A new Serpukhovian (Mississippian) fossil flora from western Argentina: Paleoclimatic, paleobiogeographic and stratigraphic implications

Palaeogeography, Palaeoclimatology, Palaeoecology 280 (2009) 517–531

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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o

A new Serpukhovian (Mississippian) fossil flora from western Argentina: Paleoclimatic, paleobiogeographic and stratigraphic implications Diego Balseiro a,⁎,1, Juan J. Rustán a,1, Miguel Ezpeleta b, Norberto E. Vaccari a a b

Centro de Investigaciones Paleobiológicas, CICTERRA (CONICET- Universidad Nacional de Córdoba), Av. Vélez Sársfield 299, X5000JJC, Córdoba, Argentina Laboratorio de Análisis de Cuencas, CICTERRA (CONICET-Universidad Nacional de Córdoba), Av. Vélez Sársfield, 1611, 2° piso, of. 7, X5016GCA, Córdoba, Argentina

a r t i c l e

i n f o

Article history: Received 15 January 2009 Received in revised form 7 July 2009 Accepted 9 July 2009 Available online 17 July 2009 Keywords: Serpukhovian Gondwana Argentina Floras Paleoclimate Paleogeography Biostratigraphy

a b s t r a c t An uppermost Mississippian (Serpukhovian) fossil flora from western Argentina is described for the first time. Its particular composition includes both, warm climate plants typical from the Pennsylvanian of Gondwana, namely pteridosperms, sphenophytes as well as arboreous lycopsids; and small lycopsids characteristic of cooler climates of pre-Serpukhovian times. A warm–temperate climate is inferred for this association, which fills a time gap between those Pennsylvanian and Mississippian classic floras previously known. The presence of Tomiodendron, Nothorhacopteris kellaybelenensis and Paracalamites allow the recognition of the Paraca Floral Realm in Argentina, supporting the existence of a nonglacial paleoclimate period, developed just before of the Bashkirian–Moscovian Gondwanan glaciation and after a latest Visean glaciation. It likewise would indicate the paleogeographic position of south west Gondwana, which would have acceded to a warm temperature latitudinal belt during Serpukhovian times. The recognition of this floral assemblage has relevant biostratigraphic and lithostratigraphic implications for the western margin of Gondwana. It defines a new zone for western Argentina, herein named Frenguellia eximia–Nothorhacopteris kellaybelenensis–Cordaicarpus cesarii (FNC) zone, which is recorded in the oldest known lithostratigraphic unit from the Paganzo Basin, the Loma de los Piojos Formation nom. nov. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The Late Paleozoic witnessed a major climatic event, known as the Late Paleozoic Ice Age (López-Gamundí, 1997; Fielding et al., 2008), which is classically depicted as a single protracted glaciation. However, novel contributions have challenged this idea, showing that the global climate dynamics were much more complex than previously envisaged (e.g. Fielding et al., 2008; Birgenheier et al., 2009). This view suggests a complex hierarchical arrangement of glacial and nonglacial periods, named “nested cyclicity” by Birgenheier et al. (2009). This is particularly true for western Gondwana, where the recognition of more (Pazos, 2007; Pérez Loinaze et al., 2008) and shorter glacial events (Isbell et al., 2008) as well as new dates for some events (Pazos, 2007; Astini and Ezpeleta, 2008; Ezpeleta and Astini, 2009) is nowadays under discussion.

⁎ Corresponding author. Tel.: +54 351 4332098x251; fax: +54 351 4332097. E-mail addresses: [email protected] (D. Balseiro), [email protected] (J.J. Rustán), [email protected] (M. Ezpeleta), [email protected] (N.E. Vaccari). 1 The order of the first two authors does not indicate seniority, and are listed in alphabetic order. 0031-0182/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2009.07.005

The Late Paleozoic Ice Age had major effects on the biota (e.g. Ziegler, 1990; Kelley and Raymond, 1991; Anderson et al., 1999; Pfefferkorn and Iannuzzi, 2005; Powell, 2007). Hence, the understanding of the complex climatic framework helps to elucidate the equally complex fossil record, and viceversa. In the case of western Gondwana two different floras have been classically recognized in the fossil record. The Mississippian flora was dominated by small endemic lycopsids, while the Pennsylvanian flora was characterized by Nothorhacopteris and Botrychiopsis (NBG zone). Between these floras existed a floral lacuna that was difficult to clarify (Anderson et al., 1999). Only recently, Iannuzzi and Pfefferkorn (2002) filled that blank based on the review of some Gondwanan floras, and defined a new floral realm: the “Paraca Floral Realm” (Iannuzzi and Pfefferkorn, 2002; Pfefferkorn and Iannuzzi, 2005). This realm was characterized by a combination of elements known from other climatic belts, and defined a new warm–temperate climatic belt for the Late Visean–earliest Serpukhovian. Specifically, the Carboniferous floras of western Argentina have been known for more than a century (Geinitz, 1876), and received much attention ever since (e.g. Kurtz, 1921; Frenguelli, 1949; Arrondo and Petriella,1978; Césari,1987; Archangelsky et al.,1987,1996; Gutiérrez et al., 1992; Gutiérrez, 1995; Vega, 1995; Carrizo and Azcuy, 1998; Césari and Pérez Loinaze, 2006; Césari et al., 2007). These contributions dealt with systematic, biogeographic and climatic aspects of both Mississippian and

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Pennsylvanian floras. However, the particular characteristics of Serpukhovian floras remained unknown due to their absence from the western Argentinian fossil record (Archangelsky et al., 1996). Here we present the study of a unique fossil flora found at Loma de Los Piojos, southwest Jáchal, San Juan Province, western Argentina. The composition of this flora is intermediate between typical Mississippian floras and Pennsylvanian floras of western Gondwana, and has important biostratigraphic implications. Given its Serpukhovian age, this assemblage may shed light on the Late Mississippian–Early Pennsylvanian biogeographic, climatic, and paleogeographic history of Gondwana. 2. Geological setting and paleobotanic background The Late Paleozoic deposits from western Argentina are usually divided into three main sedimentary basins: Calingasta–Uspallata and

Río Blanco to the west, and the Paganzo Basin to the east (Fig. 1B). The Paganzo basin is characterized by two well differentiated zones, a western region with marine influence, and an eastern region dominated by continental sedimentation. The stratigraphy of western Paganzo has been subdivided in Guandacol, Tupe and Patquía supersequences (Fernández Seveso et al., 1993). In agreement with this division, the history of this basin was summarized by Limarino et al. (2006) in three main events: 1) a Bashkirian glacial event recorded at the base of Guandacol Formation, 2) a Pennsylvanian postglacial fluvial succession with NBG megaflora zone in upper Guandacol and Tupe formations, and 3) Permian red beds with Glossopteris flora in the Patquía Formation and equivalents (Archangelsky et al., 1987). Late Paleozoic outcrops at Loma de Los Piojos, 7 km southwest of San José de Jáchal, San Juan Province, correspond to western Paganzo

Fig. 1. A) Geographic location of study area in central western Argentina. B) Paleogeographic reconstruction of Upper Paleozoic basins in central western Argentina, showing main localities. C) Detailed map of the type locality of Loma de Los Piojos Formation, and stratigraphic sections studied (A and B).

