Pergamon Journalof African Earth Sciences, Vol. 29, No. 1, PP. 243-255, 1999

Pll:SO899-5382(99)00094.9 0 1999 Elsevier Science Ltd

All rights reserved. Printed in Great Britain 0899.5362/99 S- see front matter

The Triassic taphoflora of the Paran Basin, southern Brazil: a biostratigraphical approach

M. GUERRA-SOMMER,” M. CAZZULO-KLEPZIG and R. IANNUZZI lnstituto de Geociencias, Universidade Federal do Rio Grande do Sul, C.P. 15001,

CEP. 91501-970, Porto Alegre, RS, Brazil

ABSTRACT-A Triassic taphoflora identified in the Central Region of the State of Rio Grande do Sul, southern Brazil, represents an important biostratigraphical stage in the palseofloristic succession of the Parana Basin. A megafloristic association composed of compressed leaves, fronds and seeds of a ‘Dicroidium Flora‘ shows a predominance of the Dicroidium genus, with several species and other important taxa like Neocalamites sp., Cladophlebis sp., Tetraptilon aff. heteromerum, Ginkgoites antarctica, Sphenobaiera sp., Podozamites sp., Nilssonia sp., Pteruchus sp. and Carpolithus sp. Taking into account the stratigraphical distribution of different species of the Dicroidium genus, a biostratigraphical framework was established. Considering that the recognition of this ‘Dicroidium Flora’ was based on limited outcrops belonging to one lithostratiphical level (Santa Maria Formation - Passo das Tropas Facies), it was impossible to establish a formal biostratigraphical zonation. Instead, at the present time, an informal floristic interval, named the ‘Dicroidium odontopteroides Flora’, is proposed (Late Anisian to Late Ladinian, Middle Triassic). An ‘Araucarioxylon Flora’ composed of secondary woods of the Araucarioxylon type and stems of Rhexoxylon brasiliensis was also identified several km apart. The compressed fossils of the ‘Dicroidium Flora’ and the petrified stems of the ‘Araucarioxylon Flora’ could represent different but contemporaneous communities corresponding to fluvial-lacustrine environments. o 1999 Elsevier Science Limited. All rights reserved.

RESUME-La taphoflore triasique identifiee dans la region central de I’Etat de Rio Grande do Sul, Bresil du sud, represente un &age biostratigraphique important dans la succession paleo- floristique du Bassin du Parana. L’association mega-floristique, composee de feuilles aplaties, de frondes et de semences de la ‘Flore a Dicroidium’ montre la predominance du genre Dicroidium. avec plusieurs especes, et d’autres taxa importants, comme Neocalamites sp., Cladophlebis sp., Tetraptilon aff . heteromerum, Ginkgoites antarctica. Sphenobaiera sp., Podozamites sp., Nilssonia sp., Pteruchus sp. et Carpolithus sp. Un cadre biostratigraphique a et6 Btabli en prenant en compte la distribution stratigraphique des differentes especes du genre Dicroidium. Comme la reconnaissance de la ‘Flore B Dicroiaium’ se fonde sur des affleurements limit& d’un seul niveau lithostratigraphique (Formation de Santa Maria, facies de Passo das Tropas), il est impossible d’etablir une zonation biostratigraphique formelle. Au contraire, un intervalle floristique informel, appelt! ‘Flore a Diocroidium odontopteroides’, est propose actuellement (Anisien superieur a Ladinien superieur, Trias moyen). Quelques kilometres plus loin, une ‘Flore a Araucarioxylon’ composee de bois secondaire de type Araucarioxylon et de tiges de Rhenoxylon brasiliensis a et6 identifiee. Les fossiles aplatis de la ‘Flore a Dicroidium’ et les tiges petrifiees de la ‘Flore a Araucarioxylon’ pourraient rep&enter des communautes differentes, mais contemporaines, correspondant a des environnements fluviatiles et lacustres. @ 1999 Elsevier Science Limited. All rights reserved.

