AOBPreview originally published online on May 4, 2006
Annals of Botany 2006 98(1):123-140; doi:10.1093/aob/mcl078
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Former Diversity of Ephedra (Gnetales): Evidence from Early Cretaceous Seeds from Portugal and North America
1 Department of Palaeobotany, Swedish Museum of Natural History, Box 50007, S-104 05 Stockholm, Sweden, 2 Department of Botany, Stockholm University, S-106 91 Stockholm, Sweden, 3 Department of Geology, University of Aarhus, Universitetsparken, DK-8000 Århus C, Denmark and 4 Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3AB, UK
* For correspondence. E-mail Catarina.Rydin{at}nrm.se
Received: 5 October 2005 Returned for revision: 22 December 2005 Accepted: 6 March 2006 Published electronically: 4 May 2006
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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• Background and Aims The extant species of the seed plant group Gnetales (Ephedra, Gnetum and Welwitschia) have been considered a remnant of a much greater, now extinct, diversity due to the pronounced differences in form and ecology among the genera. Until recently, this hypothesis has not been supported by evidence from the fossil record. This paper adds to the expanding information on Gnetales from the Early Cretaceous and describes coalified seeds from Barremian-Albian localities in Portugal and USA.
• Methods The fossils were extracted from sediment samples by sieving in water. Adhering mineral matrix was removed by chemical treatment. Seeds were investigated using light and scanning electron microscopy. Morphology and anatomy of the seeds were documented and compared with those of extant species.
• Key Results The fossils share characters with extant Ephedra, for example papillae on the inner surface of the seed envelope and in situ polyplicate pollen grains that shed the exine during germination. They differ from extant Ephedra seeds in morphological and anatomical details as well as in their smaller size. Two new species of Ephedra are described together with one species assigned to a new genus of Gnetales. Other Ephedra-like seeds, for which pollen and critical morphological details are currently unknown, are also present in the samples.
• Conclusions These Cretaceous seeds document that key reproductive characters and pollen germination processes have remained unchanged within Ephedra for about 120 million years or more. There is sufficient variety in details of morphology to suggest that a diversity of Ephedra and Ephedra-like species were present in the Early Cretaceous flora. Their presence in Portugal and eastern North America indicates that they were widespread on the Laurasian continent. The fossil seeds are similar to seeds of Erdtmanithecales and this supports the previously suggested relationship between Erdtmanithecales and Gnetales.
Key words: Early Cretaceous, Ephedra, Erdtmanispermum, Erdtmanithecales, Eucommiidites, fossil pollen, fossils, Gnetales, Portugal, Potomac Group, seeds
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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The three extant genera Ephedra, Gnetum and Welwitschia comprise the Gnetales, a small group of seed plants with a poorly known evolutionary history. The three genera are very different in overall form and ecology and the monophyly of the group has sometimes been questioned (Eames, 1952
; Nixon et al., 1994). However, several molecular studies (Goremykin et al., 1996; Bowe et al., 2000; Chaw et al., 2000; Magallón and Sanderson, 2002; Rydin and Källersjö, 2002; Rydin et al., 2002) support monophyly of the Gnetales, as well as the position of Ephedra as sister group to a Gnetum-Welwitschia clade. Gnetalean monophyly and this pattern of relationships was hypothesized originally based on morphology (Crane, 1985; Doyle and Donoghue, 1986, 1992; Doyle, 1996).
Arber and Parkin (1908) interpreted the marked morphological differences among the three extant genera as an indication of a long evolutionary history and, even though no unambiguous fossil representatives of the group were known at that time, they suggested that extant Gnetales constitute merely a remnant of ‘former greatness’ (Arber and Parkin, 1908). Later studies of dispersed pollen lent support to this suggestion by documenting the extensive fossil record of polyplicate pollen grains often referred to as ‘ephedroids’ that in general morphology are very similar to the pollen of extant Ephedra and Welwitschia (Wilson, 1962; Crane and Lidgard, 1989; Osborn et al., 1993; Crane, 1996; Osborn, 2000). Discoveries of well-preserved and informative mega- and mesofossils from the Early Cretaceous have provided further undisputed support for a long and rich fossil history of the group. Cratonia cotyledon (Rydin et al., 2003) and Welwitschiostrobus murili (Dilcher et al., 2005) from Brazil, and possibly also Drewria potomacensis (Crane and Upchurch, 1987) from Virginia, USA, are Early Cretaceous members of the Gnetum-Welwitschia clade. Otherwise many of the Early Cretaceous gnetalean fossils in the literature appear more closely related to extant Ephedra (Wu et al., 1986; Cao et al., 1998; Krassilov et al., 1998; Guo and Wu, 2000; Wu et al., 2000; Sun et al., 2001; Tao and Yang, 2003; Rydin et al., 2004; Yang et al., 2005). There are also Mesozoic fossils, with affinity to the Gnetales, which are difficult to assign to any of the extant lineages (Krassilov, 1982, 1986; Krassilov and Bugdaeva, 1988; Duan, 1998; Sun et al., 2001).
Extant Ephedra comprises approx. 35–40 species (Kubitzki, 1990) inhabiting arid environments of the northern hemisphere and South America. They are shrubs with opposite to whorled branching and reduced leaves that are decussate or in whorls. Female cones have fleshy or dry bracts and seeds with an integument surrounded by an outer envelope. The apical part of the integument forms a micropylar tube that extends beyond the outer seed envelope and serves as the pollen-receiving area. Male cones consist of microsporangiophores with fused microsporangia, surrounded by paired bracts. The pollen grains are distinctive with aligned ridges and valleys (polyplicate) that form alternating thicker and thinner regions of the exine (Steeves and Barghoorn, 1959; El-Ghazaly and Rowley, 1997; Osborn, 2000). The exine is discarded during germination (El-Ghazaly et al., 1998).
