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Palaeobotany Department, University of Liège at Sart Tilman—Bât B18, B-4000 Liège 1, Belgium
Received for publication March 17, 2000. Accepted for publication June 20, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONNOMENCLATURE NOTEMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: anisophylly • Archaeopteris • progymnosperm • Upper Devonian
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONNOMENCLATURE NOTEMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Meyer-Berthaud, Scheckler, and Bousquet, 2000
), are the predominant element in Upper Famennian (Late Devonian) plant assemblages in Belgium. These specimens are referred to A. roemeriana (Göppert) Lesquereux 1880
. Despite the abundance of material (Crépin, 1874
; Gilkinet, 1875, 1922
; Stockmans, 1948
) many details of the organization of the plant, particularly of fertile leaves, sporangia, and spores, remain unknown, and a contradiction exists between the morphology of the ultimate vegetative branches, as they are described in the literature, and their internal structure (Kenrick and Fairon-Demaret, 1991
). From the base towards the apex of the ultimate vegetative branches, the dorsiventrally symmetrical vascular supply shows variation in the number of protoxylem strands, changing from four to five, to four again, and reducing distally to three. Two out of the four or five protoxylem strands are more prominent than the others, giving off leaf traces two times bigger than those issued from the smaller strands. Such a difference in strand size should result from a difference in leaf size (Kenrick and Fairon-Demaret, 1991
). Archaeopteris roemeriana possesses leaves broadly obovate in shape with a characteristic radiating dichotomous venation; they appear suboppositely to alternately borne on the vegetative ultimate axes, but a helical insertion is suggested (Nathorst, 1902
; Phillips, Andrews, and Gensel, 1972
). No spectacular difference in size between two successive leaves on ultimate vegetative branches has ever been reported in A. roemeriana or in other species of the genus. The purpose of this paper is to address the paradox between the observed external morphology of the vegetative ultimate axes of A. roemeriana and their internal structure. |
NOMENCLATURE NOTE |
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TOPABSTRACTINTRODUCTIONNOMENCLATURE NOTEMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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in the sense used by Kenrick and Fairon-Demaret (1991)
, despite the fact that the status of this species is far from clear. It has been debated since 1939 when Arnold cast doubt on the significance of the characters used to separate the European A. roemeriana specimens from North American A. halliana. Both species were put in synonymy by Kräusel and Weyland (1941)
but not by Stockmans (1948)
. Apparently they share similarities in leaf form and arrangement, as well as the degree of leaf overlap and insertion on the penultimate or ultimate axes. Actually these two latter characters are highly variable according to the original growth position of the fragment when considered relative to what may have been a large tree and may also be influenced by the vagaries of transportation and the orientation of the specimens to the bedding plane of the sediment. In fact the known morphological details are not equivalent in both species. The anatomy of A. halliana has not yet been described but the morphology of its fertile leaves, arrangement of sporangia, and details of spores are known, and a reconstruction of a fertile ultimate axis of A. halliana has been proposed (Phillips, Andrews, and Gensel, 1972
). No comparable morphological information is available for A. roemeriana, but anatomically its penultimate and ultimate axes are similar to those of A. macilenta as described by Carluccio, Hueber, and Banks (1966)
, Beck (1971)
, and Scheckler (1978)
. In these two species leaf polymorphism is inferred (difference in size of leaf traces [Kenrick and Fairon-Demaret, 1991
] or of the leaf bases [Beck, 1971
]). An in-depth comparative study of A. roemeriana and A. halliana is still necessary to resolve the question of synonymy and is beyond the scope of this paper. Moreover, as already advocated by Kenrick and Fairon-Demaret 1991
, we wish to avoid the inference that the results from the Belgian material also apply to A. halliana from North America. |
MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONNOMENCLATURE NOTEMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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] who described remains of Moresnetia from this locality). The best results have been obtained from material recently collected, particularly from two specimens, parts and counterparts of penultimate branch fragments with vegetative leafy ultimate axes attached on them. They are preserved as dark to light-brown compressions in a gray micaceous fine-grained matrix.
