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A male cone of Pseudofrenelopsis dalatzensis with in situ pollen grains from the Lower Cretaceous of Northeast China
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Geobios 41 (2008) 689–698 http://france.elsevier.com/direct/GEOBIO/
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A male cone of Pseudofrenelopsis dalatzensis with in situ pollen grains from the Lower Cretaceous of Northeast China§ Un cône mâle de Pseudofrenelopsis dalatzensis avec des grains de pollens in situ du Crétacé inférieur (NE, Chine) Xiao-Ju Yang Nanjing Institute of Geology and Palaeontology, Palaeobotany and Palynology, Chinese Academy of Sciences, 39, East Beijing Road, 210008 Nanjing, Jiangsu Province, China Received 12 March 2007; accepted 29 June 2008 Available online 17 July 2008
Abstract A Classopollis-containing male cone found in close association with Pseudofrenelopsis dalatzensis from the Lower Cretaceous Dalazi Formation of Yanji Basin, eastern Jilin, China was studied using scanning electron microscopy and described as Classostrobus dalatzensis sp. nov. The small oval cone, borne on the top of a stalk (fertile short shoot), consists of helically arranged microsporophylls, each with a rhomboid and expanded distal head. The short shoot is divided into indistinct nodes and internodes with fine longitudinal striations. Cuticles of the microsporophyll and the internode of the short shoot are generally similar to Pseudofrenelopsis dalatzensis internode cuticles, all of them having papillate outer surfaces. Stomata are similar in structure and arrangement, with papillae of subsidiary cells overhanging the stomatal pit. Epidermal cells are rectangular or isodiametrical, each with a robust papilla on the periclinal wall. Hypodermal cells are present between the stomatal files. The internal structure of the in situ Classopollis pollen grains was studied in detail. Pollen grain is small, with thin sexine, poorly developed nexine and may be immature. This is the first record of in situ pollen grains Classopollis in China of which the internal structure is known in detail. # 2008 Elsevier Masson SAS. All rights reserved. Résumé Un cône mâle contenant un pollen du type Classopollis et trouvé en association étroite avec le feuillage Pseudofrenelopsis dalatzensis dans la formation Dalazi du bassin de Yanji (Crétacé inférieur, Jilin oriental, Chine) a été étudié au microscope électronique à balayage et décrit comme Classostrobus dalatzensis sp. nov. Ce petit cône ovale, porté au sommet d’un pédoncule (brachyblaste fertile), est constitué de microsporophylles insérées hélicoïdalement, chacune portant à leur extrémité une expansion rhomboïdale. Le pédoncule, indistinctement divisé en nœuds et entrenœuds, est strié longitudinalement. La cuticule des microsporophylles et des entre-nœuds des pédoncules est généralement similaire à celle des entre-nœuds de Pseudofrenelopsis dalatzensis, toutes les deux montrant une surface externe papilleuse. La structure et la disposition des stomates sont similaires, avec des papilles des cellules subsidiaires surplombant l’ostiole. Les cellules épidermiques sont rectangulaires ou isodiamétriques, chacune avec une papille robuste sur la paroi anticline. Un hypoderme est présent entre les rangées stomatiques. La structure interne des grains de pollen Classopollis trouvés in situ a été étudiée en détail. Le grain de pollen est petit, avec une sexine fine, une nexine peu développée et probablement immature. Il s’agit de la première donnée pour la Chine de grains de pollen de Classopollis trouvés in situ dont la structure interne est connue en détail. # 2008 Elsevier Masson SAS. All rights reserved. Keywords: Classopollis; Pseudofrenelopsis; Cheirolepidiaceae; Cretaceous; China Mots clés : Classopollis ; Pseudofrenelopsis ; Cheirolépidiacées ; Crétacé ; Chine
§
Corresponding editor: Marc Philippe. E-mail address: [email protected].
