R E C O N S T R U C T I N G THE GONDWANA SEED FERN DICROIDIUM : EVIDENCE

FROM THE TRIASSIC OF ANTARCTICA

BRIGITTE MEYER-BERTHAUD Laboratoire de Paldobotanique, URA 327 CNRS, Universitd des Sciences et Teclmiques du Languedoc, Place Eugene Bataiflon, P-34095 Mon~gellier Cedex 05.

EDITH L. TAYLOR & THOMAS N. TAYLOR Byrd Polar Research Cm~ter, The Ohio State Universffy, 1735 Nell Avenue, Columbus, Oil 43210-1293, USA.

ABSTRACt

The discovery, in Antarctic rocks of the Middle Triassic, of leaves with Dicroidium anatomy attached to stems with undivided vascular cylinder and pycnoxylic wood of the widespread Dadoxylon type, calls into question the long-accepted reconstruction of Dicroidium fronds attached to Rhexoxylon stems. Although Dicroidium leaves are common throughout Gondwana, Rhexoxylon is known only from Western Gondwana (Argentina, Brazil and South Africa). The anatomy of the Antarctic axes, including the presence of numerous buds, suggests a plant with more complex architecture than that proposed in the reconstruction of Dicroidium in Western Gondwana. Two hypotheses for the belated discovery of this additional "Dicroidium plant" are proposed.

DONNI~ES NOUVELLES POUR LA RECONSTRUCTION DE DICRO1D1UM, UNE PTERIDOSPERMALE DU GONDWANA, D'APP~S DES RESTES DU TRIAS D'ANTARCTIQUE.

Rt~.SUM£

La dtcouverte dans un gisement du Trias moyen d'Antarctique de feuilles gt anatomic de Dicroidiurn attachtes ~ des tiges qui prtsentent un cylindre vasculaire non divis6 et du bois pycnoxylique de type Dadoxylon remet en question l'hypothtse classique selon laquelle les frondes de Dicroidiurn 6taient portdes par des axes de Rhexoxylon. Alors que les feuilles de Dicroidium sent largement reprtsenttes dans l'ensemble du Gondwana, Rhexoxylon est restreint h la partie ouest du continent (Argentine, Brtsil et Afrique du Sud). L'anatomie des axes d'Antarctique, qui prtsentent en particulier de nombreux bourgeons, suggtre que l'architecture de ces plantes 6tait plus complexe que ceUe proposte pour la reconstruction de Dicroidium baste sur les testes du Gondwana de l'Ouest. Deux hypothtses sent propos6es pour interprtter la d6couverte rtcente de cette nouveUe plante ~ frondes de Dicroidium.

KEY WORDS : ANATOMY, ANTARCrICA, GONDWANA, PTERIDOSPERMALES, TRIASSIC

MOTS-CLt~S : ANATOMIE, ANTARCTIQUE, GONDWANA, PTERIDOSPERMALES, TRIAS

I N T R O D U C T I O N

Dicroidium is a form-genus established for leaves with a widespread distribution in Triassic rocks of Gondwana (Anderson & Anderson 1983). The genus includes more than thirty species and is considered to be a biostrati- graphic marker for the Triassic in Gondwana. Based on consistent association in the Ischigualasto Formation of Argentina, as well as at other sites, it has been assumed

Manuscrit d~pos6 le 04.12.1991 Manuscrit accept6 d6finitivement le 20.03.1992

for some time that Dicroidium leaves were borne on Rhexoxylon stems (Archangelsky 1968). Stems of Rhexo- xylon exhibit a complex, liana-like anatomy, consisting of one to several rings of vascular bundles with both centripetal and centrifugal development of secondary xylem separated by large wedges of parenchymatous tissue (Walton 1923 ; Archangelsky & Brett 1961). Trace production is complex and involves traces from several stem sympodia. The genus currently includes

eobios, 1992~ ote brtve | 25, fasc. 3 / 1-344 j

342

four species from South America (Argentina and Brazil) (Archangelsky & Brett 1961 ; Herbst & Lutz 1988) and South Africa (Bancroft 1913 ; Walton 1923). In addition, a specimen with Rheaoxylon-like secondary anatomy was recently reported from the Triassic of Antarctica (Fremouw Peak) (Taylor 1992).

Petriella (1981) reconstructed the "Dicroidium plant" as a tree-tern that bore Ptemchus pollen organs and Umkomasia ovulate structures. Retallack and Dilchcr (1988) suggested that the "Dicroidium plant" was a woodland tree based on vegetative material from Argen- tina (Archangelsky & Brett 1961) and reproductive parts (Pteruchus and Umkomasia) from the Molteno Formation of South Africa. One of the problems with these reconstructions is that, even though Dicroidium is common at certain sites in eastern Gondwana (i.e., Australia, India, New Zealand, Antarctica), Rhexoxylon is not currently known to occur there. Although Triassic floras have not been collected as extensively in Antarc- tica as elsewhere in Gondwana, Rhexoxylon also appears to have only a rare distribution on this continent.

