Submitted : July 13 th , 2020 – Accepted : August 13th , 2020 – Posted online : August 2 4th , 2020
To link and cite this article:
doi: 10.5710/AMGH.13.08.2020.3380
CLIMATE-SENSITIVE TRAITS IN THE FOSSIL FLORA OF THE RIO GENOA
FORMATION AND THE LATE PALEOZOIC PATAGONIA ISSUE
PRESENCIA DE CARACTERES CLIMATICO-SENSITIVOS EN LA FLORA DE LA
FORMACION RIO GENOA Y EL CASO DE PATAGONIA EN EL PALEOZOICO SUPERIOR
N. RUBÉN CÚNEO1
1MEF-CONICET. Avda. Fontana 140, Trelew (9100), Chubut, Argentina. [email protected]
33 PAGES, 12 FIGURES
Running Header: CÚNEO: PERMIAN CLIMATE-SENSITIVE PLANT TRAITS IN PATAGONIA
Short Description: An extended review of climate-sensitive plant traits recorded in Patagonia and their implications for late Paleozoic paleoclimate-paleography
Corresponding author: N. Rubén Cúneo. [email protected]
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Abstract. Paleoclimate traits are highly varied at the diverse paleoflora from the Río Genoa Formation in
Patagonia. They are thoroughly analyzed in order to define macroclimate parameters that ruled the terrestrial vegetational growth during the early Permian. Plant-climate relationships have been historically used for understanding the distribution of present/past vegetation since they represent a continuous feedback along the terrestrial environments on Earth. Most of the indicators found in Patagonia clearly suggest warm and humid conditions during the entire deposition of the formation. Such conditions, related to subtropical climates, cannot be correlated with paleoclimate approaches originated from other proxies. This is due to several imprecisions that affect the interpretation of the paleogeographical position of the Patagonian region. Different hypotheses are erected in order to explain the paleofloristic and paleoclimatic anomaly present in southern South America, which should encourage new efforts to better understanding the geological dynamics of SW Gondwana.
Keywords. Patagonia. Permian. Fossil plants. Paleoclimates. Paleogeography.
Resumen. PRESENCIA DE CARACTERES CLIMÁTICO-SENSITIVOS EN LA FLORA DE LA
FORMACIÓN RÍO GENOA Y EL CASO DE PATAGONIA EN EL PALEOZOICO SUPERIOR.
Numerosos indicadores climáticos pueden encontrarse en la flora fósil de la Formación Río Genoa en
Patagonia. Estos son detalladamente analizados con el fin de definir los parámetros macroclimáticos que rigieron el crecimiento de la vegetación terrestre durante el Pérmico inferior. La relación vegetación-clima ha sido históricamente utilizada para entender la distribución actual y pasada de la vegetación a partir de su permanente retroalimentación en todos los ambientes terrestres. La mayoría de los indicadores identificados en Patagonia sugieren condiciones cálidas y húmedas predominantes durante toda la depositación de la Fm. Río Genoa. Tales condiciones, relacionadas con climas subtropicales, no pueden ser correlacionadas con dinámicas paleoclimáticas originadas a partir de otras fuentes disciplinarias. Esto en parte se debe a diferentes imprecisiones que afectan finalmente la interpretación respecto de la ubicación paleogeográfica de la región patagónica. Se proponen diferentes hipótesis a los efectos de explicar la anomalía paleoflorística y paleoclimática Sudamérica austral con
2 el objeto de propender hacia nuevos esfuerzos que contribuyan a mejorar el entendimiento sobre la dinámica geológica del Gondwana sudoccidental.
Palabras clave. Patagonia. Pérmico. Plantas fósiles. Paleoclimas. Paleogeografía.
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SINCE THE BEGINING OF PALEOBOTANICAL STUDIES ON PERMIAN FLORAS from Chubut
(Patagonia), fossil plants indicative of particular paleoecological/paleoclimatical conditions were increasingly known and paleoclimatical/paleogeographical settings appeared. Thus, it did not take too long the establishing of new paleobiogeographical hypotheses that explained the presence of certain plant taxa substantially different from coeval gondwana floras, and indicative of opposite paleoclimatological conditions (Archangelsky & Arrondo, 1970, 1975). This fact invetibably conducted to questioning paleogeographical interpretations regarding the location of the Patagonian territory during the late Paleozoic.
With novel paleobotanical studies, along with geotectonical and paleomagnetical approaches, the “Patagonian issue” became more and more evident at the time that a strong geological controversy arose between authors supporting “fixist hypotheses” (Dalla Salda et al.,
1990; Rapela, 1997; Pankhurst et al., 2006, 2014) and those favoring “movilistic hypotheses”
(Ramos, 1984, 2008; Rapallini et al., 2010; Tomezzoli, 2012; Lugo et al., 2019) whether in their alloctonous or parauthoctonous versions (see Panhurst et al., 2014). A similar controversy extended to paleoclimatical hypotheses erected from paleoglacial studies (Henry et al., 2012) as well as from paleofaunistic evidence (Taboada, 2010), contrasting with the paleofloristic data
(Cúneo, 1996; Andreis et al., 1996).
