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Paleobotany |
2Laboratory of Systematic and Evolutionary Botany, and Herbarium, Institute of Botany, the Chinese Academy of Sciences, Xiangshan, Beijing 100093, People's Republic of China; 3Florida Museum of Natural History, University of Florida, Gainesville, Florida 32611-7800 USA
Received for publication March 22, 2004. Accepted for publication September 30, 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Asia • Ephedra • Gnetales • Lower Cretaceous • Yixian Formation
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Crane, 1985
; Doyle and Donoghue, 1986
; Nixon et al., 1994
; Hickey and Taylor, 1996
; Doyle, 1998
; Friedman, 1998
; Stefanovic et al., 1998
; Rydin et al., 2002
), but recent molecular studies suggest that they may be more closely related to conifers rather than angiosperms (Qiu et al., 1999
; Winter et al., 1999
; Bowe et al., 2000
; Chaw et al., 2000
; Gugerli et al., 2001
). While Gnetales are proposed to be monophyletic, with Ephedraceae occupying a basal position (Crane, 1985
; Friedman, 1998
; Stefanovic et al., 1998
; Winter et al., 1999
; Bowe et al., 2000
; Chaw et al., 2000
; Gugerli et al., 2001
), little is known about the origin, diversification, and evolution of the three families. Gnetalean macrofossils are rare and often distinct from living forms (Krassilov and Bugdaeva, 1982
; Krassilov, 1986
; Crane and Upchurch, 1987
; Crane, 1988
, 1996
; Cornet, 1996
; Krassilov et al., 1998
) , but pollen is quite common (Brenner, 1968
, 1976
; Herngreen, 1973
, 1974
, 1975
, 1981
; Mabesoone and Tinoco, 1973
; Regali et al., 1974a
, b
; Lima, 1978a
, b
, c
, 1979
, 1980
, 1981
, 1982
; Herngreen and Chlonova, 1981
; Regali and Viana, 1989
; Osborn et al., 1993
; Li and Liu, 1999
) . Several species of ephedroid macrofossils were found in the Lower Cretaceous of western Liaoning Province and Lower Jurassic of Qinghai Province of China (Wu et al., 1986
; Guo and Wu, 2000
). Unfortunately, these fossils lack important structural information and are not comparable to the extant Ephedra L. This study relates reproductive characters of extant Ephedra rhytidosperma Pachomova with the same characters in fossil Ephedra L. reproductive shoots bearing seeds from the Yixian Formation in northeast China.
Extant Ephedra L. consists of ca. 50 species in a monogeneric family (Price, 1996
). The genus can be grouped into three sections based on characters of the bracts of seed-bearing organs. These sections are Alatae Stapf, Asarca Stapf, and Pseudobaccatae Stapf (Stapf, 1889
). But this grouping is not supported when pollen and wood characters are included (Price, 1996
; Ickert-Bond et al., 2003
). Ephedra L. is widely distributed in arid and semi-arid regions in the North Temperate Zone, as well as some limited areas in South America. Plants are small trees, shrubs, subshrubs or herb-like subshrubs, and rarely liana-like shrubs. Stems are often profusely branched, having green branchlets arranged opposite or in whorls at the nodes. Shoots are terete and articulate, while the internodes characteristically have numerous longitudinal striations. Two or three reduced leaves are inserted at the nodes and are connate at the base, each having a triangular apex and two parallel veins. Plants are dioecious or rarely monoecious. One to three seeds are borne in ovoid to ellipsoid cones that may be terminal or axillary. Female cones bear bracts in 2– 20 pairs or whorls of three, of which only the uppermost are fertile. Bracts are membranous, coriaceous, or fleshy at maturity. The seed surface is smooth or variously sculptured in different species. The micropylar tube is straight, curved, or twisted.
