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Paleobotany |
Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5E2, Canada; Biology Department, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3280, USA; Nexen Inc., 2900, 240-4th Ave. SW, Calgary, Alberta, T2P 5C1, Canada
Received for publication September 4, 2001. Accepted for publication December 20, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Arctic • Bathurst Island, Canada • evolution • fossil • paleobotany • plant • Silurian
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Wellman and Gray, 2000
) and recent molecular clock estimates suggest a latest Precambrian or Cambrian origin of the plant lineage (Heckman et al., 2001
), the earliest land plant macrofossils are known only from the Silurian (Fig. 1). Remains of simply organized rhyniophyte-like plants are first recorded in the Wenlock (late Early Silurian) and become more common in the latest Silurian (Pridoli) and early Devonian (Edwards, Feehan, and Smith, 1983
). Among these early plant fossils preservation of internal anatomy is rare; thus, taxonomic characterization is largely based on general morphology, and, when available, on characteristics of the spores and cuticle. Most reported Silurian plant fossils consist of naked unbranched or isotomously branched axes a few millimeters to centimeters in length, some terminating in solitary sporangia. This simple plan has traditionally typified the lowest grades of land plant evolution (Edwards, Feehan, and Smith, 1983
; Edwards, 1996
; Edwards, Wellman, and Axe, 1998
). To date, reports of Silurian plants of the Laurussian paleocontinent have been limited to such rhyniophytoids.
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; Cai et al., 1996
), requires further investigation as to both affinity and age. Questions have also arisen about the dating of plant-bearing beds in Australia assigned a Ludlow age (Garratt, 1978
; Garratt et al., 1984
; Tims and Chambers, 1984
; Rickards, 2000
). In addition to the lycopsid Baragwanathia, the assemblage includes Salopella, Hedeia, and some as yet undescribed zosterophylls.
Basinger, Kotyk, and Gensel (1996)
briefly reported land plants from Silurian to Early Devonian sediments on Bathurst Island, Canadian Arctic Archipelago. The Ludlovian fossil figured therein was subsequently discussed by Edwards and Wellman (2001)
in a survey of Silurian and basal Devonian plant assemblages. Further collection from these rocks has revealed at least six additional types of plant fossils, many of which show morphological complexity comparable to that more commonly seen in the Lower Devonian. Bathurst Island, although remote from most other records of Silurian plants from Laurussia, occupied low latitudes of this paleocontinent in the Late Silurian. The Bathurst Island beds are distinguished from the Chinese and Australian sediments described above in that they are stratigraphically well understood and are well constrained chronologically. They thus provide the earliest unequivocal evidence of significant morphological complexity of undoubted plant macrofossils in the Silurian Period.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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) (Fig. 2). These beds represent a basinal, syntectonic succession of Ludlow to Lower Devonian age derived from coeval intraplate or plate margin deformed belts to the southeast. Interestingly, this is one of the few places on Earth where rapid transition from submarine slope facies to a fully continental facies succession is well exposed (de Freitas, Harrison, and Thorsteinsson, 1993
; de Freitas and Mayr, 1993
). Two main deformation events are recognized: one during the Ludlow and the other in the Pragian. Thus, the most remarkable suites of plant remains on Bathurst Island are of these ages. Plant remains are found in strata associated with periods of diastrophism, when rising highlands to the east shed appreciable quantities of siliciclastic material into the marine basin. Rapid burial of plants during mass flow events in an environment unfavorable for infaunal activity, minimal tectonic cleavage, and exceptional exposure of beds are factors contributing to the recovery of these early land plants.