In the present contribution an updated and revised view from a paleobotanical perspective is adavanced based on paleoclimatical sensitive indicators identified from the Río Genoa and
Mojón de Hierro formations paleofloras, which along with sedimentary evidence, launches a more empirically supported paleoclimatical hypothesis that might enlight the “Patagonian issue” during the late Paleozoic. In this context, the definition of a macroclimatical condition should contribute to the discussion in terms of correlation and causation between climate and geography and their responsability in plant distribution.
Climate, vegetation and biogeograpy
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Historically, it was naturalists at the dawn of the XIX century, who began to understand the basics of the natural systems functioning including the distribution of global vegetational types that subsequently conducted to phytogeography (von Humboldt & Bonpland, 1805). Others (Schouw, 1823; Meyen,
1846) emphasized on the importance of climatic controls in the patterns of plant distribution and their possible mechanisms. Thus, it was generally concluded that climatic controls operated over plant distribution conditioning physiological processes (Schimper, 1898; Walter, 1931; 1976; Cain, 1944), something that could explain why different species could be found in specific geographic conditions.
This additionaly derived in the influence of climatic control over taxa distribution (Box, 1981 among others), pointing to the correlation between life forms and climate (particularly temperature and humidity). As a result, it has been largely documented that plant species distribution, diversity and morphological traits correlate with climate, and reflect, and eventually predict, conditions and climatic trends (Herbert & Nixon, 2015).
Response of vegetation to climate. Mean temperature at different latitudes is a direct response to solar radiation; consequently CO2 fixation by plants shows a strong correlation with that parameter (Woodward &
Lomas, 2004), and conditions plant distribution when temperature decreases towards the poles. Humidity, on the other hand, results from the difference between precipitation and evapotranspiration and shows certain latitudinal differentiation from wet tropical belts, dry subtropics, wet temperate zones and dry polar areas).
These climate belts are controlled by atmospheric and oceanic circulation, responsible of warm transportation from the equator to the poles (Woodward, 1987). Stochastic events (vulcanism for instance) can also affect plant distribution and abundance.
The influence of climate parameters on plant distribution has occurred in Earth since the appearance of land plants by the Silurian, so that understanding plant distribution and physiognomical types represents an excellent proxy to determine past climates. Early plants physiognomy rapidly changed from a herbaceous/bushy configuration to a dominant forest one (open to close in structure), particularly from the evolution of the Progymnosperms in the late Devonian (Beerling & Woodwar, 2001). The forest structure mostly remained the same until today, with the only exception of grasslands that appeared about 30-45 million years ago
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(Bredenkamp et al., 2002). Consequently, if it is possible to understand morphoanatomical traits, and its corresponding taxonomy/life forms, it becomes likely to hypothesize on atmospheric conditions that regulated plant communities in the past, and their changes along time will denote unambiguous evidence on the climate evolution of a particular region. These inferences are necessarily based on a detailed systematic knowledge of the paleofloras, which makes appliable the principle of life conditions from the closest modern relative in an actualistic way assuming similar growth conditions for the ancestor. In this regard, the identification of a fossil taxon part of the phylogenetic stem or crown of a present clade, makes highly likely that its past growth parameters had been the same, suggesting certain ecological conservatism in the clade (DiMichele & Gastaldo, 2008). This feature looks to have been more intense in the pre-angiosperm world, a period during which to remove the phylogenetic component from ecological behavior and tolerance becomes almost unlike
(DiMichele & Gastaldo, 2008).
Finally, in terms of variation or paleoclimate dynamics in the long term results quite random, although it is possible to establish some correlation patterns linked, on one hand, to the distribution of continental and oceanic masses, and second to excentricity Mylankovitch cycles along Earth history. As a result, patterns of plant distribution will have been a response to those dynamics.
Infering paleoclimates from past vegetation. There exist several parameters to evaluate in the approach to interpret past climates, being one of them plant diversity at every taxonomical rank. As a principle, a decrease in diversity toward the poles, a fact linked to solar radiation, has been clearly established. Thus, resulting paleodiversity curves can be traced inducting their corresponding paleoclimate and paleogeographical correlation, in special when plant tolerance to minimal temperatures represents a major conditioning in the distribution. Zimmermann (1964) showed how a minimun temperature of – 1 ºC, even ocasionally occurring, may result enough to cause frosting in the xyleme of some plants, destroying conductive tissue. These plants, in case lacking of proper adaptability mechanisms (for instance latency in boreal or austral forests), will be condemned to limiting their distribution (Woodward et al., 2004). Adaptability capacity to extreme minimum temperatures is a trait developed in some gymnosperms (conifers in particular) and some angiosperm families by6 generating, for example, latency mechanisms (Harbert & Nixon, 2015). Other adaptability traits are deciduousness or evengreeness. On the contrary, groups with thermophilic characteristics have shown very low or absent tolerance to low temperatures. This is the case for groups like lycophytes, ferns and sphenophytes, which represent primitive groups in origin and evolution but well extended during the late Paleozoic, and whose life cycles are highly dependant on very favorable climate conditions, and that withouth them their distribution is dramatically restricted. Due to this lack of tolerance to low temperatures, those taxa normaly found their best growth conditions in wet subtropical or tropical belts. Therefore, its fossil record represents an excellent proxy to climate conditions prevailing at certain times. Moreover, some growth forms in those groups (i.e. arborescence) seldomly favored their distribution due to their inhability to survive in low temperatures.