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(Fig. 1B). In this locality, García (1945) was the first one to recognize strata having a possible Early Carboniferous flora. However, this author did not clearly specify the stratigraphic position of these records, nor did he differentiate them from the Guandacol Formation. Later, this interval was mapped as Middle Devonian and included into the underlying Punta Negra Formation (Furque, 1979). Most recently, a detailed map was made by Lehnert (1990) who assigned all these outcrops to the Carboniferous. The material collected by García (1945) was revised and illustrated by Arrondo and Petriella (1978), and Gutiérrez and Arrondo (1994) who agreed with its Mississippian age based on the recognition of Frenguellia eximia. These Carboniferous outcrops from Loma de los Piojos were correlated with those from La Usina and Pasleam by Rolleri and Baldis (1967), Baldis and Cané (1968), Furque (1979) and Rodríguez (1985). A detailed study of the outcrops at Loma de los Piojos allowed the recognition of a sandy-mudstone interval unconformably overlain by basal diamictite of the Guandacol Formation, and overlying in unconformity the Talacasto Formation (Lower Devonian). Based on its stratigraphic relations and its singular paleofloristic record, not recognized previously in western Argentina, we separate this interval as a different lithostratigraphic unit, the Loma de los Piojos Formation nom. nov., in allusion to the name of Loma de los Piojos, its type locality. 3. Stratigraphy of Loma de Los Piojos Formation Overall, the Loma de Los Piojos Formation consists of a coarseningupward basal section (between 35 and 130 m, Fig. 2) formed by black shales, green mudstones, fine grained thin-bedded tabular sandstone and coarse to medium graded sandstone beds, and an upper section (55 and 135 m, Fig. 2) of relatively monotonous green mudstones alternating with bedded massive sandstone. The differences in thicknesses of these two intervals between close outcrops indicate a marked paleorelief at the time of deposition as well as an intense glacial erosion prior of deposition of diamictites of the overlying Guandacol Formation (Fig. 2). The basal section of the Loma de Los Piojos Formation begins with thin conglomeradic bed followed by bioturbated black mudstones (~ 30 m) in thin intervals (b2.5 m), interbedded with tabular sandstones with carbonate matrix and showing no evidence of wave influence. The sandstone/mudstone ratio is very low (~ 1:10). These massive or graded sandstone beds (5 and 20 cm) with frequent sole marks are separated by thin (b2 cm) bioturbated mudstone beds. Locally, small hummocky cross stratification is present (Fig. 2A). A heterolithic succession (~40 m) overlies this muddy succession. This interval is formed by a rhythmic alternation of medium to fine tabular sandstone interbedded with gray-greenish bioturbated siltstones and silty-sandstones. The sandstones (10–30 cm), possess flat or irregular basal contacts, and commonly has carbonate cement that occupies more than 20% of the pore space. Locally, they develop load and flow structures in their base, and mud intraclasts are common. Internally, they are normally graded beds at the base, with parallel lamination and cross lamination with sinuous crests (out of phase and in phase) at the top. The lengths of waves are always shorter than 30 mm, these tabular and coarsening-upward sandstone beds are generally accompanied by subtle lateral granulometric changes. Sandstone/ mudstone ratio is relatively high (2:1 and more) and fine deposits seldom surpass 10 cm in thickness. Basal contacts of sandstones are sharp and flat, and their tops usually have ripples cross lamination. Overlying this interval, a coarse sandstones succession is observed (~55 m). It consists of amalgamated medium to coarse laminated sandstone with erosive bases and tabular or lenticular expanded geometries (b1,5 m thickness and ~100 m of lateral extension). Internally, planar bedded dominate and locally large-scale trough cross bedding is observed. Soft-sediment deformation and mud intraclasts are common at the base of these beds. At the top, sandstones show frequent symmetric ripples. Usually, thin (b1 cm), gray and

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bioturbated silty levels are interbedded. Sandstone/silt ratio is N10:1. Locally, lenticular and fine conglomerates of reduced thickness are interbedded. The upper section of the Loma de Los Piojos Formation, records a thin sand transition (b10 m) to a continuous and monotonous mudstone succession (Fig. 2). This interval is similar to the facies association described at the base of the unit. The dominant black shales at the base of the Loma de Los Piojos Formation suggest suspension and settling of sediments below storm wave base in anoxic conditions. Beds of relatively clean and tabular sandstones, with normal gradation and sole marks at their base, suggest deposits resulting from gravitational to granular flows. Hummocky cross-stratification in interbedded sandstones imply a storm-dominated succession, which eventually decayed into lower-energy wave oscillation. The small size of these structures suggests a proximal prodelta because of a decrease of storm wave size (Yang et al., 2006). In the heterolithic middle section of the basal succession, the high proportion of sandstones beds, their geometry, contacts and internal structures reflecting conditions of waning flow, suggest the periodic occurrence of flood-generated hyperpycnal flows (Mutti et al., 2003). Processes of suspension and settling in this interval would have increased the potential of preservation of the paleoflora described in this contribution. The coarse grained upper interval of the basal succession is interpreted as product of relatively expanded canalized flows forming distributarymouth bars that coalesced into complex bar assemblages. Interbedded lenticular fine-grained deposits are interpreted as settling in small water bodies confined between the channels. The coarsening-upward pattern of basal section of the Loma de Los Piojos Formation, leads to an interpretation of the basal sequence as part of a prodelta gradually replaced by shallower facies of a delta front and delta plain as a result of prograding deltaic lobes. The Lower facies association is interpreted as prodelta deposits (Fig. 2). Mudstones deposited from suspension accumulated at rates which are an order of magnitude slower than accumulation rates of hyperpycnal muds and consequently show much higher degrees of bioturbation (Allison et al., 2000). The Middle facies association is considered to be delta front deposits with a relatively low gradient, under the action of storm waves. The internal arrangement reflects the construction of lobes by relative fluctuations in the base level or in clastic influx (Fig. 2). The presence of distributary channels at the top of the basal section suggests a delta plain paleoenvironment (Bhattacharya, 2006, Fig. 2). This facies association includes a wide variety of brackish, paralic to wetland sub-environments including swamps, marshes, and interdistributary bays. Soft-sediment deformation features result from high sedimentation rates and are common in river-dominated deltas (Bhattacharya, 2006). The fine grained upper section of the Loma de Los Piojos Formation is similar to the facies association described at the base of the unit, and is interpreted as a prodelta facies association. Sudden overlap of another mudstone succession in the upper section of the unit, indicates a transgressive event and a change in the discharge rate (Fig. 2). The marked paleorelief prior to the deposition of the Loma de Los Piojos Formation, suggests that this delta system was confined inside an estuary. In estuarine environments the absence of marine fauna, high supply of fragmentary flora and intense bioturbation are common, due to the brackish conditions of deposition. In general, the shales of the basas Loma de los Piojos Fm. lack macroflora, while through the upper part, fossiliferous levels are more frequent and dominated by fragmentary stems. The fossils studied come mainly from three levels (LM1, LM2 and LP9, Fig. 2). The LM1 bears seeds, small and fragmentary fronds and sphenophytes remains preserved as adpressions. The LP9 is characterized by the abundance of sphenophytes, together with lycopsids and scarce pinnules of Nothorhacopteris; here the fossils appear as moulds or reddish impressions–compressions. Another level (LM2) presents some remains of Diplothmema, Cordaicarpus and Nothorhacopteris as the only identifiable fossils.

