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Paleobotany |
2Department of Environmental and Plant Biology, Ohio University, Athens, Ohio 45701 USA; 3Department of Ecology and Evolutionary Biology and Natural History Museum and Biodiversity Research Center, University of Kansas, Lawrence, Kansas 66045-7534 USA
Received for publication March 2, 2001. Accepted for publication August 2, 2001.
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ABSTRACT |
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2.5 cm in diameter and include up to 45 frond bases. Stems range from 5 to 8 mm in diameter with a xylem cylinder of 8–9 xylem segments separated by leaf gaps. Phyllotaxy is variable, approaching 2/5 or 3/8, with 10–12 frond traces in the cortex. Stipes have parenchymatous, stipular wings that are usually devoid of sclerenchyma; fronds are pinnate with alternate-subopposite pinnatifid pinnules. Although the absence of fertile pinnules and sporangia precludes assigning the fossils to a living genus, this species demonstrates that ferns with stelar architecture and histology similar to Osmunda subgenus Osmundastrum (Osmundaceae) were present in the Southern Hemisphere by the mid-Triassic.
Key Words: anatomy • Antarctica • Ashicaulis • ferns • Osmundaceae • stelar architecture • stems • Triassic
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSYSTEMATICSDESCRIPTIONDISCUSSIONLITERATURE CITED |
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, and papers therein). Fossil evidence demonstrates that, as part of the Gondwana supercontinent, Antarctic vegetation was important in the evolution of early vascular plants (Edwards, 1989
), and in the subsequent phylogenetic radiation of numerous groups, including bryophytes (Smoot and Taylor, 1986
), ferns (Tidwell and Ash, 1994
; Phipps et al., 1998, 2000
), cycads (Smoot, Taylor, and Delevoryas, 1985
), conifers (Stockey, 1989
; Yao, Taylor, and Taylor, 1997
; Axsmith, Taylor, and Taylor, 1998
), and several extinct seed plant groups (Pigg and Taylor, 1989
; Meyer-Berthaud, Taylor, and Taylor, 1993
; Taylor, Del Fueyo, and Taylor, 1994
; Yao, Taylor, and Taylor, 1995
; Doyle, 1996
; Axsmith et al., 2000
).
Specimens from permineralized peat deposits of Antarctica reveal a greater anatomical diversity of Triassic ferns than has been found elsewhere in the world (Millay and Taylor, 1990
; Delevoryas, Taylor, and Taylor, 1992
; Phipps et al., 1998, 2000
), and this suggests that Gondwana may have been a center for many early filicalean phylogenetic radiations (Tidwell and Ash, 1994
). The Osmundaceae is well represented in late Paleozoic and Mesozoic deposits of Gondwana (Tidwell and Ash, 1994
) by both permineralized trunks (Miller, 1967, 1971
; Gould, 1970
; Cantrill, 1997
) and compression specimens (Vakhrameev, 1991
; Phipps et al., 1998
). Among the more than 40 species of permineralized osmundaceous trunks that currently are assigned to Osmundacaulis, Millerocaulis, and Ashicaulis of the subfamily Osmundoideae (Tidwell and Ash, 1994
; Cantrill, 1997
; Stockey and Smith, 2000
), 34 are from the Gondwana realm, but only Ashicaulis beardmorensis (Schopf) Tidwell and A. livingstonensis Cantrill are from Antarctica. We suspect that the small number of currently known Antarctic species results primarily from the relative inaccessibility of the fossils and because long-term concentrated efforts to characterize the Antarctic vegetation are only beginning to bear fruit. The anatomically preserved floras from Permian and Triassic permineralized peats of the central Transantarctic Mountains (Taylor, 1994, and papers cited therein) have been particularly important in this regard.