The geographic factor. Interaction between climate and geography will define every pattern in plant distribution. Thus, phytogeographic units will be characterized by their taxonomic component and distribution at the time they will be bounded by the climate condition that favored them. Such biogeographical models can change with time when both geography and climate modify. So it is the case for paleobiogeographical models that have been proposed for different geological periods and global regions (see for instance Chaloner & Lacey,
1973), which implies the dynamics and complex character that has dominated biotic distributions on Earth along history. This dynamic outlined a planet with important biogeographical differentiation in the late Paleozoic, sensibly diminished during the Mesozoic and early Cenozoic because of more homogeneous global climates, to become again more intensenly differentiated in the late Cenozoic.
THE CASE OF THE RIO GENOA FORMATION
The Rio Genoa Formation (RGF) is the topmost unit of the Tepuel Group (Andreis et al.,
1986) and corresponds to the last sedimentary deposition of a forearc basin during the late
Paleozoic (Ramos, 2008). The RGF is a progradational fluvial-dominated deltaic system (Fig. 1) over shallow marine platforms of the Mojón de Hierro Formation (MHF) generating interfingering deposits into a major context of stand still conditions of the sea (Andreis & Cúneo, 1989). In those fine-grained facies deposited in interdistributary bays of the upper delta plain an enormous plant biomass was incorporated and fossilized in the form of impressions, compressions and permineralizations.
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FIGURE 1
The flora includes a high diversity of life forms (Fig. 2) from trees to herbs and a full range of intermediate growth forms that were part of different community arrangements (Cúneo,
1986). Its deep systematic/taxonomic knowledge, highly favored by the preservation of reproductive parts, has allowed the reconstruction of whole plants that were related to phylogenetically close relatives either living or extint. The high plant diversity registered at the
RGF includes more tan 100 species, many of them with morphological traits or taxonomical links that denote climate sensitiveness.
FIGURE 2
Climate sensitiveness in plant groups of the RGF
These groups are defined from authoecological characters that are recognized as climatically sensitive. Among them are lycophytes, which occur with every plant part: cormose roots, trunks, branches, leaves and reproductive organs including megasporophylls/microsporophylls with megaspores and microspores. Their reconstruction as whole plants shows tree forms up to 20 m tall (Fig. 3), which lived in dense community arrangements up to 525 individuals per hectare
(Cúneo & Andreis, 1983; Cúneo et al., 2013) that colonized both well-drained and swampy environments with hygromophic soils. Most lycophyte remains from the RGF have been assigned to the family Chaloneriaceae (Isoetales) (Tomescu et al., 2014), a particular group that mostly lived in Euroamerica and Angara. Lycophytes persisted along the entire deposition of the RGF suggesting favorable and unchanged growth conditions, and then climate parameters, for a long period.
FIGURE 3
Chaloneriaceous lycophytes occupied clastic swamps of Euroamerica under warm-humid conditions that shared with other and much bigger lycophytes and calamiteans; however their size was not larger than a couple of meters, which turns the patagonian examples as the biggest
8 representatives of the family, probably because of the lack of other large ecological competitors.
At large, the anatomy of Chaloneracieae includes a major percentage of parenchimatic tissues, development of ligules or infrafoliar blades, parichnos and other traits like their reproductive biology that directly responded to extreme humidity levels.
In summary, the combination of identified morpho/physiological traits, taxonomical affinity and geographic distribution of closest relatives, it becomes clear that climate conditions that favored the success of chaloneriaceous lycophytes in Patagonia were related to subtropical/tropical, with high vapor pressure in the air, and no chances to have survived any extreme low temperatures.
A second plant group of thermophilic characteristics were sphenophylls (Fig. 4.3), which occur diverse and abundantly along the RGF (Cúneo et al., 1993). These plants had scrambling and climbing features (Batenburg, 1981; DiMichele et al., 2001), being part of dense understoreys of wet swampy areas that favored their life cycle. Sphenophylls anatomy reveals high participation of parenchimatic tissue intolerant to extreme low temperatures.
The history of the group is mostly septentrional, with origins in the late Devonian or early
Carboniferous (Taylor et al., 2009), having strongly expanded to most of Euramerica and
Cathaysia by the late Carboniferous-early Permian, to mostly disappear by the end of the
Permian. Given the total abscence of any Gondwana ancestry, its first and oldest “gondwanic” appearance in the RGF can only be explained from a migrational hypothesis. Later, they survive and spread in Gondwana (except Antarctica) until the end Permian thanks to the development of post-glacial favorable habitats (Cúneo, 1996).
FIGURE 4
Equisetaleans, another type of sphenopsid plants, were also diverse and abundant in the paleocommunities of the RGF. The two families of the group have been recognized,
Calamitaceae and Equisetaceae (Cúneo, 2000; Escapa & Cúneo, 2005; Cúneo & Escapa,
2006). Calamitaleans (Fig. 4.1) were major participants of the tropical swamp floras of 9
Euroamerica during the late Carboniferous, in many cases reaching tree sizes with secondary growth, and intolerant to subcero temperatures. The classical foliage of these plants, i.e.