In this paper, seed surface characters of the fossils and extant Ephedra rhytidosperma are investigated using scanning electron microscopy (SEM). Relationships of the fossils are discussed by comparing their morphology with extant Ephedra L. and other ephedroid fossils.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The mean age of the Yixian Formation is ca. 125.0 My (Million years) determined by using radiometric 40Ar/39Ar (Swisher et al., 2002
), therefore the Yixian Formation is in the Lower Cretaceous. This result is basically consistent with most recent studies (Swisher et al., 1999
; Barrett, 2000
; Wang et al., 2001
), although the age of the Yixian Formation has been dated as latest Jurassic (Wang and Diao, 1984
; Lo et al., 1999
), or the Lower Cretaceous (Smith et al., 1995
). Based on the presence of the bird Confuciusornis sanctus Hou et al. and the feathered dinosaurs Sinosauropteryx prima Ji et Ji, Protarchaeopteryx robusta Ji et Ji, Caudipteryx zoui Ji et al. and Dendrorhynchus curvidentatus Ji et Ji, which are typical components in the Upper Jurassic of Germany and England, the Yixian Formation has been considered late Jurassic by some paleontologists (e.g., Chen, 1988
; Ji and Ji, 1997
; Chen et al., 1998
; Ji et al., 1998
; Sun et al., 1998
; Wu, 1999
). In the present paper, we agree with a date of the Barremian epoch of the Lower Cretaceous for the Jianshangou Bed, in the lower part of the Yixian Formation.
Specimens
Over 40 species of extant Ephedra L. and two specimens of extinct Ephedra L. were examined for seed sculpture characters (Schnarf, 1937
; Corner, 1976
; Werker, 1997
). The two fossil specimens examined, 053726, and 053711, are deposited in the National Museum of Plant History, Institute of Botany, the Chinese Academy of Sciences, Beijing, People's Republic of China.
Methods
Air cleaned specimens were photographed with light microscopy (LM). For SEM, the impressions were coated with gold- palladium alloy. Observations and photographs were made with a Hitachi S-800 SEM operating at 30 KV.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Holotype
053726 (fertile reproductive shoots; Fig. 1).
Paratype
053711a–b (seed impression; Figs. 7–9).
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Etymology
The specific epithet archaeorhytidosperma is derived from Latin archaeo- (ancient) and rhytidosperma (seed surface ornamented); the former indicates that the species is a fossil while the latter refers to the distinctive characters of the seed surface ornamentation, which are similar to those of the extant Ephedra rhytidosperma.
Type locality
Huangbanjigou village (approximately 41°41' N, 120°59' E), Shangyuan district, Beipiao City, western Liaoning Province, northeast China.
Age
Barremian stage of the Lower Cretaceous, Jianshangou Bed, lower part of the Yixian Formation.
Number of specimens examined
Two specimens were studied. One had vegetative shoots and cones with young seeds, and the other was part and counterpart of an isolated mature seed.
Description
The fertile reproductive shoots are dichasial, and 37 mm long (Fig. 1). Branchlets are terete, erect or suberect, opposite at the nodes, articulate, and ramifying 2–3 times (Figs. 1, 4). Internodes are longitudinally striated, slender, 8–14 mm long, with a diameter of 0.5–1 mm (Figs. 1, 2, 4). An isolated leaf, preserved near the nodes of the reproductive shoots, is incomplete, and caducous. The leaf is triangular and acute, ca. 5 mm long, ca. 2 mm broad at the base, with two parallel veins (Fig. 2). Almost every branchlet of the fertile shoot has a terminal female cone (Fig. 1). Ovules/seeds number 1–2 per branchlet, are ovoid to elongate-ovoid, have a rounded base and cuspidate apex, 1.5–4 mm long, 1–1.6 mm broad, possibly immature and ornamented with minute transverse, undulating protuberances. The dorsal sides bear a longitudinal ridge (Figs. 1, 5, 6), and the surface shows fine longitudinal striations and furrows (Figs. 5, 6). There is an incomplete bract adnate to the basal half of one of the ovules/ seeds (Figs. 1, 3). The bract is long and