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, denotes a late Ludlow to Pridoli age (Nowlan, 1996
). Two types of graptolites found stratigraphically upsection at US385, Monograptus formosus and Pseudomonoclimacis richardsonensis, indicate a Late Ludlow or possibly Early Pridoli age (Jackson, Lenz, and Pedder, 1978
; A. C. Lenz, University of Western Ontario, personal communication). Brachiopods of a new species of the genus Shaleria from US383 are most similar to Shaleria gilpeni from latest Ludlow to Lochkovian rocks of Nova Scotia (Jin, 1997
). A late Ludlow (Ludfordian) age is consistent with both the fossil invertebrates and the stratigraphic relationships as described by de Freitas, Harrison, and Thorsteinsson (1993)
. Extensive plant collections from Lower Devonian (Pragian) rocks on Bathurst Island are well separated geographically and stratigraphically from these Silurian localities. In addition, the structural context is not complex, the land surface is of low relief and stream cuts are shallow, and beds upstream from the Silurian localities are increasingly older, so that mixing of younger material into the Silurian localities is precluded. Furthermore, all the plant localities occur within a continuous, measured section spanning the Lower Silurian through Lower Devonian, with diagnostic invertebrate fossils in stratigraphic succession.
Small pieces of rock overlaying the fossil remains were chipped away from some of the specimens using the dégagement technique (Fairon-Demaret, Hilton, and Berry, 1999
). A sporangial fragment taken from Macivera gracilis was oxidized in Schulze's solution for approximately 4 h and treated with 10% NH3OH for 1 h. It was then dissected in distilled water on a glass slide and mounted in CMCP-9 mounting medium (Polysciences Inc., Warrington, Pennsylvania, USA) and viewed with a compound microscope.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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cf. Bathurstia sp
Specimens US600-6791 and US600-6788 are poorly preserved spikes up to 45 mm long and 1.0–1.1 cm wide (Fig. 4). The width of the strobilar axis is 5 mm. Vegetative axes were not recovered. Sporangia are densely borne in two rows on opposite sides of the axis. Sporangial stalks were not seen and are probably very short or absent. The sporangia appear elongate in the direction perpendicular to the axis. On US600-6791 the sporangia are 1.7–2.2 mm (mean = 1.9 mm) wide and the length varies from 3.5–4.5 mm near the base of the spike to 2.8–3.0 mm near the apex (Fig. 4).
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Although incomplete and poorly preserved, these strobili are similar in their overall morphology to those reported for the vascular plant genera Bathurstia Hueber, Barinophyton White, and Protobarinophyton Ananiev. Bathurstia is known solely from the Early Devonian (Pragian) of Bathurst Island (Kotyk and Basinger, 2000
), while species of Barinophyton are Late Devonian in age (see Brauer, 1980
for a review of this genus), and those of Protobarinophyton, from Early Devonian (Ananiev, 1955
) to Early Carboniferous (Scheckler, 1984
). The Silurian specimens described above have a more compact arrangement of sporangia than is known from Barinophyton. Also, no evidence for long clasping stalks typical of Barinophyton or Protobarinophyton (Brauer, 1981
) was noted, but the poor preservation prevents conclusion that they did not exist. Affinity to any of these genera can not be completely ruled out; however, the compact arrangement of sporangia and apparent lack of stalks make it most likely that all three specimens belong to the genus Bathurstia.
Zosterophyllum sp. (subgenus Zosterophyllum)
US384-8137 shows a fragment of an axis measuring 1.0 mm wide bearing a cluster of at least 27 reniform sporangia (Fig. 6). A second smaller fertile fragment with approximately five sporangia can be observed near the apex of the larger fragment. Sporangia are arranged either helically or randomly and terminate narrow stalks diverging from the main axis at an acute angle. Stalks measure 1.0–3.0 mm (mean = 1.6 mm) long and 0.3–0.5 mm wide, flaring slightly at the junction with the sporangium. The sporangia have moderately developed basal lobes and are 1.1–1.7 mm (mean = 1.3 mm) long and 1.9–2.1 mm (mean = 2.0 mm) wide. Distal dehiscence zones are visible on some sporangia. The distal terminus of the larger axis appears as a number of thin, short stalks where the sporangia have presumably become detached.
Although the original diagnosis of Zosterophyllum is "outdated and based predominantly on plesiomorphic characters" (Zhu and Kenrick, 1999
, p. 116), the morphology of this specimen coincides with what has come to be understood as Zosterophyllum, in its possession of lateral sporangia borne on stalks inserted on an isotomously or pseudomonopodially branching naked axis. The helically arranged sporangia indicate affinity of US384-8137 with the subgenus Zosterophyllum Hueber. It is immediately distinguishable from most members of this subgenus, except for Z. bifurcatum Li & Cai, Z. yunnanicum Hsü, Z. deciduum Gerrienne, Z. rhenanum Kräusel & Weyland, and a new species of Zosterophyllum from Pragian sediments of Bathurst Island, in its possession of distinctly reniform rather than elliptical or fan-shaped sporangia. Zosterophyllum bifurcatum, although it has sporangia of similar shape, differs in bearing its sporangia on extremely short stalks from a relatively wide axis (Li and Cai, 1977
).