Annularia and Asterophyllites have been found in the RGF (Cúneo, 1987a; 2000), while the
Equisetaceae family (Fig. 4.2, 4.5) is even more diverse with several genera and species
(Escapa & Cúneo, 2005; Cúneo & Escapa, 2006), some of them having developed tall forms as suggested by stems more than 20 cm wide (Cúneo, 1987a). Growth forms in this group included root systems with extended underground rhyzomes (Fig. 4.4), which, due to their anatomy, are excellent climate indicators since they could not have survived frozen soils
(permafrost). Nonetheless, in some cases more freezing tolerant ecotypes might have developed (Retallack, 1999a,b).
Ferns are another plant group with climatic implications, in particular some families with extensive phylogenetic histories associated with restricted climatic condictions. For example, the extint primitive family Zygopteridaceae described from the RGF (Archangelsky & Cúneo, 1986) recognizes its origins and subsequent evolution in the Euroamerican region during the
Carboniferous (Galtier & Scott, 1984), with its last representatives in the early Permian of
Cathaysia (Taylor et al., 2009); both regions were part of the late Paleozoic humid tropical belt.
Then, the anomalous Patagonian record (Fig. 5.2), given its high taxonomic/morphological similarities, clearly must have grown under a similar spectrum of temperature tolerance.
Marattialean ferns is also a possitive group that shows climate sensitiveness. These are the oldest ferns that reach current times (Murdock, 2008), having a panequatorial tropical/subtropical distribution associated with very warm and humid climates with shrubby forms. However, during the late Paleozoic marattialeans were able to developed tree forms up to 20 m high culminating in a top frond crown; this feature was probably the result of dominant competition when the group replaced lycophytes in the communities of Euroamerica during the late Pennsylvanian (DiMichele
& Gastaldo, 2008). Of particular paleoclimate interest is the morphology/anatomy of the marattialean stem with enormous amounts of aerenchimatic/parenchimatic tissue, and scarce secondary growth, a combination suitable for highly humid tropical soils (DiMichele et al., 2009). 10
Coincidently, the same forms have been recorded from Patagonia including fronds with diverse taxonomy (Archangelsky & de la Sota, 1960; Cúneo et al., 2000; Archangelsky et al., 2004;
Lundgren et al., 2019), and most importantly stems that confirm the tree growth (Cúneo &
Archangelsky, 1987).
Patagonian marattialeans find the closest relatives in their Euroamerican or Cathaysian relatives (Lundgren et al., 2019). It is of interest to underline the record of similar marattialean tree ferns in some portions of paleotropical South America during the mid-late Permian (Herbst,
1999; Tavares et al., 2014; Negerato et al., 2015), which might represent a relict of the tropical
Euroamerican swamps that survived into the Permian in northern Gondwana (DiMichele et al.,
2009), with interesting ecological parallelisms with Patagonia.
Other fern families with termophilic traits in the RGF correspond to primitive gleichenaceous (Césari et al., 1998), schizaceous (Cúneo, 1989) and osmundaceous (Cúneo, 1987 a) (Fig. 5.4) ferns, all of them with life cycles associated with at least subtropical conditions. Also noteworthy is the presence of especialized structures developed by ferns named aphlebiae, which in modern relatives are a response to excesive humidity levels helping in the transpiration of the ferns.
FIGURE 5
Some morpho/anatomical characters in gymnosperms, which result as an evolutionary consequence in some enviornments under particular climate conditions, can be used as seasonality indicators. Among these features are leaf fall (deciduous vs evergreen), branch growth, foliar size, foliar margins, presence/absence of growth rings, etc. Basically, a latitudinal gradation is verified in these traits from the equator to the poles: macrophyll to microphyll leaf size, entire to dentate margins, evergreen to deciduous, abscence or very wide to thin growth rings.
In the RGF, where at least five of the major late Paleozoic gymnosperm clades are registered, several of those morpho/anatomical features have been identified from different groups. Among them, and of major importance, it is the record of permineralized conifer wood with no growth rings (Archangelsky, 1960; Fig. 6.4). Its presence clearly suggests that at some
11 point during the depositional time of the RGF climate conditions were mostly constant year- round, favoring continuous (non seasonal) growth, characteristic of humid tropical weather
(Zamuner, 1986).
Indications of seasonality also come from conifer branchlets with differential growth
(Cúneo, 1986b; Archangelsky & Cúneo, 1987) as well as deciduosness suggested from leaves of ginkgoals, cordaitaleans or glossopterids, although in the latter two cases the presence of pseudowhorls would also suggest that shedding was not always the rule (Retallack, 1980; Cúneo
1987a).
It is also noteworthy the presence of a particular group of gymnosperms such as the seedferns that include two phylogenetic links: one with affinities to classical euroamerican clades that yielded huge seeds (Archangelsky, 1996), some of them up to 5 cm long. These are identical to those produced by Lyginopteridales and Medullosales from the Carboniferous of the USA
(Figs. 6.3; 6.5) but absolutely unknown from Gondwana floras. A second group, of gondwanic affinities (Vega & Archangelsky, 2001), is known in the Carboniferous and Permian through the foliage genera Botrychiopsis and Nothorhacopteris (Figs. 6.1; 6.2) (Archangelsky & Arrondo,
1965; Archangelsky & Cúneo, 1981). Both were herbaceous plants that grew probably as the
“grass” of the time or understorey of major forest formations of conifers, glossopterids, ferns and cordaitals. It is interesting to note that these same taxa probably developed different ecotypes such the so called Botrychiopsis “tundra” (Retallack, 1980;1999 a; Cúneo, 1996), a periglacial vegetational formation from the end Carboniferous to early Permian of Australia and Antarctica.
