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One of the most fascinating groups of fossil plants are the so-called Mesozoic seed ferns, a loose collection of principally impression/compression remains characterized by seeds produced in cupules. Although some of the vegetative and reproductive organs of each of the four orders (Caytoniales, Peltaspermales, Corystospermales, and Petriellales) are reasonably well documented, there is little agreement as to the origin(s) of the group and even less consensus as to their evolutionary relationships with modern seed plants. Since the initial description of the group by Thomas (1925), numerous authors have offered various interpretations regarding the nature of the reproductive organs, especially the cupule. This organ has been the focal point in most discussions that relate the Mesozoic seed fern groups with the flowering plants, because it has been variously interpreted as the precursor or homologue of the angiosperm carpel (e.g., Rothwell and Serbert 1994 and references therein). Even within the Petriellales, a group based on permineralized cupules from the Triassic of Antarctica (Taylor, Del Fueyo, and Taylor 1994), little evidence suggests a relationship between the cupule and carpel because the cupules of Petriellaea demonstrate a folding pattern that is different from the conduplicately folded carpel of flowering plants. In addition to differing opinions as to the nature of the seed-bearing reproductive organs, even less information is available about how and where these structures were borne and the overall habitat of the plant.
Fieldwork during the 1995-1996 season in the Shackleton Glacier region uncovered an exceptional outcrop of the Upper Fremouw-Falla Formation containing well-preserved impression/compression plant remains and a few fragments of silicified peat. The section is approximately 42 meters thick and includes six plant levels of late middle to early late Triassic age ( figure 1). The site occurs on "Alfie's elbow," an unnamed ridge southeast of Schroeder Hill at approximately 85°23'717"S 174°49'916"W. The rocks at this locality represent fluvial deposits that include several cycles of channel, levee, floodplain, crevasse splay, and swamp facies. A few paleosols present are associated with the floodplain facies. It appears that the fluvial system at this site was dominated by low-energy braided streams, allowing for the preservation of fine-grained siltstones and shales that contain well-preserved plant remains, including delicate reproductive organs attached to the axes that bore them.
Of particular significance is the specimen illustrated in figures 2 and 3. It consists of a stem approximately 1.5 centimeters in diameter from which arise evenly spaced, short lateral axes up to 3 centimeters long. On the surface of these axes are closely spaced lenticular scars that may represent the former position of leaves. Attached to the tip of some of these branches are elongate slender axes each of which terminates in a cupule. Each cupule is flattened and appears to consist of several units that correspond to either cupule lobes or seeds. Present at the same site as the attached cupules are leaves of Heidiphyllum, a form genus of elongate, broadly sessile leaves with parallel veins that show some anastomoses. Anderson and Anderson (1989) suggest that Heidiphyllum is the leaf of the plant that produced the coniferlike seed cone Telemachus. We speculate that Heidiphyllum leaves may have been produced by the seed fern that bore the cupulate organs described above. We are unaware, however, of any Mesozoic seed fern cupulate structures that are attached to delicate petiolelike axes like those from the Shackleton site. Morphologically, these reproductive axes appear similar to the short or spur shoots of some ginkgophytes, a group that was well represented by Triassic time. We are hopeful that the Shackleton specimens will reveal sufficient detail so that the affinities of this interesting fossil can be determined.
This research was supported by National Science Foundation grant OPP 93-15353 to the University of Kansas. Fieldwork was made possible by the logistic support of Helicopters New Zealand and U.S. Navy squadron VXE-6. We are especially appreciative of the excellent field support provided by David Buchanan.
References
Anderson, J.M.,and H.M. Anderson. 1989. Palaeofloraof southern Africa Molteno Formation (Triassic). 2. Gymnosperms (excluding Dicroidium). Rotterdam, Netherlands: A.A. Balkema.
Rothwell, G.W., and R. Serbert. 1994. Lignophyte phylogeny and the evolution of spermatophytes: A numerical cladistic analysis. Systematic Botany, 19, 443-482.
Taylor, T.N., G.M. Del Fueyo, and E.L. Taylor. 1994. Permineralized seed fern cupules from the Triassic of Antarctica: Implications for cupule and carpel evolution. American Journal of Botany, 81, 666-677.
Thomas, H.H. 1925. The Caytoniales, a new group of angiospermous plants from the Jurassic rocks of Yorkshire. Philosophical Transactions of the Royal Society of London, 213B, 299-363.