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4. Systematic paleontology Materials are housed in the collection of the CIPAL (Centro de Investigaciones Paleobiológicas) (CEGH-UNC), Facultad de Ciencias Exactas Físicas y Naturales, Universidad Nacional de Córdoba, Córdoba, Argentina. Materials from the Museo de Paleontología (CORD-PB), Universidad Nacional de Córdoba, have also been revised when necessary. The terminology used in the description of the Lycophyta follows Meyen (1976), while the phyllotaxis angle was measured as indicated by Gutiérrez and Arrondo (1994). The nomenclature defined by Archangelsky (2000) is used for the description of the seeds. Frenguellia Arrondo, Césari and Gutiérrez, 1991 Type species: Protolepidodendron eximium Frenguelli, 1954, Malimán Formation, Mississippian, San Juan Province, Argentina. Frenguellia eximia (Frenguelli) Arrondo, Césari and Gutiérrez, 1991 Fig. 3I–J, N–O Materials assigned and stratigraphic position: LP9: CEGH-UNC 23552–23554, 23556–23559; LM3: CEGH-UNC 23555 Description The assignable material corresponds to several remains of stems, preserved as impressions and moulds in siltstones and sandstones. The best preserved (CEGH-UNC 23552) is a stem mould, conserved in medium-grained sandstone. For the observation of morphological details we took into account the latex mould of this specimen and used the originals of the other specimens. This specimen lacks bifurcations and is 51 mm in length and 13 mm in width. The decurrent bases of the leaf cushions are not clearly defined, and exhibit false leaf scars in its apical part. The leaf cushions are distributed in a 20° lepidodendroid phyllotaxy with orthostichies and horizontal lines clearly visible. The false leaf scars are approximately 0.6 mm in the maximum width, and they are within a horizontal distance of 1.5 mm and a vertical distance of 5.2 mm of each other. Strong striations run longitudinally through the stem in a discontinuous way, connecting the leaf cushions. The interference of the decurrent base of the leaf cushion with these longitudinal lines gives the specimen a distinctive rhomboidal ornamentation pattern. A false leaf scar is located in each vertex of the rhomboidal areas formed. The disposition of the leaf cushions and the false leaf scars is somewhat irregular (especially in the upper and the right sections). There are also some variations in the orientation of the horizontal lines and in the definition of the longitudinal striation, which may be due to taphonomic processes. Taking this into consideration, all the measurements were obtained from the lower right section which, to our understanding, has maintained the original disposition of the structures. On the upper left margin of the fragment there is a falcate leaf which is partially covered by sediment. It is 4 mm in length and 0.4 mm in width. This leaf is separated from the stem by an angle of 38°, and it presents an inflection in its distal half and an acuminated end, where a fine line indicates a partition. Discussion The material is assigned to F. eximia based on its size (interpreted here as evidence for a herbaceous growth habit) and its false leaf scars arrayed in a lepidodendroid phyllotaxy, and its adaxially recurved distally segmented falcate leaves. All of these are diagnostic characters for F. eximia (Arrondo et al., 1991). The false leaf scars and the longitudinal striation are characters that are present in several specimens undoubtedly assigned to this taxon (e.g.: Morel et al.,1993 Plate I figure A; Gutiérrez and Arrondo, 1994). Although none of the studied materials show dichotomous branching, the specimen LP-PB469 does (illustrated by Arrondo and

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Petriella, 1978 Plate I figures B and C). This specimen was collected by Dr. E. García as part of his PhD thesis (1945) from the same levels and locality of the materials studied here; and Gutiérrez and Arrondo (1994) assigned it to F. eximia in their revision of the genus Frenguellia. Likewise, the five segments of the leaf are not clearly observed in our materials. However, many specimens lack this diagnostic character (e.g.: Morel et al., 1993 Plate I figure A), so this does not impede the specific assignment. It is worth mentioning that the material here studied bears a strong similarity to those from Mina la Negra (Sierra de Maz, La Rioja Province), which come from the strata assigned to the Guandacol Formation. These materials were illustrated by Arrondo and Petriella (1978, Plate I figures A and D), who referred them as Lepidodrendopsis sekondiensis Mensah and Chaloner 1971. We think that they should also be referred to Frenguellia eximia. Malanzania Archangelsky et al., 1981 Type species: Malanzania nana Archangelsky et al., 1981, Malanzán Formation, Pennsylvanian, La Rioja Province, Argentina. Malanzania sp. Fig. 3K–L Materials assigned and stratigraphic position: LP9: CEGH-UNC 23560–23561, 23563–23565; LM1: CEGH-UNC 23562; LM2: CEGHUNC 23566 Description The best preserved material (CEGH-UNC 23563) is an adpression of a fragmented stem. It is 23.4 mm long and 13 mm wide and it has excrescences distributed in a 40° lepidodendroid phyllotaxy. There is a 70° angle between whorls and the orthostichies are not evident. The excrescences are approximately 0.74 mm in size measured normally to the stem and 0.5 mm longitudinally. They exhibit a flat surface in its apical part, a well defined outline and no evidence of a leaf trace. Their shapes vary from rounded to sub-rhombic in cross section. The excrescences on the margins of the stem are short and truncated ending abruptly in a plane. The distance between them can be variable, possibly due to deformation (12–18 mm) but it is never less than the length of their maximum diameter. The extensive interareas between the false leaf scars are smooth, with some irregular and discontinuous lines which may be explained by taphonomic effects. Since the stem fragment we have is deformed, measurements were taken from the upper left part, where the deformation is lesser. Discussion The placement in the genus Malanzania is based on the following diagnostic characters: the excrescences are irregular and sub-circular outline, they have lepidodendroid arrangement, and are separated by smooth and ample interareas. The holotype description of M. nana Archangelsky et al., 1981 from the Pennsylvanian of Malanzán includes some of the characters observed in our material, such as sub-rhombic section of the false leaf scar's sub-rhombic section, and its arrangement in a lepidodendroid phyllotaxy without apparent orthostichies. Nonetheless, in this new material the false leaf scars show well delimited and clear borders. There are neither spiniform excrescences nor longitudinal lines in the interareas as there are in the specimens from Malanzán. M. ottonei Carrizo and Azcuy, 1998, from the Mississippian (Visean) of Cerro Mudadero and Punilla Formation (Quebrada de la Jarilla), La Rioja Province, resembles our material in the following characteristics: the false leaf scar has a sub-circular and wide base, they show similar separation distance between leaf scars (1.5–3 times the maximum diameter), and a similar phyllotaxy angle (in the vertical

Fig. 2. Stratigraphic columns for the Loma de Los Piojos Formation. See map in Fig. 1 to location of columns A and B. Paleoenvironments are associated to the sea level curve. Note the coarsening-upward basal section and sudden transgressive event in the upper section of the unit. A) Black shales interbedded with tabular fine sandstones in a prodelta setting. B) Details of hummocky and swalley cross stratification. C) Turbiditic deposits in sandstones beds generated by sudden decelerating unidirectional currents. D) Details of flute marks. E) Coarse sandstone-filled channel eroding into laminated fine wave sandstone. F and G) Wave laminated fine sandstones. H) Green mudstone interbedded with tabular fine sandstones. I) Erosive discordance between Loma de Los Piojos Fm and glacial diamictites of Guandacol Fm.