(Received l/7/98: revised version received 214199: accepted 2514199)

* Corresponding author

Journal of African Earth Sciences 243

M. GUERRA-SOMMER et al.

INTRODUCTION

The present study aims to update biostratigraphical information about plant fossils present in the Meso- zoic sequences of Rio Grande do Sul state, southern Brazil, and to contribute to a better interpretation of those deposits within the chronostratigraphical framework of the Parana Basin (Fig. 1). Results ob- tained gave an overall picture of the knowledge of the Rio Grande do Sul Mesozoic floras, providing correlations with other Triassic Gondwanic floras.

Leaf and frond impressions can be found in the Passo das Tropas Member of the Santa Maria For- mation which are widely known for its herpetofauna (Barberena, 1977; Barberena eta/., 1993). Permine- ralised stems, according to Andreis et a/. (19801, could be related to the upper part of the Caturrita Formation, or to another overlying stratigraphical unit now completely eroded (Faccini, 1989) (Fig. 2).

The first references to Mesozoic plant fossils in the state of Rio Grande do Sul were made by natu- ralists at the end of the last century (Ave-Lallemant, 1880; Isabelle, 1883). Until the 1950s these fossil plants were just mentioned, together with reptiles, as biostratigraphical markers in studies that tried to establish the stratigraphy of the Gondwanic series in southern Brazil (White, 1908; Moraes Rego, 1930; Huene and Stahlecker, 1931; Fiuza da Rocha and Scorza, 1940; Gordon, 1947; Beurlen et al., 1955). Rau (1933) noted the presence of fossil trunks in the Santa Maria area (central region of the state of Rio Grande do Sul), identifying them as Araucari- ox ylon (Dadox ylon I.

Gordon and Brown (1952) described a collection of fossil plants, preserved as impressions, collected by Gordon and Price in 1947, and discussed aspects relating to the Triassic flora in the south of Brazil. Dicroidium, Path ypteris, Zuberia, Thinn feldia, Schizo- neura or Neocalamites and Baiera were identified. Pinto (1956) noted the presence of some fossil plant impressions in the Santa Maria Formation, identifying Sewardia, Pteruchus, Stenorachis and Samaropsis. From 1965, studies on frequency, localisation and stratigraphical horizons were developed (BeltrZo, 1965; Bortoluzzi and Barberena, 1967; Bortoluzzi, 1975). Bortoluzzi and Barberena (op. cit.), for the first time, interpreted megaplant associations as representatives of the so called ‘Thinnfeldia-Dicroi- dium Flora’, nowadays named the ‘Dicroidium Flora’. In the last two decades, systematic studies were undertaken either in fossils preserved as impressions of ‘Dicroidium Flora’ (Bortoluzzi et al., 1983, 1984, 1985; Guerra-Sommer et al., 1985; Mastroberti, 1997) or in silicified woods (Herbst and Lutz, 1988; Guerra-Sommer and Gamermann, 1985). The taxo- nomic classifications adopted by these authors are

244 Journal of African Earth Sciences

still accepted and are used in the characterisation of the Mesozoic flora of Rio Grande do Sul.

After 1990, important taphonomic, biostratigra- phical and palaeoecological contributions concerning the Mesozoic floras of Rio Grande do Sul were carried out (Bolzon, 1993; Bolzon and Guerra-Sommer, 1994; Minello, 1994; Silverio da Silva et al., 1997). Recently, lannuzzi and Schultz (1997) registered a new megafloristic association related to stratigra- phical levels (Alemoa Member, Santa Maria Forma- tion) that had so far only been characterised by the herpetofauna (Barberena et al., 1993).

MATERIAL

Siltstone sediments containing the impressions are located close to the city of Santa Maria in outcrops named by Bortoluzzi (1974) as Passo das Tropas and Dom Antonio Reis, both linked to the same stratigraphical level. This level corresponds to a conglomerate with interbedded mudstone lenses deposited in a fluvial-lacustrine environment. Mega- floristic associations in these two outcrops are similar, the Dicroidium genus being dominant with complementary forms related to Coniferophyta (Po- dozamites sp.1, Pteridophylla (Cladophlebis sp., Tetraptilon aff. heteromerum) and lncertae Sedis (Taeniopteris sp., Sewardia sp.).

