-


Figure 3: By the end of the Triassic until the end of the Sinemurian, the studied region (Neuqunbasin) was at its southernmost position (c. 50 S). From then on, it moved northward at about 20cm yr-1 and reached its northernmost location (c. 25 S) in the Pliensbachian. Subsequently, the areamoved again to the south at 10 cm yr-1 and eventually attained c. 30 S by the end of the Jurassic,which is similar to its present-day position (from Iglesia Llanos et al. 2006).

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sons are represented in a thickness of 19cm of sediment. This points to a seaso-nal sedimentation rate of 1 mm or less.Other evidence, as growth bands, pointsto wet-dry cycles in the water bodies in-habited by the conchostracans studied byTasch (in: Tasch and Volkheimer 1970).Cabaleri et al. (2005) studied a saline pale-olake of the Caadn Asfalto Formationnear the village of Cerro Condor, middlevalley of the Chubut River. Within a la-custrine facies association they observedan extensive biohermal belt (500 m longand 39 m thick), showing three growth-stages of microbial communities: a corefacies with discontinuous stromatolite la-minae, formed by green filamentous al-gae, interbedded with mudstone; a flankfacies (up to 22 m thick), made up bystratiform stromatolites with intercala-tions of Magadi-type chert laminae and atop facies assemblage, consisting of thickhemispheroidal stromatolites, interbed-ded with Magadi-type chert and affectedby desiccation processes, presentingcrack systems and caliche covers, indica-ting arid conditions and a gradual transi-tion from eulittoral to supralittoral envi-ronments (Cabaleri and Armella 2005).The biohermal belt favoured the forma-tion of the pan lake of Caadn Carrizal.Cycles present a sedimentation patternthat responds to climatic variations. Thepaleoenvironmental history of the lake isdefined by a succession of five cyclesthat lead to the progressive reduction ofthe water body, each cycle being shallo-wer than the previous one and endingwith evaporites.The same authors emphasize that thementioned succession of different cyclesof expansion-contraction during the evo-lution of the lake is a consequence of thetectonic history of the basin, which wasinterpreted as a pull-apart structure (SilvaNieto et al. 2002), characterized by diffe-rential movements of small blocks thatdelimit horsts and grabens.Neuqun basin: During late Bajocian times,in a context of marine regression anddrying out of the basin, widespread sul-phatic rocks (the gypsum of the Tbanos

Formation) were deposited. During theCallovian a return to moister conditionsoccurred, probably related to a marineingression. In the southern part of theNeuqun basin thin coal seams are wi-dely distributed, bearing an abundant pa-lynoflora with many representatives ofthe hygrophile ferns (Volkheimer 1972).A hypothetic scenario for the Middle Ju-rassic in the southern part of the Neu-qun basin was proposed by Quattro-cchio et al. (2001), on the basis of sedi-mentary facies and palynological data ob-tained from the study of eight stratigra-phic sections. The analysis of the terres-trial palynological assemblages (102 spe-cies of sporomorphs) shows that certainplant groups dominated the scenario, inparticular the Cheirolepidiaceae, Arauca-riaceae and Podocarpaceae and to a lesserdegree Cyatheaceae, Osmundaceae, Ma-rattiaceae, Dipteridaceae, Lycopodiaceae,Schizaeaceae, Anthocerotaceae, Riccia-ceae, Cycadales/Bennettitales, Caytonia-ceae and Gnetales. In addition to theconventional approach using percentagevalues, principal component analysis(Martnez et al. 1996) proved to be anexcellent and complementary tool, be-cause it reduced the data set into definitegroups of representative taxa.Quattrocchio et al. (2001) used modernanalogues, the Chilean-Argentinian fo-rest and the Planalto of southern Brazil,to explain the paleoclimatic and paleoen-vironmental conditions during the Mid-dle Jurassic. In the Planalto (about 1000m above sea level) the Araucariaceae andPodocarpaceae grow together, but thelatter disappear with an increase of tem-perature and humidity farther north inBrazil. The influence of acid effusives ofthe Serra Geral Formation conditions thelow pH of the soils. In the Neuqun ba-sin, rhyolites of the Choiyoi Group (Per-mian-Triassic) and other acid rocks ofthe basement could have been the subs-tratum of the ancient Araucariaceae. Ac-cording to the above mentioned assess-ment, they proposed a plateaus scenariowith warm to temperate conditions andvariable precipitation rates through the

Middle Jurassic.The sedimentary facies and the palynolo-gic assemblages of the Middle Jurassicdepositional sequences (Garca et al.1994) at Sierra de Chacaico are conside-red, taking into account the Classopolliscontent. This genus is dominant in all theassemblages, with increasing frequenciesfrom the offshore-prodelta deposits, to amaximum in the stream mouth bar. Thisgenus is here considered as a coastal pro-xy related to extraordinary flooding ofthe delta plain. In this scenario Classopo-llis acts as a clastic component, sensitiveto increments of the sedimentary input.Classopollis reaches up to 90 %, associatedwith bars of the prodelta deposits.