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and between whorls). The excrescences have, however, very different morphologies, being much longer in M. ottonei. M. antigua Archangelsky,1983, from the Middle Devonian of Malvinas Islands and La Rioja Province, shares the following characteristics with the Loma de Los Piojos material: the shape of the excrescence, their clear definition, and their truncated apex. Because Archangeslky's (1983) and our description are based on latex molds, our Serpukhovian species shares many characteristics with Archangeslky's Middle Devonian species. The poor quality of the natural moulds of the rock matrix of M. nana and M. ottonei may explain the absence of clear borders in the false leaf scars of these species. The phyllotaxy with well defined vertical and horizontal lines of M. antigua (Archangelsky, 1983, Figs. 3 and 4) were not observed in the studied material. Although the material studied shares characters with all three species of the genus, it also differs in many others. Consequently these specimens possibly correspond to a new taxon, however, as the quality of the material is not sufficient to erect a new species the nomenclature is left open. Tomiodendron Radczenko, 1955, enmd. Meyen, 1972 Type species: Lepidodendron ostrogianum Zalessky in Zalesskij and Čirkova, 1935; Kuznetsk Basin, Serpukhovian, Angara, Russia. Tomiodendron sp. Fig. 3 A–B, E, H Materials assigned and stratigraphic position: LP9: CEGH-UNC 23551 Description The only specimen assignable is an impression/compression of bark, which presents part and counterpart. The most informative of these shows the stalk from the inside, thus our description will be based on that fragment and its latex mould. This specimen is 54 mm in length and 28 mm in width. The leaf cushions show different degrees of preservation: some are clearly defined, others are represented by their outline and the sediments filling the ligule pit, and others are absent. The phyllotaxy is a lepidodendroid of 37°, with scalariform parastichies and without othostichies or horizontal lines. The leaf cushions are 5 mm in length and 2 mm in width, with an inverted flame appearance, conveyed by their fusiform shape and their asymmetry. In the latex mould the right side of the leaf cushions is slightly concave in its lower part while the left side is smoothly convex. From the upper third and onwards the leaf cushions became progressively acuminate downwards, losing relief towards the interarea, and they do not show a heel. In at least two leaf cushions the upper margin, subtly preserved, is acuminate; nevertheless in most cases it is sub-rounded in appearance due to compaction. The presence of some variability in the outline of some leaf cushions can be explained by the rupture of the sedimentary matrix below the wings. The leaf cushions do not have evident leaf scars or associated elements such as leaf traces and parichnos. The axilar line of a false leaf scar is hard to appreciate and the infrafoliar bladder cannot be seen, but this may be due to poor preservation. The leaf cushions are flat and widely separated (3.7 mm), defining interareas with a slight ornamentation consisting in longitudinal striations that cover the interarea. There are some fine lines densely arranged, and other wider lines more separated. The disposition of these striations is irregular; they are sinuous and discontinuous. There is no evidence for anastomosis or convergence towards the leaf cushions. Discussion The generic placement is based on the presence of the following items: lepidodendroid phyllotaxy, fusiform and spaced leaf cushions, existence of a ligule pit, ample interareas, absence of leaf scars, and

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inferred presence of wings. They are all characters mentioned in the genus enmended diagnosis by Meyen (1972, 1976). The apparent absence of an infrafoliar bladder may be attributed to taphonomic processes. Decomposition of the specimen under study prior to burial is inferred from the differential preservation of leaf cushions and other structures (such as the striation of the interarea surface). The studied material bears a great resemblance with T. kemeroviense (Chachlov) Radczenko, 1955 from the Mississippian (Serpukhovian) of Angara (Russia) and with T. peruvianum (Gothan) Pfefferkorn and Alleman in Ianuzzi and Pfefferkorn, 2002, from the Mississippian (Visean) of Paracas (Peru). The lepidodendroid phyllotaxy, the leaf cushions asymmetrically narrowed and downwards attenuated, and the ample and ornamented interareas are characters shared with T. kemeroviense. Nonetheless, the leaf cushions with a narrower and flame inverted appearance, the absence of a conspicuous infrafoliar bladder and the disposition of the ornamentation in the interareas differentiates our material from the latter species. Our material is even more similar to T. peruvianum. The main characteristics are the likeness in the outline, the leaf cushion's asymmetry, the greater extension of the interareas and the very conspicuous ligule pit. However, the leaf cushions of the material from Loma de Los Piojos are less rhomboidal, with their maximum width in the upper third. The striated ornamentation of the interareas is subparallel rather than anastomosing or convergent to the leaf cushions. It is interesting to point out that T. peruvianum infrafoliar bladder is less noticeable than that of T. kemeroviense. This characteristic makes it more similar to our material, where that structure is not observed. In spite of the remarkable similarity of the Loma de Los Piojos material with T. peruvianum, a clear placement can not be made due to the quality of the preservation of the only specimen available and therefore the nomenclature is left open. The genus Tomiodendron has been compared to the genus Bumbudendron Archangelsky et al., 1981. Cogeneric or even cospecific relations between B. paganzianum Archangelsky et al., 1981 and T. kemeroviense have been particularly discussed, based on the possibility that the absence of the leaf scar in the latter is a result of the decortication of the material. Yet, the occurrence of the ligula pit in T. kemeroviense, which is diagnostic for the genus, eliminates the possibility to relate this species to Bumbudendron, which does not posses a ligule pit in any of its species. The material from Paracas (Peru) assigned to Tomiodendron peruvianum by Pfefferkorn and Alleman (in Ianuzzi and Pfefferkorn, 2002) was first referred to as Lepidodendron rimosum Sternberg by Berry (1922), then as Lepidodendron peruvianum by Gothan (1928), and finally in 1954 as Lepidodendropsis peruvianum by Jongmans (1954). In Argentina, materials identified as “Lepidodendron/Lepidodendropsis peruvianum” are known for the Carboniferous (Frenguelli, 1949; Arrondo and Petriella, 1978; Furque, 1979) and many of them have been revised (Archangelsky et al., 1981; Gutiérrez, 1996). These authors showed that the previous material is hard to assign to a genus owing to the poor preservation of diagnostic characters and consequently this material will not be dealt with in this discussion. This is the first record of a ligulated lycopsid and particularly of this genus in Argentina and has important paleoclimatic and paleobiogeographic implications that will be discussed below. Bumbudendron Archangelsky et al., 1981 Type species: Bumbudendron paganzianum Archangelsky et al., 1981, Lagares Formation, Pennsylvanian, La Rioja Province, Argentina. Bumbudendron millani (Arrondo and Petriella, 1978) Arrondo and Petriella, 1985

Fig. 3. A, B, E and H) Tomiodendron sp. (CEGH-UNC 23552A) A) General view, scale bar = 1 cm. B) Detail of ligule pit, scale bar = 0.5 mm. E) Leaf cushion, scale bar = 1 mm. H) Detail of the phyllotaxis, note the poor preserved leaf cushions just between well preserved ones, scale bar = 5 mm. C, D, F and G) Bumbudendron millani (CEGH-UNC 23567A) C) Detail of leaf scar, scale bar = 2 mm. D) General view, scale bar = 1 cm. F) Leaf cushion, scale bar = 5 mm. G) Detail of the phyllotaxis, scale bar = 5 mm. I and J) Frenguellia eximia (CEGH-UNC 23554A) I) Detail of leaf, scale bar = 1 mm. J) General view, scale bar = 5 mm. K and L) Malanzania sp. (CEGH-UNC 23563) K) General view, scale bar = 1 cm. L) Detail of the excrescence in lateral view, scale bar = 1 mm. M) Paracalamites sp. (CEGH-UNC 23570A) scale bar = 5 mm. N) General view of Frenguellia eximia (CEGH-UNC 23553), scale bar = 1 cm. O) General view of Frenguellia eximia (CEGH-UNC 23552), scale bar = 1 cm.