The current investigation adds to the known floristic diversity of a Middle Triassic permineralized peat from the Transantarctic Mountains by describing small osmundaceous trunks and frond remains as Ashicaulis woolfei n. sp. This taxon displays essentially modern features of plant form, stem anatomy, and frond structure, thus providing additional evidence for the early evolution of derived Southern Hemisphere Osmundaceae. It also adds to our understanding of stelar architecture and evolution among filicalean ferns and provides further evidence for the fern understory in Triassic communities of high latitude Gondwana in which Dicroidium seed ferns were the apparent canopy vegetation.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSYSTEMATICSDESCRIPTIONDISCUSSIONLITERATURE CITED |
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). The material was collected from the Middle Triassic Fremouw Formation near Fremouw Peak, in the Beardmore Glacier area of the Transantarctic Mountains, Antarctica (Smoot, Taylor, and Delevoryas, 1985
). Specimens were identified on outer surfaces of the chert blocks and on faces of 2 cm thick slabs. Slabs containing ten trunk specimens and numerous frond segments were chosen for detailed study after serial sectioning by a modification of the cellulose acetate peel technique for silicified permineralizations (Basinger and Rothwell, 1977
). Peels for microscopic examination and image capture were mounted on standard microscope slides with Eukitt mounting medium (O. Kinder, Freiburg, Germany). Images were captured with a Microlumina digital scanning camera (Leaf Systems, Bedford, Massachusetts, USA) and processed with Adobe Photoshop for Windows NT version 5.5. All chert blocks, slabs, peels, and microscope slides are housed in the the Division of Paleobotany, Natural History Museum and Biodiversity Research Center, University of Kansas. |
SYSTEMATICS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSYSTEMATICSDESCRIPTIONDISCUSSIONLITERATURE CITED |
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Family
Osmundaceae L.
Subfamily
Osmundoideae
Genus
Ashicaulis Tidwell
Species
Ashicaulis woolfei sp. nov.
Specific diagnosis
Morphospecies of permineralized osmundaceous plants, trunk 2.5 cm in diameter with alternate-subopposite, pinnate pinnatifid fronds. Cortex with 10–12 frond traces in cross section and 30–45 frond bases surrounding stem. Stems 5–8 mm wide; pith heterogeneous with sclerenchyma and interspersed parenchyma; xylem cylinder 1.2–1.5 mm wide, 0.2–0.3 mm (7–9 tracheids) thick with 8–9 xylem segments separated by leaf gaps. Inner cortex parenchymatous, 0.5–0.6 mm thick; outer cortex 0.7–1.0 mm thick, of homogeneous sclerenchyma fibers. Frond divergence varying from 2/5 to 3/8 helix. Frond trace C-shaped with one endarch protoxylem strand proximally, dividing in two in outer cortex, developing enrolled margins distally. Sclerotic ring homogeneous, extending into center of rachis from adaxial surface and forming T-shaped central bundle. Sclerotic nests absent from parenchyma inside sclerotic ring. One dark (secretory?) patch occasionally present adaxial to trace within sclerotic ring, dividing distally to form two lateral patches on convex side of xylem trace. Stipular wings typically uniformly parenchymatous, infrequently with 1–3 small sclerenchyma patches toward periphery of trunk. Mesophyll of pinnules homogeneously spongy. Sporangia and spores unknown.
Holotype hic designatus
Trunk specimen with attached stipe bases and adventitious roots, including slabs, slides and peels from chert block 12 825. Figs. 1, 3, 4, 7–10, 12.
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).
Stratigraphic position and age
Fremouw Formation, Beacon Supergroup; early Middle Triassic.
Etymology
The epithet woolfei is proposed in memory of Dr. Ken Woolfe, an extraordinary field geologist who generously shared information and ideas about the stratigraphy and sedimentation of the Transantarctic Mountains.
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DESCRIPTION |
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The stele consists of a ring of eight to nine xylem segments and diverging leaf traces that are separated by parenchymatous gaps (Figs. 7, 9, 10) and a continuous cylinder of phloem (Figs. 8–10). Several of the xylem segments are round-oval in cross section, comprising only metaxylem tracheids (Figs. 9, 10); others have a centrally located, mesarch protoxylem strand (Fig. 10, upper left). Still others are U-shaped with endarch protoxylem (Fig. 9). Metaxylem tracheids are angular in cross section (Figs. 9–11), 26–74 µm in diameter and display uni-multiseriate scalariform pitting on all walls (Fig. 5). Phloem consists of small, axially elongated cells that lie adjacent to the xylem segments (Figs. 5, 10, 11). In comparison to living osmundaceous stems (Hewitson, 1962
), the latter represent sieve cells. A prominent zone of isodiametric (phloem?) parenchyma cells of variable diameters characterizes the phloem outside the sieve cells (Figs. 5 [at far left], 8–10). Endodermis has not been identified in most stems, but is present as an indistinct zone at the outer margin of the phloem (Fig. 8, arrows) that is comparable to the endodermis of living osmundaceous species (Hewitson, 1962
).