Silicified stems are widely distributed along a 200 km east-west range, although the most significant fossils occur in the SBo Pedro do Sul, Mata and Santa Maria Regions (Bolzon and Guerra-Sommer, 1994). Fragments of silicified wood can be found embedded in red sandstones considered to be of fluvial origin. These sandstones were dated as Tri- assic (Gamermann, 19731, Upper Triassic (Bottoluzzi, 1973; Andreis et al., 19801, Upper Triassic/Rhaetic (Faccini, 19891, and even Jurassic (Minello, 1994) (Fig. 2). Minello selected the outcrops of Xiniqua and Ermida for anatomical and petrological studies on fossil wood. At the Xiniqua outcrop, silicified wood fragments occur in sediments from the Alemoa Member of the Santa Maria Formation (Andreis et al., 19801 or in the top of Sequence II of Faccini (1989). Tetrapods were collected in this outcrop, characterising the ‘Local Fauna’ of Xiniqua (Bar- berena et al., 1993). At the Ermida outcrop, silicified fragments occur in sandy sediments and subordinate pelites, the reddish colours being predominant. Ana- tomical studies made on subsurface samples indi- cated the presence of Araucarioxylon Kraus. Rhex- oxylon brasiliensis, a new species described by Herbst and Lutz (1988) from the neighbourhood of S5o Pedro do Sul, represents an important record for biostratigraphical studies in Triassic sequences.

The Triassic taphoflora of the Parand Basin, southern Brazil

600

I 600

I 400

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vvvvvvvv

\ I

v v v v v v v v v v v v

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600 60”

I J 1200 Km

f0J‘J Mesozoic Sequences

m Coostol Plain

m Bosoltic Rocks

m Poloeozoic Sequences

m Bosement 0 40Km

Figure 1. Map of the Paran. Basin with location of the Triassic sequences in southern Brazil.

Journal of African Earth Sciences

M. GUERRA-SOMMER et al.

0

v, co a a t-

Permian

GAMMERMANN 1973

L

Botucatu Fm.

BORTOLUZZI 1974

Botucatu Mb.

Caturrita Mb.

Alemoa Facies

Pass.0 das Tropas Facies

Rosario do Sul Fm.

Passa Dois Group

ANDREIS et al. 1980 c

Botucatu Fm.

Caturrita Fm.

Sanga do Cabral Fm.

FACCINI 1989

Sequence IV

Sequence II

Sequence I

Figure 2. Different stratigraphical schemes for the Triassic of the southern ParanB Basin. Fm.: Formation; Mb.: Member.

Besides the material described to date by different authors, this study also presents information obtained from the preliminary analysis of permin- eralised wood material coming from the palaeon- tological sites of SBo Rafael, Agua Boa, Faxinal, Capeleto and Piscina (Santos and Moreira, 19871, collected at the same stratigraphical level. Except for Rhexoxylon brasiliensis, the holotype of which is in the Walter llha Museum of SZo Pedro do Sul, all material mentioned in this paper is kept at the Palaeobotany section, Departamento de Paleon- tologia e Estratigrafia, lnstituto de Geociencias, Universidade Federal do Rio Grande do Sul (IG/ UFRGS).

BIOSTRATIGRAPHY

Based on the above mentioned studies the Dicroi- dium Flora in the Rio Grande do Sul State is com- posed of the following elements, according to the taxonomic studies of Bortoluzzi et a/. (1983, 1984, 1985) and Guerra-Sommer et al. (I 985): SPHENOPHYTA - Neocalamites sp.