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Fig. 4. A) Pinnule of Nothorhacopteris kellaybelenensis (CEGH-UNC 23587), scale bar = 5 mm. B) Paracalamites sp. (CEGH-UNC 23573), scale bar = 2 mm. C) Diplothmema bodenbenderi (CEGH-UNC 23592A), scale bar = 1 cm. D) Pinnule of Nothorhacopteris kellaybelenensis (CEGH-UNC 235878) scale bar = 5 mm. E and G) Nothorhacopteris kellaybelenensis (CEGH-UNC 235) E) Fragment of pinnule, scale bar = 5 mm. G) Detail of the distal margin, note the lobes delimitated by incisions, scale bar = 1 mm. F) Pinnule of Nothorhacopteris kellaybelenensis (CEGH-UNC 23588) note the differentiated petiole, scale bar = 5 mm. H) Cordaicarpus cesarii (CEGH-UNC 23574A), scale bar = 1 mm.

Fig. 3C–D, F–G Materials assigned and stratigraphic position: LP9: CEGH-UNC 23567 Description The material is an impression of a stem fragment preserved in sandstones, which length is about 54 mm in length and 22 mm wide. The specimen shows leaf cushions clearly defined in all along its surface in a 25° lepidodendroid phyllotaxis without evident ortostichies nor horizontal lines, but shows scalariform parastichies. The lower leaf cushions are fusiform and 9 mm long and 3 mm wide, while the upper ones tend to be equidimensional and smaller, being 4.3 mm long and 1.7 mm wide. This variation in size and shape of the leaf cushions allows the recognition of a growth zone. In general the leaf cushions insinuate a leaf scar, show an infrafoliar bladder and a leaf trace. Although these characteristics allow inferring a rather high decortication state as shown in the Fig. 7d–e and Fig. 8f–g of Archangelsky et al. (1981), the absence of a ligule pit and wings can be taken for sure. The leaf cushions are separated by a deep furrow of constant width (0.5 mm) with a thin striation, which clearly delimits the leaf-cushion contour. The preservation does not allow recognition of detailed patterns in the furrows striations.

Discussion The placement in the genus Bumbudendron is based on the presence of fusiform leaf cushions in a lepidodendroid phyllotaxis, separated by deep narrow furrows with thin striations and only one leaf trace in the middle of the leaf scar. All these characters are mentioned in the diagnosis of the genus (Archangelsky et al., 1981). Due to the preservation, it is not possible to see other diagnostic characters. As has been discussed above, the absence of a ligule pit and wings in Bumbudendron, differentiates this genus from Tomiodendron where this structures are diagnostic (Meyen, 1972, 1976). The studied material shows great similarity with the known species of the genus, particularly with B. nitidum Archangelsky et al., 1981 and B. millani. The variation in shape and size of the leaf cushion due to the presence of a growth zone in our material makes it difficult to place the specimens in either species. Nonetheless, the general small size of the leaf cushions, their length/width ratio not higher than 4:1, the thin furrows of constant width delimitating the leaf cushions and the phyllotaxis angle, suggest that our material belongs to B. millani (Arrondo and Petriella) Arrondo and Petriella, 1985. It has to be said that the holotype of B. millani shows growth zones just like the ones in our material (Arrondo and Petriella, 1978, 1985).

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The finding of remains undoubtedly assigned to Bumbudendron millani, extends the stratigraphic range of the genus in general, and this species in particular, up to the Late Mississippian (Serpukhovian). Paracalamites Zalessky, 1927 Type species: Phyllotheca sibiricus Zalessky, 1927 Paracalamites sp. Fig. 3M and Fig. 4B Materials assigned and stratigraphic position: LP9: CEGH-UNC 23568, 23571–23573; LM1: 23569–23570 Description The materials we assigned to this form genus comprise compressions and pith casts of articulate stems of up to 20 mm in length and 7.5 mm in width. The internodes are notably longer than the stem's diameter. The ribs are usually opposite in nodes and on some cases they are delicately striated. The ribs are 0.5 mm in width while the senus are 0.75 mm wide. There are neither leaf nor branch scars in any of the specimens we studied. Discussion The form genus Paracalamites was defined by Zalessky (1927) in order to include the remains of sphenophytes from Angara. These were pith casts with a continuous ribbing. Later, Rigby (1966a) included in the genus not only the pith casts, but also the stem impressions with the characteristic ribbing and recommended the use of this genus for materials from the Late Paleozoic of Gondwana. Finally, Naugolnykh (2002) distinguished Paracalamites from Paracalamitina Zalessky, 1934 based on the presence of leaf scars in the latter. He suggested the usage of Paracalamites for pith casts. The genus Archaeocalamites Stur, 1875 refers to a whole plant in which there are Paracalamites-like stems in organic connection to leaves (Mamay and Bateman, 1991). Yet, several pith casts, impressions or compressions from the Mississippian of Euramerica that had continuous ribbing and lacked foliage were assigned to this genus (Mamay and Bateman, 1991). Accordingly, Iannuzzi and Pfefferkorn (2002) referred to Archaeocalamites all the material from the Mississippian of Gondwana that had been originally assigned to Paracalamites. The material under study is assigned to Paracalamites based in the following characters: its continuous ribbing, absence of leaf/branch scars, and the presence of fine striations. These characters are diagnostic of the form genus. Given the characteristics of the material, we prefer to assign it to a form genus instead of a whole plant genus such as Archaeocalamites. In addition, at least some genera from the Pennsylvanian of Paganzo Basin, such as Gondwanites Césari and Pérez Loinaze, 2006, have Paracalamites-like stems. Paracalamites australis Rigby, 1966a,b has been reported in the Carboniferous of Argentina (e.g.: Archangelsky and Archangelsky, 1987; Ottone and García, 1990; Gutiérrez, 1995); probably due to its broad diagnosis. We preferred to leave the nomenclature open because the material from Loma de Los Piojos is morphologically very varied, which suggests the presence of more than one taxon at the species level. This is the first report of sphenophytes in the Mississippian of Argentina and as such, it has important paleobiogeographic and paleoclimatologic implications that will be discussed later. Although sphenophyte fossils from Loma de Los Piojos Fm. are abundant and vary in size, unfortunately, we are unable to provide a more precise systematic given the quality of our material. Further studies will be needed considering the relevance of this flora. Nothorhacopteris Archangelsky, 1983 Type species: Otopteris argentinica Geinitz, 1876, Cuesta Colorada, Pennsylvanian, La Rioja Province, Argentina. Nothorhacopteris kellaybelenensis Azcuy and Suárez-Soruco, 1993 Fig. 4A, D–G Materials assigned and stratigraphic position: LP9: CEGH-UNC 23589–23591; LM1: CEGH-UNC 23578–23584; LM2: CEGH-UNC 23585–23588