All extant species of Ephedra are very similar in gross morphology and have few informative characters in gene regions investigated so far [18S, 26S, ITS, rbcL, trnL, rps4 (Rydin et al., 2004); rps4, ITS1 (Ickert-Bond and Wojciechowski, 2004); rbcL, matK, ITS1 (Huang et al., 2005)]. Phylogenetic analyses indicate that major groups of species within Ephedra correspond to geographic regions, but relationships among these groups are not well supported (Ickert-Bond and Wojciechowski, 2004; Rydin et al., 2004; Huang et al., 2005). Within the genus, the absence of obvious morphological synapomorphies for extant subgroups, and the apparently homoplasious nature of many characters, have hampered traditional classification as well as phylogenetic reconstruction of relationships and molecular dating analyses. Groups defined on morphological characters (Stapf, 1889; Mussayev, 1978; Freitag and Maier-Stolte, 1994) are not fully consistent with the phylogenetic history of the genus indicated by molecular data. Huang and Price (2003) suggested that extant species of Ephedra evolved recently (Oligocene to Miocene) based on the paucity of base substitutions in the rbcL gene and an assumption of a molecular clock calibrated by landmark events. The current lack of morphologically well-defined subgroups impedes a placement of the fossils in any of the subgroups and thus also makes it difficult to date subgroups by fossil calibration.
In this paper a set of fossils are described from the Early Cretaceous Buarcos and Torres Vedras localities, Portugal and from Drewry's Bluff, Virginia, USA. The material consists of coalified Ephedra-like seeds, which based on differences in detailed morphology are assigned to several new species. The fossil seeds add to knowledge of the evolutionary history of the Gnetales and provide clear evidence for the early occurrence of several unique characters of extant Ephedra.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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The fossils were collected from Early Cretaceous localities in Portugal and USA. Samples from Portugal were collected from fluviatile sands and clays at two localities: Buarcos and Torres Vedras, in the western Portuguese (Lusitanian) Basin. Both localities have now been obliterated by town development.
The Buarcos locality was a partly overgrown road cut in the town of Buarcos, situated north of Figueira da Foz (Beira Litoral region), 40 °09'54''N; 8 °52'11''W, Carta Geológica de Portugal 19C Figuera da Foz (Rocha et al., 1981). Samples were collected by E. M. Friis and K. R. Pedersen in 1992–1997. A small palynoflora was reported from the Buarcos locality by Pais and Reyre (1981). Subsequently, rich mesofossil assemblages have been extracted from the sediment sequence including many angiosperms (Friis et al., 1997, 1999, 2000a, b). The plant-bearing sequence from Buarcos is included in the lowermost member (Calvaria Member) of the Figueira da Foz Formation, thought to be of late Aptian to early Albian age (Dinis, 2001) or early Albian (Heimhofer et al., 2005).
The Torres Vedras locality was an open-cast clay pit on the north-eastern outskirts of Torres Vedras (Estremadura region), about 1 km north-east of Forte de Forca on the road to Sarge, 39 °06'13''N; 9 °14'47''W, Carta Geologica de Portugal Torres Vedras 30C, (Zbyszewski et al., 1955). The sample studied here (Torres Vedras 43) was collected in 1989 by E. M. Friis and K. R. Pedersen, in a sandy, lignitic horizon referred to the basal member of the Almargem Formation, at the same level as sample 44 described in Friis et al. (2004). According to Rey (1993), this part of the sequence is of late Barremian to early Aptian age. A late Barremian to early Aptian age of the Torres Vedras samples is also supported by mesofossil floras and in situ pollen grains (E. M. Friis, K. R. Pedersen and P. R. Crane, unpubl. res.).
Samples from the eastern USA were collected by P. R. Crane, A. N. Drinnan, E. M. Friis and K. R. Pedersen in 1988, from the Potomac Group sequence at the Drewry's Bluff locality along the James River, south-east of Richmond, Chesterfield County, VA (37 °25'30''N; 77 °25'15''W). All specimens examined here are from reworked clayballs sampled in the lower part of the exposed fluvial sequence, about 1–2 m above river level (Drewry's Bluff samples 163, 164 and 198). The Early Cretaceous sequence at the Drewry's Bluff locality belongs to the Patuxent Formation and the sediments are referred by Doyle and Hickey (1976) to the upper pollen Zone I of Brenner (1963), which according to Doyle (1992) may be of middle Aptian age. The clayballs are older than the sediments in which they occur and are referred by Brenner (1963) to the lower part of pollen Zone I. They were considered by Brenner (1963) to be of Barremian age, while Doyle (1992) indicated an early Aptian age for the lower part of pollen Zone I in the Potomac Group sequence.
The fossils were extracted from the sediment samples by sieving in water. Adhering mineral matrix was removed by treatment with hydrofluoric acid (40 %) and hydrochloric acid (10 %), followed by rinsing in water, following standard methods (Friis et al., 1988). The fossils were investigated initially using a stereomicroscope. For scanning electron microscopy (SEM), specimens were mounted on cleaned aluminium stubs, coated with gold for 90 s in a sputter coater, and examined with a Hitachi field emission-scanning electron microscope at 2 kV. Selected seeds were macerated for light microscope studies, using nitric acid followed by ammonia or by using sodium hypochlorate. Specimens and preparations are housed in the Swedish Museum of Natural History, Stockholm (S) and in the Field Museum, Chicago (PP).
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RESULTS |
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Formal descriptions
SPERMATOPHYTA
Order Gnetales
Genus Ephedra L.
Ephedra portugallica sp. nov. (Figs 1A–K and 2A–F).
Specific diagnosis
Fossil seeds and in situ pollen with their general organization and structure as in modern Ephedra (nucellus surrounded by integument and outer sclerenchymatous envelope, long micropylar tube extending beyond the envelope, papillae on inner apical part of envelope, pollen exine shed during germination), and with the following defining features:
Seeds small, 0·85–1·2 mm long, 0·7–0·8 mm wide, ovoid with rounded base and pointed apex. Attachment scar prominent, rounded to slightly squared, approx. 0·25 mm in diameter. Sclerenchymatic cells of outer seed envelope narrowly rectangular, approx. 10–30 µm long, longitudinally arranged. Pollen approx. 33–40 µm long, 20–25 µm wide, approx. 10–15 ridges on exposed side (inferred total of 20–30 ridges).