elliptic, length ca. 2 mm, and extending to just below the middle of the ovule. At the dorsal surface of the same ovule, there is a bract fragment (scar) with an irregular shape (Fig. 3). The micropylar tube is exerted, straight, or sometimes slightly curved at the top, is short, ca. 1–2 mm long (Figs. 1, 3). Isolated mature seeds occurring with the reproductive shoots in the same bed are narrowly obovate or elongate-ovoid, ca. 7.5 mm long, 2.2 mm broad, narrow at the base and broadening upwards to a cuspidate apex, and showing a longitudinal ridge (Figs. 7, 8). Seed surface is ornamented with numerous regular transverse, lamellar, undulating protuberances that are almost perpendicular to the seed surface. Both the upper and lower sides of the lamellar protuberances have many striations and furrows, while their ends are moniliform (Figs. 7–9).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Jongmans and Dijkstra, 1973
). Later, authors used this name for Mesozoic and Tertiary fossils probably related to Ephedraceae, some poorly preserved (Wu et al., 1986
; Guo and Wu, 2000
). However, the genus is inappropriate for ephedraceous fossils because its type species was demonstrated to be a member of Loranthaceae (Conwentz, 1886
). Guo and Wu (2000)
proposed to conserve the generic name "Ephedrites Saporta" and selected "Ephedrites antiquus Heer" as its type, but the new type species was also suggested to consist of Welwitschia-like bracts, samaras, and pollen (Krassilov and Bugdaeva, 1988
). Accordingly, "Ephedrites Saporta" is not appropriate for fossils of Ephedraceae.
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), and E. chenii Guo et Wu (Guo and Wu, 2000
) in particular. Ephedrites chenii, also from the Yixian Formation is characterized by multinoded shoots that ramify several times, internodes with many fine striations, branchlets opposite and decussate at nodes, opposite linear leaves, and ovulate cones with 4–8 pairs of bracts terminal on branchlets (Guo and Wu, 2000
). The vegetative characters of E. chenii are similar to those of certain extant ephedras and Ephedra archaeorhytidosperma (Table 1). However, ovulate reproductive structure, especially seed surface sculpture, two pairs of bracts, and micropylar tube distinquish E. archaeorhytidosperma and living E. rhytidosperma from E. chenii.
Chaoyangia liangii Duan was published as the oldest tricarpous fruit of a flowering plant from the lower part of the Yixian Formation, Chaoyang City of Liaoning Province, China (Duan, 1998
). Later it was suggested to have an affinity with Gnetales (Sun et al., 1998
; Wu, 1999
; Guo and Wu, 2000
). Reproductive shoots of C. liangii are multinoded and dichasially branched. Internodes have many fine longitudinal striations. Leaves are small and parallel veined. An ovulate cone, with radiating thin and forked appendages, terminates each branchlet and bears three seeds. Each seed has a long (ca. 3 mm) and straight micropylar tube. All the characters of this species, except the forked appendages of the ovulate cones, coincide with those of extant Ephedra L. As a consequence, C. liangii might be another extinct species of Ephedraceae.
There are several similarities between Chaoyangia liangii and Ephedra archaeorhytidosperma, such as multi noded, striated shoot and branchlets, terminal reproductive organs, leaves with parallel veins, and a seed with a micropylar tube. However, the differences between them are also obvious. First, unlike C. liangii, the cones of E. archaeorhytidosperma are not surrounded by thin, forked appendages. Second, each cone of C. liangii bears three seeds, while those of E. archaeorhytidosperma have only one or two seeds. Third, the micropylar tube is longer and straight in C. liangii, rather than shorter and sometimes slightly curved at the top in E. archaeorhytidosperma. Finally, the leaf is linear and fine in C. liangii, but is triangular in E. archaeorhytidosperma. At present, we cannot compare the seed surface characters of E. archaeorhytidosperma with those of C. liangii, because the condition of the latter is unknown.