Specimen US384-8137 differs from Zosterophyllum yunnanicum in that the junction between the stalk and sporangium is more distinct. Furthermore, the sporangia of Z. yunnanicum are circular, elliptical, or reniform, with poorly developed lobes (Cai and Schweitzer, 1983
). Specimen US384-8137 has reniform sporangia with moderately well-developed lobes.
While bearing sporangia and stalks of similar shape, the Silurian specimen differs from Zosterophyllum rhenanum in its much smaller dimensions, less dense or orderly arrangement of sporangia into four rows, and more narrow sporangial dehiscence zone (Schweitzer, 1979
).
US384-8137 bears smaller sporangia than most species within the subgenus Zosterophyllum except for Z. deciduum (Gerrienne, 1988
). Zosterophyllum deciduum exhibits deciduous sporangia, a condition that could also occur in this specimen; however, the sporangia of that species are commonly elliptical rather than reniform, and the stalks tend to be shorter and are less densely clustered than the Bathurst specimen.
Except for its small size and smaller sporangial width-to-height ratio, the shape and arrangement of the sporangia and stalks is most similar to one of the new species of Zosterophyllum from the Pragian flora of Bathurst Island (Kotyk, 1998
). Notably, specimens from both horizons apparently had deciduous sporangia. This specimen may represent a diminutive form of the Pragian Bathurst Island species, may be related to Z. deciduum, or may represent a new species altogether. Considering that information on this taxon is based on a single, incomplete specimen, it is best referred to as Zosterophyllum sp.
Aff. Zosterophyllum sp. A (subgenus Platyzosterophyllum)
US688-8152 bears sporangia in at least two diffuse subopposite rows along the entire length of a dichotomously branching axis (Fig. 7). Although no evidence for additional rows of sporangia has been found, they cannot be ruled out. The axis is 2.1 mm wide and shows four isotomous branchings. One isotomy is located near the base of the specimen, and each of the daughter axes divides again at nearly the same level, approximately 1.5 cm distal from that point. At least one of the daughter axes of the left hand branch divides again near the distal end. Sporangia are borne on minute, obliquely inserted stalks about 0.4 mm long and 0.9–1.0 mm wide. The sporangia are discoid, measuring 1.2–1.6 mm long and 1.8–2.2 mm wide.
This plant's extremely short, obliquely inserted sporangial stalks arranged in subopposite rows distinguish it from most of the known zosterophylls except Demersatheca Li & Edwards, Distichophytum Mägdefrau, Serrulacaulis Hueber & Banks, Trichopherophyton, Lyon & Edwards, and one other undescribed species from the Pragian deposits of Bathurst Island.
Demersatheca contigua (Li & Cai) Li & Edwards from Yunnan, China (Li and Edwards, 1996
) differs in having sporangia that are sunken into the surface of the stem and are more regularly and closely arranged on all sides of the axis.
Distichophytum (see next section for more details) has sporangia arranged into an unbranched spike and directed towards one side of the axis. The sporangia of US688-8152, in contrast, are diffuse along the axis, appear to be discoid rather than ovoid, and the fertile region is branched.
Serrulacaulis, known from the late Devonian of New York and the mid-late Devonian of Venezuela, has a similar arrangement of sporangia, yet it differs from US688-8152 in having distinctly reniform sporangia and axes with conspicuous, triangular, prism-shaped emergences along their lengths (Hueber and Banks, 1979
; Berry and Edwards, 1994
).