FIGURE 6
Glossopterids, considered at large as part of the seedferns (Taylor et al., 2009), have been considered to have initially developed as a Permian “taiga-like” formation that replaced the former
“tundra” (Retallack, 1980); primitive representatives yielded trunks with very narrow growth rings and yielded Gangamopteris leaves that conformed “autum Banks” suggesting strong deciduosness after a short growth season following glacial conditions (Cúneo et al., 1993). 12
FIGURE 7
Taxonomy and paleocommunity diversity as climate proxies
These two factors work following approximately latitudinal and altitudinal patterns (Clements,
1936). Then, it is possible to grade from tropical rain forests of extreme high diversity to very low subpolar diversity taigas and tundras around the pole (N). Something similar occurs at the paleocommunity structure with highly variable and complex arrangements at low latitudes to much simpler high latitude (and altitude) ones.
If we extrapolate this basic pattern to the time and space of the RGF, where more than 100 megaplant species, 25 genera and at least 15 families are recognized (Cúneo, 1996), and more than 150 palinomorphs from only locality! (Gutiérrez et al., 2007; Vergel & Cúneo, 2007), as well as several insect species and their traces (Pinto, 1972; Cúneo, 1986 a, Gallego et al., 2014), a pattern of high taxonomic diversity and a variable paleocommunity structure arises. In the latter, at least 10 possible paleoecological arrangements have been recognized following a model of water table dependance (Remy & Remy, 1977) from hygrophytic to mesophytic types
(Archangelsky, 1981; Cúneo, 1983, 1986a; 1987b; Cúneo & Andreis, 1983).
This model results extremely contrasting with coeval examples from the rest of Gondwana (Fig.
8), where very low diversity and structuraly simple plant formations such as the Botrychiopsis “tundra”
(Retallack, 1980; 1999) or the impoverished Gangamopteris “taiga”in Antarctica and Australia (Cúneo et al., 1993; Retallack, 1980; 1999 a, b) have been interpreted associated to at least seasonal frozen soils (permafrost), a feature also extensive to other Gondwana regions (Cúneo, 1996).
FIGURE 8
Abiotic climate indicators from the RGF
In essence, the process of phytomass accumulation deriving in the formation of peatlands requires of a hydrological balance with precipitations prevailing over evapotranspiration (DiMichele et al.,
2006); this originates a swampy anaerobic habitat with acidic pH. If erosional process do not affect such habitats, then final burying and diagenesis will result in coal formation, whose 13 composition will have depended on the constituent macerals and prevailing temperature/pressure parameters.
The recorded presence of coal seams (Fig. 9.1) up to 70 cm in thickness are remarkable in the RGF (Andreis & Cúneo, 1989), although somehow limited (and eroded) due to the dynamics of the sedimentary process, where short fluvial high-energy systems were dominant (Andreis &
Cúneo, 1989) The maceral composition defines médium-rank sub-bituminous coals with a dominant presence of vitrinite and lack of fusinite (Cúneo, 1983), the last normally linked to forest fires during very dry seasons (Scott, 1979).
Coal sean plant source mostly derived from hydro-hygrophitic communities dominated by lycophytes growing in dense forests on hydromorphic soils (Cúneo, 1983; Andreis & Cúneo, 1983; Cúneo et al.,
2013); these conditions succeded repeatedly along the deposition of the RGF suggesting that climate conditions allowing peat formation prevailed for many million years. Interestingly, there is an inconsistency in terms of time of appearance of coal seams in Patagonia (much earlier) than the rest of
Gondwana (Fig. 9.2), suggesting a quite different paleoclimate setting. This inconsistency also extends to the plant source in each case, with a phytomass mostly derived from glossopterid forests in other
Gondwana subcontinents (Cúneo et al., 1993; McLoughlin, 2011).
FIGURE 9
Other abiotic factors that could be indirectly considered as climate indicators have to do with the regional context during the deposition of the RGF deltaic system associated with shallow platforms in a standstill sea condition (Andreis & Cúneo, 1989). This is relevant in terms of guarranteeing the regional hydrological cycle that most probably included heavy rain seasons (monsoonal?), which resulted for instance in high energy deposits associated with fluvial and distributary channels. In this regard, the bigger the accomodation sedimentary space the higher the potential preservation of biomass to different sedimentary facies (DiMichele & Gastaldo, 2008). Therefore, in a spectrum from very humid to semiarid conditions, the preservational potential will diminished accordingly. This is directly related to the relative position of the watertable, where a stable one will contribute to a higher
14 preservation of the paleobiota. The enormous amounts of biomass fossilized in the RGF is suggesting these highly favorable variables.
DISCUSSION
Paleoclimate hypothesis. Based on previous direct and indirect empirical evidence, macroclimate patterns that regulated the biota and deposition of the RGF in Patagonia should have included consistent temperature and humidity parameters. In this regard, the different lines of evidence suggest minimum paleotemperatures of the air masses never below the freezing point, perhaps with mean anual temperatures in the range of 15-20 ºC.
Future approaches on this subject should consider quantitative information originated in carbon and oxigen isotopes that could be obtained from carbonatic and coal intercalations present in the RGF.
