Sommerxylon spiralosus from Upper Triassic in southernmost Paraná Basin (Brazil): a new taxon with taxacean affinity

EARTH SCIENCES

Sommerxylon spiralosus from Upper Triassic in southernmost Paraná Basin (Brazil): a new taxon with taxacean affinity

Etiene F. Pires^I; Margot Guerra-Sommer^II

^IBolsista CAPES-PPG-GEO, Departamento de Paleontologia e Estratigrafia, Instituto de Geociências, Universidade Federal do Rio Grande do Sul - 91501-970 Porto Alegre, RS, Brasil

^IIDepartamento de Paleontologia e Estratigrafia, Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Campus do Vale, Bairro Agronomia, Prédio 43127, sala 201 - 91501-970 Porto Alegre, RS, Brasil

^{Correspondence}

ABSTRACT

The anatoical description of silici?ed Gymnospermae woods from Upper Triassic sequences of southernmost Paraná Basin (Brazil) has allowed the identi?cation of a new taxon: Sommerxylon spiralosus n.gen. et n.sp. Diagnostic parameters, such as heterocellular medulla composed of parenchymatous and sclerenchymatous cells, primary xylem endarch, secondary xylem with dominant uniseriate bordered pits, spiral thickenings in the radial walls of tracheids, medullar rays homocellular, absence of resiniferous canals and axial parenchyma, indicate its relationship with the family Taxaceae, reporting on the first recognition of this group in the Triassic on Southern Pangea. This evidence supports the hypothesis that the Taxaceae at the Mesozoic were not con?ned to the Northern Hemisphere.

Key words: fossil wood, taxacean affinity, Upper Triassic, Paraná Basin.

RESUMO

A descrição anatômica de lenhos silicificados de Gymnospermae em seqüência do Triássico Superior no sul da Bacia do Paraná (Brasil), possibilitou a identificação de um novo taxon: Sommerxylon spiralosus n.gen. et n.sp. Parâmetros diagnósticos tais como medula heterocelular, composta por células parenquimáticas e esclerenquimáticas, xilema primário endarco, xilema secundário com pontoações areoladas unisseriadas dominantes, espessamentos espiralados nas paredes radiais dos traqueídeos, raios lenhosos homocelulares, ausência de canais resiníferos e de parênquima axial, indicam a sua vinculação à família Taxaceae, constituindo-se em reconhecimento inédito da presença deste grupo no Triássico Superior no sul do Pangea. Esta evidência suporta a hipótese de que a família Taxaceae não estava confinada ao Hemisfério Norte durante o Mesozóico.

Palavras-chave: lenho fóssil, afinidade com taxaceae, Triássico Superior, Bacia do Paraná.

INTRODUCTION

The petrified woods from several paleontological sites in the central portion of Rio Grande do Sul State (Brazil) have been ascribed to distinct ages and correlated to different stratigraphic units, such as the Rosário do Sul Formation - Triassic according to Gamermann (1973), the Caturrita Member of the Botucatu Formation with an age Jurassic as suggested by Bortoluzzi (1974), the Caturrita Formation of a Upper Triassic age according to Andreis et al. (1980) and the Mata Sandstone referred to the Rhaetian by Faccini (1989). Fossil woods occur as silicified fragments that are usually found as rolled pieces in sedimentary rocks, although they seldom occur included within the sedimentary deposits. The fossil record comprises mainly conifer-related gymnosperm forms and possibly represents a mesophytic flora originated when climate changes took place during the Meso-Neotriassic transition. The so called Conifer Flora is supposed to correspond to a younger association that the Dicroidium Flora, represented by impressions referred to the Asselian-Ladinian sedimentary sequences of the Santa Maria Formation (sensu Bortoluzzi 1974).

These occurrences of petrified wood in the Mesozoic of southernmost Paraná Basin have been know from more a century. They have been generally included by different authors in the genus Araucarioxylon Kraus (1870) and have been considered as indicative of Coniferales. According to Grambast (1960), Greguss (1967), Lepekhina (1972) and Mussa (1982), this genus is broad defined, encompassing almost all the possible variation in characters, and thus embracing a large group of plants. Consequently this taxon doesn't have taxonomic and phytostratigraphic relevance. Bamford and Philippe (2001) ratify that Araucarioxylon is illegitimate, and consider that most of wood species included of this genus should be transferred to Agathoxylon Harting.