In Langlier quarry an exposed series of stacked sequences of Famennian "Condroz Sandstones" or "Psammites du Condroz" group includes lenticular sandstones, the overall architecture of which corresponds to a point bar or estuary channel (Thorez et al., 1994
). The filling of the channel (with accretion structures) consists of gray to dark mudstones containing on some of their bedding planes a rich concentration of plant debris usually preserved as compressions. The plant horizon belongs to the VCo (Diducites versabilis–Grandispora cornuta) spore Biozone (Maziane, Higgs, and Streel, 1999
) and is late Famennian (Fa2c, Upper but not uppermost Devonian) in age.
The main method of study of this compression material has been that of dégagement (Fairon-Demaret, Hilton, and Berry, 1999
) whereby sediment is removed from the fossil by using sharpened triangular needles struck with a small hammer under the binocular microscope. Photographs were taken (using polarized light) and camera lucida drawings made to record the successive stages of uncovering of critical leaves on the axis. Attempts at transfers of the ultimate vegetative axis were not successful; the thin compression remains crumbled to dust when freed from the matrix support.
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RESULTS |
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TOPABSTRACTINTRODUCTIONNOMENCLATURE NOTEMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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On the specimens the fracture plane of the rock matrix usually splits through the thickness of the compression remains so that both complementary pieces, part and counterpart, appear as mirror-images of each other. As a consequence only the lateral sides, right and left, of the compressed axis remains and the leaves attached along them are usually seen. If supplementary, smaller leaves were to be attached to the adaxial and/or abaxial sides of an ultimate branch, they might still be embedded in the sediment, on the part and/or the counterpart. In order to settle the question the axes of three ultimate branch compressions were uncovered on two specimens, ULg 13012a-b and ULg 13177a-b. These are not the better preserved specimens, nor the most impressive ones. They were, nevertheless, chosen on account of the low density of their visible leaves, which were not overlapping, thus avoiding confusion between successive ones when uncovering.
Specimen ULg 13012, a and b (Figs. 1, 3–9)
Along the preserved fragment of a penultimate branch 17 cm long, three pairs of subopposite ultimate leafy axes are attached at an acute angle of 
30° and gently curve abaxially 1 cm above their base. Two ultimate axes are complete with the apex preserved. The basalmost one, measuring 14.5 cm long, is partly fertile; the next branch immediately above it is shorter, 11 cm long, and wholly vegetative. On each of these two ultimate branches, the three most distal, terminal leaves, 1–0.9 cm long, are shorter than the others that reach 1.9–2 cm in length, and the change to smaller sized terminal leaves is abrupt. The third distalmost ultimate branch was uncovered by dégagement. It is broken by a fracture in the rock 5.8 cm above its base, and its apex is not preserved. It was chosen because the eleven alternating leaves (Fig. 1a, a–k) were not overlapping. Three only (Fig. 1a, e, which was uncovered, f and h) are well preserved enough and show the usual fan-shaped outline with the crenulate to entire margin of the expanded blade that is progressively attenuated proximally into a slightly decurrent leaf base. They are 20, 19, and 22 mm long, respectively, with a maximum exposed width of 10 mm. On one piece, arbitrarily designated as the part (ULg 13012a), the second one being regarded as the counterpart, occurrence of 12 additional small leaves was revealed by removing the remains of the axis compression (1–12 on Figs. 1b, 3). These small leaves appear irregularly distributed along the whole preserved length of the ultimate axis. Occasionally two of them (3–4, on Figs. 1b, 3) are observed grouped between two successive bigger, "regular" leaves. As a rule the small leaves appear poorly preserved, twisted (Figs. 4, 5, 7) or torn into strips with only the veins remaining visible (Figs. 8, 9). They are rarely well enough preserved to ascertain their length when uncovered; the longest is 7 mm long (Fig. 7), measuring only about one-third of the length of the "regular" leaves. The reduced blade expands from a narrow base of up to 1 mm maximum width when seen in face view (Fig. 6). The venation of these small leaves is apparently organized as in standard Archaeopteris leaves: two veins that fork distally enter the blade (Figs. 8, 9).
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Specimen ULg 13177, a and b (Figs. 2, 10–16)
This specimen shows a fragment of penultimate branch 14.5 cm long bearing four pairs of subopposite ultimate leafy axes. These appear rigid, inserted at an angle of 45°, and regularly spaced every 3 cm. Remains of a fifth pair, the attachment of which is not preserved, are also present. Two of these ultimate axes have been uncovered on the part and the counterpart.