0016-6995/$ – see front matter # 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.geobios.2008.03.001
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1. Introduction The Cheirolepidiaceae is a Mesozoic conifer family that was recorded from the Late Triassic to the Late Cretaceous around the world. Since the 1970, information about this group, its biology, structure and palaeoecology, have increased based on both palynological studies and megafossil investigations. One of the distinctive and reliable characters for this family is the possession of distinctive and unusual pollen grains of the genus Classopollis Pflug (Watson, 1988). A discovery of a permineralized fossil cone Classostrobus crossii Rothwell et al. from the Middle Jurassic of southern England provides the newly evidence of male cones contained Classopollis (Rothwell et al., 2007). In the Early Cretaceous, frenelopsid forms reached the peak of radiation more than 20 species have been recorded around the world based on the numerous limited and/or ambiguous materials of vegetative shoots. Although vegetative shoots are of rather frequent occurrence, male cones with in situ pollen grains are rare. Among the small number of male cones attributed to Frenelopsis and Pseudofrenelopsis, Classostrobus arkansensis (Axsmith et al., 2004), C. cathayanus (Zhou, 1983), C. comptonensis (Alvin et al., 1978, 1994), C. turolensis and C. ugnaensis (Gomez et al., 2002), and male cone of Frenelopsis alata (Kvacˇek, 2000) are well preserved. In China, only two forms of Classopollis-containing cones have been found: Classostrobus cathayanus (Zhou, 1983) and the male cone attributed to Pseudofrenelopsis papillosa (Zhou, 1995). In the Yanji area, eastern Jilin Province, several cheirolepidiaceous conifers were reported (Chow and Tsao, 1977; Zhang et al., 1980; Zhang, 1986; Zhou, 1995), but they are all fragmentary vegetative shoots. During a recent field trip to the same area, the present author collected a Classopollis-bearing cone associated with numerous Pseudofrenelopsis dalatzensis (Zhou, 1995) remains from the type locality of the Dalazi (Dalatze) Formation, Zhixin, Longjing county. Pseudofrenelopsis dalatzensis was described based on a few fragmentary vegetative shoots from the Early Cretaceous Dalazi Formation (Chow and Tsao, 1977; Zhou, 1995) and recently was found in the Lower Cretaceous Xiagou Formation of Jiuquan Basin, Gansu, northwestern China (Deng et al., 2005), but its reproductive organs have never been reported. This article presents new data regarding the male cone with in situ pollen grains, and cuticles of the microsporophyll and associated vegetative shoots. One of the most interesting and fortuitous results of this research is that some clumps of broken pollen grains reveal detailed internal structure of the in situ Classopollis pollen grains.
Fig. 1. Sketch map, showing the location of Longjing, Jilin Province, Northeast China.
oil shale beds and yields abundant animal and plant fossils (Zhang, 1986; Tao, 2000). Pseudofrenelopsis dalatzensis was the most common plant collected from the Dalazi Formation, especially in some dark gray-yellow fine-grained siltstones. The male cone is a compression with a short stalk. Pieces of cuticles and pollen masses were removed from the specimens, first treated with hydrofluoric acid (HF) for 12 hours, followed by maceration with Schulze’s solution (nitric acid and potassium chlorate). The time for oxidation depended upon the degree of coalification of the compressed specimen, usually about three to five hours for cuticle specimens and five to eight hours or more for pollen mass. After the specimens became yellow and translucent, they were rinsed with water and were treated for 0.5 to one minute with 5% ammonia and rinsed
2. Material and methods The material was collected from the middle part of the upper Lower Cretaceous Dalazi Formation, well exposed along a road near Zhixin town, Longjing county, Jilin province, northeastern China (Fig. 1). The Dalazi Formation, which is one of early angiosperm-bearing deposits in China, consists of gray-yellow conglomerates, sandstones and black shales, intercalated with
Fig. 2. a: C. dalatzensis sp. nov., holotype, PB20208, showing male cone borne on a short shoot and spirally arranged microsporophylls. Scale bar: 2 m; b: vegetative shoot of Pseudofrenelopsis dalatzensis, showing spirally arranged leaves and distinct longitudinal ridges and grooves, PB20207. Scale bar 4 mm.