The present study aims to describe and identify an association of silicified wood composed of specimens not decorticated, complete, recovered at the Linha São Luiz outcrop, district of Faxinal do Soturno at Rio Grande do Sul State southernmost Paraná Basin (Brazil). The sedimentary sequence which the outcrop is included, according to Rubert (2003) using litostratigraphic criteria, in the base of Caturrita Formation (sensu Andreis et al. 1980). Pires (2003) included the outcropping rocks in the Carnian-Eonorian Sequence according to Scherer et al. (2000). Besides silicified wood, different fossils were identified at this outcrop, represented by vertebrates (fish-scales, procolophonides, dinosaurs, mammals, cinodonts), invertebrates (conchostraceans and insects), icnofossils, shoots and reproductive structures of gymnosperms (Ferigolo and Ribeiro 2000, 2001). Impressions of leaves are included in the genus Brachiphyllum Brogniart by Bolzon et al. (2002) and the silicified woods are identified as Araucarioxylon Kraus. On the other hand, Dutra and Crisafulli (2002) characterized two different patterns of leaves: type-Cyparissidium and type-Pagiophyllum.

MATERIALS AND METHODS

The 31 fragments of wood on which the present study is based, were collected from the same stratigraphic horizont in the Linha São Luiz outcrop (UTM: 22J0262516E/66277528N) in different missions, by researchers of UFRGS, UNIVATES and Fundação Zoobotânica do Rio Grande do Sul. Preservation is usually good, specimens are mostly silicified. Dense impregnation by iron oxide is a common feature. The type material is deposited in the Paleobotanical Sector of the Departamento de Estratigrafia e Paleontologia of the Instituto de Geociências of the Universidade Federal do Rio Grande do Sul, Rio Grande do Sul State, Brazil. The specimens were cut in transversal, radial and tangential planes; observations were made on polished surfaces, with incident light; anatomic details were observed from petrographic slides mounted in Canada balsam, in transmitted light.

DESCRIPTION

Sommerxylon spiralosus n.gen. et n.sp.

Holotip - PB 3784.

Paratips - MCN PB 338; MCN PB 339; MCN PB 340; MCN PB 357; MCN PB 366; PB 3779; PB 3789; PB 3790; PB 3810; PB 278.

Locality - Linha São Luiz outcrop, (UTM: 22J0262516E/66277528N), Faxinal do Soturno, Rio Grande do Sul State, Brazil.

Horizon - basal section of Caturrita Formation (sensu Andreis et al. 1980).

Age - Upper Triassic.

Derivatio Nominis - generic name: is attributed in honor to Dr. Margot Guerra Sommer; specific epithet - to derive from the presence of spiral thickenings in the radial wall of the tracheids.

DIAGNOSIS

Sommerxylon n.gen.

Gymnospermous wood consisting of pith, primary xylem, secondary xylem, phloem and cortex. Pith solid, circular to sub-circular, with occasional large gaps, irregularly shaped heterocellular. Thin walled parenchymatous cells of the pith, rounded or polygonal in cross section, grouped in solid nests. Thick walled sclerenchymatous cells, polygonal, walls heavily pitted, single and isolated or in small nests, not oriented. Protoxylem endarch, centrifugal,wedge shaped. Growth rings distinct, large, with a gradual transition from early to late wood; early wood wide. Bordered pits in radial walls of secondary xylem uniseriate (99%), isolated or contiguous, locally biseriate (1%) alternate or opposite. Medullary ray homocellular, uniseriate. Cross-field pits single, small, 1-4 pits per cross-field, irregularly disposed. Radial spiral thickenings single in radial walls, inclination against the wall of tracheids 40-45º. Wood parenchyma and resinous canals absent. Phloem not differentiated in primary and secondary phloem, showing parenchymatous cells with dark contents and phloematic rays; resinous canals absent. Cortex with parenchymatous cells and canals.

Sommerxylon spiralosus n.gen. et n.sp.