The cortex is differentiated into an inner parenchymatous region 0.5–0.6 mm in radial thickness and an outer sclerenchymatous zone that is 0.6–1.0 mm thick (Figs. 1, 12). Parenchyma cells of the inner cortex are typically shrunken together and incompletely preserved (Figs. 1, 2, 9, 10). The most well-preserved cortical parenchyma cells are thin-walled, 40–60 µm in diameter, and typically devoid of internal contents (Fig. 9). No sclereids are preserved within the inner cortex. The outer sclerenchymatous cortex is relatively homogeneous, but the cell walls are somewhat thicker toward the periphery and at positions where adventitious roots diverge from the stem (Figs. 1, 12). Cortical sclerenchyma fibers are round to oval, measuring 24–52 µm in cross section.
Vascular architecture and leaf trace divergence
As in living osmundaceous species (Hewitson, 1962
), short internodes and helically arranged frond traces produce a relatively consistent and repeating configuration of the cauline xylem and leaf traces in A. woolfei (Figs. 1, 9). This is illustrated by a stem with 3/8 phyllotaxis. Frond traces and gaps in one stem are labeled in Fig. 9 to help illustrate the pattern. Because there is no continuous system of cauline protoxylem, some xylem segments display only metaxylem (Figs. 9, 10). Mesarch xylem segments represent the most proximal levels at which incipient frond traces can be identified (i.e., bundles 1–3 of Fig. 9). Progressing distally from the level where protoxylem is lacking (i.e., bundle 0 of Fig. 9), a mesarch xylem strand first becomes evident (bundles 1–3 of Fig. 9) and then the centripetal metaxylem is replaced with parenchyma to produce an endarch frond trace (bundles 4–5 of Fig. 9). At more distal levels the xylem of the frond trace expands radially (bundle 6 of Fig. 9), and then separates to form a frond trace and two cauline bundles that flank a leaf gap (i.e., traces and gaps 7–8 of Fig. 9). One of the bundles (e.g., bundle 0 of Fig. 9) develops a protoxylem strand distally (i.e., bundle 1), and this reveals the position where a more distal leaf trace will be produced. The other bundle (e.g., to the left of trace 6 in Fig. 9) fuses with an adjacent bundle at more distal levels, thus narrowing (gaps at arrows 10–11 of Fig. 9), and finally closing the gap (arrow 12 of Fig. 9) produced by trace divergence at a more proximal level.
Leaf traces diverge from the stele at longitudinal angles of 16–20° (Fig. 2). In cross section they are C-shaped at the level of divergence from the stele, with a single endarch protoxylem strand (trace 6 of Fig. 9). Extending distally, the traces continue through the inner parenchymatous cortex and outer sclerenchymatous cortex (Fig. 1). Within the sclerenchymatous cortex, each trace is surrounded by a zone of parenchyma cells that is continuous with the inner cortex and is generally incompletely preserved. Just below the levels where stipe bases diverge from the cortex, the protoxylem divides to form two strands. Protoxylem strand number remains constant at more distal levels (Fig. 9).
Stipe bases
As is characteristic of osmundaceous trunks, the stipe bases of A. woolfei are stipulate and tightly packed at the stem periphery (Figs. 1, 3, 4). There are 35–45 stipe bases surrounding the stem, and the zone of stipe bases is often thicker on one side of the specimen than the other (Fig. 1). In cross section stipes are 3.8–6.5 mm wide and 1.3–2.6 mm thick, increasing in size toward the periphery of the trunk (i.e., distally). Stipes consist of a C-shaped xylem strand that is surrounded by a narrow zone of tightly packed thin-walled cells. To the outside of this region is a zone of loosely packed parenchyma devoid of sclereids, an oval-elliptical sclerenchymatous sheath, and an outer cortex of thin-walled cells (Figs. 1, 3, 4). Frond traces become somewhat more deeply C-shaped distally, with the margins of the most peripheral traces enrolling slightly (Fig. 1 at top).