PTERIDOPHYLLA - Cladophlebis sp. Tetraptilon aff. T. heteromerum Frenguelli 1950

PTERIDOSPERMOPHYTA - Thinnfeldia spp. Dicroidium lancifolium (Morris) Gothan 1912

Dicroidium (Johnstonial stelzneriana (Gein.) Frenguelli 1941 Dicroidium iXylopteris) argentinum (Kurt.?) Arrondo 1972 Dicroidium lancifolium Petriella 1978 Dicroidium (XylopterisJ elongatum (Carruthers) Arch- angelsky 1968 Dicroidium odontopteroides (Morris) Gothan 1912 Dicroidium odontopteroides var. odontopteroides (Morris) Gothan 19 12 Dicroidium odontopteroides var. remotum (Szajnocha) Retallack 1977 Dicroidium odontopteroides var. moltenense Retallack 1977 Dicroidium zuberi var. zuberi (Szajnocha) Archangelsky 1968 Dicroidium zuberi var. papillatum (Townrow) Retallack 1977 Dicroidium zuberi var. brasiliensis Petriella 1978 Dicroidium zuberi var. feistmantelii (Johnston) Retallack 1977 Dicroidium dubium Jacob and Jacob 1950 Dicroidium aff. narrabeenense (Dun in Walkom) Jacob and Jacob 1950 Pteruchus sp .

GINKGOPHYTA - Ginkgoites antartica (Saporta) Shirley 1898 Sphenobaiera sp. Stenorachis sp.

CONIFEROPHYTA - Podozamires sp.

246 Journal of African Earth Sciences

Figure 3. (a) D. lancifolium - Pb 1481 (Pass0 das Tropas outcrop). (61 D. aff. narrabeenense - Pb 1361 (Dom AntGnio Reis outcrop). (c) D. zuberi var. papillatum - Pb 2 178 (Pass0 das Tropas ootcropl. Id) D. zuberi var. brasiliensis - Pb 3454 (Pass0 das Tropas outcrop]. (e) 0. aff. narrabeenense - Pb 3453 (Dam AntGnio Reis outcropl. IfJ D. odontopteroides - Pb 1569 (Dom Ant&Co Reis outcropl.

Figure 4. (a) D. zuberi var. moltenense - Pb 1369 (Pass0 das Tropas outcrop). ib, c/ D. zuberi var. brasiliensis - Pb 1502/ 2674 (Pass0 das Tropas outcrop). (d, ej D. zuberi var. zuberi - Pb 1491/345? (Pass0 das Tropas outcropl. (f, h) D. zuberi var. feistmantellii - Pb 1800/1230 (Pass0 das Tropas outcrop). (g) D. zuberi var. odontopteroides - Pb 7365 (Pass0 das Tropas outcrop/.

The Triassic tapho flora of the Parana Basin, southern Brazil

INCERTAE SEDIS - Taeniopteris sp. Sewarda sp. Nilssonia sp . Carpolithus sp.

Figures 3, 4, 5 and 6 show the most important forms used in this biostratigraphical study. Dicroidium acutum, recorded by Bortoluzzi et al. (1983), is con- sidered here as Dicroidium lancifolium, according to Petriella’s (1978) criteria.

Retallack (1977) presents a chart with stratigraphi- cal ranges of selected species in some better known Gondwanic sequences in Australasia, spanning at least one oppel-zone boundary. Criteria used are those referring to the evolution of Dicroidium and similar forms (e.g. Johnstonia, Xyjopteris), with re- stricted stratigraphical distribution. In relation to the Triassic floras of Argentina, Petriella (1983) conclu- ded that the stratigraphical distribution of certain genera and species was too limited to represent relevant chronological elements. Using Retallack’s (1977) and Stipanicic’s (1979) data, Petriella pre- sented a chart with the stratigraphical distribution of the most important South America genera, using the classic divisions of the European Triassic. The analysis of this chart makes it evident that the species registered for Rio Grande do Sul, in common with those from Argentina, occur only since the Neo-Anisian (Dicroidium odontopteroides var. remo turn, Dicroidium (Johns tonia) s telzneriana, Dicroidium (Xylopteris) argentinum and Dicroidium (Xylopteris) elongatum). On the other hand, Dicroi- dium odontopteroides var. remotum had its last record in the Neo-Ladinian.