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again. Cuticles of the stalk of cone and microsporophylls were observed with Nikon Eclipse E600 light microscopy and photographed with Nikon Coolpix 950 in the Department of Palaeobotany and Palynology of Nanjing Institute of Geology
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and Palaeontology, and then, together with cuticles of the vegetative shoot and pollen masses, were mounted respectively on stubs using double-sided adhesive tape, air dried and coated with gold, then observed and photographed with the LEO-1530
Fig. 3. a, b: cuticles from the cone stalk of C. dalatzensis sp. nov. (SEM); a: outer view of internode cuticle, showing papillae and holes that spread all over the outer surface. Scale bar 20 mm; b: inner view of internode cuticle, showing the well-defined stomatal and nonstomatal files. Note the presence of cutinized hypodermis in the nonstomatal files. Scale bar 50 mm; c, d: cuticles from the microsporophyll of C. dalatzensis sp. nov. (SEM); c: outer view of cuticle, showing papillose surface. Note the stoma (arrows). Scale bar 50 mm; d: inner view of cuticle, showing the stomatal and nonstomatal files. Note the stoma (arrows). Scale bar 50 mm; e, f: cuticles from the vegetative shoot of Pseudofrenelopsis dalatzensis (SEM); e: outer view of the abaxial cuticle, showing the papillate outer surface and the distinct stomatal files. Note each epidermal cell bears a papilla. Scale bar 10 mm; f: inner view of abaxial cuticle, showing the well-defined stomatal and nonstomatal files. Note the cutinized hypodermis consisting of elongate cells, and the thick anticlinal wall of polygonal or isodiametrical epidermal cells. Scale bar 10 mm.
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scanning electron microscope in the State Key Laboratory of Palaeobiology and Stratigraphy, Chinese Academy of Sciences, Nanjing, China. A vegetative shoot of Pseudofrenelopsis dalatzensis was macerated in order to compare with the cuticles of the stalk of the male cone and to verify the relationship between them (Figs. 2(b) and 3(e, f)). A detailed study of vegetative shoots will be the subject of a future contribution. All the specimens and stubs are housed in the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. 3. Systematic palaeontology Genus Classostrobus Alvin et al., 1978 Type species: Classostrobus comptonensis Alvin et al., 1978. Classostrobus dalatzensis sp. nov. Etymology: The specific epithet ‘‘dalatzensis’’ is coined after the type locality name and refers to the vegetative shoots of Pseudofrenelopsis dalatzensis with which the male cone is associated. Holotype: Specimen PB 20208. Repository: All described specimens are housed in the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China. Type locality: Dalazi, Zhixin, Longjing County, Jilin Province, Northeast China. Stratigraphic horizon: Dalazi Formation. Age: Early Cretaceous (Aptian-Albian). Diagnosis: Male cone borne terminally at the top of a stalk with fine longitudinal ridges and grooves on its surface. Microsporophylls helically arranged expanded distal head rhomboidal with an acuminate to acute apex. Microsporophyll cuticle similar to that of cone stalk. Each epidermal cell bearing a papilla on the outer surface of the periclinal wall. Stomatal files well-defined. Stomata elliptical to rounded, transversely or obliquely oriented. Guard cells slightly sunken, surrounded by five to six papillate subsidiary cells. Epidermal cells rectangular, polygonal or isodiametric. Cutinized hypodermal cells present between stomatal files, longitudinally elongated. In situ pollen of the Classopollis type, small, with thickened equatorial band bearing six to 11 striae. External sculpture of pollen baculate, internal sculpture verrucose, granulate or cerebrelloid. Description: General morphology. The Classopollis-containing male cone (Fig. 2(a)) is preserved isolated, borne at the top of a stalk (fertile short shoot). Stalk consists of distinct nodes and internodes about 11 mm long, 2.4 mm wide at the lower portion and 3.8 mm wide at the top. Outer surface of the stalk is covered with parallel, fine longitudinal ridges and grooves which represent the stomatal and nonstomatal files respectively. Male cone is ovoid in shape, 10 mm wide at the widest part and 14 mm long (not complete) with a central axis bearing numerous, spirally arranged, imbricate microsporophylls. Microsporophylls arise from the axis at wide angles