Some stipes show an indistinct epidermis, but in most specimens this tissue is hard to identify (Figs. 3, 4). The thin-walled cells accompanying the xylem bundle within the sclerenchyma sheath usually lack contents, but a few stipes have a dark patch adaxial to the bundle (Fig. 11, top center). At more distal levels some stipes possess two dark patches on the concave face of the bundle. When present, this feature is most easily recognized in stipes that are near the periphery of the trunk (Fig. 1, top). The sclerenchymatous sheath is homogeneous, with the fibers becoming smaller in diameter at the periphery (Figs. 3, 4). The outer parenchymatous cortex surrounds the sclerenchyma sheath and is expanded into the stipular wings laterally (Figs. 1, 3, 4). In most stipes the cells of this zone lack contents (Fig. 4), but a few toward the periphery of the zone display one or more small dark patches near the lateral margins of the stipular wings (Fig. 3). These probably represent sclerenchyma patches that are comparable to those of most other species of Ashicaulis and other osmundaceous ferns; they are known to be taxonomically important characters among living and fossil species of the family (Hewitson, 1962
; Tidwell and Ash, 1994
).
Roots
Adventitious root traces typically diverge from the stele in association with frond trace production in all osmundaceous species. Just proximal to the level where a frond trace separates from the stele, a root trace diverges from the position where the frond trace distally separates from one of the cauline xylem segments (Fig. 12, lower left). In a small percentage of nodes two traces are produced, one from each side of the diverging frond trace (Fig. 12, arrows). Root traces are round-oval in cross section (Fig. 9), and when they diverge, no gap in the xylem is produced (Fig. 2, upper right). At this level protoxylem is not apparent in the root traces (Fig. 9); however, the roots become distinctly diarch at more distal levels (Fig. 6). Each trace extends through the cortex, diverges in a more-or-less transverse plane (Figs. 1, 12), and extends among the stipe bases to the outside of the trunk.
Beyond the periphery of the trunk, most roots are characterized by diarch primary xylem surrounded by an empty space and several layers of relatively sclerenchymatous cortical cells with moderately thickened walls (Fig. 6). In the best preserved roots the zone between the xylem and sclerotic cortex is filled with thin-walled cells that represent phloem and a thin zone of parenchymatous cortex.
Fronds
Numerous frond segments occur in the same chert blocks as the trunk specimens and can be identified as belonging to A. woolfei by the shape of the xylem bundle and by characteristic histological features (Figs. 13–18). Stipes are up to 3 mm wide and consist of a somewhat involuted C-shaped vascular bundle, sclerotic sheath, and outer parenchymatous cortex (Figs. 13–15). The parenchymatous cortical tissue is typically thinner than in the more proximally attached stipes, but small stipular wings are still visible (Fig. 14, at arrows). At these and more distal levels the sclerotic sheath of the stipe/rachis extends to the center of the axis from the adaxial side and occupies the area within the xylem of the vascular bundle (Figs. 13–15). This central sclerenchyma strand is distinctly T-shaped in cross section. Primary pinna traces diverge in alternate arrangement at proximal levels (Fig. 15), becoming subopposite distally and nearly opposite near the tip of the frond (Fig. 13, center). The primary pinna bundle forms a slightly adaxially concave band at the level of divergence (Fig. 15, left) and is more clearly C-shaped distally (Fig. 16).
Primary pinnae are smaller (0.9 mm wide excluding pinnatifid extensions) than all but the most distal levels of the rachis, and the sclerotic sheath does not extend into the center of the axis as clearly as it does in the rachis (Fig. 16). The parenchymatous outer cortex displays distinct air spaces, with shrunken dark contents in many of the mesophyll cells. (Figs. 16, 17). The epidermis is clearly differentiated as a uniseriate layer of closely packed cells, 22–60 µm in diameter (Figs. 16–18). Some primary pinnae are oval in cross section, but most have lateral extensions that demonstrate a pinnatifid structure (Fig. 16). Other, somewhat smaller frond segments show a midvein surrounded by a sclerenchyma sheath (Fig. 13, top) and prominent pinnule laminae. These features indicate that the frond of A. woolfei has a pinnate pinnatifid architecture.