Ultimate branch 1 is 9.5 cm long and 2 mm wide; its apex is missing. Twenty alternating leaves (Fig. 2a, a–t) are observed along the margins; when completely exposed they measure 19 mm long and possess a slightly undulate margin (e.g., Fig. 2a, leaf o). Uncovering of the part revealed the occurrence of 16 additional smaller leaves originally covered by the axis compression (Figs. 2b, 12). As on specimen ULg 13012a, they are poorly preserved with a distorted blade that sometimes appears fringed with only the veins still present (Figs. 10, 11). These additional leaves are also reduced in size and are much smaller and narrower than the readily visible "regular" leaves. Along the proximal 5 cm they are irregularly disposed on the axis, two successive ones being sometimes very close together, forming a pair of small appendages between two successive bigger leaves (6–7 on Figs. 2b, 12). Along the distal 4 cm these smaller leaves are more regularly disposed, being observed about every 7 mm. Owing to their poor preservation their length is difficult to ascertain; they nevertheless appear progressively longer from the base towards the apex, passing from 7.5 mm long for the first proximal one to 9 mm for the 16th distal one. One of these small leaves, seen in face view (7 on Figs. 2b, 12), is 7.5 mm long. Actually three of these additional small leaves (7, 9, 10 on Fig. 2a) were already visible before working out the specimen, but owing to their segmented, distorted aspect they were at first regarded as detached bits of other organs and were not accounted for.
Ultimate branch 2 is distal to branch 1 along the same margin of the penultimate branch. Its preserved length is 7.5 cm and its axis is 1.7 mm wide. It bears 16, occasionally overlapping, alternating leaves (Fig. 2a, a–p). On the part 12 small additional leaves (1–12 on Figs. 2b, 13) were embedded in the matrix and originally concealed by the axis compression, but have been uncovered. Leaf 8 is well preserved; the blade, 7.5 mm long, is twice bifurcated, ending in four short vascularized segments (Fig. 15). The two veins entering the leaf also bifurcate twice (Figs. 14–16). As on the adjacent ultimate branch, the distribution of the small leaves appears more regular on the distal 3.5 cm of the preserved length of the axis than along the proximal 4 cm where they are locally disposed in pairs (3–4 on Figs. 2b, 13). Their length also increases towards the tip of the ultimate branch; the distalmost uncovered one (12 on Figs. 2b, 13) reaches 9 mm. Uncovering of the counterpart of these two ultimate branch axes did not reveal any supplementary small leaves.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONNOMENCLATURE NOTEMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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the marked difference in leaf trace size in the vascular strand of the vegetative ultimate axes of Archaeopteris roemeriana is actually expressed externally by a marked difference in leaf size. Small leaves are about one-third the size of the bigger "standard" ones readily seen on the compression specimens. These small leaves are also different in shape with a reduced lamina that appears twice forked. With very few exceptions they are not obvious on the usual fracture surfaces. They remain concealed in the matrix under the remains of the axis compression, and technical work is necessary to demonstrate their occurrence. Unworked specimens show only the bigger leaves that are suboppositely to alternately arranged in two ranks along the axis margins. This explains why neither Stockmans (1948)
nor Nathorst (1902)
mentioned a variation in leaf size on different sides of the ultimate axes of the A. roemeriana specimens they described, nor did Phillips, Andrews, and Gensel (1972)
in their review of the literature about possible leaf variation in this species.
These smaller leaves are clearly restricted to one side of the ultimate axes. Unfortunately, from the compression material studied, we cannot decide on which surface, adaxial or abaxial, of the ultimate axes they occur. Re-examination of the transverse section through a penultimate axis of Archaeopteris roemeriana illustrated as Fig. 1, Plate 3, of Kenrick and Fairon-Demaret (1991)
was informative in this regard. In addition to several leaf traces, a larger ultimate branch trace is seen, detached from the vascular strand but still in the cortex of the penultimate branch. This protostelic but already dorsiventrally flattened ultimate branch trace has a peculiar tangentially elliptical, slightly curved outline with the concavity abaxially oriented. Despite the poor preservation, two prominent protoxylem poles located at the tips of the abaxial side of the branch trace are observed. They produce traces that are in the size range of the bigger ones and are thought to vascularize the two proximal large, "standard" leaves. Even if this ultimate branch trace shows a sequence of change in the number of sympodia from base to apex, its dorsiventrally flattened outline remains unchanged, with the bigger leaf traces developing from the abaxially oriented extremities of the curved strand. We conclude that the larger leaves are attached to the abaxial, lower, side of the ultimate axes and the smaller ones that differ in size and shape, are disposed on the adaxial, upper, surface, as hypothesized by Kenrick and Fairon-Demaret (1991)
. The determinate plagiotropic ultimate vegetative axes of Archaeopteris roemeriana are thus characterized by anisophylly coupled with shoot dorsiventrality, a combination thought to represent an adaptation maximizing light interception (Dengler, 1999
). In A. roemeriana these characteristics actually appear superimposed on radial symmetry and isophyllous leaf development as exemplified by arrangement and morphology of the identical terminal leaves coupled with a progressive length increase of the smaller leaves towards the apex of the ultimate axes.