Gymnospermous wood consisting of pith, primary xylem, secondary xylem, phloem and cortex. Pith almost circular, with irregular boards, small to medium size (2,5-7,75 mm in diameter), heterocellular, composed by parenchymatic and sclerenchymatic tissues. Medullar parenchyma comprising thin walled cells irregularly rectangular to sub-circular grouped in nests. Sclerenchymatous cells thick walled, heavy pitted, polygonal interposed singly or in irregularly nests with parenchymatic tissue. Large mesh gaps of irregular shape not orientated dispersed in the pith. Primary xylem easily distinguished in transverse section, endarch, dispersed in wedge shaped bundles at the periphery of the pith composed by cells thin walled. Secondary xylem centrifuge, radially disposed. Distinct growth rings 1,12-6 mm wide, showing a gradual transition from early to late wood. Early wood 58-150 cells (1-4 mm), tracheids wide, polygonal lumen almost rectangular. Late wood narrow 6-20 cells (0,08-0,95 mm), compressed radially with a compressed lumen, rectangular in shape. False growth rings not continuous all over the diameter are frequent. Radial walls of tracheids with bordered pits, mainly uniseriate (99%), circular, isolate or contiguous flattened; partially biseriate (1%), alternate or opposite. Medullary rays homocellular, uniseriate, cells oval in tangential view, 1-22 cells high (average height 6 cells). 1-4 circular and small pits per cross-field; pit pore is usually centric, circular to sub circular. Spiral thickenings in radial walls of tracheids, distance between the bands 10 to 28 mm, running parallel, about 2 mm in thickness; individual bands are flat, inclination against the walls 40º to 45º. Wood parenchyma and resinous canals absents. Phloem not differentiated in primary and secondary phloem, showing parenchymatous cells with dark contents and phloematic rays; resinous canals absent. Cortex with circular cells (parenchymatous cells) and canals.

The material corresponds of fragments of petrified wood, measuring 2-10 cm in length and 2-8,5 cm in diameter (Fig. 1,a), cylindrical, sometimes slightly compressed in transversal view.

All the specimens are composed by pith, primary xylem, secondary xylem, phloem and cortex. The pith is circular (Fig. 1,d), centric or eccentric, small to medium (2,5-7,75 mm), solid (Fig. 1,d), or with mesh gaps in some levels, which are irregularly shaped and disposed without any kind of organization, probably originated by cellular decay (Fig. 1,c; Fig. 3,e).

The pith is heterocellular, composed by parenchymatic and sclerenchymatic tissues (Fig. 1,d). Parenchymatous cells are circular to sub-rectangular or polygonal in shape, of different sizes. Large cells occur dispersed trough the pith (D: 24-40 mm) and smaller ones forms solid clusters (D: 18-22 mm) (Fig. 3,e).

The sclerenchymatous cells are either single or grouped in nests dispersed all over the pith without any kind of arrangement (Fig. 1,c). These cells vary in size (42-180 × 32-150 mm) are usually isodiametric in transverse section (Fig. 1,b) and rectangular in longitudinal view. Their walls show several layers of thickenings and are heavy pitted, radial canals can be observed running from the external radial walls to the lumen (Fig. 1,b).

The primary xylem encircles the pith disposed in wedge shaped bundles, and in some places projects into the pith. It is endarch and can be distinguished from secondary xylem in transverse section having thinner walls and tracheids being circular to sub-circular in shape (Fig. 1,c). The endarch disposition was not confirmed in the radial longitudinal section, where primary xylem was not preserved.

The secondary xylem is centrifuge, radially disposed; growth rings are distinct (Fig. 1,a), with gradual transition from early to late wood (Fig. 4,a). The circular cross-section of the growth rings indicated a possible relation of the specimens with vertical stems according the criteria of Schweingruber (1996). Rings 62-170 tracheids wide, 1,12-6 mm broad.

Early wood in each ring is wide, 58 to 150 cells deep; the width of the early wood varies from 1 to 4 mm; the tracheids are rectangular in transverse section (20-50 mm) with circular lumen (4-32 mm).

Late wood is narrow, 6 to 20 cells deep and the tracheids are radially compressed (10-30 mm) and have a small lumen (2-30 mm), rectangular in shape. The width of the late wood varies from 0,08 to 0,95 mm.

False growth rings are common along different levels of the secondary xylem, characterized by being not uniform and continuous rings around the transverse section.

Radial walls of tracheids have bordered pits showing dominantly uniseriate disposition (99%), closely spaced (Fig. 2,b,c) or contiguous slightly flattened (Fig. 2,e). When the pits are separated, they are more or less circular (D: 8-11 × 26-50 mm) with a spacing of 2 to 4 mm. The disposition partially biseriate altern or opposite were very rarely observed (1%) (Fig. 2,a,d). The pit pore is usually centric, circular to sub-circular (D: 2-4 mm) (Fig. 2,e).

Spiral thickenings are common on radial walls of tracheids (Fig. 3,a,b,c) rarely in longitudinal walls. The individual bands are flat, wide, single, running parallel; the distance between the bands of the spiral is about 10-28 mm. In some tracheids the individual bands of the spiral thickenings are superposed, resembling as double (Fig. 3,c). The inclination of different spirals against the wall of the tracheids is 40 to 45º. Tangential walls of tracheids are unpitted.

