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Dicroidium Foliage from Mount Falla, Central Transantarctic Mountains

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Dicroidium Foliage from Mount Falla, Central Transantarctic Mountains Dicroidium foliage from Mount Falla, ' central Transantarctic Mountains WIT 1L I EDITH L. TAYLOR, LISA D. BOUCHER, AND THOMAS N. TAYLOR Byrd Polar Research Center and Department of Plant Biology Ohio State University Columbus, Ohio 43210-1293 ¶4 Dicroidium is a widespread foliage type found in Triassic rocks throughout Gondwana. The genus is characterized by basal forking of the frond rachis. The majority of species are once pinnate with ovate pinnules. The first appearance of the genus, V- along with the disappearance of Glossopteris, has been used as a biostratigraphic indicator for the base of the Triassic, although these two genera are known to coexist in Indian strata (Bose et al. 1990). Dicroidium has been found in close association with several reproductive organs (e.g., Llmkomasia and Pteruchus) and is as- signed to the Corystospermales, an extinct group of Mesozoic seed ferns. Samples of Dicroidium were collected from the north side of RD = the western ridge of Mount Falla, near the Beardmore Glacier in the central Transantarctic Mountains (8421 S 16442 E). The fossils occur as compressions 135 meters above the base of the type section of the Falla Formation (section F2 of Barrett 1969; Figure 1. Dlcroidium odontopteroides. Barrett et al. 1986). These rocks are considered to be late Triassic (Camian-Norian) based on palynomorph assemblages (Farabee et al. 1989). Most of the specimens have a cuticle and represent one of the few reports of preserved cuticle known from Antarctica. Thus it was possible to examine both morphological and cuticular fea- tures in order to determine systematic affinities. Three different species are represented in the Mount Falla flora—Dicroidium odontopteroides (Townrow 1957), D. lancfolium (Retallack 1977), and D. dubium (Retallack 1977). There are some complete pinnate .yI fronds that show the forked rachis typical of the genus, but most specimens represent incomplete fronds. Pinnule shape ranges from ovate to lanceolate, and their attachments range from alter- nate to opposite. Venation patterns are odontopteroid (D. odontopteroides) or alethopteroid-like (D. !anczfolium, D. dubium) A and are dependent upon pinnule size. Most cuticular features are similar in all specimens. Epider- Jot mal cells are generally isodiametric, but may be elongated over ... the rachis and major veins. Usually more than one papillae are •-r present on each cell. Stomatal distribution is amphistomatic and • ( ' random; over the rachis the guard cells are parallel to the long -p •••,••• axis. Adjacent to the guard cells are two lateral subsidiary cells D. that may sometimes be divided transversely. Two polar subsid- ád.i iary cells are usually present. Sometimes, additional subsidiary cells are present, and they form an incomplete outer ring. Most •. j:•: stomata are slightly sunken and some have papillae arching over IX the stomatal aperture. Trichome bases are rare. In addition to the three species from Mount Falla, Dicroidium has been described from other localities in Antarctica, including 43. southern Victoria Land (D. dubium and D. zuberi; Rigby 1985), the Allan Hills (D. dutoitii; Townrow 1967; Taylor et al. 1990), the Shackleton Glacier area (D. dutoitii, D. feistmantelii; Townrow 1967) and Livingston Island (Dicroidium sp.; Banerji and Lemoigne Figure 2. Dicroidium lancifolium. 2 ANTARCTIC JOURNAL References Banerji, J. and Y. Lemoigne. 1987. Significant additions to the Upper Triassic flora of Williams Point, Livingston Island, South Shetlands (Antarctica). Géobios, 20: 469-487. Barrett, P. J. 1969. Stratigraphy and petrology of the mainly fluviatile Permian and Triassic Beacon rocks, Beardmore Glacier area, Antarc- tica. Institute of Polar Studies Report, 34:1-132. Barrett, P. J., D. H. Elliot, and J. F. Lindsay. 1986. The Beacon Super- group (Devonian-Triassic) and Ferrar Group (Jurassic) in the Beardmore Glacier area, Antarctica. Geology of the Central Transantarctic Mountains (Antarctic Research Series), 36(14):339428. Bose, M. N., E. L. Taylor, and T. N. Taylor. 