Etymology
Named from Portugal, where the fossils were found.
Holotype
S107680 (Buarcos sample 157, Figs 1A, 2A–C).
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Paratypes
S107678–S107679, S107681, S136808 (Buarcos sample 157); S107682, S107684–S107686, S148070–S148072, 148102 (Buarcos sample 209); S107687–S107689, S148077–S148078 (Buarcos sample 211); S107683, S134042, S148073, S148087–S148088, S148090, S148092, S148094–S148096 (Buarcos sample 243); S148060 (Buarcos sample 267).
Locality
Buarcos, North of Figueira da Foz, Portugal (40 °09'54''N; 8 °52'11''W).
Stratigraphic position and age
Calvaria Member, Figueira da Foz Formation. Early Cretaceous (late Aptian or early Albian).
Description and comments on Ephedra portugallica
This seed type is the most abundant and informative of the fossil ephedroid seeds from the Buarcos locality and is known from more than 50 specimens.
Shape and size of seeds
The seeds are small, about 0·85–1·2 mm long and 0·7–0·8 mm wide. They are often compressed to various degrees but have a basically ovoid shape with a rounded base and pointed apex (Fig. 1A–G). In cross-section they are almost circular, to faintly four-angled but they are more distinctly four-angled towards the apex (Fig. 2A). In some specimens the four-angled form is emphasized by distinct longitudinal ridges that probably mark the position of vascular bundles along the four edges (Figs 1A, B and E). The seeds sometimes split apically into four valves along the ridges (Fig. 1G). The basal attachment scar is distinct and circular to four-angled, approx. 0·25 mm in diameter (Fig. 1B, C and F).
Organization and structure of seeds
The seeds are composed of three distinct parts: an innermost membranous nucellus preserved as a delicate cuticle, a membranous integument and a thick sclerenchymatic outer seed envelope. The nucellus has been observed only in macerated specimens. It is ovate to elliptic in longitudinal outline with an irregular apical area. The integument is ovate to elliptic in longitudinal outline and extended at the apex into a long, narrow micropylar tube (Fig. 2E) that is circular in cross-section (Fig. 2A). The outer seed envelope consists of an outer epidermis, a middle sclerenchyma layer, and an inner epidermis. In one specimen the outer epidermis is well preserved (Fig. 1B), but otherwise this layer is usually abraded over most of the seed surface except for small areas in the apical region. The shape and size of the epidermal cells are unclear. The sclerenchyma layer constitutes the bulk of the envelope and the sclerenchyma cells are generally well preserved. They are narrowly rectangular, about 10–30 µm long, and longitudinally arranged (Fig. 1H and I).
The inner epidermis of the seed envelope has, in the apical region adjacent to the micropylar tube, a distinct and well-preserved zone of papillae extending from the top of the seed envelope and about 0·17 mm downwards. The papillae are observed in specimens where the envelope is split apically (Fig. 1G, J and K), and sometimes in well-preserved intact specimens where the micropylar area is not filled in with debris or collapsed cells. Each papilla is about 10 µm long, but they decrease in size and prominence towards the base of the papillate zone.
In-situ pollen grains
Pollen grains were found in situ in the micropylar tube of three specimens. In two specimens (Fig. 1A and D) pollen grains were observed at the top of the micropyle of seeds mounted for SEM (Fig. 2A–D). The pollen grains are ellipsoid, approx. 33–40 µm long, and approx. 20 µm wide, with 10–15 ridges on the exposed surface; the inferred number of ridges for the whole grain is thus 20–30. There is no visible aperture. The surface of the pollen wall is indistinctly rugulate (Fig. 2C). In the macerated specimen (Fig. 2E and F) pollen grains were observed inside the integument in the micropylar tube near the apex of the nucellus. These pollen grains show upcurled pollen exines (Fig. 2F).
Ephedra drewriensis sp. nov. (Figs 3A–K and 4A–D)
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Specific diagnosis
Fossil seeds and in situ pollen with their general organization and structure as in modern Ephedra (nucellus surrounded by integument and outer sclerenchymatous envelope, long micropylar tube extending beyond the envelope, papillae on inner apical part of envelope, pollen exine shed during germination), and with the following defining features:
Seeds small, 1·2 and 1·4 mm long, 0·8–0·9 mm wide, ellipsoid with rounded base and short pointed apex. Attachment scar indistinct, rounded, approx. 0·15 mm in diameter. Sclerenchymatic cells of outer seed envelope narrowly rectangular, up to 70 µm long, longitudinally arranged. Pollen approx. 50 µm long, 20 µm wide, narrowly ribbed, six or seven ridges on exposed side (inferred total of 12–14 ridges).
Etymology
Named from Drewry's Bluff, where the fossils were found.
Holotype
PP44843 (Drewry's Bluff sample 198), (Fig. 3A and J).
Paratypes
PP44844-PP44845, PP44878 (Drewry's Bluff sample 198); PP44854, PP44866–PP44867 (Drewry's Bluff sample 163); PP44874–PP44875 (Drewry's Bluff sample 164).
Locality
Drewry's Bluff on the James River, south of Richmond, Chesterfield County, VA. USA.
Stratigraphic position and age
Clayballs in Patuxent Formation, Potomac Group. Pollen Zone I, Early Cretaceous (early Aptian).
Description and comments on Ephedra drewriensis
About 10–15 seeds of this type were found in the samples from Drewry's Bluff.
Shape and size of seeds
The seeds are about 1·2–1·4 mm long and 0·8–0·9 mm wide. They are compressed to various degrees but have a basically ellipsoid shape with a rounded base and pointed apex. They are often flattened and faintly four-angled in cross-section, but they are more distinctly four-angled towards the apex (Fig. 3A–G). Weak longitudinal ridges correspond to the position of vascular bundles (Fig. 3B, C, E and H). The seed envelope sometimes split apically into four valves (Fig. 3G). The basal attachment scar is indistinct, circular, and approx. 0·15 mm in diameter.