Anderson and Anderson (1970) established a correlation between the Santa Maria Formation (Bortoluzzi, 1974) and the Triassic floras of Argen- tinian basins: Buena Vista, Los Rastos, Cortaderita, Potrerillos, Tranquilamal and El Tranquilo. The data presented by the authors (op. cit.) indicated an affin- ity between the floras of the Santa Maria Formation and those of the Molten0 beds (Karoo Basin), mainly an abundance of the Dicroidium genus and the presence of Ginkgoites antarctica. Anderson and Anderson (1985) presented a chart with the floristic contents of the Molten0 Formation, in which the Dicroidium genus, represented by various species, is abundant, associated with Neocalamites, Taeniop- teris, Nilssonia, Sphenobaiera and Ginkgoites. This diversified association is more complex than the macroflora studied here, which is more homoge- neous. The authors presented two assemblage zones in the South African Triassic: Dicroidium hughesii and Dicroidium odontopteroides. The first assem- blage corresponds to the top of the Lower Triassic (top of the Beaufort Group) and the second to the base of the Upper Triassic (Molten0 Formation).

However, according to the zonation suggested by Retal-lack (1977), the assemblages related to the second biozone of Anderson and Anderson (1985) would be equivalent to the top of the Mid-Triassic and to the base of the Upper Triassic.

According to Guerra-Sommer et al. (19851, the taphoflora of the Parand Basin corresponds to the ‘oppel-zone’ Dicroidium odontopteroides (Neo-Ani- Sian to Ladinian), according the biostratigraphical criteria of Retallack (1977). This zone differs from the lower zones on the basis of the presence of Dicroidium odontopteroides (Morris) Gothan, 1912 and its reproductive structures, like Pteruchus john- stonii (Feist.) Townrow, 1962 and Pilophorosperma spp. (Thomas, 1933). The presence of Dicroidium dubium and Dicroidium af f . Dicroidium narrabeen- ense has recently been added by Mastroberti (1997) to the flora described by Guerra-Sommer eta/. (1985). Comparing these taxa with those of the zonation suggested by Retallack (op. cit.1 for Australasia, Mastroberti confirmed that the association of the Santa Maria Formation would correspond to the Dicroi- dium odontopteroides ‘oppel-zone’. This coexistence of forms, according to Mastroberti allows restriction of association to the Meso-Anisian/Neo-Anisian.

The biostratigraphical criteria used by Guerra- Sommer eta/. (19851, including the Dicroidium Flora of Rio Grande do Sul in ‘oppel-zones’ of Retallack (1977), cannot be applied since the outcrops corres- pond to the same stratigraphical level. Thus, despite the very restricted geographical and stratigraphical occurrence of Triassic megafloristic associations in Rio Grande do Sul, it is necessary to use other criteria to define the biostratigraphical parameters.

Barberena (1977) suggested a preliminary biostrati- graphical zonation for the Santa Maria Formation with two tetrapod biozones: the Therapsida Ceno- zone and the Rhyncocephalia Cenozone, with an ‘intermediary biozone’ of Dicroidium. Barberena et al. (1993) kept the Therapsida and Rhyncocephalia Cenozones, but suggested abandoning the Dicroi- dium Cenozone.

PHYTOSTRATIGRAPHY: LEAVES

Aiming at the phytostratigraphical refinement of the Triassic sequence in southern Brazil, the stratigraphi- cal distribution of different species of the genus Dicroidium has been taken as the main reference for the present study. A distribution chart of the taxa was constructed based on the forms identified at the Passo das Tropas and Parque Dom Antonio Reis outcrops. This chart was built based on the max- imum stratigraphical range presented by Retallack (1977) and Petriella (1983) for species of Dicroidium

Journal of African Earth Sciences 249

M. GUERRA-SOMMER et al.

Figure 5. (a, c) D. argentinum - Pb 1550/1451 (Pass0 das Tropas outcrop). Ibl D. dubium - Pb 1474 (Pass0 das Tropas outcrop/. (dj D. zuberi var. zuberi - Pb 1934 (Pass0 das Tropas outcrop/.