When humidity parameters are considered it becomes clear that high levels where dominant, with precipitations probably during at least 4-7 months favoring a hydrological balance with precipitations ≥ evaporation. If such levels of available atmospheric humidity combined with water saturated hydromorphic soils and minimum temperatures below 0 ºC (as in modern cold temperate biomas, Ziegler, 1990: Ziegler et al., 2003), a strong disruption in the vegetation would have been produced, especially on thermophilic elements abundantly preserved in the RGF. Thus, necessary minimum temperatures above freezing point should have yearly prevailed during the entire deposition of the RGF.
In order to establish analogies with modern climates certain basic approximations can be traced with the Strahler System (Strahler, 1984) that, with minor modifications, follows the traditional Köpen System (Köpen, 1936). Strahler (1984) basically divided climates into three big groups: a) low latitude climates regulated by tropical and equatorial air masses; b) middle latitude climates regulated by tropical and polar air masses, and c) high latitude climates regulated by polar air masses. As such, the macroclimate for the time of deposition of the RGF could have been subtropical humid (Cfa in the Köpen System). This climate occurs between 20 and 35 degrees of north and south latitude being characteristic of regions of the Gulf of Mexico, southern
Brazil or southern China, where it is controlled by maritime humid and unstable air tropical masses, which collide (but dominate) polar air masses. In subtropical climates the most variable 15 parameter, and responsible for seasonality, is temperature; air polar masses can influence, however tropical ones dominate during most of the year. Interestingly, regions dominated by this climate conditions have forest vegetation with high participation of conifers, a group clearly dominant in Patagonia during the early Permian.
In the same sense, Rees et al. (2002) suggested that the anomalous vegetation of the RGF could be linked to their Biome 5 as previously defined by Ziegler (1990) for modern geographic/climate configurations. Biome 5 was characterized by a warm-temperate climate in mid latitudes (36 to 46 degrees South), with a prevailing evergreen vegetation, although minimum temperatures negative during the coldest month. In this regard, Ziegler (1990) suggested that climate, and not geography –due to the amalgamation of the Gondwana supercontinent- was the controlling factor in the distribution of Permian vegetation.
At this point, another clear inconsistency is arising between the Patagonian climate dynamics and the Gondwana glaciation timing. According to Taboada (2010) and Henry et al.
(2012) a series of six glacial pulses ocurred in the Tepuel-Genoa basin between the late
Mississipian and the Artinskian; this interpretation is the stratigraphical occurrence of diamictite beds. In particular, since the begining of the Permian, two glacial episodes were suggested and identified in the Mojón de Hierro Formation, which surprisingly records the same paleoflora as the RGF. These glacial episodes in Patagonia would be coincident with two large and approximately coeval Gondwanan events interpreted from O2 isotopes and CO2 paleoconcentrations, which suggest an intense event during the Asselian, and a second one less intense by the Sakmarian-Artinskian (Montañez & Poulsen, 2013). Interestingly, Gulbranson et al. (2010) and Montañez & Poulsen (2013) discard any evidence of glaciation in western South
America from the late Carboniferous onwards, although without including Patagonia. Here, according to Henry et al. (2012) a glacial dispersion center was located on eastern (current)
Patagonia, and remained active until de Artinskian (Taboada & Shi, 2011). To this have occurred, and in order to introduce a glacial accomodation space in the Tepuel-Genoa basin, both the RGF and MHF needed to be “rejuvenated” in approximately 10 m.y. due to the presence in them of 16 subtropical climate indicators. Thus, a reinterpretation of the local biostratigraphy based on invertebrate faunas forced a time “displacement” by which both units (again including the same floral content) are now Asselian-Sakmarian (MHF) and Artinskian (RGF) in age (Taboada, 2010;
Taboada y Pagani, 2010; Henry et al., 2012). In conclusion, it results evident that climate, glacial and paleovegetational dynamics lack a confident temporality (see discussion below) precluding detailed interpretations for the existing phenomena.
Paleobiogeography hypothesis. A key question arises from the previous hypothesis: could the suggested paleoclimate model have taken place during the early Permian under a “fixist” paleogeography that is with
Patagonia added to the western Gondwana margin? Paleogeographical configurations for this time (see for instance Scottesse, 2004) indicate that Patagonia (Tepuel-Genoa basin) occupied a location between 45º and 60º
S (Fig. 10) or even more austral (Blackey, 2008). These are paleolatitudes similar to other early Permian regions such as Southern Victoria Land in Antarctica or South Africa, and higher in comparison with Australia, India,
Brasil and northern Argentina.
FIGURE 10
Then, the next question is: how is that Patagonia records the highest plant diversity that differentiates it as a separate biogeographic unit from the rest of Gondwana (Cúneo, 1996; Fig. 11 this paper), and bears subtropical climate indicators, while at the same or lower latitudes other Gondwana regions register an absolutely more pauperized vegetation? Answers to this question could be twofold: Patagonia was influenced by a very special climate condition, i.e. a cuasi-tropical climate at high latitudes, or well its paleographical location was different.