Organization and structure of seeds
The seeds are composed of three distinct tissues: an innermost nucellus surrounded by a membranous integument, and a thick sclerenchymatic outer seed envelope. The nucellus has been observed only in macerated specimens. It is ovate to elliptic in shape and with an irregular apical area. The integument is elliptic in shape and apically extended into a micropylar tube that is circular in cross-section (Fig. 4A, B and D). The outer seed envelope consists of an outer epidermis, a middle sclerenchyma layer and an inner epidermis. The outer epidermis is partly preserved in some specimens (Fig. 3B and D), and consists of more or less isodiametric cells approx. 9–13 µm across. The sclerenchyma cells are generally well preserved, narrowly rectangular up to 70 µm long, and longitudinally arranged (Fig. 3H and I).
Papillae occur on the inner surface of the outer seed envelope in an area extending from the apex to approx. 0·17 mm downwards. The papillae are 10 µm wide and 10–20 µm long, but the lowermost papillae are less well developed. Papillae can be observed in split open seed envelopes (Fig. 3G and K), in some intact seeds (Fig. 3A, F and J) and in macerated specimens (Fig. 4D).
In-situ pollen grains
Specimen PP44875 (Figs 3C and 4A–D) was macerated and at least five polyplicate pollen grains were found in situ inside the integument (Fig. 4C). They are narrowly elliptic, nearly 50 µm long and 20 µm wide, ribbed with six or seven ridges on one side. The total number of ridges inferred for pollen grains of Ephedra drewriensis is 12–14. There is no visible aperture in the pollen grains. Some grains are intact, but there are also several upcurled pollen exines (Fig. 4C).
Ephedrispermum gen. nov. (Fig. 5A–N)
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Generic diagnosis
Fossil seeds with their general organization and structure similar to modern Ephedra. Seeds ovoid to almost spherical with a short pointed apex. Nucellus and integument surrounded by an outer sclerenchymatous seed envelope, seed envelope distinctly four-angled towards apex. Sclerenchyma cells rectangular and in two layers, outer layer cells transversely arranged, inner layer longitudinally arranged. Papillae on seed envelope not observed. Pollen grains narrowly ribbed.
Etymology
These Ephedra-like seeds have been named from Ephedra and spermum meaning seed.
Type species
Ephedrispermum lusitanicum gen. et sp. nov.
Comments on the new genus Ephedrispermum
The seeds are similar in organization and structure to seeds of Ephedra, but discarded pollen exines and papillae on the seed envelope, characters that are diagnostic of extant Ephedra, have not been detected. Seeds of Ephedrispermum further differ from Ephedra in the cross-wise pattern of sclerenchyma cells on the seed envelope. As far as is known, this pattern is not present in Ephedra and, therefore, a new genus within the Gnetales has been established for these seeds.
Ephedrispermum lusitanicum gen. et sp. nov. (Fig. 5A–N)
Specific diagnosis
As for the genus with the following additions. Seeds minute, 0·6–0·9 mm long, 0·6–0·8 mm wide, ovoid to spherical with short pointed apex. Attachment scar prominent, rounded, approx. 0·1 mm in diameter. Outer layer of sclerenchyma cells rectangular, up to 30 µm long, transversely arranged. Inner layer cells up to 30 µm long, longitudinally arranged. Pollen grains approx. 23 µm long, 10 µm wide, approx. six ridges on the exposed side (inferred total of up to 12 ridges).
Etymology
From Lusitania, name of Roman province covering central Portugal where the fossils were collected.
Holotype
S148062 (Buarcos sample 244; Fig. 5A and L).
Paratypes
S148062–S148064 (Buarcos sample 244); S148079–S148083 (Buarcos sample 157); S148097, S148099 (Buarcos sample 209); S148076 (Buarcos sample 211); S148074, S148085–S148086, S148091 (Buarcos sample 243); and S148106–S148109, S148111–S148112, S148114–S148120, S148125, S149286–S149289 (Torres Vedras sample 43).
Locality
Buarcos, North of Figueira da Foz, Portugal (40 °09'54''N; 8 °52'11''W).
Stratigraphic position
Calvaria Member, Figueira da Foz Formation. Early Cretaceous (late Aptian or early Albian).
Description and comments on Ephedrispermum lusitanicum
About 35 seeds of this type have been found in samples from Torres Vedras and Buarcos. They are typically less well preserved than seeds of Ephedra portugallica and Ephedra drewriensis. This is particularly true for seeds from the Buarcos locality.
Shape and size of seeds
Seeds are minute, 0·6–0·9 mm long and 0·6–0·8 mm wide, broadly ovoid to almost spherical in shape (Fig. 5A–K) and faintly four-angled in cross-section except at the apex where they are distinctly four-angled (Fig. 5A–K). The seeds are rounded at the base and have a circular attachment scar, about 0·1 mm in diameter, and a short, pointed apex (Fig. 5E and F).
Organization and structure of seeds
The seeds are apparently identical to other ephedroid seeds from Buarcos in organization and structure, although they are smaller, nearly spherical and not as well preserved. Maceration of seeds failed to provide information on the nucellus, the shape of micropylar tube, and the inner epidermis of the outer seed envelope. The outer epidermis of the outer seed envelope is poorly preserved in most specimens. The sclerenchymatic part of the seed envelope consists of at least two cell layers. Cells in the outer layer are rectangular, up to 30 µm long and arranged in transverse rows (Fig. 5 M and N). Cells of the inner layer are more narrowly rectangular, up to 30 µm long and longitudinally aligned. The micropylar region is generally poorly preserved and papillae have not been observed.