Medullary rays are homocellular, parenchymatous, smooth, uniseriate, 1 to 22 cells high (average 6 cells) (Fig. 3,d); their thick varies from 7 to 26 mm and their height from 114 to 496 mm. Ray cells are oval to sub-circular in tangential view. The density of the rays is 4 to 6 per millimeters square.

Cross-field pits are badly preserved (Fig. 2,f), varying from 1-4 pits per field (Fig. 2,g), and seems to be ordered in irregularly rows; they are circular, small (2 to 4 mm in diameter) with a round central pore (Fig. 2,h).

Axial parenchyma and resinous canals were absent in the secondary xylem.

Cellular elements of the cambium have collapsed, giving the appearance of a continuous, all around gap, in transversal section.

Due to poor preservation not much is know about the phloem and cortex. The phloem and cortex have partially decomposed and only little information is available about its structures. The phloem is not differentiated in primary and secondary phloem; parenchymatous cells with dark contents are observed and phloematic rays are represented as dark bands (Fig. 4,b); resinous canals and fibers were not visualized. The cortex is characterized by circular cells (parenchymatous cells?) and the presence of canals (Fig. 4,c).

DISCUSSION

The main features presented by the studied specimens are heterocellular pith with sclerenchymatic nests in a parenchymatic matrix, endarch protoxylem, uniseriate pits in radial walls of tracheids, medullary rays homocellular uniseriate, spiral thickenings in radial walls of tracheids, absence of axial parenchyma and resiniferous canals.

Taxopitys Kräusel (1928), a Permian gonduanic morphogenus &91; Taxopitys Africana Kräusel (1928), South Africa; Taxopitys alves-pintoi Kräusel and Dolianiti (1958) and Taxopitys jolyi Mussa (1982), Permian, Paraná Basin, Brazil&93; besides heterocelular medula with sclerenchymatous nests, presents protoxylem centripetal, mesarch, medullary rays uniseriate, cross-field with little pits, horizontal spiral thickenings and araucarioid pitting in secondary xylem. The two last characters are different from the specimens here analyzed. At the same way, diagnostic parameters of the morphogenus Parataxopitys, (P. brasiliana Maniero (1951) and P. Americana Milanez and Dolianiti (1950) of Permian Paraná Basin, Brazil) as araucarioid pitting and the structure of spiral thickenings show distinct in relation to the specimens studied in this paper (Table I-A).

Dutra and Crisafulli (2002) cited Kaokoxylon zalesskyi (Sahni) Maheshwari, from the Linha São Luiz outcrop at the same level from were collected the present specimens. Taxonomic diagnostic features are neither cited, nor described or figured. However the characterization of Araucarioxylon type of secondary wood and the non-reference of spiral thickenings, indicates distinct taxonomic affinity from the specimen named as Kaokoxylon by Dutra and Crisafulli (2002).

Descriptions of homoxylous morphogenera restrict to secondary wood prolififerate in literature mainly in Mesozoic and Cenozoic periods. Table I-B gives complete data and important xylotomical features of this genus compared with the new taxon here proposed. The relevant character for comparison was the presence of true spiral thickenings in secondary xylem. According to Barefoot and Hankins (1982), spiral thickenings are genetic feature and so have diagnostic importance.

Prototaxoxylon (= Spiroxylon) Kräusel and Dolianiti (1958) correspond to a morphogenus defined originally from Upper Paleozoic at that time represented by P. indicum (= Spiroxylon) (Metha) Prakash and Srivastava (1961) (Permian- India), P. (= Spiroxylon) africanum (Walton) Kräusel and Dolianiti (1958) (Permian- Karoo Basin - South Africa), P. brasilianum Kräusel and Dolianiti (1958) (Upper Permian - Paraná Basin, Brazil), Prototaxoxylon (Spyroxylon) intertrappeum Prakash and Srivastava (1961) (Upper Cretaceous- India) and Prototaxoxylon andrewsii Agashe and Chitnis (1971). This wood, besides to be characterized by true spiral thickening on the radial walls of tracheids, differs from the specimens under examination by having radial walls of tracheids characterized by 1-3 seriate circular - slightly horizontally compressed bordered pits. The specimen identified as P. intertrappeum by Lutz et al. (1999), from the Upper Triassic of North then Chile, seems to correspond by the characters figurate, to Taxaceoxylon Kräusel (1949) instead of Prototaxoxylon.

The presence of bars of Sanio and axial parenchyma are important and distinctive characters in the genus Platyspiroxylon Greguss (1961) described from the Jurassic of Hungry (P. heteroparenchymatosum Greguss 1961) and P. parenchymatosum from the Permian of Hungry (Greguss 1967) and Upper Cretaceous of Canada Ramanujam (1972). A comparison with Torreyoxylon boureaurii Greguss (1967), Lower Cretaceous from Hungry, make evident important differences, such as the presence of axial parenchyma and spiral thickenings in doubles bands.