Anisophylly combined with dorsiventrality might characterize the whole lateral branching system of Archaeopteris. As inferred from a difference in leaf base size, leaves that are smaller occur on one side, hypothesized to be adaxial, on the penultimate branches of A. macilenta (Beck, 1971
). However, dorsiventral symmetry of these penultimate branches was never put forward. It might, however, exist since ultimate axis traces, which are larger than leaf traces, usually arise from two larger protoxylem ribs on opposing sides of the stele of the studied penultimate branches (Carluccio, Hueber, and Banks, 1966
; Beck, 1971
); larger branches bear three or more orthostiches of ultimate axes in an irregular but still dorsiventral pattern (Scheckler, 1978
).
According to Meyer-Berthaud, Scheckler, and Wendt (1999)
, Archaeopteris is the oldest known "modern" forest tree that evolved a branching syndrome similar to the axillary branching of early seed plants. It had a single radially symmetrical trunk bearing lateral structures of three different types. It is not yet known whether the determinate plagiotropic lateral branching systems described above are to be equated to one of the apically initiated branch types described in Archaeopteris, and probably shed as units (Beck, 1971, 1981
; Scheckler, 1978
; Beck and Wight, 1988
; type A branches of Meyer-Berthaud, Scheckler, and Wendt [1999
] that are compared to ultimate branches), or whether they correspond to the adventitious organs that Trivett (1993)
interpreted as branches allowing for reparation of damaged portions of the crown and increase of photosynthetic area. They also may have been borne on "branch B" of Meyer-Berthaud, Scheckler, and Wendt (1999)
, which are long-lived structures producing regularly arranged appendages and already showing developmental features common in derived seed plants (Meyer-Berthaud, Scheckler, and Bousquet, 2000
). In any case the lateral anisophylly (sensu Goebel, 1923
) of Archaeopteris would still emphasize the "modernity" of the Archaeopteris vegetative body. A note of caution is nevertheless to be added: presently it cannot be ascertained that lateral anisophylly characterizes all the members of the genus nor that it is invariant in expression (as lateral anisophylly in many modern trees may switch over to isophyllous development again, depending on changing environmental constraints on the growing shoot; Dengler, 1999
).
Lateral anisophylly has been reported in many modern woody genera with a variety of phyllotaxis, from spiral to decussate (Dengler, 1999
, and references therein). Phyllotactic series could not be established with certainty for the whole length of the studied ultimate vegetative axis compressions of Archaeopteris roemeriana. The small leaves are so poorly preserved that several could have been missed and/or destroyed during the uncovering (the width of the veins, which most of the time are the only part of these smaller leaves to remain, is the same size as the grain of the matrix). Nevertheless the occurrence of two small leaves close together between two successive larger ones allows us to infer a local 2/5 organotactic fraction for the short length of axes where the vascular strand has five sympodia. Distally, above the proximal 4–5 cm of the axes, the small leaves appear more regularly borne every 6–7 mm, a change that most probably reflects another phyllotactic pattern in an area where the vascular bundle has four sympodia again. On Archaeopteris ultimate axes leaves are thought to be borne according to an anomalous series of fractions, most of which lie outside the usual Fibonacci series of numbers (Scheckler, 1978
). A four-ranked or spiral disposition in A. halliana (Phillips, Andrews, and Gensel, 1972
) and a decussate phyllotaxis in A. macilenta (Beck, 1971
; Beck and Wight, 1988
) were tentatively suggested. A diagonal decussate pattern, resulting in a four-ranked phyllotaxis with small leaves in two adaxial ranks that is common in modern anisophyllous species (Dengler, 1999
) cannot be ruled out for the studied specimens of A. roemeriana. A local helical phyllotaxis with differential growth of leaves in different position on the axis (as in modern conifers, e.g., Tsuga canadensis, Dengler, 1999
) may be hypothesized as well, especially since, towards the apex of the ultimate axes, the identically sized leaves are most probably borne in a 1/3 helical pattern in accordance with the three-lobed vascular strand. Presently it is not clear whether the sequence of changes in the phyllotactic pattern on the vegetative ultimate axes characterizes only the species A. roemeriana or the genus as a whole.