In-situ pollen grains
Two specimens (Fig. 5A and B) have pollen grains in the micropyle. The pollen grains are polyplicate and small, only approx. 23 µm long and 10 µm wide. The grains in the holotype (Fig. 5A and L) have up to six exposed ridges (the total number of ridges is probably less than 12). The poorly preserved pollen grain in the micropyle of the paratype S148064 (Fig. 5B, grain not shown in close up) is only 15 µm long and has two or three ridges. This is likely to be a foreign grain.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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Systematic and nomenclatural comments
The fossil seeds described here share many characters with each other and with seeds of extant Ephedra. These include a sclerenchymatous outer seed envelope that surrounds the nucellus and integument, a long micropylar tube that extends beyond the outer envelope, and in situ polyplicate pollen grains. Based on differences in pollen features, in the shape and size of the seeds and in the cellular details of the seed wall (Table 1), they have been described as three new species. Two of the species have papillae on the inner, apical part of the seed envelope and pollen that shed the exine during germination. This character combination is only known for seeds of Ephedra and they have been assigned to two fossil species of the extant genus: Ephedra portugallica and Ephedra drewriensis.
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The third species also has general characters of Ephedra including a sclerenchymatic seed envelope and polyplicate pollen in situ in the micropyle. However, two diagnostic characters of Ephedra (discarded pollen exines and papillae on the seed envelope) have not been observed in any specimen of this species. Further, these seeds differ from Ephedra in the cross-wise pattern of sclerenchyma cells on the seed envelope, a pattern that as far as is known is not present in Ephedra and, therefore, a new genus has been established, Ephedrispermum, to accommodate these seeds.
Interpretations of the new fossils and comparison with extant species
Seed characters
Seeds of the three new fossil species are all small, ranging in length between 0·6 and 1·6 mm, and all possess a slightly four-angled sclerenchymatic outer seed envelope with a rounded base and a pointed apex (Figs 1–5). The shape of the fossil seeds varies from ovoid in Ephedra portugallica, ellipsoid in Ephedra drewriensis to more globose in Ephedrispermum lusitanicum. Many extant species have ovoid seeds, but more ellipsoid and globose seeds also occur. The apex can be distinctly or less prominently pointed, in the fossils as well as in extant seeds. In the fossils, four longitudinal ridges extend from the apex and the seed envelope is sometimes split from the apex into four valves. Xylem cells with a spiral to helical wall pattern have been observed on the longitudinal ridge of one fossil (Figs 3C and H) and the ridges probably correspond to the position of vascular bundles. The four-angled nature is evident in all the fossils, whereas the shape of extant seeds varies from two to three to four angles (four-angled seed of Ephedra californica shown in Fig. 6A). The shape (the number of angles) of extant seeds is partly related to the number of seeds in the cone, a character that varies among and within species.
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The outer seed envelope in Ephedra seeds consists of an outer epidermis, a middle sclerenchyma layer, and an inner epidermis. In the fossils, the outer epidermis is typically abraded over most of the seed surface except for small areas in the apical region, but the sclerenchyma cells that comprise the bulk of the envelope are generally well preserved in all three species. In Ephedra drewriensis the sclerenchyma cells are very long and narrow, up to 70 µm long, while those of the Portuguese species Ephedra portugallica and Ephedrispermum lusitanicum are 30 µm or shorter. In Ephedra portugallica and Ephedra drewriensis, these cells are longitudinally arranged. The seeds of Ephedrispermum lusitanicum are often preserved slightly differently and the shape and size of the sclerenchyma cells are not always clear. Species with well-preserved cell patterns reveal an outer layer of transversely arranged sclerenchymatic cells covering an inner layer of longitudinally arranged cells, which together give the seeds a chequered appearance (Fig. 5G–J and M–N), not known from seeds of extant Ephedra. This pattern is less clear in some specimens (Fig. 5A–C and E), but is still obvious on parts of these seeds upon careful examination (Fig. 5 M).
The shape and arrangement of sclerenchymatic cells of the seed envelope were investigated for nine extant species from all major subgroups of Ephedra (see Table 1). Removal of the epidermis revealed a layer of longitudinally arranged sclerenchymatic cells that range in length from approx. 50 µm (Fig. 6B) to 200 µm or more. These cells gradually decrease in length towards the apex. From this limited sample it seems as if Old World species on average have shorter cells than New World species. The same pattern is observed in the fossils.
Maceration of the seeds to isolate acid resistant cuticles and pollen walls, shows both the cuticles of the outer seed envelope and the thin cuticles of the integument and nucellus (Figs 2E and 4A and B). The main body of the integument is ovate to elliptic in lateral view and tightly adheres to the nucellus. Apically it is extended into a long micropylar tube that protrudes beyond the outer seed envelope (Figs 2E and 4A and D). The protruding part of the tube breaks easily, which is obvious in the fossils as well as in extant seeds. Inside the envelope, the micropylar tube is supported by papillae (Fig. 6C and D) located on the inside of the upper part of the outer seed envelope (Thoday and Berridge, 1912; Rydin et al., 2004). Occurrence of papillae on the outer seed envelope was investigated for 25 extant Ephedra species representing major subgroups (Rydin et al., 2004); all possessed this character.
The fossils Ephedra drewriensis and Ephedra portugallica also have papillae, which may be observed in specimens where the envelope is split apically (Figs 1G, J and K and 3G and K), in some well-preserved intact specimens (e.g. Fig. 3J), and also in macerated specimens (Fig. 4D). In extant and fossil Ephedra, the papillae adhere to the micropylar tube (Fig. 6C) and are gradually transformed downwards into ordinary cells of the inner epidermis of the envelope (Fig. 3G and K). The papillae of the fossil seeds are identical in shape and position to those of extant Ephedra. Papillae were not found in Ephedrispermum lusitanicum, but no seed of this type had split open prior to preservation, and papillae can be difficult to see in intact seeds.
