An unusual plant organ from the Triassic of Antarctica LISA D. BOUCHER, THOMAS N. TAYLOR, and EDITH L. TAYLOR, Department of Plant Biology and Byrd Polar Research Center, Ohio State University, Columbus, Ohio 43210-1293

s a result of the discovery of silicified plant material at floral elements can be placed within major groups of vascular remouw Peak several years ago, the Triassic flora of plants, there are several organs for which taxonomic assign- Antarctica today represents the most completely known ment remains problematic. One of these organs is represented assemblage of fossil plants providing anatomical detail. The by axes that display unusual vascular tissue organization. The flora currently include representatives of the following major larger axis possesses secondary vascular tissue, and the diame- plant groups: sphenophytes (for example, Osborn and Taylor ter ranges from 1.2 to 2.2 centimeters (cm). All specimens are 1989), several types of ferns (for example, Millay and Taylor incomplete, the largest being approximately 12 cm long. 1990; Delevoryas, Taylor, and Taylor 1992), cycads (for exam- Vascular tissue distribution is best described as polyarch, ple, Smoot, Taylor, and Delevoryas 1985), conifers (for exam- in which 10-16 arms radiate toward the periphery of the axis ple, Meyer-Berthaud and Taylor 1991), and seed ferns (for (figure 1). Each of these vascular segments contains a small example, Meyer-Berthaud, Taylor, and Taylor 1993). Dis- amount of secondary xylem, which surrounds a narrow pri- persed pollen grains and spores preserved in the silicified mary body. Thin-walled cells, topographically in the position of peat indicate that during the early Middle Triassic this region a vascular cambium, and presumed phloem tissue make up the of Antarctica supported a diverse flora (for example, Farabee, remainder of the vascular tissue. Circular-elliptical bordered Taylor, and Taylor 1990). Various types of fungi are also asso- pits are present on the radial walls of the tracheids. Some speci- ciated with plants preserved at the Fremouw Peak site mens possess a distinct periderm. Possible root traces extend (840 16S 164021E) (for example, White and Taylor 1989). from the periphery of several vascular segments (figure 2). The plant remains occur in silicified blocks that are Associated with the larger axis are several smaller stems believed to represent levee deposits that were undercut during that appear immature. These range from 0.2 to 0.5 cm in flooding (Taylor, Taylor, and Collinson 1989). Although many diameter; all are fragmentary and extend just a few millime- ters in the matrix. In transverse section, they contain cortical lacunae incompletely surrounded by vascular tissue; some tracheids project into the lacunae (figure 3). These axes consistently lack secondary tissues, however. In addition, the individual vascular seg- ments are separated in the cortex and lack continuity. In these axes, the primary xylem is mesarch. Some cortical cells contain opaque materials. Based on both the histology and arrangement of the vascular seg- ments, we believe that the small axes represent imma- ture stages of the larger polyarch stem. One of the most interesting aspects of this Triassic axis is the anatomical similarity with the Cladoxylales, a group of Devonian plants. The vascular tissue arrangement of this group includes radiating arms of mesarch xylem, which are interconnected longitudi- nally (Stein and Hueber 1989). One apparent differ- ence between the cladoxylaleans and the Fremouw Peak axis is the consistent presence of peripheral loops in the former. Such loops are represented by thin-walled parenchyma cells that are associated with protoxylem elements. Most Cladoxylales are considered relatively small plants that branched irregularly and bore ultimate, planated appendages thought to have functioned as leaves. Although the Cladoxylales have been included in several taxonomic categories in the past, today most regard them as a type of fern. At least one proba- t-igure 1. I ransverse section ot the larger axis showing vascular tissue ble Devonian sphenophyte, however, possessed a cla- organization. (x 6.5) doxylaleanlike vascular system (Schweitzer 1973).

ANTARCTIC JOURNAL - REVIEW 1994 28 Despite the absence of peripheral loops, we believe that the to adventitious root traces in both their origin and distribu- Fremouw axis is related to some group of Mesozoic ferns. The tion. Although both marattialean and filicalean ferns are abundant traces in some specimens are anatomically similar known to have been present in rocks from this site, to date th ere is no character that can be used to associate the above-ground portions with their subterranean organs. Finally, despite an increasing understanding of the vegetative and reproductive parts of the Meso- zoic seed ferns, such as the corystosperm Dicroidium, there is nothing known about the rooting organs of these plants. It is not beyond reason to suggest that this interesting axis from the Fremouw Peak site may, in fact, represent some component of the under- ground system of a Mesozoic seed fern. We hope that as these studies progress, especially concerning the developmental anatomy of the immature specimens, it may be possible to relate this "cladoxylalean" anatomy with either the ferns or seed ferns. This research was supported by National Science Foundation grant OPP 91-18314.

References

Delevoryas, T., T.N. Taylor, and E.L. Taylor. 1992. A marat- tialean fern from the Triassic of Antarctica. Review of Palaeobotany and Palynology, 74(1-2), 101-107. Farabee, M.J., E.L. Taylor, and T.N. Taylor. 1990. Correlation of Permian and Triassic palynomorph assemblages from Figure 2. Transverse section showing possible root traces originating irom tne the central Transantarctic Mountains, Antarctica. Revie of 65(1-4), 257-265. periphery of vascular segments. (x 6.5) Palaeobotany and Palynology, Meyer-Berthaud, B., and T.N. Taylor. 1991. A probable conifer with podocarpacean affinities from the Triassic of Antarctica. Review of Palaeobotany and Palynology, 67(3-4),179-198. Meyer-Berthaud, B., T.N. Taylor, and E.L. Taylor. 1993. Petri- fied stems bearing Dicroidium leaves from the Triassic of Antarctica. Palaeontology, 36(2), 337-356. Millay, M.A., and T.N. Taylor. 1990. New fern stems from the Triassic of Antarctica. Review of Palaeobotany and Paly- nology, 62(1-2), 41-64. Osborn, J.M., and T.N. Taylor. 1989. Structurally preserved sphenophytes from the Triassic of Antarctica: Vegetative remains of Spaciinodum, gen. nov. American Journal of Botany, 76(11), 1594-1601. Schweitzer, H.-J. 1973. The Middle Devonian flora of Lindlar (Rhineland), 4. Filicinae-Calamophyton primaevum Kräusel and Weyland. Palaeontographica B, 140, 117-150. (In German) Smoot, E.L., T.N. Taylor, and T. Delevoryas. 1985. Struc- turally preserved fossil plants from Antarctica. I. Antarcti- cycas, gen. nov., a Triassic cycad stem from the Beard- more Glacier area. American Journal of Botany, 72(9), 1410-1423. Stein, W.E., and F.M. Hueber. 1989. The anatomy of Pseu- dosporochnus: P. hueberi from the Devonian of New York. Review of Palaeobotany and Palynology, 60(3-4), 311-359. Taylor, E.L., T.N. Taylor, and J.W. Collinson. 1989. Deposi- tional setting and paleobotany of Permian and Triassic permineralized peat from the central Transantarctic Mountains, Antarctica. International Journal of Coal Geol- ogy, 12, 657-679. White, J.F., and T.N. Taylor. 1989. An evaluation of sporocarp Figure 3. Transverse section of the immature stem illustrating trie organization ci structure in the Triassic fungus Endochaetophora. Review Palaeobotany and Palynology, 61(3-4), 341-345. vascular and cortical tissues. (x 15) of

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