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Description The study material includes thirteen fragments of adpressions with coaly remains of isolated rhacopteridean pinnules. They are preserved in dark grey siltstones (LM1) and light gray-greenish sandstones (LP9). The specimen that is most complete (CEGH-UNC 23578) shows an impression of a flabelliform pinnule, and is 20 mm long and 25 mm wide. It is attached to a fragment of rachis by means of a short, wide petiole. The catadromic and anadromic margins of the lamina are clearly differentiated and they are slightly curved, defining an obtuse angle. The distal margin is curved and has regular lobes which show distal incisions. In the lamina, large veins leave the petiole with a radial disposition. The vein pattern is clearly dichotomous. Each vein delimits a cuneiform segment. There are 18 in total, each corresponding to a lobe in the margin of the pinnule. The veins end in the incision between each segment. There are fine interveinal striations parallel to the veins. Discussion The placement in the genus Nothorhacopteris is based on the nonlacinated flabelliform rhacopteridean pinnules. Their margins have some smooth lobes and inter-venal striations, as mentioned in the diagnosis by Archangelsky (1983). We did not use the number of veins to determine the placement at the species level, given that Archangelsky and Archangelsky (1987) showed the intraspecific variability of this character. N. argentinica (Geinitz) Archangelsky, 1983 differs from our material in the general morphology of its pinnules, which are sub-circular and slightly rhomboidal, according to the original description and examination of the holotype (CORD PB 869) and the paratype (CORD PB 867). It also differs in the following: absence of a defined petiole, its bigger length–width relation, curved and sinuous veins that don't define clearly cuneiform segments, absence of lobes and incisions in the distal margin, and curved undefined catadromic and anadromic margins. N. chubutiana (Archangelsky and Arrondo) Archangelsky, 1983, is characteristic of the Lower Permian of Nueva Lubecka Formation, Chubut province, Argentina. In contrast to the material from Loma de Los Piojos, the distal margin of the sessile pinnules do not show lobes or incisions and the catadromic and anadromic margins are curved and not clearly distinguishable. N. kellaybelensis from the Mississippian of the Ambo Group of Bolivia and Perú is similar to our material in that the distal margin of the lamina is regularly lobed and has incisions in between the segments, the anadromic and catadromic margins are well defined and straight, the petiole is conspicuous, and the veins define cuneiform segments. In the Loma de Los Piojos material there is a short and wide petiole, and a variable angle between the anadromic and catadromic margins (Fig. 4A, D). On the Peruvian species the petiole is longer and thinner, and there is a normal angle between the anadromic and catadromic margins. The shape of the pinnules in N. kellaybelenensis has been described as typically cuneiform; however, the pinnules of the basal portion of the fronds have a flabelliform shape (as it can be seen in Plate 1, Fig. 2 from Azcuy and SuárezSoruco, 1993). Taking this into account, the pinnules from Loma de Los Piojos have the same range of morphology as the Peruvian material, and consequently they can be considered cospecific. This is the first record of Nothorhacopteris for the Mississippian of Argentina, and the first mention of N. kellaybelenensis, in this country. The biostratigraphic, paleobiogeographic and paleoclimatic implications of this taxon will be discussed below. Diplothmema Stur, 1877 Type species: Diplothmema schuetzei Stur, 1877 Diplothmema bodenbenderi (Kurtz, 1921) Césari, 1987 Fig. 4C Materials assigned and stratigraphic position: LM2: CEGH-UNC 23592

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Description Compression of a third order pinnule, preserved in medium grained sandstone. It is 21.5 mm length and 11.6 mm wide. The rachis is 1.18 mm wide and carries a fourth order pinnule of suborbicular shape. This pinnule is attached in a 90° angle, and its lamina is deeply lobated. Each lobe defines an apparently entire segment which are long, cuneiform, and have rounded margins. Each segment is nearly 3.5 mm in length. The vein pattern is not clearly observable, but in certain regions only one vein can be seen running along the rachis. In the distal region fragments of another pinnule can be seen, which seems to be attached at an acute angle. Discussion The material is attributable to D. bodenbenderi because it presents fourth order pinnules of suborbicular shape, with deeply lobated lamina having subrounded margin that are attached at nearly 90° angles. These are all characteristics described in the enmended diagnoses (Césari,1987). It differs from D. gothanica (Dolianiti) Iannuzzi in Iannuzzi and Pfefferkorn, 2002, which has fourth order pinnules attached at acute angles, which have cuneiform narrow shape, and are never suborbicular (Iannuzzi et al., 2006). The material, due to its fragmentary nature, does not allow the observation of fourth order pinnules in alternate disposition; nevertheless, this does not prevent its specific assignment. Cordaicarpus Geinitz, 1862 enmd. Archangelsky, 2000 Type species: Cordaicarpus cordai Geinitz, 1862 ex Seward, 1917 Cordaicarpus cesarii Gutiérrez et al., 1992 enmd. Archangelsky, 2000 Fig. 4H Materials assigned and stratigraphic position: LM1: CEGH-UNC 23574–23576; LM2: CEGH-UNC 23577, 23593 Description Compression/impression of flat seeds, bilaterally symmetric (platispermic). It has an oval to circular shape, lacks wings, and its maximum length is 5.8 mm and maximum width is 4.25 mm. The base is wide, rounded and without umbilicus or rafe. The apex presents two rounded projections which define an acute and triangular incision corresponding to the micropylar zone. The sclerotesta has thin longitudinal striations and an acuminate apex that does not project outside the seed's body. In some specimens, preserved in sandstone, a wide straight longitudinal carena can be observed. Following the shape of the nucula, there is a smooth sarcotesta, which is thin at the base and thickens distally, reaching its maximum width near the distal end. The maximum width of the sarcotesta is 0.4 mm. The sarcotesta width/nucula width ratio is about 1:12. Due to the nature of the preservation, it is impossible to observe the endotesta or any other structure. Discussion The material is assigned to Cordaicarpus because it shows bilateral symmetry, a sclerotesta with an acuminated apex that does not project outside the body, and a narrow sarcotesta. All these characteristics have been mentioned by Archangelsky (2000) in the emended diagnoses of the genus. Oliveira and Da Silva Pontes (1976) proposed the sarcotesta width/nucula width ratio as a criterion to differentiate Cordaicarpus from Samaropsis Goeppert, 1864. This ratio is smaller than 1:5 in Cordaicarpus. The specimens from Loma de Los Piojos have a ratio of 1:12, which is clearly within the range of Cordaicarpus. At a specific level, the material is assigned to C. cesarii because it has a subcircular shape, rounded base, and narrow sarcotesta (less than 1 mm), which is wider in the apical region. All of these are diagnostic characters according to Gutiérrez et al. (1992) and Archangelsky (2000). It is worth mentioning that the 1:12 value of the sarcotesta width/ nucula width ratio coincides with the value given by Gutiérrez et al. (1992) for this species. The size of the studied materials is also within the size range given by Archangelsky (2000) for this species. This is the first Mississippian record of this genus in western Argentina, consequently it can't be considered restricted to NBG zone, as previously postulated (Archangelsky, 1999).