Pollen characters
Polyplicate pollen grains (Fig. 6E and F) occur in all extant species of Ephedra, and have also been found in situ inside the fossil seeds. The surface of the pollen wall is indistinctly rugulate in the holotype of Ephedra portugallica (Fig. 2C), but this feature is not seen in other specimens and it is unclear if this is a result of the fossilization process. The pollen grains are narrowly ribbed and about 33–40 µm long in Ephedra portugallica, about 50 µm long in Ephedra drewriensis, and only about 23 µm long in Ephedrispermum lusitanicum. Pollen of extant Ephedra ranges between 30 and 70 µm in length, 40–50 µm being common in many species. The pollen grains of Ephedrispermum lusitanicum are thus smaller than what is usually seen in extant taxa.
The number of ridges on pollen grains of extant Ephedra differs among species. According to Kubitzki (1990), the range is from 6 to 18, but Steeves and Barghoorn (1959) report up to 20 ridges for Ephedra alata. Intraspecific variation has been reported for grains of extant Ephedra (Steeves and Barghoorn, 1959; El-Ghazaly and Rowley, 1997; Ickert-Bond et al., 2003). In the fossils, pollen grains are always preserved inside the micropyles and are often folded or overlapping. It is not possible to get a three-dimensional view of these grains that exposes all sides, and it is therefore difficult to assess the exact number of ridges. Therefore, the number of visible ridges (generally the number of ridges on one side) and the total number of ridges (inferred to be about twice as many) are reported. Grains of Ephedra portugallica have 10–15 exposed ridges (inferred total of 20–30) (Figs 2A, B and D). Pollen grains of Ephedra drewriensis have fewer ridges, only six or seven on the exposed side (inferred total of 12–14) (Fig. 4C). The numbers of ridges on the fossil pollen grains of Ephedra drewriensis are well within the range of variation seen in modern species, whereas the inferred total of 20–30 ridges in Ephedra portugallica (Fig. 2A, B and D) is higher than what is generally seen in extant taxa. In Ephedrispermum lusitanicum, one of the grains in the holotype has six visible ridges (total number probably <12) (Fig. 5L). The other grain conceivably has fewer ridges, but it is partly covered and the number of ridges is difficult to assess. A single pollen grain observed in paratype S148064 (Fig. 5B, not shown in close up), is only 15 µm and seems to have two or three ridges. This grain is covered in a gluey substance and is considered to be foreign. Thus, the variation in size and number of ridges seen in grains of Ephedrispermum lusitanicum is probably an artefact owing to one partly covered and one foreign grain.
The exine of extant Ephedra pollen grains splits up during germination and is shed, leaving the male gametophyte naked in a characteristic way (El-Ghazaly et al., 1998). Pollen grains were found inside the micropyles at the top of the megasporangium in specimens of Ephedra drewriensis and in Ephedra portugallica. In both species these included grains with curled up, shed exines (Figs 2E and F and 4B and C), which completely match the shed, upcurled, transversely striated exines described for extant Ephedra (El-Ghazaly et al., 1998). Pollen grains of Welwitschia are similar to those of Ephedra in size and shape but Welwitschia grains possess a sulcus through which germination occurs and the exine remains on the gametophyte (Rydin and Friis, 2005). The unusual habit of shedding the pollen exine prior to male gametophyte growth and pollen tube formation is evidently unique to Ephedra among the polyplicate pollen-producing genera of the Gnetales (Rydin and Friis, 2005).
Comparison with other fossil seeds and pollen
Fossil Gnetales
The fossils described here are currently the only coalified and three-dimensionally preserved ephedroid seeds reported from the fossil record. In addition to the three newly described species, a diversity of other Ephedra-like seeds has been found in samples of Early Cretaceous age from Portugal and the Potomac Group sequences (Fig. 7A–L). However, these seeds differ from those described here in key details and lack diagnostic features such as in situ pollen grains and papillae on the seed envelope. This material requires further study to establish their exact systematic position, but they provide additional evidence that a diversity of ephedroid species were present in the Early Cretaceous vegetation.
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The only other Ephedra fossil in which details of seeds are preserved is Ephedra archaeorhytidosperma, from the Early Cretaceous Yixian Formation of north-eastern China (Yang et al., 2005
). It is a compression fossil of a fertile shoot that has strong, vegetative similarities to Ephedra. Female cones are terminal on the branches. Seeds are partly enclosed by remnants of bracts, and there are weak impressions of an extended micropylar tube. The seeds of Ephedra archaeorhytidosperma resemble the fossils presented here in their size, ovoid shape with a pointed apex and the presence of an extended micropyle, but are clearly different in having a prominent, longitudinal, dorsal ridge and a seed surface characterized by wavy transverse ridges.
A variety of dispersed polyplicate pollen types have previously been recorded from the Drewry's Bluff locality (Brenner, 1963). Many are clearly ephedroid but others are more similar to Welwitschia, e.g. those found in association with Drewria potomacensis (Crane and Upchurch, 1987). The pollen grains associated with Drewria are clearly distinct from those found in situ in Ephedra drewriensis. Grains of Ephedra drewriensis appear inaperturate as in pollen of extant Ephedra, and they shed the pollen exine during germination.
In addition, there are dispersed and in situ grains of Eucommiidites (Erdtmanithecales) at the same locality (further discussed below).
Fossil Erdtmanithecales
Among seeds of other gymnosperms only those of the extinct Erdtmanithecales (Fig. 8A–D) are comparable in gross morphology to the fossil Ephedra seeds described here. Erdtmanithecales is an order of seed plants established by Friis and Pedersen (1996) to encompass dispersed pollen organs and seeds with Eucommiidites pollen in situ in the pollen sacs and inside the micropyles of the seeds.
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The pollen genus Eucommiidites (Fig. 8D) was established by Erdtman (1948)
for pollen grains from the Early Jurassic of Scania, Sweden, characterized by three elongated slits. Erdtman described the pollen as tricolpate and compared it with pollen of the angiosperm genus Eucommia. It was later shown that the colpi are unevenly distributed and the grains were re-interpreted as non-angiospermous pollen grains with one distal colpus and two subsidiary, lateral colpi (Couper, 1956, 1958). A non-angiospermous affinity of the grains was later demonstrated by the discovery of pollen grains in situ in the micropyle of small seeds (Hughes, 1961; Brenner, 1963, 1967) (Fig. 8D).