Pinnules in various planes of section are dispersed among the other frond parts in several chert blocks (Fig. 13). Pinnules typically show a midvein from which laminar tissue extends from one or both sides (Fig. 13, top) and smaller veins that lack a sclerenchymatous bundle sheath (Fig. 18, arrows). The smaller pinnule veins consist of a few tracheids surrounded by a lacuna and mesophyll (Fig. 18, arrows). Pinnule laminae are 70–100 µm thick in cross section, increasing in thickness around veins (Fig. 18). The mesophyll is spongy throughout and consists of parenchyma cells and large air spaces like those beneath the epidermis and in the pinnatifid wings of the primary pinnae (Figs. 16, 18). No palisade mesophyll has been found. Many parenchyma cells contain dark material that suggests shrunken cellular contents (Figs. 13, 18).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSYSTEMATICSDESCRIPTIONDISCUSSIONLITERATURE CITED |
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; Tidwell and Ash, 1994
). Likewise, among genera of Osmundoideae the combination of all these characters is found only in Osmunda and Ashicaulis (Table 1).
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; Hewitson, 1962
; Miller, 1967, 1971
), but sclerenchyma distribution has proven to be less useful for revealing systematic relationships. Although there is some similarity in the sclerenchyma of related living species (Hewitson, 1962
), subgenera, genera, and subfamilies are based primarily on distinctive combinations of other characters (Table 1; Hewitson, 1962
; Miller, 1967, 1971
; Tidwell, 1986, 1994
; Tidwell and Ash, 1994
). Living genera and subgenera of Osmundaceae are characterized primarily by a combination of frond structure, fertile pinnule structure, sporangium structure, and mode of frond trace production (Table 1; Hewitson, 1962
). Unfortunately, some of these characters are not known for most permineralized specimens of extinct species (Table 1). As a result, the fossils have been assigned to one of several morphogenera (Table 1; Tidwell and Ash, 1994
), unless they are either (1) Cenozoic and have characters that compare closely to a living genus (Miller, 1967, 1971
) or (2) display diagnostic characters of a living species (Serbet and Rothwell, 1999
).
The generically diagnostic fertile pinnule and sporangial characters of living osmundaceous species are not known for A. woolfei. Although we suspect that the species is actually an Osmunda, this cannot be demonstrated unequivocally from the data currently available (Table 1). As a result we assign this species to the morphogenus (Greuter et al., 2000
) Ashicaulis Tidwell on the basis of the dictyoxylic stele with consistent leaf gaps and mesarch-endarch protoxylem, small stem size, absence of medullary bundles, and frond traces that diverge from a single cauline bundle with one protoxylem strand. As reviewed by Tidwell and Ash (1994)
, Cantrill (1997)
, and Stockey and Smith (2000)
, other morphogenera of the Osmundoideae (i.e., Aurealicaulis, Millerocaulis, Osmundacaulis, and Palaeosmunda) do not display this combination of characters (Table 1).
Including A. woolfei, there are currently 24 recognized species of Ashicaulis, and these have numerous characters that intergrade from species to species (Cantrill, 1997
). Individual species are best differentiated by unique combinations of several characters (Tidwell, 1992
), many of which are tabulated by Cantrill (1997)
for 21 of the species. Among these, there are ten species that potentially could be compared to A. woolfei on the basis of overall morphology of the trunk, numbers of cauline meristeles, size of the stem and stele, and/or distribution of sclerenchyma in the stem and stipe (Table 2).
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). Ashicaulis beardmorensis also has a larger number of cauline xylem segments and more leaf traces in the cortex than A. woolfei (Table 2). Ashicaulis spinksii and A. swanensis are both from Australian rocks of unknown age (Tidwell, Munzing, and Banks, 1991
). Ashicaulis spinksii and A. woolfei share many features of sclerenchyma tissue distribution and frond trace numbers in the cortex, but A. spinksii has a homogeneous pith and frond trace protoxylem that divides in the rachis, whereas A. woolfei has a heterogeneous pith and frond trace protoxylem that divides in the outer cortex (Table 2). Ashicaulis spinksii also has uniformly parenchymatous stipular wings, whereas a small number of stipular wings near the outer margin of the trunk in A. woolfei contain a few small patches of sclerenchyma.