c) Late Jurassic paleoclimates

During Kimmeridgian times, approxima-tely 150 to 155 My ago, as a consequenceof the Araucanian Movements, the pale-ogeography and climate of South Ame-rica underwent fundamental changes. Aseries of marginal basins began to subsi-de near the eastern border of the conti-nent.Neuqun basin and related areas: The world-


Figure 4: The South American arid belt duringthe Late Jurassic (adapted from Volkheimer,1969). Paleolatitudinal position of the conti-nent during the Late Jurassic is from IglesiaLlanos et al. (2006).

wide marine transgression was reflectedby the important marine ingression ofthe Tithonian sea in the Neuqun basin.The Tithonian Berriasian microfloras ofthe Neuqun basin are very similar andcorrespond to the Equisetosporites -Trisac-cites Assemblage (Volkheimer and Pthede Baldis 1982). The only difference bet-ween Tithonian - Berriasian microflorasis the lower diversity of the later (Vol-kheimer 1980, Quattrocchio et al. 2003).The sporomorph assemblages show mar-ked change only at the Berriasian/Valan-ginian boundary, by the appearance ofthe conspicuous and abundant Cycluspha-era psilata (Riccardi et al. 1990).A large region of southern South Ame-rica was subject to extremely arid condi-tions during the Late Jurassic (Fig. 4) andparts of the Early Cretaceous. In theChaco-Paran basin a large paleodesert,documented by eolian sandstones, exten-ded from Minas Gerais (Brazil) to Uru-guay (Bigarella and Salamuni 1964) andcentral Argentina (Padula and Min-gramm 1968, Volkheimer 1967). The pa-leoclimatic scenario of an extremely aridregion was complemented by thick gyp-sum and anhydrite deposits of LateOxfordian age, which extended fromZapala (Neuqun Province, AuquincoFormation) to Mendoza and San Juanprovinces and continued to southernPer. Again marine regression (increa-sing continentality) was accompanied byan increase of aridity.Biohermal limestones of Middle Oxfor-dian age (La Manga Formation, Neuqunbasin) indicate warm and shallow oceanicwaters in large areas of this basin.

CONCLUSIONS

1) The use of vertebrates as paleoclima-tic proxies in the Jurassic of Argentina iscurrently severely restricted. One reasonfor this is the very patchy Jurassic recordof especially terrestrial vertebrates inArgentina, with the only good faunabeing that of the Mid-Jurassic CaadnAsfalto Formation of Chubut. The otherproblem in using Jurassic vertebrates as

climate proxies in general concerns thefact that all groups represented belong toextinct lineages with no close living rela-tives, and, thus, their climatic preferencesand necessities remain speculative (seeOstrom 1970). This is especially true forgroups such as ichthyosaurs or plesio-saurs, which do not have any living rela-tives, and for the dinosaurs, the livingrepresentatives of which (the birds) arehighly specialized physiologically. Al-though also not unproblematic, othergroups that have more similar living rela-tives, such as anurans, turtles, and croco-diles might help to interpret general as-pects of Jurassic climates, although cau-tion is advisable in interpreting their im-plications. Thus, abundant marine croco-diles in the Middle and Late Jurassic ofthe Neuqun basin indicate warm watertemperatures in this basin, and the pre-sence of turtles and other ectothermicvertebrates, such as frogs, in the CaadnAsfalto Formation of Chubut also indi-cate a rather warm climate for this unit.In both cases, these interpretations are inagreement with other lines of evidence,both geologic and palaeontologic, so thatthe vertebrates seem to reliably reflectthe climatic conditions.2) Beside the thermophilic character pre-viously postulated for the Cheirolepidia-ceae, dominant in almost all the palyno-logical assemblages through the Jurassic,it is important to analyze the facies inwhich these pollen grains occur. Forexample, in marine deposits of the Mid-dle Jurassic (Neuqun basin) they showincreasing frequencies from the offsho-re-prodelta deposits, to a maximum inthe stream mouth bar. Principally theyare considered as a coastal proxy relatedto extraordinary flooding of the deltaplain. In this case Classopollis acts as aclastic component, sensitive to incre-ments of the sedimentary input. Classo-pollis reaches up to 90 % associated withbars of the prodelta deposits.For paleoclimatic inferences it is impor-tant to consider also the relationships be-tween palynomorph concentrations andvegetation types.

For example the income of a new groupof the Araucariaceae in the Toarcian ispostulated to have been associated withmoist conditions.There is also an increasing need to un-derstand the taphonomic processes thatact upon the palynomorphs, in order toobtain accurate paleoclimatic and paleo-environmental interpretations.

ACKNOWLEDGEMENTS

We wish to acknowledge the ConsejoNacional de Investigaciones Cientficas yTcnicas (CONICET) for economicsupport (PIP 5222 and PIP 5297); IANI-GLA-CCT-CONICET-Mendoza forlaboratory facilities, to the SecretaraGeneral de Ciencia y Tecnologa de laUniversidad Nacional del Sur (SEGCyT),Baha Blanca, for economic support, toDrs. E. Schrank and Z. Gasparini for ma-ny useful comments and corrections andto Lic. Paula Narvez for correction ofthe manuscript.

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Recibido: 5 de junio, 2008 Aceptado: 15 de agosto, 2008


jurassic paleoclimates of argentina (volkheimer et al, 2008)

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