250 Journal of African Earth Sciences

The Triassic taphoflora of the Paranri Basin, southern Brazil

Figure 6. (a, b) D. (Johnstonia) stelzneriana - Pb 1464/1324 (Pass0 das Tropas outcrop). /cl D. zuberi var. brasiliensis - Pb 1813 (Pass0 das Tropas outcrop). fdl D. zuberi var. zuberi - Pb 7958 (Pass0 das Tropas outcrop/. (el D. elongatum - Pb 1354 (Pass0 das Tropas outcrop). If) D. odontopteroides var. remotum - Pb 1466 /Pass0 das Tropas outcrop).

Journal of African Earth Sciences 251

M. GUERRA-SOMMER et al.

D.

D.

0.

D.

D.

D.

D.

0.

D.

D.

D.

D.

E. M. 1 L.LIE:I~L:/E.i.L.

zuberi var. papilla turn

zuberi var. feistmantelii

aff. narrabeenense

dubium

zuberi var. zuberi

lancifolium

(X ylop teris) elonga turn

odon topteroides var. remo turn

odon top teroides var. odon top teroides

(Johns tonia) s telzneriana

IXylop teris) argen tinum

odontopteroides var. moltenense

LATE

Figure 7. Biostratigraphical distribution of different species of Dicroidium in the Parand Basin.

in Australasia and Argentina, respectively. The species of Dicroidium occurring in southern Brazil indicated a time interval corresponding to Neo-Ani- sian/Neo-Ladinian (Mid Triassic). Dicroidium odontop- teroides var. remotum is the taxon of greater bio- stratigraphical value in the association. Other taxa of biostratigraphical importance are: Dicroidium odon top teroides var. odon top teroides, Dicroidium odontopteroides var. moltenense, Dicroidium (John- stonia) stelzeneriana and Dicroidium (Xylopteris) argentinum. The maximum age suggested for this flora corresponded to the Neo-Anisian interval. On the other hand, Dicroidium zuberi var. papillaturn, Dicroidium zuberi var. feistmantelli and D. aff. narrabeenense define the minimum age correspon- ding to the Neo-Ladinian (Fig. 7).

The characteristics presented by the associations studied here, such as diversity of species and abun- dance of specimens associated with the limited geographic and stratigraphical occurrence, are insuf- ficient for the establishment of a formal zonation. An informal palaaofloristic interval named the ‘Dicroi- dium odontopteroides Flora’ is therefore suggested.

PHYTOSTRATIGRAPHY: FOSSIL WOOD

The secondary xylem, identified as Araucarioxylon Kraus by Minello (1994) from samples collected in

252 Journal of African Earth Sciences

sandstones at the Ermida outcrop, provided little support for biostratigraphical studies. This genus, dominant in the association, corresponds to a col- lective taxon representing secondary xylem. This pattern is common in gymnosperms of the Palaeo- zoic, Mesozoic and Tertiary, having no exclusive asso- ciation with Araucariaceae.

Baieroxylon cicatricum Lele. was identified by Min- ello (1994) as fragments in sequences that contain the local fauna of Xiniqua. Minello had considered only superficial leaf scars on stems as indicating patterns of generic affinity. In this paper, therefore, Minello’s designation has not been considered as valid, because these characteristics are considered insufficient for such a taxonomic link.

The record of Rhexoxylon brasiliensis Herbt and Lutz is the most important biostratigraphical para- meter supplied by the lignitaphoflora. The holotype was refered to as having been collected at “North- western Surroundings of Sao Pedro do Sul”. Although the location is not known precisely, only Triassic sediments outcrop in the area. This exclu- sively Gondwanic genus is related to the Dicroidium Flora, with species described from many localities of the Triassic of South Africa, Argentina, Brazil and Antarctica. Five species are recognised, according to the criteria of Meyer-Berthaud et al. (1993): R. africanum (Bancroft) Walton 1923 (Cape region,

The Triassic taphoflora of the Parana Basin, southern Brazil

South Africa), R. tetrapteridoides Walton, 1923 (South Africa), R. priestleyi Walton, 1956 (South Africa), R. pianitzkyi Archangelsky and Brett, 1961 (Provincia de la Rioja, Agentina) and R. brasilensis Herbst and Lutz, 1988 (Brazil).