Ashicaulis swanensis is characterized by two sclerotic masses on the concave side of the rachis bundle; these appear similar to the two dark patches occasionally present in A. woolfei. However, A. swanensis can be distinguished by a smaller homogenous pith, fewer frond traces in the cortex, and by the occurrence of one large mass of sclerenchyma in each stipular wing (Table 2). Ashicaulis guptai (Sharma) Tidwell, from the Jurassic of India, is the only other species in which sclerenchyma is absent from the stipular wings (Cantrill, 1997
). This species is easily distinguished from A. woolfei by much larger stem size, more cauline xylem bundles, homogeneous pith, and fewer frond traces in the cortex of the former (Table 2). Ashicaulis hebeiensis (Wang) Tidwell from the Jurassic of China has about the same stem diameter and xylem thickness as A. woolfei, and there are a few small sclerenchyma patches in the stipular wings (Cantrill, 1997
). However, a larger stele, mixed pith, and sclerenchyma lining the trace concavity distinguish this species from A. woolfei (Table 2). Other species of Ashicaulis differ from A. woolfei in even larger numbers of characters (Table 2; Tables 1 and 2 of Cantrill, 1997
).
Ashicaulis woolfei is the first permineralized species of extinct osmundaceous ferns in which frond structure has been determined. Almost all permineralized osmundaceous species occur as isolated trunks that preserve only stipe bases. The holotype of Osmundicaulis janii Tidwell and Pigg has croziers of dissected fronds at the stem apex, but the overall frond structure was not determined for that species (Tidwell and Pigg, 1993
). The occurrence of A. woolfei in permineralized peat makes it possible to examine distal frond parts that document the pinnate pinnatifid frond structure. This frond type characterizes living species of Osmunda, subgenus Osmundastrum (Hewitson, 1962
). In contrast, all other living osmundaceous species have either pinnate (i.e., Osmunda subgenus Plenasium) or bipinnate (i.e., Osmunda subgenus Osmunda, Todea, and Leptopteris) frond architecture (Hewitson, 1962
; Table 1).
Fertile pinnule and sporangial characters needed to assign it to a living genus in the Osmundaceae have yet to be discovered for Ashicaulis woolfei. This species also has sclerenchyma distribution that differs from all extant species (Hewitson, 1962
). Nevertheless, A. woolfei does display several characters that are comparable to some living species and that suggest possible phylogenetic relationships. Ashicaulis woolfei shares homogeneous cortical sclerenchyma with species of Osmunda and other fossil genera in the family, rather than the heterogeneous cortical sclerenchyma that characterizes species of Todea and Leptopteris (Hewitson, 1962
; Table 1). As noted above, the pinnate pinnatifid fronds of A. woolfei are similar to Osmunda subgenus Osmundastrum, but differ from the pinnate fronds of Osmunda subgenus Plenasium and the bipinnate fronds of Osmunda subgenus Osmunda. Subgenus Plenasium also differs from A. woolfei by having frond traces that originate from two adjacent cauline bundles (Hewitson, 1962
; Table 1).
Although the homogeneous sclerotic ring in the stipes of A. woolfei differs from the heterogeneous ring that typifies living species of Osmundastrum (Hewitson, 1962
; Table 2), other features discussed above suggest that A. woolfei may be most closely related to this subgenus (Table 1). In this regard it is perhaps significant that species of Osmundastrum have the longest geologic history of all living ferns, including the most ancient living species, Osmunda cinnamomea from the Late Cretaceous onward (Maastrichtian Stage; Serbet and Rothwell, 1997
). Perhaps even more significant is that the oldest extinct species of the genus Osmunda, O. claytoniites Phipps et al., occurs in only slightly younger Upper Triassic rocks from southern Victoria Land, Transantarctic Mountains, Antarctica (Phipps et al., 1998
), demonstrating that the subgenus Osmundastrum was well established in Gondwana by Triassic time.
The branching rhizomes of O. claytoniites are narrower than the unbranched rhizomes of A. woolfei, but both species have pinnate pinnatifid fronds. More detailed comparisons of these species are difficult because the compression specimens of O. claytoniites lack internal anatomical characters, whereas pinnule shape, fertile pinnules and sporangia have not been discovered for A. woolfei. Nevertheless, both species probably grew as understory vegetation in forests of Dicroidium seed ferns (Taylor and Taylor, 1989
; Phipps et al., 1998
), and this further strengthens our understanding that ferns in general (Millay and Taylor, 1990
), and Osmundaceae in particular, were important components of high-latitude, Southern Hemisphere ground cover by early Triassic time.