The distorted, poorly preserved aspect of the small adaxial leaves on the material studied is remarkable. It might be the result of the vagaries of the fossilization processes. In Upper Famennian times the Dinant Synclinorium was located in a paleotropical belt at 15° S paleolatitude with long, dry intervals alternating with shorter, more humid ones that were characterized by episodic strong flooding (Streel and Scheckler, 1990
). Recent investigations precisely indicate growth of Archaeopteris trees in such contrasting climatic conditions (Bateman et al., 1998
). The highly fluctuating fluvial regimes could have induced violent rafting of the A. roemeriana remains that were fossilized in the mud fill of a channel they reached after a transport of unknown length and duration. The smaller adaxial leaves clearly appear more delicate than the larger laterally oriented, less damaged ones.
As in "modern" plants, the reduced adaxial leaves most probably result from dissimilarities in growth rate of primordia that were induced and regulated by competition between ab- and adaxial surfaces of the axes for growth regulators or nutrients (Dengler, 1999
). As a consequence reduced adaxial leaves are not only shorter in length but are also reduced in other allometric constants (as, e.g., thickness of the blade). Nevertheless, despite their limited development, by their mere presence on the upper surface of an ultimate branch, these small adaxial leaves would have contributed to the photosynthetic role of this unit and also to a reduction of water loss by transpiration during dry, stressful periods by producing a boundary layer "whose thickness resists the transfer of mass as well as heat" (Niklas, 1994
, p. 123).
In summary, for the first time occurrence of anisophylly is demonstrated in a member of the Upper Devonian genus Archaeopteris. As demonstrated in the present study, that only one type of leaf is described on ultimate axis compressions of Archaeopteris halliana, a contemporaneous species that shares many morphological traits with A. roemeriana, may not be regarded as indicative of nonoccurrence of anisophylly. Obviously careful uncovering of leaves on ultimate axis compressions of A. halliana from North America is the very next step to address this issue.
Shifts in mode and tempo of patterns of growth rate are an acting power of evolutionary change (Basile and Basile, 1993
), but the phylogenetic implication of such a differential leaf development is difficult to evaluate in a member of the Archaeopteridales regarded as the closest seed-plant sister group (Nixon et al., 1994
; Rothwell and Serbet, 1994
). It most likely occurred in response to selection pressure primarily induced by optimization of photosynthesis in a plagiotropic branching system. The dorsiventral ultimate vegetative axes of A. roemeriana, which still possess, in their distal part, a radial symmetry reminiscent of an ancestral aneurophytalean anatomy, are not to be oversimply equated with a compound leaf. They may nevertheless illustrate an evolutionary step towards such a structure (Bateman et al., 1998
) that already was selected for in the vegetative body of early seed plants such as Elkinsia (Serbet and Rothwell, 1992
) and Moresnetia (M. Fairon-Demaret and I. Leponce, unpublished data).
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FOOTNOTES |
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONNOMENCLATURE NOTEMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Basile, D. V., and M. R. Basile. 1993 The role and control of the place-dependent suppression of cell division in plant morphogenesis and phylogeny. Bulletin of the Torrey Botanical Club 25: 63–84
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Thorez, J., R. Dreesen, M. Fairon-Demaret, and E. Goemare. 1994 The Condroz Sandstone (latest Devonian): an overview of the main characteristics of the group, especially in the Bocq valley area, 4th European Paleobotanical and Palynological Conference. Heerlen/Kerkrade, The Netherlands, 19–23 September 1994. Excursion Guidebook 1—Devonian-Carboniferous, Part 3: 21–35
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