Regarding the first assertion, Rees et al. (2002), even though recognizing that climate conditions for the Tepuel-
Genoa basin were warm-temperate, explain the “floristic anomaly” as the result of a paleoceanic circulation model (Ziegler, 1998). According to this, warm ocean currents would have dominated the Patagonian paleopacific shoreline and were responsible for heat transportation towards high latitudes. This hypothesis however is somehow opposed from the marine paleofaunas that appartently reflect cooler sea temperatures
(Taboada, 2010). In this regard, it must be emphasized that the only evidence of warm waters in South America during late Carboniferous-early Permian times comes from the Copacabana formation in Bolivia-Peru
(DiPasquo, 2017).
FIGURE 11 17
The Patagonian floristic “anomaly” of the early Permian was also explained from a migrational hypothesis (Archangelsky & Arrondo, 1969; 1975). The process involved a paleofloristic influx from northern Gondwana that reached southern latitudes finding suitable climate conditions. This supposed a Patagonia location adjacent to South Africa that would favor the migracional process. Somehow, this hypotheses was supported from a tectonical perspective by Rapela (1997). However, when possible migration routes are analized the presence of hard climatical barriers expressed for instance in the development of permafrost (Retallack, 1980) would have been impassable. Additionally, it was hard to explain that the proposed migration did not arrive first to more septentrional regions in both South
America and South Africa, where fossil floras show the typical impoverished low diversity spectrum of the basal Permian.
In the case of Patagonia having been displaced under the “movilistic” hypothesis, this finds its grounding in plate tectonics, which suggests an allochtonous origin for one or several
Patagonian microplates that wealded to the SW Gondwana margin by the late Paleozoic (Ramos,
2008; Arzadún et al., 2018, among others). In order to explain the identified paleoclimatic variables, the allochthonous plate should have derived from lower latitudes. Thus, in biogeographic terms, it could have received the numerous extra-gondwanic plant groups recorded in the RGF favored by an ecotonal location close to a dispersal center, which allopatrically emited plant clades with no gondwanic ancestry. Perhaps, the best answer to sustain this position should have come from paleomagnetic studies; however, even though they validated the allochtonous hypothesis, no clear paleogeographic locations at low latitudes for Patagonia are interpreted (see for instance Rapallini et al., 1994; 2010). Similar conclusions occur when the collision process is reconstructed, which indicates an approximation from the SW (present) that is higher paleolatitudes in the late Paleozoic (Fig. 12.2).
FIGURE 12
As seen, several factors remain unanknown. Limarino et al. (2014), in their analysis on the climate evolution of southern South America during the late Paleozoic, expressed that “for now
Patagonia can not be considered in such analyses since it is affected by an imprecised 18 paleogeography”. This is a correct statement from a geological viewpoint, but not from the paleoclimate indicators above described, which demostrate to be unquestionable.
Problems that conspire against a definitive paleogeographical hypothesis. In the first place, it is evident that the lack of a confident chronostratigraphical framework for the entire Tepuel-Genoa basin represents a major weakness in terms of defining a clear temporality to include the major events that occurred there. So far, chronostratigraphical proposals based on invertebrate faunas or micro/macrofloras have demonstrated to be contradictory. In this regard, along the last 60 years of proposals suggested from the faunal analisis have resulted somehow unstable, apparently due to taxonomic bias (see Amos, 1960; González, 1981;
Simanauskas & Archbold, 2002; Taboada & Pagani, 2011). On the other hand, micro/macrofloristic proposals, nonetheless influenced by the complexity of correlating continental units (Lucas et al., 2015), seem to have been more stable through time, and correlation charts that arose decades ago are still useful (Archangelksy & Cúneo,
1985). On this subject, it must also be emphazised that, even though the palynological record of the RGF is restricted due to strong Jurassic volcanic activity, palynomorph associations from the Ferraroti locality
(uppermost RGF) show high diversity and abundance, reflecting a classical earliest Permian spectrum (Asselian to Sakmarian) (Vergel & Cúneo, 2006). Besides, no younger biomarkers such as striate pollen grains have been found, excepting for some very dubious records (Gutiérrez et al., 2007). And even though this were true, then perhaps the uppermost part of the RGF could have reached the Artinskian, meaning the previous more than 1000 m deposits are definetely pre-Artinskian, and initial deposition probably started by the late Carboniferous based on suggested subsidence rates (Andreis & Cúneo, 1989). A similar situation accounts for the MHF, in special when its first recorded plant event in the lower half of the unit was interpreted as a late Carboniferous association (Cúneo, 1991).
The evolution of the bio and chronostratigraphical knowledge of the late Paleozoic in
Argentina has been somehow erratic. In this regard it is notheworthy the case of western
Argentina Paleozoic basins, where once a time controller based on high-precission radiometric ages was available (Gulbranson et al., 2010), it turned out evident that biostragraphical schemes supported from macro/microfloras sources (and their relative ages) resulted not so distorted regarding the more accurate U-Pb chronostratigraphy (Césari et al., 2011). On this ground, it becomes unavoidable to reach a similar framework in Patagonian basins, and then a confident
19 resolution context, which allows a better time adjustment of the paleclimatical and paleogeographical events that outlined Patagonia. Efforts in this regard have started and the first mínimum U-Pb ages from detrital zircons have emerged, in one case with ages of around 307 m.a. for the uppermost part of the MHF, and 305 m.a. also for the uppermost part of the RGF (Cúneo &
Ramezzani, in prep.).
