An unusual plant organ from the Triassic of Antarctica
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 remouw Peak several years ago, the Triassic flora of Antarctica today represents the most completely known assemblage of fossil plants providing anatomical detail. The flora currently include representatives of the following major plant groups: sphenophytes (for example, Osborn and Taylor 1989), several types of ferns (for example, Millay and Taylor 1990; Delevoryas, Taylor, and Taylor 1992), cycads (for example, Smoot, Taylor, and Delevoryas 1985), conifers (for example, Meyer-Berthaud and Taylor 1991), and seed ferns (for example, Meyer-Berthaud, Taylor, and Taylor 1993). Dispersed pollen grains and spores preserved in the silicified peat indicate that during the early Middle Triassic this region of Antarctica supported a diverse flora (for example, Farabee, Taylor, and Taylor 1990). Various types of fungi are also associated with plants preserved at the Fremouw Peak site (84 0 16'S 164 0 21'E) (for example, White and Taylor 1989). The plant remains occur in silicified blocks that are believed to represent levee deposits that were undercut during flooding (Taylor, Taylor, and Collinson 1989). Although many
floral elements can be placed within major groups of vascular plants, there are several organs for which taxonomic assignment remains problematic. One of these organs is represented by axes that display unusual vascular tissue organization. The larger axis possesses secondary vascular tissue, and the diameter ranges from 1.2 to 2.2 centimeters (cm). All specimens are incomplete, the largest being approximately 12 cm long. Vascular tissue distribution is best described as polyarch, in which 10-16 arms radiate toward the periphery of the axis (figure 1). Each of these vascular segments contains a small amount of secondary xylem, which surrounds a narrow primary body. Thin-walled cells, topographically in the position of a vascular cambium, and presumed phloem tissue make up the remainder of the vascular tissue. Circular-elliptical bordered pits are present on the radial walls of the tracheids. Some specimens possess a distinct periderm. Possible root traces extend from the periphery of several vascular segments (figure 2). Associated with the larger axis are several smaller stems that appear immature. These range from 0.2 to 0.5 cm in diameter; all are fragmentary and extend just a few millimeters 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 segments 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 segments, we believe that the small axes represent immature 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 longitudinally (Stein and Hueber 1989). One apparent difference 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 probat-igure 1. I ransverse section ot the larger axis showing vascular tissue ble Devonian sphenophyte, however, possessed a claorganization. (x 6.5) doxylaleanlike vascular system (Schweitzer 1973).
ANTARCTIC JOURNAL - REVIEW 1994 28
Despite the absence of peripheral loops, we believe that the Fremouw axis is related to some group of Mesozoic ferns. The abundant traces in some specimens are anatomically similar
to adventitious root traces in both their origin and distribution. Although both marattialean and filicalean ferns are 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 Mesozoic 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 underground 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
Figure 2. Transverse section showing possible root traces originating irom tne periphery of vascular segments. (x 6.5)
Delevoryas, T., T.N. Taylor, and E.L. Taylor. 1992. A marattialean 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 the central Transantarctic Mountains, Antarctica. Revie of Palaeobotany and Palynology, 65(1-4), 257-265. 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. Petrified 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 nology, 62(1-2), 41-64.
of Palaeobotany and Paly-
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. Structurally preserved fossil plants from Antarctica. I. Antarcticycas, gen. nov., a Triassic cycad stem from the Beardmore Glacier area. American Journal of Botany, 72(9), 1410-1423. Stein, W.E., and F.M. Hueber. 1989. The anatomy of Pseudosporochnus: 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. Depositional setting and paleobotany of Permian and Triassic permineralized peat from the central Transantarctic Mountains, Antarctica. International Journal ogy, 12, 657-679.
Figure 3. Transverse section of the immature stem illustrating trie organization ci vascular and cortical tissues. (x 15)
White, J.F., and T.N. Taylor. 1989. An evaluation of sporocarp structure in the Triassic fungus Endochaetophora. Review of Palaeobotany and Palynology, 61(3-4), 341-345.