Eoanthus zherikhinii Krassilov was collected from the Lower Cretaceous at the upper reaches of the Vitim River, east of Lake Baikal. It might be related to gnetophytes on account of the ovule containing Ephedripites pollen grains in the pollen chambers (Krassilov, 1986
). The ovulate structure of the species shows a conspicuous midrib and transverse protuberances, which are similar to the seed surface sculpture of Ephedra archaeorhytidosperma (Fig. 8). Because the reproductive organs were not well-preserved, and no vegetative organs were found, additional characters are still needed to determine the relationships of Eoanthus Krassilov. In the absence of additional information, it is difficult to evaluate whether the resemblance of ovulate structures between Eoanthus Krassilov and E. archaeorhytidosperma is due to close affinity or to convergence.
Allicospermum retemirum Harris, Erdtmanispermum balticum Pedersen et al., Drewria potomacensis Crane et Upchurch, and Dinophyton spinosum Ash are fossil seeds or reproductive shoots supposed to have gnetalean affinities (Reymanówna, 1968
; Crane and Upchurch, 1987
; Krassilov and Ash, 1988
; Pedersen et al., 1989
). Allicospermum retemirum, E. balticum, and D. spinosum have no reproductive branches and thus lack sufficient characters to be compared with Ephedra archaeorhytidosperma. Seed ornamentation is unknown for D. potomacensis, and the species differs from E. archaeorhytidosperma in having loose, axillary, spike-like reproductive structures and bracts with prominent chevron-like crossveins between the parallel veins (Crane and Upchurch, 1987
).
Although no pollen was found with Ephedra archaeorhytidosperma, or within the sediment adjacent to the fossil, ephedroid pollen (Ephedripites sp.) has been found in the same locality as the fossil (Li and Liu, 1999
).
Comparisons between Ephedra archaeorhytidosperma and extant species of Ephedra
The morphology of vegetative organs of Ephedra L. is distinctive and easily distinguished from that of all other plant taxa. However, characters such as stem length, thickness and appendicular structures are of little value in classifying various Ephedra L. species. The reproductive structures, such as cone characters, seed morphology and surface sculpture, are diagnostic for certain Ephedra species. A comparison between Ephedra archaeorhytidosperma and extant Ephedra L. species suggests that E. archaeorhytidosperma shows many similarities to the extant Ephedra rhytidosperma.
Ephedra rhytidosperma (Fu et al., 1999
) has a growth habit of mat-like subshrubs with strongly lignified stems (Fig. 13). Its woody stems are short and stout, multinodular, with thick nodes and a knotty appearance because the green branchlets are densely aggregated at the nodes of dichasial branches. The green reproductive branchlets are ramified 1–3 times (Fig. 14). Internodes are 1–1.5 cm long with 2–3 connate leaves at each node. Cones are solitary or opposite. One or two ovules are enclosed in two or three pairs of bracts (Fig. 15), in which the apical pair is much longer than the other pairs and connate for about half of their length, and the lower pair is minute and usually less than half the size of the apical pair. The bracts are red and fleshy at maturity. The micropylar tube is straight or slightly curved at the top, 1–2 mm long (Fig. 15). The seeds are ovoid or oblong-ovoid, prominently longitudinally ridged, with densely arranged, tiny transverse lamellar projections (Fu et al., 1999
) (Figs. 10–12).