The genus Trichopherophyton was based on permineralized specimens from the Rhynie Chert of Scotland (Lyon and Edwards, 1991
). Although the sporangial distribution and arrangement along the stem is not known, the sporangia are reported to be broadly oval in face view, sessile, and upright on the axis. Trichopherophyton also bears hair-like emergences on its axes and sporangia. Although no emergences were seen on US688-8152, the preservation of the specimen is not adequate to rule out the possibility that it once possessed fine or delicate emergences. Based on these characteristics, US688-8152 cannot be definitively excluded from Trichopherophyton, largely because of the inadequacy of the information available for both.
The undescribed Pragian Bathurst Island species noted above has nearly sessile, discoid to elliptical sporangia 1.0–2.0 mm long and 1.6–2.2 mm wide borne in two diffuse rows (Kotyk, 1998
). US688-8152 appears to be most similar to this species; however, the specimens from the Pragian flora have not been shown to branch as commonly as this specimen.
The features of US688-8152 ally it most closely with the Pragian Bathurst Island species, which has not yet been formally described. These plants are distinguishable from all other described taxa and merit the erection of a new genus. Because US688-8152 itself is incompletely preserved, we will defer systematic description of this new taxon to publication of the Pragian materials. This specimen is thus tentatively referred to as aff. Zosterophyllum sp. A (subgenus Platyzosterophyllum).
Distichophytum sp
Two fragmentary specimens, US600-8144 and US600-8140, bear sporangia in dense, two-rowed spikes. The axis (known only from US600-8144) measures 1.1–1.2 mm wide near the junction with the spike and is poorly preserved proximally (Fig. 8). No evidence of epidermal ornament is discernable. Spikes are 8.2–10.0 mm long and 2.2–3.8 mm wide, with sporangia upright and oriented towards one side of the stem (Figs. 9 and 10). Each complete spike on specimen US600-8144 is composed of 10–14 sporangia (5–7 per row) (Fig. 9). As a result of dorsiventral orientation of the spikes, sporangial stalks, if present, are not visible. Sporangia are circular or somewhat elliptical in face view, measuring 1.4–2.1 mm (average 1.9 mm) long and 1.2–1.9 mm (average 1.6 mm) wide.
Arrangement of sporangia into a two-rowed dense spike oriented towards one side of the axis indicates that these fossils are a species of Distichophytum Mägdefrau, a genus with a complicated history. In 1933, Dorf described plants bearing two dense rows of round, sessile "appendages" at the end of a naked slender axis from Pragian beds of Wyoming as Bucheria ovata Dorf. Shortly thereafter, Mägdefrau (1938)
established the species Distichophytum mucronatum Mägdefrau from contemporaneous beds in Germany with a description nearly identical to Bucheria Dorf. It seems unlikely that Mägdefrau would have created Distichophytum had he been aware of Bucheria. A complicating factor is that Bucheria is a junior homonym of an extant plant. In 1970, Hueber renamed Bucheria as Rebuchia (Dorf) Hueber, with no mention of the genus Distichophytum. Hueber (1972b)
later redescribed Rebuchia ovata and argued that Distichophytum mucronatum was a synonym of this species. Schweitzer (1979)
argued that Distichophytum mucronatum and Rebuchia ovata belong to the same genus but not the same species. As the name Distichophytum has priority over Rebuchia, whether one species or two are recognized, the appropriate generic name is Distichophytum.
A number of additional species from the Early Devonian have been ascribed to Bucheria or Rebuchia; however, only the type species appears acceptable. Stockmans (1940)
assigned a single Belgian fossil to Bucheria pendula Stockmans, but Gerrienne (1994)
concluded that the specimen is too poorly preserved to be placed in the genus with confidence. Bucheria longa Høeg was described from Spitzbergen by Høeg (1942)
, who, realizing that all specimens ascribed to Bucheria to that date were poorly preserved, recommended use of the name as a form genus for poorly preserved spikes. Høeg (1967)
later transferred Bucheria longa to Zosterophyllum. Bucheria dawsonii Ananiev (1960)
represents use of Bucheria as a "garbage pail genus." Ananiev claimed that his single specimen appears like Protobarinophyton Ananiev, but is not well enough preserved to be placed in that genus. Little information on Bucheria dawsonii was provided, and none has been added since. Rebuchia capitanea Hueber from Bathurst Island (Hueber, 1972a
) is considered synonymous with Bathurstia denticulata Hueber (Kotyk and Basinger, 2000
).