5. Discussions 5.1. Biostratigraphy and age The classic continental biostratigraphic scheme proposed for the Carboniferous of Argentina, based on plant macrofossils, has been summarized in the Fig. 5 (Archangelsky et al.,1987; Azcuy et al., 2007a,b; Césari et al., 2007). A gap encompassing the Serpukhovian is present between the Frenguellia–Paulophyton zone (Mississippian) and the NBG zone (Pennsylvanian), however its stratigraphic limits had never been clearly defined. The studied assemblage is unique due to the presence of Tomiodendron and Nothorhacopteris kellaybelenensis, in association with elements from the NGB and Frenguellia–Paulophyton zones (i.e. Frenguellia, Bumbudendron and Cordaicarpus). Because of these compositional characteristics, this assemblage corresponds neither to the Frenguellia– Paulophyton zone, nor to the NBG zone, and defines a new intermediate zone, herein named the Frenguellia eximia–Nothorhacopteris kellaybelenensis–Cordaicarpus cesarii zone (FNC zone). Recently Rustán et al. (2008) assigned a Visean age to the flora from the Loma de Los Piojos Formation, based on the presence of Frenguellia eximia. However, new collections have allowed the adjustment of this age taking into account new evidence. The age assigned to the FNC zone in this contribution is latest Mississippian (Serpukhovian). This age is based on the presence of Tomiodendron, Frenguellia eximia and Nothorhacopteris kellaybelenensis, associated with Cordaicarpus cesarii. The presence of Tomiodendron is fundamental because it occurs almost exclusively in the Serpukhovian, with scarce Late Visean records (Meyen, 1976; Iannuzzi et al., 1998; Iannuzzi and Pfefferkorn, 2002). The genus Nothorhacopteris has been classically considered characteristic of the Pennsylvanian. Nevertheless, assemblages characterized

Fig. 5. Biostratigraphic chart of Mississippian–Lower Pennsylvanian zones from Argentina, showing the age of the NFC zone. Modified from Archangelsky et al. (1996) and Azcuy et al. (2007b). Note the uncertainty in the limits of the FNC zone.

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by Nothorhacopteris kellaybelenensis together with Tomiodendron and sphenophytes have been assigned to the Late Visean–Lower Serpukhovian on basis on marine invertebrates, palynomorphs, and isotopic data (Roberts et al., 1995; Iannuzzi and Rösler, 2000; Iannuzzi and Pfefferkorn, 2002; Azcuy and di Pasquo, 2005; Fasolo et al., 2006 and references therein). This is particularly significant in Bolivia, where the Nothorhacopteris kellaybelenensis–Triphyllopteris boliviana zone was erected for Latest Visean–Early Serpukhovian (Iannuzzi et al., 2003). Due to the compositional similarity, it is possible to assign a similar age to the FNC zone (Fig. 5), and correlate it, at least partially, with N. kellaybelenensis–T. boliviana zone from Bolivia. Finally, the FNC zone is unique due to the presence of seeds assignable to Cordaicarpus. This genus has been related mainly to groups of gymnosperms (Cordaitales and Coniferales) whose earliest records are Serpukhovian (Gorelova, 1978; Wagner et al., 1979; Wagner, 1984; Rothwell, 1988; Trivett and Rothwell, 1991; Scott et al., 1997). As previously explained, the fossiliferous strata studied lie immediately under the glacial beds that characterize the base of the Guandacol Formation, dated as not older than earliest Pennsylvanian (Bashkirian) (Pérez Loinaze et al., 2008). Provided that the association of Tomiodendron and N. kellaybelenesis with Cordaicarpus would characterize an age not older than Early Serpukhovian, and the studied strata underlie the Guandacol Formation, we assign a Serpukhovian age to the FNC zone. Unfortunately, neither the precise moment of the Serpuhkovian nor the extent of time represented by the FNC floral zone can be yet known. The FNC zone, represents a particular moment in the Late Mississippian (Serpukhovian) previously undocumented by means of fossil plants in Argentina, and fills the stratigraphic gap between the Frenguellia–Paulophyton and NBG zones (Fig. 5). Although other authors have proposed a Mississippian history for the Paganzo Basin (Férnandez Seveso et al., 1993; Limarino et al., 2006), the only unit previously recognized underlying the Guandacol Formation was the Aguas Blanquitas Formation (Pazos, 1993). This unit crops out both at the Cerro Guandacol and Cerro Bola (Sierra de Maz, La Rioja Province) (Pazos, 1993; Caselli and Limarino, 2002).

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However, recent interpretations related the Aguas Blanquitas Formation to Glacial Event II (Pazos, 1997, 2007 Fig. 3), making its correlation with the Loma de los Piojos Formation impossible (Fig. 6). Sediments at Pasleam, San Juan Province (Volkheimer, 1962) and the south of Sierra de Maz, La Rioja Province (Frenguelli, 1949; Andreis and Arrondo, 1974; Arrondo and Petriella, 1978), may have FNC zone fossils and correlate with the Loma de los Piojos Formation (Fig. 6). However, these strata need more investigation. In this scenario, the Loma de los Piojos Formation represents the only certain pre-Bashkirian unit. This unit not only has precise stratigraphic relationships, but carries fossils that indicate a Serpukhovian age. Stratigraphic relationships suggest that the deposits of Loma de los Piojos Formation belong to the Paganzo Basin, rather than to a possible earlier basin, known as the Protopaganzo Basin (Limarino et al., 2006). Instead, we refer the Loma de los Piojos Formation to the initial structuring and functioning of the Paganzo Basin. Future studies of this new unit will allow a better understanding of the magnitude, development and early tectono-stratigraphic evolution of this Basin. 5.2. Paleobiogeography and paleoclimatology The Mississippian floras from western Gondwana are characterized by the presence of herbaceous lycopsids, and they have been considered to belong to a cold climate. Conversely, the Pennsylvanian floras contain Nothorhacopteris and Botrychiopsis, which are indicative of a warmer climate (Archangelsky et al., 1987; Anderson et al., 1999; Césari et al., 2007). Only recently, have associations of intermediate composition been identified. Floras of Late Visean–Early Serpukhovian age, and intermediate composition fill a void between Early to Middle Visean and Bashkirian floras (Iannuzzi and Pfefferkorn, 2002). These Late Visean– Early Serpukhovian floras are characterized by the presence of pteridosperms (distinctively Nothorhacopteris kellaybelenensis), together with arboreous lycopsids (particularly Tomiodendron) and tropical elements such as Archaeocalamites (= Paracalamites); while endemic lycopsids from Argentina such as Frenguellia and Malanzania are absent. These floras have been recognized in South America, Africa,

Fig. 6. Stratigraphic columns and correlation of principal South American Mississippian–Lower Pennsylvanian localities. Note the temporal and geographic distribution of the Paraca Floral Realm and the stratigraphic position of Loma de los Piojos Formation and possible coeval strata at Sierra de Maz. AG, Aguas Blanquitas Fm. Modified from Iannuzzi and Rösler (2000), López-Gamundí and Martínez (2000), Iannuzzi and Pfefferkorn (2002), Fasolo et al. (2006), Azcuy et al. (2007b) and Pazos (2007).