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). This kind of ridge is also present in E. archaeorhytidosperma (Figs. 1, 8). The seed of E. archaeorhytidosperma is narrowly obovate or elongate-ovoid, ca. 7.5 mm long, 2.2 mm broad, narrow at base, broadening upwards, apex cuspidate (Fig. 8), thereby comparable to that of E. rhytidosperma in its size and overall morphology; 3) E. archaeorhytidosperma is characterized by an ornamented ovule wall and seed surface with numerous regularly arranged, wavy, transverse ridges (Figs. 1, 3, 5–9), which is similar to E. rhytidosperma (Figs. 10–12, 15). An isolated mature seed, co-occurring with fertile shoots at the same locality as E. archaeorhytidosperma, is also covered by many regular transverse ridges (Figs. 7–9). Superimposed across these ridges and the intervening areas are numerous fine, closely spaced longitudinal striations, observed in both E. archaeorhytidosperma (Figs. 6, 8, 9) and extant species (Figs. 10–12); 4) The micropylar tube of E. archaeorhytidosperma is straight or slightly curved at the top (Figs. 1, 3), which resembles that of E. rhytidosperma (Fig. 15); 5) Cones of E. archaeorhytidosperma have relatively few pairs of bracts (Figs. 1, 3). One bract is prominent at the side of the two seeds of the cones in E. archaeorhytidosperma (Figs. 1, 3). We infer that a bract exists on the dorsal side of the cone because the fossil has decussate phyllotaxy. In later cone development in extant Ephedraceae, the proximal bracts tend to be smaller and shorter than half of the ovule. In E. archaeorhytidosperma, the ornamented ovules suggest that the ovulate cone is nearly mature (Figs. 1, 3). The bract of this fossil cone is small and hardly half the length of the ovules, which indicates that the bract may represent the proximal pair of bracts. Ovules of extant Ephedraceae are always axillary to a bract. The only bract that we could observe in E. archaeorhytidosperma is at the side of the seeds within the cone. Thus, another pair of bracts would be expected to be present on the dorsal side of each seed. A bract-scar is observed in this position on the same ovule mentioned above (Fig. 3), so there might be altogether two pairs of bracts per cone in E. archaeorhytidosperma, which is similar to E. rhytidosperma (Fig. 15). Ephedra archaeorhytidosperma differs from E. rhytidosperma in two characters. First, E. archaeorhytidosperma has a reproductive branch that ramifies 2–3 times (Fig. 1) and does not have a woody stem, whereas E. rhytidosperma usually has a short, woody stem with reproductive branches that ramify only one or two times (Fig. 14). Second, the cones of E. rhytidosperma are usually sessile and solitary or oppositely inserted at nodes; however, in E. archaeorhytidosperma, the cones are usually terminal on branchlets. With regard to the fossil leaves, their overall morphology and two parallel veins show a general resemblance to leaves of extant Ephedra L., but we could not compare it with that of E. rhytidosperma in some details, such as the number of leaves at a node, or the degree of connation of leaves, because the fossil did not preserve such characters. Seed surface of E. frustillata Miers were described as reticulatly sculptured. But we had no seed source for comparitive material. Thus, future material of E. frustillata is needed for proper seed comparison.
Phylogenetic relationships of the fossils
Both vegetative and reproductive characters of E. archaeorhytidosperma are diagnostic for extant Ephedraceae. Based on the comparisons presented above, and in Table 1, E. archaeorhytidosperma is more closely related to extant Ephedra than to other ephedroid fossils. Ephedra archaeorhytidosperma is most similar to E. rhytidosperma in its reproductive structures and differs in only two vegetative organ characters. This close resemblance suggest that E. archaeorhytidosperma is closely related to E. rhytidosperma and that the similarities of reproductive structures are symplesiomorphic. Ephedra archaeorhytidosperma can reasonably be assigned to the same section of Ephedra as E. rhytidosperma, section Pseudobaccatae.
Systematic and evolutionary significance of Ephedra archaeorhytidosperma
The suggestive mat-like habit, strongly lignified, and multinodular stems with 2–3 pairs of bracts in the cone, coupled with the uni- or sometimes bi-ovulate cones, the large seed, and the distinct seed surface sculptures, suggest that Ephedra rhytidosperma is a derived form of Ephedra. Pachomova (1971)
and Mussayev (1978)
considered this species specialized and grouped it with other specialized types, such as E. equisetina Bunge within the Ephedra section Pseudobaccatae Stapf. The occurrence of E. archaeorhytidosperma in the Lower Cretaceous is of special interest because of its morphological similarities to the extant E. rhytidosperma.