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FOOTNOTES |
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4 Author for correspondence (Tel: 740-593-1129; FAX: 740-593-1130; rothwell{at}ohio.edu
).
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSYSTEMATICSDESCRIPTIONDISCUSSIONLITERATURE CITED |
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Axsmith B. J. T. N. Taylor E. L. Taylor 1998 Anatomically preserved leaves of the conifer Notophytum krauselii (Podocarpaceae) from the Triassic of Antarctica. American Journal of Botany 85: 704-713[Abstract]
Barrett P. J. D. H. Elliott 1973 Reconnaissance geologic map of the Buckley Island Quadrangle, Transantarctic Mountains, Antarctica. U.S. Geological Survey Antarctic Geologic Map A-3
Basinger J. F. G. W. Rothwell 1977 Anatomically preserved plants from the middle Eocene (Allenby Formation) of British Columbia. Canadian Journal of Botany 55: 1984-1990
Cantrill D. J. 1997 The pteridophyte Ashicaulis livingstonensis (Osmundaceae) from the Upper Cretaceous of Williams Point, Livingston Island, Antarctica. New Zealand Journal of Geology and Geophysics 40: 315-323
Delevoryas T. T. N. Taylor E. L. Taylor 1992 A marattialean fern from the Triassic of Antarctica. Review of Palaeobotany and Palynology 74: 101-107
Doyle J. E. 1996 Seed plant phylogeny and the relationships of Gnetales. International Journal of Plant Sciences 157: (6 Supplement) S30-S39
Edwards D. 1989 Silurian-Devonian paleobotany: problems, progress and potential. In T. N. Taylor and E. L. Taylor [eds.], Antarctic paleobiology: its role in the reconstruction of Gondwana, 89–101. Springer-Verlag, New York, New York, USA
Gould R. E. 1970 Palaeosmunda, a new genus of siphonostelic osmundaceous trunks from the Upper Permian of Queensland. Palaeontology 13: 10-28
Greuter W. et al 2000 International code of botanical nomenclature. Koeltz Scientific Books, Königstein, Germany
Hewitson W. 1962 Comparative morphology of the Osmundaceae. Annals of the Missouri Botanical Garden 49: 57-93
Kidston R. D. T. Gwynne-Vaughan 1907 On the fossil Osmundaceae, Part I. Transactions of the Royal Society of Edinburgh 45: (27)759-780
Meyer-Berthaud B. T. N. Taylor E. L. Taylor 1993 Petrified stems bearing Dicroidium leaves from the Triassic of Antarctica. Palaeontology 36: 337-356[ISI]
Millay M. A. T. N. Taylor 1990 New fern stems from the Triassic of Antarctica. Review of Palaeobotany and Palynology 62: 41-64
Miller C. N. 1967 Evolution of the fern genus Osmunda. Contributions from the Museum of Paleontology, University of Michigan 21: 139-203
Miller C. N. 1971 Evolution of the fern family Osmundaceae based on anatomical studies. Contributions from the Museum of Paleontology, University of Michigan 23: 105-169
Phipps C. J. B. J. Axsmith T. N. Taylor E. L. Taylor 2000 Gleichenipteris antarcticus gen. et sp. nov. from the Triassic of Antarctica. Review of Palaeobotany and Palynology 108: 75-83
Phipps C. J. T. N. Taylor E. L. Taylor N. R. Cúneo L. D. Boucher X. Yao 1998 Osmunda (Osmundaceae) from the Triassic of Antarctica: an example of evolutionary stasis. American Journal of Botany 85: 888-895[Abstract]
Pigg K. B. T. N. Taylor 1989 Permineralized Glossopteris and Dicroidium from Antarctica. In T. N. Taylor and E. L. Taylor [eds.], Antarctic paleobiology: its role in the reconstruction of Gondwana, 164–172. Springer-Verlag, New York, New York, USA
Schopf J. M. 1978 An unusual osmundaceous specimen from Antarctica. Canadian Journal of Botany 56: 3083-3095[CrossRef]
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