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India and Australia, pre dating the Glacial Episode II (Bashkirian), and they have been referred to as the Paraca Floral Realm (Iannuzzi and Pfefferkorn, 2002). This realm was put forward to describe floras that developed along a Gondwanan belt of warm climate, located approximately between 30° and 60° of paleolatitude. It comprises a preglacial period of high temperatures which developed during the Late Visean–Early Serpukhovian (Iannuzzi and Pfefferkorn, 2002). The high temperatures proposed for these associations are based on the presence of arborescent lycopsids, sphenophytes and paleosinecological characteristics of a warm temperate vegetational zone (Walter, 1985; Ziegler, 1990; Pfefferkorn, 1997; Iannuzzi and Rösler, 2000; Iannuzzi and Pfefferkorn, 2002). The paleoclimatic and paleobiogeographic models of Iannuzzi and Rösler (2000) and Iannuzzi and Pfefferkorn (2002) suggest it would be reasonable to find Paracan floral elements in the Mississippian of Argentina. Nonetheless, these records had not been found up to the present, and previous work suggested cold climates for Late Mississippian from Argentina (López-Gamundí et al., 1992). Particularly, palynological data in relation to glacial diamictites from the Cortaderas Formation, San Juan Province suggested a global cooling event for the Late Visean (Limarino et al., 2006; Césari et al., 2007; Pérez Loinaze, 2007). The flora from Loma de los Piojos acquires a significant importance in this discrepancy. The presence of the arboreous lycopsid Tomiodendron associated with Nothorhacopteris kellaybelenensis and Paracalamites stems in the Loma de los Piojos Fm. allows the recognition of the Paraca Floral Realm in Argentina. Consequently, it confirms the existence of warm–temperate climates for the Late Mississippian (Serpukhovian) of Argentina (Fig. 6). Nevertheless, the Paracan flora from Loma de los Piojos is an impoverished assemblage, containing small lycopsids (Frenguellia and Malanzania) from colder environments. Thus, the climate of the FNC zone could be interpreted as slightly colder than the Paraca sensu strictu, but surely warmer than earlier Visean climates of western Argentina. Hence, the flora from Loma de los Piojos Fm. may represent a restructured paleocommunity, mixing relict elements from the cold

climate floras from the Visean (Frenguellia and Malanzania) and elements of the warmer floras from the Paraca Floral Realm (Tomiodendron, N. kellaybelenesis and Paracalamites). The unusual characteristics of this flora can be explained considering the superimposition of paleogeographic and paleoclimatic effects. On the one hand, a clockwise rotation of Gondwana has been suggested for the Carboniferous (Scotese et al., 1979; Scotese and McKerrow, 1990). This implies a consequent migration of South America to lower latitudes, during the Carboniferous. According to this idea, the central region of Argentina would have entered a warm and humid climate belt during the Serpukhovian (Scotese, 2000), while during earlier times, the higher paleolatitudinal position allowed the development of glaciations (Fig. 7A). This would have permitted the flora from the Paraca Floral Realm to move towards the south of South America, as postulated by Iannuzzi and Rösler (2000), but not before Serpukhovian times (Figs. 6 and 7B). The fact that the Australian record has evidence of warm climates during the Missisippian (Fielding et al., 2008: 134), while western Gondwana registers glaciations (Isaacson et al., 2008), is in agreement with this clockwise rotation. Similar evidence is found in the fossil record. As explained by Iannuzzi and Pfefferkorn (2002 and references therein), in Australia the fossil record shows an increasingly colder climate through the Carboniferous up to the Permian. Meanwhile in western Argentina the evidence shows cold climates for the Mississipian and milder climates for the Pennsylvanian and Permian (López-Gamundí et al., 1992; Vega, 1995; Iannuzzi and Rösler, 2000; Iannuzzi and Pfefferkorn, 2002). On the other hand, the identification of a warm–temperate Serpukhovian climate implies the recognition of a nonglacial interval for this moment, and the differentiation of a Bashkirian glacial interval (López-Gamundí, 1997) from the glacial interval proposed for the Upper Visean of Argentina (Pérez Loinaze, 2007). The FNC is recognized as a nonglacial flora developed during this interval. The identification of such glacial and nonglacial intervals is in agreement with the dynamics of the glaciations put forward by Fielding et al. (2008). Particularly, these intervals are broadly correlationable with their C1 and C2 glaciations, and the nonglacial period in-between

Fig. 7. Paleogeographic reconstructions for the Mississippian–Lower Pennsylvanian. A) Position of America in Gondwanan context. The arrow points towards central western Argentina. Note that this area enters the 60°–30° climatic belt during the Serpukhovian. B) Detail of South America and Argentina, showing the known relevant fossil floras and localities for each time interval. Modified from Iannuzzi and Rösler (2000) and Scotese (2000).

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(Fielding et al., 2008; Birgenheier et al., 2009). This correspondence is remarkable, as Fielding et al. (2008) model is based on the record of eastern Australia, while our findings are from western Argentina, two opposite extremes of Gondwana. Furthermore, the finding of a warm– temperate climate in the Serpukhovian of western Gondwana coincides with the recent identification of a mid-Serpuhkovian warm interval in Euramerican paleotropics (Gastaldo et al., 2009: 362). This coincidence may reflect that this warming had important consequences throughout the globe, at least for plant assemblages. We must emphasise, however, that this simple correlation should be done with caution, because it is not known the precise timing and duration of the FNC zone relative to this warm interval. Overall, regarding its Serpukhovian age, the Loma de los Piojos flora agrees with the climatic and biostratigraphic models put forward by Iannuzzi and Rösler (2000) and Iannuzzi and Pfefferkorn (2002), and the glaciations model proposed by Fielding et al. (2008). New Visean–Serpukhovian data along with detailed biostratigraphic studies at western Argentina will be needed to test these hypotheses.

6. Conclusions 1) Mississippian fossiliferous sedimentary rocks are recognized at Loma de Los Piojos, southwest Jáchal, San Juan Province, Argentina. Clear stratigraphic relationships show that they unconformably underlie the diamictites of the base of Guandacol Formation (Bashkirian), and unconformably overlie the Talacasto Formation (Lower Devonian). This sequence is named the Loma de los Piojos Formation. 2) The fossil flora is compositionally intermediate between typical Mississippian floras and Pennsylvanian floras of western Gondwana, and is indicative of a Late Mississippian (Serpukhovian) age. 3) This new flora comes from a time interval previously unrecognized in western Argentinean basins. It overlies the Frenguellia–Paulophyton zone and underlies the NBG zone and correlates, at least partially, with the Nothorhacopteris kellaybelenensis–Triphyllopteris boliviana zone (Iannuzzi et al., 2003). We here name this as the Frenguellia eximia–Nothorhacopteris kellaybelenensis–Cordaicarpus cesarii zone (FNC zone). 4) The association of the FNC zone, due to its unique composition, belongs to the Gondwanan “Paraca Floral Realm” (Iannuzzi and Pfefferkorn, 2002). The paleoecological characteristics of these floras, suggest a warm temperate climate (Iannuzzi and Pfefferkorn, 2002 and references therein). 5) This finding is in agreement with the existence of a nonglacial interval developed between a Late Visean glaciation and another one of Bashkirian age (Glacial Episode II). 6) The Loma de los Piojos Formation records the initial functioning and structure of the Paganzo basin and establishes a minimum Serpukhovian age for this event. This will allow a better understanding of the magnitude, development and earliest tectonostratigraphic evolution of this basin.

Acknowledgments The manuscript was greatly improved by the excellent reviews of two anonymous reviewers. The authors wish to thank Dr. E. Morel for reviewing the earlier versions of the manuscript and providing advice. Dr. A. Sterren has specially contributed with comments and bibliography. Dr. C. Rodríguez Amenábar and Lic. F. Degrange have kindly provided useful bibliography. Ms. J. Martinelli has collaborated invaluably for the English version. Financial support for this study was provided by ANPCyT (Agencia Nacional de Promoción Científica y Tecnológica) PICT-2006-01272 grant to Dr. Claudia Rubinstein.

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