The affinity of Rhexoxylon with the Corystosper- males, and more specifically with Dicroidium, was suggested by Archangelsky and Brett (1961). Con- clusive evidence was later given by Archangelsky (1968) and Petriella (1978, 1981). A permineralised trunk, with branches organically connected to D. femouwensis, was described much later by Meyer- Berthaud et al, (1993) from the Fremow Formation, Transantarctic Mountains, Antarctica. It was con- sidered to be Early to Mid-Triassic, based on palyno- logical data (Farabee et a/. , 1990) and identified as Kykloxylon sp. The authors suggested that the Dicroidium/Rhexoxylon plant must have been dom- inant in West Gondwana, whereas the Dicroidium/ Kykloxylon plant might have had a wider geographi- cal distribution in Gondwana. Thus, Antarticoxylon, Kykloxylon and Rhexoxylon would be components of a natural group, the Corystospermales, and were distributed in different Gondwanic provinces.

According to Archangelsky and Brett (19611, the presence of Rhexoxylon in the same beds from which the rich herpetofauna of the lschigualasto Formation, was identified in Argentina, allowed correlations with sequences from South Africa, at the top of the Beaufort Group (Mid Triassic). For the sequence that contains Rhexoxylon in Argentina, an Upper Ischigua- lastian Reptile age is defined (Bonaparte, 1973). Herbst and Lutz (1988) correlated these sequences with those where Rhexoxylon brasiliensis occured in Rio Grande do Sul.

Permineralised gymnospermic wood fragments from the central region of Rio Grande do Sul State (SZio Rafael, Agua Boa, Faxinal, Capeleto and Pis- cina), were also studied. The pith, when present, was generally small and parenchymatic, suggesting an affinity with the Coniferophyta (Boureau and Mar- guerier, 1985). The occurrence of Rhexoxylon asso- ciated with this flora of conifers, indicates that the Corystospermales were associated with this conifer flora as complementary elements.

CONCLUSION

The pattern of Triassic vegetation in southern Brazil was fairly uniform, and the most diversified and abundant plant group was the genus Dicroidium. Elements of the ‘Dicroidium Flora’ included trees (e.g. conifers, ginkgos), woody shrubs (Mssonia), ground cover (e.g. ferns) and swamps growths (e.g. sphenophytes). An assemblage composed of silicified

woods was also identified at the same location as the ‘Dicroidium Flora’. This association is domin- ated by secondary wood of Araucarioxylon type, and petrified stems of Rhexoxylon brasiliensis which are related to fronds of Dicroidium. This lignitapho- flora is considered here as ‘Araucarioxylon Flora’.

The levels containing the Dicroidium Flora could represent hygro-mesophilous environments where Corystospermales were dominant, and Ginkgoales and Coniferophyta occurred as complementary forms. On the one hand, the conifer lignitaphoflora would represent a meso-xerophilous association where hygrophilous Corystospermales would represent periferic forms. Based upon this concept, the Dicroi- dium Flora preserved as impressions, and the conifer lignitaphoflora represented by fossil wood, would represent different contemporaneous communities preserved in distinct facies within the same depo- sitional site, corresponding to a fluvial-lacustrine environment. On the other hand, the conifer ligni- taphoflora could correspond to an association youn- ger than the Dicroidium Flora. This concept agrees with Bolzon (19931, who suggested that a progres- sive development of the conifer flora in the Parana Basin was caused by climatic changes which would have started in the Mid/Upper Triassic. According to Bolzon, the conifer floras were restricted to the Carnian-Norian interval.

Even though there are difficulties in defining bio- stratigraphical parameters using fossil wood, it is possible to suggest a Triassic age to the conifer lignitaphoflora, mainly by the presence of the Rhex- oxylon genus.

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