Simultaneously, it turns necessary to carry out geochemical analyses for C and O isotopes that could rescue a more precise quantitative information regarding climate variables affecting the biota and sediments. This would offer an excellent opportunity to better understand the atmospheric conditions in Patagonia. In this regard, results from the Permian of Antarctica are promising in outlining paleoatmospheric parameters from carbon isotopes in permineralized wood
(Gulbranson et al., 2012).
A second major important element to understand the relative paleogeographical location of Patagonia is the lack of an apparent wandering curve that would surge from poles obtained from the more of 5000 m sedimentary pile of the Tepuel-Genoa basin. In this respect, it is valid to mention that among the first clues to suggest an allochtony for Patagonia it came from this field (Vilas & Valencio, 1982). Several contributions followed that pioneering work (see
Rapalini et al., 2010; Tomezzoli, 2012 among others), although all of them were the result of exotic data bases. The pursuing on this matter will for sure contribute to improve further more accurate interpretations of the “Patagonian case”.
CONCLUSIONS
Based on the above data and discussion, it is clear that the paleogeographical location of
Patagonia should have been related with a paleoclimatical latitudinal belt that supported warm- temperate or subtropical conditions. If movilistic hypotheses were to be confirmed, a W or NW drifting approximation of Patagonia to the Gondwana margin should have been the pattern as somehow Tomezzoli (2012) and Luppo et al. (2019), or earlier Dalymayrac et al. (1980), have proposed (Figs. 12.1; 12.3). In this scenario, Patagonia at some place and some time was at a
20 nearby location with other paleofloristic regions, which allowed the necessary phylogenetic flow that explained extra-gondwanic clades. It means, in phytogeographic terms, that Patagonia acted as an ecotonal region, where clades of gondwanan affinity (glossopterids, southern pteridosperms or conifers) coexisted with paleotropical clades. The latter, interpreted as migrants, allopatrically dispersed and evolved under non-limiting climate parameters, at the time that species interaction was favored (Woodward & Lomas, 2004). These adjacent regions were influenced by in average temperate marine currents, with warm waters prevailing in summer and viceversa, perhaps as an analogous to the present south Pacific pattern, where the cold Humboldt current reaches the Peru latitude, or in the Atlantic with Malvinas cold current reaching southern Brazil, and their warm counterparts.
As evidenced by paleofloristic data maximum biogeographical separation between
Patagonia and SW Gondwana probably occurred during the latest Carboniferous and earliest
Permian. Towards the end of the early Permian (Cisuralian) and the late Permian, a maximum approximation takes place and the florstic exchange intensifies as demonstrated by mid-late
Permian floras from southern Patagonia that show a more gondwanic alliance (La Golondrina
Formation, Santa Cruz).
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Figure captions
Figure 1. Río Genoa Formation Paleocelta, early Permian, Patagonia. Modified from Andreis &
Cúneo (1989).
Figure 2. Compositional spectrum of the RGF total flora (macroclades at the order category).
Figure 3. Lycophyte forest from La Casilda locality, Chubut. 1–4, chalonerian cormose bases
(hammer scale equals 28 cm); 5, reconstruction of to forest stands (profile and plan) with scaled individuals density.
Figure 4. Sphenophyllean plants from the RGF. 1, Annularia mucronata (calamitalean); 2,
Cruciatheca feruglioi (equisetalean); 3, Sphenophyllum patagonicum (sphenophyllalean); 4,
Cruciaetheca rhyzome; 5, Peltotheca furcata (equisetalean). Scale bars equal 1 cm. 32
Figure 5. Ferns from the RGF. 1, Caulopteris sp., marattialean stem; 2, Corynepteris australis, zygopteridalean; 3, Floratheca apokaliptica, marattialean; 4, Oligocarpia patagónica, gleichenialean. Scale bars equal 2 cm (1), 5 cm (2), and 1 cm (3, 4).
Figure 6. Gymnosperms from the RGF. 1, Botrychiopsis valida, southern Pteridosperm; 2,
Nothorhacopteris chubutensis, southern Pteridosperm; 3, giant medullosan seed, northern
Pteridosperm; 4, Abietopitys patagonica, permineralized wood showing continuous growth; 5,
Patagosperma, lyginopteridalean northern Pteridosperm. Scale bars equal 1 cm (1–3, 5) and 50
µm (4).
Figure 7. Gimnosperms from the RGF. Fm. 1, Glossopteris wilsonii, glossopteridal; 2,
Ferugliocladus sp., coniferophyte; 3, Ginkgoites patagonica, proto-ginkgoal; 4, Cordaites sp., cordaitalean leaf. Scale bars equal 1 cm.
Figure 8. Generic diversity curves for different Gondwanan regions during the Permian.
Figure 9. 1, Coal sean of the RGF; 2, Coal seams and red beds participation and evolution along Permian sequences in Gondwana.
Figure 10. Phytogeographic units during the early Permian of Gondwana showing a clear
Patagonian (A) differentiation from the remaining regions (Brazil, South Africa,
Antarctica, Australia, India).
Figure 11. Gondwana paleomap with the approximate location of Patagonia at the
Carboniferous-Permian boundary (“fixist interpretation”).
Figure 12. Patagonia location and drifting (“movilistic hypotheses”). 1, according to Dalmayrac et al. (1980); 2, sensu Ramos (1984); 3, Tomezzoli (2013).
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