Dispersed Eucommiidites pollen grains are widespread in Mesozoic palynofloras from the northern hemisphere, ranging in age from the Early Jurassic to the early Late Cretaceous, but all pollen organs and seeds discovered so far are from the Cretaceous. Pollen organs that can be assigned to the order include Erdtmanitheca (Pedersen et al., 1989), Eucommiitheca (Friis and Pedersen, 1996) and Bayeritheca (Kvacek and Pacltová, 2001). Seeds of Erdtmanithecales include fossils assigned to Erdtmanispermum (Pedersen et al., 1989) as well as two unnamed fossils (Hughes, 1961; Brenner, 1963, 1967). Erdtmanitheca is from the early Late Cretaceous (early Cenomanian) of North America (Texas), while all the other taxa are from the Early Cretaceous.
Seeds of Erdtmanispermum balticum (Erdtmanithecales) are coalified and comparable in preservation to the fossil Ephedra seeds described here. They were described from the earliest Cretaceous (Berriasian-Valanginian) sediments of Bornholm, Denmark (Pedersen et al., 1989) (see also Fig. 8A–D). They are ovoid in outline with a rounded base and a long, distinctly pointed micropyle (Fig. 8A and B). The seed wall is composed of a membranous integument and an outer sclerenchymatic seed envelope. The sclerenchyma cells are small and isodiametric (approx. 10 µm; Fig. 8C). The envelope is three-parted and gives the seed a weakly triangular cross-section. The envelope sometimes splits from the apex into three valves. In apical view the micropylar tube is circular and surrounded by the triangular envelope. Pollen grains of Eucommiidites-type were found inside the seeds by maceration (Pedersen et al., 1989) and were also observed by SEM in the top of the micropylar tube (Fig. 8D). The two unnamed seed types with Eucommiidites pollen in situ are preserved only by their seed cuticles and the nature of the seed wall is unknown, but the specimen from Drewry's Bluff figured by Brenner (1963) clearly shows a triangular form.
Seeds of Erdtmanispermum balticum strongly resemble the seeds described here, in size, in their ovoid shape with a prominently pointed apical region, in the angled envelope, probably derived from bracts, and in having a long micropylar tube. Erdtmanispermum (Erdtmanithecales) differs from fossil Ephedra seeds in the three-angled rather than four-angled nature of the envelope, the absence of papillae on the envelope, and in the much smaller, iso-diametric sclerenchymatic cells under the epidermis. They also differ in having a distinct megaspore membrane, which is unknown for Ephedra. Eucommiidites pollen (Erdtmanithecales) is smaller than Ephedra grains and distinct with only three ridges.
The microsporangiate units of Erdtmanitheca are described from the Early Cretaceous Vila Verde locality (Pedersen et al., 1989), not far from Buarcos, in sediments that may be of approximately the same age as the Buarcos locality, and plants of Gnetales and Erdmanithecales occur together, e.g. in the clayballs at the Drewry's Bluff locality. The possibility that small angular seeds from Buarcos and Drewry's Bluff could belong to Erdtmanithecales cannot be excluded. However, seeds of Ephedra portugallica and Ephedra drewriensis have clear affinity to Ephedra based on their relatively large pollen grains with many ridges and the presence of discarded exines in the micropyle. Ephedrispermum lusitanicum is more problematic as it has small pollen grains with few ridges. Ephedrispermum is, however, clearly distinct from Erdmanithecales in the four-angled nature of the seeds, the shape of the sclerenchyma cells and in the presence of pollen grains with more than three ridges.
The evolutionary history of Ephedra
Fossil evidence
Polyplicate pollen grains that may resemble those of extant Ephedra and Welwitschia are common in the fossil record and extend back to the Permian (Wilson, 1962). They become more common in the Triassic (Traverse, 1988), decline again in the Jurassic and expand rapidly in both diversity and abundance in the Early Cretaceous (Crane and Lidgard, 1989; Osborn et al., 1993; Crane, 1996). Many of the Cretaceous dispersed polyplicate pollen types most likely represent the Gnetales (e.g. Osborn et al., 1993), but the systematic position of pre-Cretaceous polyplicate pollen is more uncertain. Some of the Permian pollen grains, such as Vittatina, are known to be produced by extinct seed plants (peltasperms), while others are of unknown origin (Meyen, 1987).
The earliest megafossil possibly representing the Gnetales, is the Permian cone Palaeognetaleana auspicia that has in situ polyplicate pollen (Wang, 2004), but details of the organization and structure of this plant are not fully established. Pre-Cretaceous megafossils associated with the Gnetales are often morphologically distinct from extant groups and difficult to interpret. The Late Triassic plant Dechellyia gormani (Ash, 1972) and the early Jurassic Piroconites kuespertii (van Konjinenburg-van Cittert, 1992) are other examples of fossil plants that have been compared with Gnetales but are of uncertain relationship.
A study of Cretaceous floristic diversity based on palynological assemblages suggest that there was a striking increase in gnetalean diversity simultaneously with the initial angiosperm radiation in the Early Cretaceous, but that the Gnetales underwent a significant decline again through the Late Cretaceous (Crane and Lidgard, 1989). Unequivocal gnetalean megafossils are now known from Early Cretaceous strata. The Crato flora from Brazil (Mohr and Friis, 2000; Mohr and Eklund, 2003; Mohr and Bernardes-de-Oliveira, 2004; Mohr et al., 2004; Dilcher et al., 2005) contains several ephedroid plants. The Yixian Formation in western Liaoning China, also famous for its feathered dinosaurs and other exquisitely preserved vertebrates, has proven particularly rich in gnetalean fossils. The lower part of the formation has been radiometrically dated to approx. 125 Myr, corresponding to the Barremian (Early Cretaceous), and the fossil-bearing sediments extend into the Aptian (for a review of the geology and biology of the Yixian Formation, see Zhou et al., 2003).