(sometimes at about 908). The microsporophyll consists of a proximal stalk (up to 4 mm long) and a distal rhomboidal head. The apex of the head is acuminate to acute and incurved. Fragments of pollen sacs containing Classopollis pollen grains have been found in maceration, but there is no definite information about their number, size and their attachment on the microsporophyll. Cuticle of the stalk of the male cone. Cuticle from the stalk of the male cone is thick. Outer surface is covered with numerous tiny holes and a robust papilla, about 12–16 mm in size, is almost invariably present on the outer surface of each epidermal cell (Fig. 3(a)). Stomatal files are well defined; stomata are rounded to elliptical, commonly about 35–40 mm in diameter and transversely or obliquely oriented; guard cells are slightly sunken, the outline of subsidiary cells is indistinct (Fig. 3(b)). Epidermal cells in stomatal files are polygonal or isodiametric, about 20–30 mm in diameter with about 1.3–2.5 mm thick anticlinal wall (Fig. 3(b)). Hypodermal cells between stomatal files are elongated and longitudinally arranged, about 7–25 mm 15–42 mm in size (Fig. 3(b)). There are no stomata in the marginal area that consist entirely of longitudinal elongated cells. Cuticle of the microsporophyll of the male cone. Cuticle of the microsporophyll is thick with cutinized hypodermis. The outer surface is covered with numerous tiny holes (Fig. 3(c)). Each epidermal cell bears one rounded papilla 9–13 mm in diameter on the outer periclinal wall, the aperture of stoma is surrounded by papillae of the subsidiary cells (Fig. 3(c)). Hypodermal cells between stomatal files are elongate, 3–8 mm wide and arranged in regular longitudinal files (Fig. 3(d)). Stomata are elliptical, typically 22 mm 30 mm in size (Fig. 3(d)). Outlines of subsidiary cells surrounding the two guard cells are indistinct (Fig. 3(d)). Guard cells are semilunar in outline, 15 mm long and 2.8 mm wide at the widest part, forming a transversely or obliquely oriented aperture. Outlines of epidermal cells are also not well marked, especially in stomatal rows (Fig. 3(d)). Pollen grains. Pollen grains are of the Classopollis type, more or less spherical, about 16–21 mm in diameter (mean 18 mm, 50 grains measured). Equatorial band is distinct (Figs. 4(b–f), 5(b, d, f)) and about 3.9–5.8 mm wide. In the inside view the band consists of eight to 11 transverse flanges and grooves. These flanges and grooves form the characteristic parallel equatorial striae of the pollen grains. The flanges are massive with a smooth or slightly wavy edge. In some case, the flange edges are uneven with flutes and papilla-like teeth (Fig. 4(d)). Occasionally, the flutes of adjacent flanges may form a common flute perpendicular or oblique to the equatorial band (Fig. 5(f)). Subequatorial circular furrow (rimula), situated at the distal edge of an equatorial band, is about 2–4 mm in width (Fig. 5(b)). At the distal pole, the cryptopore is typically 3.2 mm in diameter (Fig. 4(f)). At the proximal pole, the triradiate laesurae are typically 3.2 mm long (Fig. 5(a)). Wall of pollen grains consists of two layers: nexine and sexine. The nexine is a nonsculptured layer that does not exist in most pollen grains. In fact, the nexine layer can occasionally be observed in a few pollen grains. The nexine is very thin, up to 143 nm in thickness and roughly follows the inner contour of
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Fig. 4. In situ pollen grain of C. dalatzensis sp. nov. (SEM). a: a pollen mass. Note pollen was broken and in small size. Scale bar 10 mm; b: a clump of broken pollen grains, showing internal morphology. Scale bar 2 mm: c: three fractured pollen grains, showing internal morphology. Scale bar 2 mm; d: a fractured pollen grain, showing portion of the striate band. Note the uneven edge of flanges with papilla-like teeth (arrows). Scale bar 1 mm; e: one fractured pollen grain. Note the striae of pollen grain, and the tectate layer and baculate layer of the sexine (arrows). cp: cryptopore. Scale bar 1 mm; f: inner view of distal pole showing the cryptopore (cp). Scale bar 3 mm.