D E S C R I P T I O N O F T H E A N T A R C T I C S P E C I M E N S A N D C O M P A R I S O N W I T H R H E X O X Y L O N

The present material comes from silicified peat collec- ted at a site adjacent to Fremouw Peak in the central Transantarctic Mountains (Taylor et el. 1989). This site is considered to be early Middle Triassic in age, based on vertebrates within the Fremouw Formation (Hammer 1990), and pollen and spores recovered from the peat itself (Farabee et al. 1990). This peat has yielded a number of plant organs (Taylor & Taylor 1990), including anatomically preserved Pteruchus pollen organs (De Vore & Taylor 1988). In addition, Pigg (1990) recently described leaves of Dicroidium (assigned to a new species, D. frernouwensis) from the peat at Fremouw that provide the first data on the internal anatomy of this important form genus.

The stems described here consist of eight twigs with attached leaf bases (fig. 2.2) that are similar in size and structure to D. fremouwensis, and four larger stems devoid of leaves (fag. 2.1). Leafy twigs are at most one year old, but the larger axes show 2-5 growth rings and measure up to 15 mm in diameter. Most axes bear buds. The vasculature in the leaf bases consists of several bundles arranged in an adaxial line and two abaxially disposed, incomplete rings of bundles (fig. 1). This structure is similar to D. fremouwensis, which consists of a horizontal line of bundles toward the adaxial face and a ring of bundles abaxially. In addition, the leaf bases possess secretory cavities of the same diameter as resinous bodies described in D. fremouwensis (Pigg 1990).

Imm

Figure 1 - Drawing of a leaf base in transverse section. Adaxial face upwards. Blackened areas represent vascular bundles and empty circles, secretory cavities. (10,628 ElbT. 82). Oessin d'une basefoliaire en section transversale. Face adaxiale vers le baut. Les suoCaces noircies reprd- sentent les faisceaux vasculaires ; les surfaces vides, des caviMs sdcrdtrices.

The stems from Fremouw Peak possess a solid cylinder of vascular tissue and no centripetal wood (fig. 2.1) and, in these features, they are unlike Rhexoxylon stems. However, characteristics of the pith, wood and leaf traces show similarities to this genus. Like Rhexoxylon, the Antarctic axes possess secretory structures, sclerotic nests (fag. 2.3) and, eventually, bands of periderm-like tissue in the pith and cortex The wood in all axes is pycnoxylic with uniseriate rays. Radial pitting on the tracheids is 1-3 seriate with predominantly hexagonal and contiguous Caraucarian") bordered pits. Cross field pits are simple and variable in shape and number ; small oval pits tend to fuse into larger units (fag. 2.4). Leaf traces have been studied in detail only in R. piatnitskyi (Archangelsky & Brett 1%1), but these too, are similar to the Antarctic specimens. Traces are emitted in pairs and originate from several sympodia. They follow a low-angled course through the wood and cortex before dividing to form the complex, three-dimensional pattern present in the leaf bases (fig. 1).

D I S C U S S I O N

The "Dicroidium plant" from Fremouw Peak is composed of axes with dense, pycnoxylic wood with no centripetal development, bearing leaves of the D. fremouwensis type. This plant might also have borne the Pteruchus-like pollen organs that were reported earlier from this locality (De Vore & Taylor 1988). This relationship is based not only on association within the peat, but also on anatomical similarities between the leaves and the pollen organs (i.e., the presence of characteristic secretory structures in both organs). The leaves were attached to the distal-most twigs and were apparently deciduous. The extensive occurrence of buds

343

Figure 2 - 1, Transverse section of a five-year-old stem. Scale bar = 1 mm. (10,891 F2aT.4). 2. Oblique longitudinal section of a leafy twig. Scale bar = 200/2m. (CB 545 A). 3, Pith in transverse section showing 2 sclerotic nests and 2 secretory cavities. Scale bar = 100 }tm. (10,891 F2aT.5). 4, Secondary xylem in radial section showing pits on the radial walls of the tracheids and within cross fields. Scale bar = 20 ,tZm. (10,891 F2IIS.1). 1, Section transversale d'un axe de 5 arts. 2, Section longitudinale oblique d'un axe feuilld 3, Section transversale de la moelle montrant 2 nids scl~reux et 2 caviMs sdcrdtrices. 4, Section radiale du xylkme secondaire montrant les poncBzations sur la paroi des trachdides et clans les champs de croisaments.

suggests that this plant was branched and architecturally more complex than a typical tree-fern.