The genus Taxaceoxylon Kräusel (1949) was proposed in substitution of Taxoxylon Kraus (1870) originally described as Taxites scalariformis (Goeppert) {= Taxoxylon Goepperti Unger = Taxaceoxylon escalariforme (Goeppert) Seward (1919)} which was invalidated.

This taxon includes specimens of secondary homoxylic wood with true spiral thickenings and abietinean type of pitting in the radial walls of tracheids, medullary rays homocellular, absence of resiniferous cells or ducts and axial parenchyma. The identification was based only on the characters of the secondary wood; some distinguishing features of the pith corresponding to thick walled polygonal cells dispensed in the parenchymatous tissue, were mentioned by Kräusel (1949), but not included in the diagnosis of the genus.

Kräusel and Jain (1963) have shown that only a few woods described as Taxaceoxylon (Taxoxylon) really belongs to the Taxaceae, corresponding to Taxaceoxylon torreyanum Shimakura (1936) from Pliocen of Japan, Taxaceoxylon (=Taxoxylon) antiquum (Boeshore and Gray) Kräusel (1949) from the Upper Cretaceous of North Caroline (EUA) and Taxaceoxylon rajmahalense (Bhandwaj) Kräusel and Jain (1963) from Jurassic of India.

After the revision of Kräusel and Jain (1963) new species were described T. cupressoides Sharma (1970), from Jurassic Sequences of India, T. japonomesozoicum Nishida (1973), from Cretaceous of Japan e T. mcmurrayensis Roy (1972) registered from Lower Cretaceous of Canada.

Bamford and Philippe (2001) revised the generic names of Jurassic/Cretaceous homoxylons wood, applying the roles of the International Code of Botanical Nomenclature (Greuter et al. 1999), and indicate that Taxoxylon should be abandoned and Taxaceoxylon preferred.

A detailed comparison with regard to the nature of bordered pits, spiral thickenings, medullary rays, number of cross-field pits and absence of resin ducts and axial parenchyma (Table IA-B) shows that the studied specimens are more closely comparable to the genus Taxaceoxylon. The identification however is impossible because the diagnosis of Taxaceoxylon is based exclusively in characters of secondary wood. On the other hand, the specimens, under study presents distinguished features of the pith, primary xylem, phloem and cortex. Consequently, considering the restrictions of the International Code of Botanical Nomenclature (Greuter et al. 1999), a new taxon is proposed: Sommerxylon spiralosus nov. gen. et. nov. sp. The present wood possesses all the general anatomical characters of a taxacean wood, as it was referred by Kräusel (1949), and Kräusel and Jain (1963) based in Florin (1940). It's important to observe that, even not including pith studies in the diagnosis, Kräusel (1949) refers to the similarity between the pith nature of Taxaceoxylon and that one of the extant genus Torreya. The same structures can be found in the pith of the specimens under the examination, and this evidence ratifies the botanical affinity between extant and fossil forms.

The occurrence of fossil representatives of the extant Taxaceae are mainly concentrated on the bases of leaf branches, the oldest one corresponding to Paleotaxus rediviva Nathorst (1908), from Upper Triassic of South Sweden; on the other hand, the oldest dated Taxacean petrified wood up till now was Taxaceoxylon rajmahalense (Bardwaj) Kräusel and Jain (1963) from Middle Jurassic from India. The identification of Sommerxylon spiralosus nov. gen. et nov. sp. in Upper Triassic sequences of southern Pangea supports the hypotheses of Kräusel and Jain (1963) based on Florin (1940) and Studt (1926) that in the Mesozoic the Taxaceae were not confined to the Northern Hemisphere.

ACKNOWLEDGMENTS

This work received financial support from the CAPES-MEC, Universidade do Vale do Taquari (UNIVATES) and Fundação Nacional de Desenvolvimento do Ensino Superior Particular (FUNADESP). the authors thank Prof. Dr. Robson Tadeu Bolzon (Universidade Federal do Paraná) and Prof. Dr. Jorge Ferigollo (Fundação Zoobotânica do Rio Grande do Sul) for loan of specimens.

Manuscript received on September 25, 2003; accepted for publication on January 5, 2004;

presented by ALCIDES N. SIAL

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Publication Dates

• Publication in this collection
  26 Aug 2004
• Date of issue
  Sept 2004

History

• Accepted
  05 Jan 2004
• Received
  25 Sept 2003