ANTARCTIC JOURNAL - REVIEW 1994 29
of Coal Geol-
Biology and Byrd Polar Research Center, Ohio State University, Columbus, Ohio 43210-1293
s a result of the discovery of silicified plant material at remouw Peak several years ago, the Triassic flora of Antarctica today represents the most completely known assemblage of fossil plants providing anatomical detail. The flora currently include representatives of the following major plant groups: sphenophytes (for example, Osborn and Taylor 1989), several types of ferns (for example, Millay and Taylor 1990; Delevoryas, Taylor, and Taylor 1992), cycads (for example, Smoot, Taylor, and Delevoryas 1985), conifers (for example, Meyer-Berthaud and Taylor 1991), and seed ferns (for example, Meyer-Berthaud, Taylor, and Taylor 1993). Dispersed pollen grains and spores preserved in the silicified peat indicate that during the early Middle Triassic this region of Antarctica supported a diverse flora (for example, Farabee, Taylor, and Taylor 1990). Various types of fungi are also associated with plants preserved at the Fremouw Peak site (84 0 16'S 164 0 21'E) (for example, White and Taylor 1989). The plant remains occur in silicified blocks that are believed to represent levee deposits that were undercut during flooding (Taylor, Taylor, and Collinson 1989). Although many
floral elements can be placed within major groups of vascular plants, there are several organs for which taxonomic assignment remains problematic. One of these organs is represented by axes that display unusual vascular tissue organization. The larger axis possesses secondary vascular tissue, and the diameter ranges from 1.2 to 2.2 centimeters (cm). All specimens are incomplete, the largest being approximately 12 cm long. Vascular tissue distribution is best described as polyarch, in which 10-16 arms radiate toward the periphery of the axis (figure 1). Each of these vascular segments contains a small amount of secondary xylem, which surrounds a narrow primary body. Thin-walled cells, topographically in the position of a vascular cambium, and presumed phloem tissue make up the remainder of the vascular tissue. Circular-elliptical bordered pits are present on the radial walls of the tracheids. Some specimens possess a distinct periderm. Possible root traces extend from the periphery of several vascular segments (figure 2). Associated with the larger axis are several smaller stems that appear immature. These range from 0.2 to 0.5 cm in diameter; all are fragmentary and extend just a few millimeters 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 segments 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 segments, we believe that the small axes represent immature 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 longitudinally (Stein and Hueber 1989). One apparent difference 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 probat-igure 1. I ransverse section ot the larger axis showing vascular tissue ble Devonian sphenophyte, however, possessed a claorganization. (x 6.5) doxylaleanlike vascular system (Schweitzer 1973).
ANTARCTIC JOURNAL - REVIEW 1994 28
Despite the absence of peripheral loops, we believe that the Fremouw axis is related to some group of Mesozoic ferns. The abundant traces in some specimens are anatomically similar
to adventitious root traces in both their origin and distribution. Although both marattialean and filicalean ferns are 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 Mesozoic 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 underground 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
Figure 2. Transverse section showing possible root traces originating irom tne periphery of vascular segments. (x 6.5)
Delevoryas, T., T.N. Taylor, and E.L. Taylor. 1992. A marattialean 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 the central Transantarctic Mountains, Antarctica. Revie of Palaeobotany and Palynology, 65(1-4), 257-265. 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. Petrified 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 nology, 62(1-2), 41-64.
of Palaeobotany and Paly-
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. Structurally preserved fossil plants from Antarctica. I. Antarcticycas, gen. nov., a Triassic cycad stem from the Beardmore Glacier area. American Journal of Botany, 72(9), 1410-1423. Stein, W.E., and F.M. Hueber. 1989. The anatomy of Pseudosporochnus: 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. Depositional setting and paleobotany of Permian and Triassic permineralized peat from the central Transantarctic Mountains, Antarctica. International Journal ogy, 12, 657-679.
Figure 3. Transverse section of the immature stem illustrating trie organization ci vascular and cortical tissues. (x 15)
White, J.F., and T.N. Taylor. 1989. An evaluation of sporocarp structure in the Triassic fungus Endochaetophora. Review of Palaeobotany and Palynology, 61(3-4), 341-345.
ANTARCTIC JOURNAL - REVIEW 1994 29
of Coal Geol-