Thus, our concept of the "Dicroidium plant" must be expanded to include not only the Dicroidiurn/ Rhexo- xylon all iance known from South Amer i ca and South Africa, but also the present reconstruct ion based on mater ia l f rom Antarc t ica . Perhaps the Rhexoxylon- type p redomina t ed in Wes te rn Gondwana and the Antarc t ic type in eastern regions. It is possible that the "Dicroi- d ium plant" with Rhexoxylon stems was also secondari ly establ ished in Eas tern Gondwana , based on the rare occurrence of this s tem genus in Antarct ica . The plant which we p ropose here, with Dicroidium leaves and pycnoxylic wood of the Dadoxylon type, has not yet been found in other par ts of Gondwana . However , due to the rari ty of anatomical ly preserved mater ia l throughout Gondwana, it is impor tan t to consider the possibility that the plant recons t ruc ted here occur red throughout Gondwana, but has not yet been recognized at many sites due to preservat ional differences.

Acknowledgements - This research was supported in part by funds from the National Science Foundation (DPP-8815976) and by an Ohio State University post-doctoral fellowship to Brigitte Meyer- Berthaud. Contribution n ° 782 of the Byrd Polar Research Center.

REFERENCES

ANDERSON J.M. & ANDERSON H.M. 1983 - Paleoflora of Southern Africa. Molteno Formation (Triassic). Balkema, Rot te rdam, 1 : 227 p.

ARCHANGELSKY S. 1968 -Studies on Triassic fossil plants from Argentina. IV. The leaf genus Dicroidium and its possible relat ion to Rhexoxylon stems. Palaeontology, 11 : 500-512.

ARCHANGELSKY S. & BREYr D.W. 1961 - Studies on Triassic fossil plants from Argent ina . I. Rhexoxylon from the Ischigualasto Format ion. P h i l Trans. R. Soc. London , B, 244 : 1-19.

BANCROFT N. 1913 - Rhexoxylon africanurn, a new meduUosean stem. Trans. Lin. Soc. 2nd Ser., 8 : 87-103.

344

DE VORE M.L. & TAYLOR T.N. 1988 - Mesozoic seed plants : a pollen organ from the Triassic of Antarc- tica. Amer. J. Bot., 75 : 106.

FAIL,XBEE M.J,, TAYLOR E.L. & TAYI,OR T.N. 19¢X) - Correlation of Permian and Triassic palynomorph assemblages from the Central Transantarclic Moun- tains, Antarctica. Rev. Palaeobot. Palynol., 65 : 257- 265.

HAMMER W.R. 1990 - Triassic terrestrial vertebrate faunas of Antarctica. In TAYLOR T.N. & TAYLOR E.L. (eds) : Antarctic paleobiology and its role in the reconstruction of Gondwana. Spzinger-Verlag : 42-50.

HERBST R. & LUTZ A.I. 1988 - Rhexoxylon brasiliensis n. sp. (Corystospermaceae, Pteridospermales) from the Upper Triassic Caturrita Formation, Brazil, with comments on biology and environment. Meded. Rijks Geol. Dienst, 42 : 21-28.

PEI~ELLA B. 1981 - Sistematica y vinculaciones de las Corystospermaceae H. Thomas. Ameghiniana, 18 : 221-234.

PIG(; K.B. l~)0 - Anatomically preserved Dicroidium foliage from thc Central Transantarctic Mountains. Rev. Palaeobot. Palynol., 66:129-145.

RI~I'AH.ACK (}. & DIL('Ht,;R D.L. 1988 - Reconstruction of selected sccd ferns. Ann. Missouri Bot. Gard., 75 : 1010 -1057.

TAYLOR E.L. 1~)2 - The occurrence of a Rhexoxylon- like stem in Antarctica. Cour. Forschung. Inst. Sen&.

TAYLOR E.L. & TAYLOR T.N. 1990 - Structurally pre- served Permian and Triassic floras from Antarctica. In TAYLOR T.N. & TAYLOR E.L. (eds) : Antarctic paleobiology and its role in the reconstruction of Gondwana. Springer-Verlag : 149-163.

TAYLOR E.L., TAYLOR T.N. & COLLINSON J.W. 1989 - Depositional setting and palaeobotany of Permian and Triassic permineralized peat from the central Transantarctic Mountains, Antarctica. Int. J. Coal Geol., 12 : 657-679.

WALTON J. 1923 - On Rhexoxylon BANCROFT - a Trias- sic genus of plants exhibiting a liane-type of vascular organization. Trans. Lin. Soc. London, 2, 8 : 79-109.