At present, only two well-described species of Distichophytum exist: D. ovatum (Dorf emend Hueber) Schweitzer and D. mucronatum Mägdefrau. Distichophytum ovatum is a profusely branched plant with numerous vegetative axes. It has wider axes and larger, thicker sporangia than D. mucronatum. Distichophytum mucronatum branches rarely and has ellipsoid sporangia with a mucronate tip. As mentioned previously, Hueber (1972b)
thought that the difference in sporangial shape between D. ovatum and D. mucronatum was not real, but a result of compression and argued that the two names are synonymous.
Although the Silurian specimens described here are significantly smaller in every respect than the type specimens of Distichophytum ovatum from Wyoming (sporangia 2.5–3 mm in diameter in face view [Hueber, 1972b
]), they fall within the size range of the D. ovatum specimens from the Pragian flora of Bathurst Island (Kotyk, 1998
). In comparison to the Distichophytum mucronatum specimens from Germany, the sporangia are only slightly smaller. Because neither the spikes of specimen US600-8140 nor US600-8144 are laterally compressed, it is not possible to determine whether the sporangia were ovate or mucronate. While it is possible that this difference is taxonomically unimportant, we choose to be conservative and refer to these specimens as Distichophytum sp.
Macivera gracilis n. gen., n. sp
Specimen US385-2398, including both part and counterpart, is 8.1 cm long and incomplete at its base (Fig. 11). The axes are naked and range from 0.7 to 1.0 mm wide, becoming narrower towards the apex. Branching is isotomous at an acute angle (between 32° and 40°). At the base of the axis there is some evidence for downwardly directed branches. The most proximal of the apically directed branches does not bear sporangia at its apex and appears to taper to a point. One of the daughter axes of the next isotomy appears to have been fertile, bearing 1–3 sporangia near its apex, while the other dichotomizes to produce two apical clusters of sporangia that are 3.4 and 5.5 mm long. Some sporangia are preserved as carbonaceous compressions, whereas others are discerned by an impression left in the rock. Sporangia are sessile or borne on short indistinct stalks and appear to be inserted obliquely on the main axis. They do not appear to be aligned into rows (Figs. 12, 13, and 16), suggesting that they are either helically arranged or have no clear organotaxy. Sporangia measure 1.3–2.0 mm long, 1.1–1.3 mm wide, and exhibit a distal dehiscence zone.
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Because the sporangia appear to be borne at more than one level at the terminal portion of the axis, and there is a specialized transverse dehiscence zone, a zosterophyll affinity is suggested. Nevertheless, most zosterophylls reported are larger and bear sporangia over a longer region of the axis than shown by this specimen. To date, there exists no description of plants with longitudinally elongate sporangia, with extremely short or no subtending stalks, and which are not borne in rows. The closest comparison can be made to Distichophytum, which bears sessile, elongate sporangia with a distal dehiscence borne in two subopposite rows. Members of the subgenus Zosterophyllum, genus Zosterophyllum, have sporangia that are helically arranged on the axis (or at least not in clear rows), and some Chinese species, such as Z. sinense Li & Cai and Z. spathulatum Li & Cai (Li and Cai, 1977
), have sporangia that are somewhat longer than wide; however, most species of this genus bear distinct sporangial stalks and are considerably larger, with a more extensive fertile region. Similarly, while Hicklingia edwardii Kidston & Lang (Kidston and Lang, 1923
; Edwards, 1976
) from Scotland bears sporangia that are longer than wide and not confined to rows, they are two to three times larger than the Bathurst specimen and are found on stalks 2.3–2.7 mm long and arranged diffusely along the axis. Another contrast to Hicklingia is that the Silurian specimen does not exhibit tufted growth, and it is not clear whether the axis was terminated by a sporangium.
This specimen cannot be ascribed to any known taxon; therefore, a new genus and species is established.
Order
Zosterophyllales sensu Gensel and Andrews 1984
Genus
Macivera Kotyk, Basinger, Gensel and deFreitas gen. nov.