Several Ephedra-like impression fossils have been discovered from the Yixian Formation including currently unpublished material (C. Rydin, S. Q. Wu and E. M. Friis in progress). Eragrosites changii and Liaoxia chenii (Cao et al., 1998) from the Yixian Formation both have opposite branching and small cone-like reproductive structures. They were originally described as angiosperms but are clearly ephedroid and Liaoxia and Eragrosites were later combined and re-described jointly as Ephedrites chenii (Guo and Wu, 2000). Ephedra archaeorhytidosperma (Yang et al., 2005), also from the Yixian Formation, has seeds with transverse laminar protuberances, comparable to those of extant Ephedra rhytidosperma. Ephedrites? elegans (Sun et al., 2001) from the Early Cretaceous of Liaoning, China, Ephedrites sinensis and Ephedra exhibens (Wu et al., 1986) from the Jurassic Xiaomeigou Formation, Qinghai, China, and Alloephedra xingxuei (Tao and Yang, 2003) from the Early Cretaceous Dalazi Formation in Jinlin, China, also belong to the Gnetales and are probably most closely related to Ephedra.
The fossils presented here provide details of the reproductive parts of Cretaceous Ephedra-like plants that have not been available in previously described material. They document that plants with characters such as apical papillae on the outer sclerenchymatic envelope, and polyplicate pollen grains that discard the exine during germination were already established in the Early Cretaceous. Such plants were widespread across Laurasia and were present in what is now the Old World, as well as in the New World. Together with other material that is not preserved at the same level of detail, these fossils suggest that a diversity of Ephedra and Ephedra-like plants were present in the Early Cretaceous. The only difference in general appearance between the fossil seeds and those of extant Ephedra is size. Seeds of modern Ephedra are much larger than their Cretaceous relatives. Whereas the fossils are generally <1·5 mm long, the length of extant Ephedra seeds is often up to 6–7 mm or more. Increase in seed size through time has been observed also for several angiosperm taxa, e.g. Spirematospermum (extinct Zingiberales) (Friis, 1988) and Stratiotes (Hydrocharitales) (Mai, 1985).
Phylogenetic implications
The information currently available on Cretaceous ephedroids is insufficient for detailed phylogenetic analysis, and the phylogenetically isolated nature of extant Gnetales further complicates outgroup comparison. It is interesting, however, that seeds and pollen of Erdtmanithecales are very similar to those of Ephedra. Based mainly on the granular structure of the exine, and the possible homology between the two lateral colpi in Eucommiidites with the furrows of Ephedra and Welwitschia grains, it has been suggested that Eucommiidites may have been produced by plants related to the Gnetales (Doyle et al., 1975; Trevisan, 1980; Crane, 1985; Pedersen et al., 1989). The decussate arrangement of the microsporangiate units of Eucommiitheca is in agreement with the opposite, decussate or whorled phyllotaxis of the Gnetales, but the peltate microsporangia-bearing units of Erdtmanithecales are very different from those in extant Gnetales. Peltate structures are, however, present in Aegianthus sibiricus, supposedly gnetalean male cones from the mid-Jurassic of East Siberia (Krassilov and Bugdaeva, 1988).
Seeds of Erdtmanithecales and Gnetales are also similar in overall structure. An origin of the outer seed envelope from bracts has previously been suggested for extant Gnetales, e.g. by Crane (1985) and by Pedersen et al. (1989). This is consistent with ontogenetic studies of extant Ephedra, which show that the inner integument is initiated as a circular protrusion, while the seed envelope initiates as two, three or four protrusions and is apparently of foliar nature (Pearson, 1929; Takaso and Owens, 1994; Yang, 2001, 2004). The angled nature of the seed envelope is prominent in the fossil forms and it is very likely that the seed envelopes of Ephedra, Ephedrispermum (Gnetales) and Erdtmanispermum (Erdtmanithecales) are derived from paired or whorled bracts. It seems probable that the Cretaceous forms more evidently reflect this evolutionary origin. In conclusion, some but not all features of the Gnetales are established in the extinct plant group Erdtmanithecales, and whether this indicates a position of Erdtmanithecales as sister group to Gnetales needs to be tested as more information accumulates on Gnetales and their fossil relatives.
Ephedrispermum lusitanicum, described here, has rather small pollen (23 µm) and seems to lack papillae on the seed envelope, as do seeds of Erdtmanithecales, but Ephedrispermum is clearly distinct from Erdtmanithecales with its polyplicate pollen and four-angled seed envelope with rectangular sclerenchyma cells. The similarities between Erdtmanispermum (Erdtmanithecales) and Ephedrispermum lusitanicum (Gnetales) are probably plesiomorphies and not indications of a sister relationship between the two.
Seeds of Ephedra portugallica, Ephedra drewriensis and extant Ephedra are distinguished from those of Erdtmanispermum and Ephedrispermum by the presence of apical papillae on the seed envelope and pollen grains that discard the exine during germination. Resolving relationships among these fossil and extant species is difficult, because the extant genus comprise a homogenous group of species in which morphological characters are often more or less identical. Moreover, some characters are evidently homoplasious while others are sometimes not uniform within species or even within an individual (El-Ghazaly and Rowley, 1997; Huang, 2000; Ickert-Bond et al., 2003; Ickert-Bond and Wojciechowski, 2004; Rydin et al., 2004; Huang et al., 2005). Currently, the variation in anatomical details and reproductive characters among extant species of Ephedra is evidently complex and poorly understood [also see Ickert-Bond and Wojciechowski (2004) and Huang et al. (2005) for a discussion of extant morphology].
Evidence from molecular data provides limited information on relationships among extant species of Ephedra. A previous study (Rydin et al., 2004), investigated six gene regions from the chloroplast and the nuclear DNA, but all contained very few informative characters (Table 2). Most of the information came from the internal transcribed spacer (ITS) of the nuclear ribosomal DNA. As previously reported (Maggini et al., 1998), the ITS1 region of Ephedra is very long compared with ITS1 in angiosperms, and has thre