1990. Gondwana floras of India and Antarctica-A survey and appraisal. In T.N. Taylor and E.L. Taylor (Eds.), Antarctic Paleobiology and its Role in the Reconstruc- tion of Gondwana. New York: Springer-Verlag, 118-148. Farabee, M. J., T. N. Taylor, and E. L. Taylor. 1989. Pollen and spore assemblages from the Falla Formation (Upper Triassic), central Transantarctic Mountains, Antarctica. Review of Palaeobotany and Pa- lynology, 61: 101-138. Lacey, W. S. and R. C. Lucas. 1981. The Triassic flora of Livingston Island, South Shetland Islands. British Antarctic Survey Bulletin. 53:157-173. Pigg, K. B. 1990. Anatomically preserved Dicroidium foliage from the central Transantarctic Mountains. Review of Palaeobotany and Palynol- ogy, 66:129-145. Plumstead, E. P. 1962. Fossil floras of Antarctica. Trans-Antarctic Figure 3. Cuticle preparation of D. odontopteroides showing oUtlines of epidermal cells and stoma (center) flanked by guard Expedition 1955-1958, Scientific Reports, 9:1-154, pl. 1-28. cells and subsidiary cells. (X 400) Retallack, G. J. 1977. Reconstructing Triassic vegetation of eastern Australasia: A new approach for the biostratigraphy of Gondana- land. Alcheringa, 1:247-277. 1987 and D. cf. lancfolium, D. cf. elongata, D. cf. spinifolia; Lacey Rigby, J. F. 1985. Some Triassic (Middle Gondwana) floras from South and Lucas 1981). Dicroidium odontopteroides is the most common Victoria Land, Antarctica. New Zealand Geological Survey; Hornibrook species, known from southern Victoria Land (Plumstead 1962; Symposium (Christchurch, New Zealand), Abstracts: 78-79. Rigby 1985), the Allan Hills (Townrow 1967), the Shackleton Taylor, E. L., T. N. Taylor, B. Meyer-Berthaud, J . L. Isbell, and N. R. Glacier area (Townrow 1967), and northern Victoria Land Cüneo. 1990. A Late Triassic flora from the Allan Hills, southern (Tessensohn and Madler 1987), as well as Mount Falla. Dicroidium Victoria Land. Antarctic Journal of the U.S., 25(5):20-21. Tessensohn, F., and K. Mädler. 1987. Triassic plant fossils from North fremouwensis was described from a permineralized peat deposit Victoria Land, Antarctica. Geologische Jahrbuch, B66:187-201. at Fremouw Peak (Middle Triassic) and is the only species based Townrow, J. A. 1957. On Dicroidium, probably a pteridospermous leaf, on anatomically preserved material (Pigg 1990). The remaining and other leaves now removed from this genus. Transactions of the specimens are preserved as impression/ compressions. The ant- Geological Society of South Africa, 60:21-60. arctic taxa are most similar to Dicroidium described from Austra- Townrow, J. A. 1967. Fossil plants from the Allan and Carapace Nuna- lia, however, the Mount Falla material is noteworthy in that taks, and from the Upper Mill and Shackleton Glaciers, Antarctica. biprnnate frond types, such as D. zuberi, are not known. New Zealand Journal of Geology and Geophysics, 10:456-473. This work was supported in part by National Science Founda- tion grant DPP 88-15976. A glimpse of early Miocene antarctic from beneath the Ross Ice Shelf (see figure 2 in Harwood et al. 1989). Problems associated with palynomorph mixing and re- forests: Palynomorphs from RISP working due to glacial processes limited the ability of palynolo- diatomite gists to distinguish in situ grains from recycled grains, and prevented the unequivocal assessment of terrestrial fossil floras in Antarctica. In most studies of Antarctic Cenozoic palynology, recycled pollen dominate the assemblages (Truswell and Drewry XINHE JIANG AND DAVID M. HARWOOD 1984; Truswell 1986; Mildenhall 1989; Truswell 1990), and con- fuse paleo-floral reconstructions. These problems are greatly Department of Geology minimized in this study (Jiang and Harwood 1993), because we University of Nebraska-Lincoln focus our attention on marine diatomaceous sediments that show Lincoln, Nebraska 68588-0340 little, if any, glacial influence, and lack reworked Paleozoic and Mesozoic palynomorphs. We report on a palynomorph assemblage recovered from a A clearer view of mid-Cenozoic antarctic terrestrial vegeta- lower Miocene diatomite that was deposited in the south central tion is available through the recovery of rich palynofloras in Ross Embayment at a time of minimal glaciation (Harwood et al. lower Miocene diatomite clasts from RISP Site J/9 sediment cores 1989). We believe that most, if not all, of the pollen was delivered 1992 REVIEW.
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