Ephedra rhytidosperma might be considered an example of character stasis through geologic time as was observed for a Ceratophyllum L. species from the Eocene (Herendeen et al., 1990
). Some characters may be fixed early and remain in a population or taxon over many millions of years. This is true for characters of Mesozoic conifers (Miller, 1999
), ferns, Ginkgo L. (Tralau, 1968
) and cycads, and it may be the case in Gnetales.
The occurrence of Ephedra archaeorhytidosperma in northeast China demonstrates that species of Ephedra with an ornamented seed surface may have been more widespread in the past than the present limited area of Helan Mountain (Fu et al., 1999
). Ephedra archaeorhytidosperma co-occurs with Archaefructus in humid or aquatic sites (Wu, 1999
; Sun et al., 2001
, 2002
), while extant E. rhytidosperma is endemic to an extremely arid area (open places at alt. 1400–1600 m, annual average temperature: ca. 8°C, annual precipitation: 200–300 mm) . Wu (1999)
suggested that the habitat for northeast China was warm and humid in the early and middle Jurassic, but became arid in the early Cretaceous. However, plants in the Yixian Formation may represent a mesic and not an arid environment.
There have been widely different opinions about the origin of the genus Ephedra L. Based on the analysis of the distribution and relationships of modern species, the genus has been inferred to date back to the Tertiary or late Cretaceous (Gadach, 1964
; Vasil'chenko, 1965
) or to the Triassic (Mussayev, 1978
). The occurrence of ephedroid pollen as early as the Permian (Wilson, 1959
) indicates an ancient record for the family, although a recent study using a molecular clock suggests that Ephedra dates from approximately 8–32 Mya (Huang and Price, 2003
). Our evaluation of megafossil records suggests that the extant genus extends back to the Lower Cretaceous. Eames (1952)
, Hickey and Taylor (1996)
, and Yang (2001)
inferred that the ancestral female cones of early Ephedraceae bore secondary reproductive shoots that gave rise to the female reproductive units in the extant Ephedra L. by fusion and reduction. However, the basic pattern of female cones of E. archaeorhytidosperma and other ephedroid fossils is the same as that of extant Ephedra L., indicating that the hypothesized fusion and reduction of reproductive units would have already occurred by the Lower Cretaceous. According to Wu et al. (1986)
, ephedroid female cones appeared as early as the early Jurassic. Consequently, the ancestral seed cone with secondary reproductive shoots as inferred by Eames (1952)
, Hickey and Taylor (1996)
, and Yang (2001)
must date back to an age earlier than the Yixian Formation. Our inference coincides with the evidence that the ephedroid pollen was recorded as early as middle Permian (Wilson, 1959
) and was widely distributed in fossil palynofloras in the Northern Hemisphere during the Triassic (Crane, 1996
).
The ephedraceous fossils described from the Yixian Formation are herb-like, with shoots that ramify several times. More specimens are needed to define the woody versus herbaceous nature of ephedroid fossils from the Yixian Formation. These species have terminal cones on branchlets, which suggests that terminal cones maybe less derived than the pedunculate and sessile cones. This inference is important for determining polarity of characters, such as cone terminal/pedunculate/sessile, because all three character states are present in living species and taxonomically valuable in many cases.
Gnetalean megafossils are rare and often distinct from living forms (Krassilov and Bugdaeva, 1982
; Krassilov, 1986
; Crane and Upchurch, 1987
; Crane, 1996
; Cornet, 1996
; Krassilov et al., 1998
), or lack enough structural information compared with the extant Ephedra L. (Wu et al., 1986
; Guo and Wu, 2000
). Ephedra archaeorhytidosperma is an early fossil species with morphological characters comparable to extant Ephedra. We expect that the evolutionary history of the Ephedraceae will be further clarified with the discovery of additional fossils.
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FOOTNOTES |
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4 Author for correspondence (e-mail: dilcher{at}flmnh.ufl.edu
or zhiduan{at}ns.ibcas.ac.cn
)
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