Generic diagnosis
Axes naked, thin; branching isotomous. Sporangia borne in small cluster at distal end of axis. Arrangement of sporangia not in rows. Sporangia sessile, elliptical, longer than wide, oriented at an angle oblique to the axis. Dehiscence zone distal. Homosporous.
Derivation of name
In memory of Dr. Elisabeth E. McIver, distinguished paleobotanist and co-collector of Bathurst Island plant fossils.
Species
Macivera gracilis Kotyk, Basinger, Gensel and deFreitas sp. nov.
Specific diagnosis
As for generic diagnosis. Plant about 8 cm tall. Axes naked, 0.1–1.0 mm wide, branching isotomous at an acute angle, either tapering distally to a point or terminating in a small cluster of 2–10 sporangia; fertile region 2.5–5.5 mm long. Sporangia sessile or on short indistinct stalks and inserted obliquely. Sporangia 1.3–2.0 mm long, 1.1–1.3 mm wide, with a distal dehiscence zone. Immature spores are round, approximately 50 µm in diameter.
Derivation of name
From the Latin gracilis, referring to the gracile appearance of the axis.
Holotype
US385-2398.
Type locality
University of Saskatchewan Paleobotanical Locality 385; 75°50'49.2'' N, 98°35'34.0'' W; beds at water level immediately below large olistostrome, along unnamed stream, about 14 km northeast of Polar Bear Pass research station, Bathurst Island, Nunavut, Canada.
Horizon
Dark grey shales of the lower member of the Bathurst Island beds, Ludfordian.
Aff. Zosterophyllum sp. B
Specimen US383-2385 consists of a moderately well-preserved axis that is incomplete both proximally and distally and bears four sporangia laterally (Fig. 14). Some pyrite permineralization is evident along the axis and sporangial stalks and, although not extensive enough to reveal much anatomical detail, shows the elongate nature of the cells making up the axis. The main axis is naked and at its base is approximately 1.9 mm in width, gradually increasing to approximately 2.4 mm below the sporangium second from the apex. The sporangia are stalked and borne singly and terminally. Sporangial stalks are approximately 2.0 mm wide, 4–5 mm long, and are arranged either alternately in one plane or helically on the main axis (the precise arrangement of sporangia cannot be determined due to the compression of the axis). The distance between successive stalks is, on average, 11 mm. On this specimen, the lowermost branch is poorly preserved and does not bear an evident sporangium at its end. The remaining four stalks bear sporangia that are of variable width but are between 4.9 and 5.6 mm in length. The lower two sporangia appear to be between 3.6 and 3.8 mm wide, whereas the penultimate sporangium is 5.5 mm wide and slightly wider than long. The uppermost sporangium is at least 3.2 mm wide, but is hidden behind the central axis and could be up to 4.0 mm wide. While the outline of the penultimate sporangium appears the most complete of the four, and a hypothesis of edge-on flattening of the others could be offered, it cannot be determined if the variability in sporangial width is actual or preservational. A distal dehiscence zone is evident on the upper two sporangia. No spores were isolated.
Like zosterophylls, this specimen bears sporangia laterally on stalks (reduced branches). The stalks of US383-2385, however, are not as subordinate to the main axis as is typical of the sporangial stalks of other zosterophylls and instead are nearly equal in width to the main axis. In this respect, this taxon is comparable to Renalia Gensel, considered to be transitional between rhyniophytes and zosterophylls because it bears large, reniform sporangia terminally on lateral isotomously branching axes (Gensel, 1976
). The branching of the lateral axes in Renalia distinguishes it from US383-2385.
Although this specimen is distinct from all other known early land plants, the paucity of information available regarding sporangial shape and the presence of only one specimen prevent us from assigning it to a new genus. The characteristics of this plant ally it most closely with the genus Zosterophyllum; thus, it is designated aff. Zosterophyllum sp. B.
Rhyniophytoid
Only one of the fertile specimens, US600-6774, appears to be a rhyniophytoid. This fragmentary fossil is composed of naked axes that appear to have originated from an isotomy, although the branching point is not preserved (Fig. 15). Each axis bears a large (3.0–3.2 mm diameter) round sporangium terminally. The junction between the axis and sporangium is not clear. One of the two sporangia shows the remnants of a narrow dehiscence zone.
If one assumes that the proximal portions of this plant exhibited exclusively isotomous branching, as seems reasonable, then this fossil would fit well within the genus Cooksonia Lang. Nearly all known species of Cooksonia, however, bear sporangia much smaller than those of this specimen. Cooksonia crassiparietilis Yurina (1964)
is the only species with sporangia of similar size, but they are ribbed. It is also possible that this fragment may represent the terminal end of a plant that did not have exclusively isotomous branching, so that in the absence of proximal portions, a generic assignment is not possible.
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DISCUSSION |
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, pp. 233–234), in her discussion of Silurian plant geography, reminds us that "the most critical appraisal of age determination is necessary where the composition of assemblages is at variance with their presumed stratigraphic position." We have already shown that multiple lines of evidence, including stratigraphic context and three groups of invertebrates, point to a late Ludlow age. The Bathurst Island fossils thus provide the first firm evidence for the existence of substantial morphological diversity, complexity, and stature of vascular land plants for this time.
While the Australian "Lower Plant Assemblage" exhibits a similar level of complexity, its significance has long been ambiguous due to controversy with respect to its putative Ludlow age. This age has been repeatedly called to question (Chaloner and Sheerin, 1979
; Edwards, Bassett, and Rogerson, 1979
; Banks, 1980
; Hueber, 1983, 1992
; Cleal and Thomas, 1999
) and its equivocal nature is often cited (Edwards, 1990
; Gensel, 1992
; Stewart and Rothwell, 1993
; Kenrick and Crane, 1997
). Although Rickards (2000)
argues that the evidence for the Ludlow age of the Lower Plant Assemblage is very strong, his paper serves only to reiterate previous statements and fails to strengthen the claims of previous work. It remains that the graptolites on which the Ludlow age is largely based (Garratt, 1978
, 1981
, 1983
; Garratt and Rickards, 1984
, 1987
; Garratt et al., 1984
) are poorly preserved (and as such subject to misidentification).
If anything, the Ludlow flora from Bathurst Island makes the Silurian age of the Baragwanathia flora less anomolous. The zosterophyll-rich Silurian flora of Bathurst Island appears to have been at least somewhat paleogeographically localized. Even though the island(s) on the eastern margin of Bathurst Island on which the Silurian plants grew were relatively remote from the northern extension of the Old Red Sandstone continent (Scotese, 2001
): the nearest land mass, North Greenland bears Ludlow-aged plant collections that have produced only remains of naked plant axes and the simple rhyniophytoid Salopella (Larsen, Edwards, and Escher, 1987
; Edwards, 1990
). With the addition of the Bathurst data, hypotheses as to the origins of early land plants based on the paleogeographic distribution of non-rhyniophytoid Silurian plant assemblages becomes far less straight forward.
The Silurian flora of Bathurst Island is notable not only for the considerable number of fertile zosterophyll specimens preserved, but also in the morphological patterns that prevail. Of the six types of zosterophylls described above, four bear sporangia in compact terminal clusters and only aff. Zosterophyllum sp. A and aff. Zosterophyllum sp. B have diffusely borne sporangia. Also of note, stalks of cf. Bathurstia sp., Distichophytum sp., aff. Zosterophyllum sp. A (subgenus Platyzosterophyllum), and Macivera gracilis are either minute or absent. These observations raise questions such as the following: Are clustering or sessile placement of sporangia primitive characters in the zosterophylls? Do these features confer selective advantage? Could these characters be linked to abiotic or biotic factors affecting early land plants? Raymond (1987)
hypothesized that clustering of sporangia could have been an adaptation to dry or dry and cool conditions. Dry climatic conditions are supported by more recent paleoclimatic models for the Late Silurian (Scotese, 2001
).
Clearly, this fossil flora is significant in its implications for the diversification and elaboration of early land floras prior to the Early Devonian and reiterates the need for continued, global exploration of Silurian sediments in order to further elucidate the emergence of vascular land floras.
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FOOTNOTES |
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5 Current address: Biology Department, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3280, USA
6 Author for reprint requests (pgensel{at}bio.unc.edu
)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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