the sexine (Fig. 5(c)). The sexine consists of a tectate layer and a baculate layer (Fig. 4(e)). The tectate layer is homogeneous enveloping the entire pollen grain, thicker at the equatorial area, usually about 0.5 mm at the grooves and 1.4 mm at the flanges (Fig. 5(d, f)), thinner at the cryptopore, subequatorial circular furrow and at the trifid tetrad scar about 0.14 mm. The baculate layer is absent on the cryptopore, the trifid tetrad scar and the subequatorial canal.
Surface sculpture of the pollen grains (the external sculpture of sexine) consists of bacula with a density of about 78/mm2. The bacula are very tiny, usually 0.08–0.15 nm high and no more than 0.1 mm in diameter (Fig. 5(e)). The infratectal sculptural elements may be absent or reduced. The various arrangements and fusion of these sculptural elements create various patterns of sculpture: gemmate (Fig. 5(a, b)), verrucose or verrucate (Fig. 5(e)), cerebrelloid or vermiculate (Fig. 5(d)).
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Fig. 5. In situ pollen grains of C. dalatzensis sp. nov. (SEM). a: inner view of proximal pole, showing the trifid tetrad scars. Scale bar 3 mm; b: a fractured piece of pollen grain, showing the striae at the equatorial band and the subequatorial circular furrow (arrow). Scale bar 2 mm; c: a fractured piece of pollen grain, showing the nexine (arrow). Note the nonsculptured nexine roughly follows the inner contour of the sexine. Scale bar 2 mm; d: a fractured piece of pollen grain, showing the cerebrolloid or vermiculate sculpture on the inner surface. Scale bar 1 mm; e: detail of pollen surface sculpture, showing the bacula of the outer surface (center) and the verrucose of the inner surface (upper). Scale bar 1 mm; f: fractured piece of inner surface, showing portion of striate band. Note the flutes (arrows) on flanges formed a common flute perpendicular or oblique to equatorial band. Scale bar 1 mm.
4. Discussion 4.1. A general comparison and discussion Like many other male cones of fossil plants (Table 1), the male cone C. dalatzensis associated with Pseudofrenelopsis dalatzensis was not completely known. The specimen is compressed oblique to the bedding plane. The generic identification appears, however, secure as characteristic Classopollis pollen grains have been found in situ.
It is difficult to determine the number of pollen sacs per microsporophyll in compressed specimens. According to Watson (1988), the pollen sac number by the different Cheirolepidiaceae species varies from two to eight. No well-preserved pollen sacs were found in the present species. Some pollen masses (not figured) obtained by oxidation of the coalified compression are cylindrical in shape. They are generally similar to other intact pollen masses so far obtained in a single sac (Watson, 1988; Barale et al., 1988). It is unfortunate that when treated with ammonia these pollen masses broke and left clumps of broken
Table 1 Comparison of some male cone of or associated with Frenelopsis and Pseudofrenelopsis
Associated
C. cathayanus
C. comptonensis
C. dalatzensis
C. turolensis
C. ugnaensis
Pseudofrenelopsis parceramosa Round to ovoid 6–14 9–20
Pseudofrenelopsis intermedia Spherical 20 23
Pseudofrenelopsis parceramosa Spherical 4–5
Pseudofrenelopsis dalatzensis Ovoid 10 14
Frenelopsis turolensis Oval to elliptical 4.5–4.9 2.8–3.6
Frenelopsis ugnaensis — 3.5 2.5
Elliptical 6–12 4–8
Pseudofrenelopsis papillosa Oval to elliptical Ellipsoidal 11–13 7–10 12 15
2?
–
3?
4?
–
–
–
–
–
Spherical to oval 12.5–37.5
Spheroidal with flattened poles 26–41
Spheroidal with flattened poles 30–40
Spheroidal with flattened poles 16–20
Spheroidal with fattened poles 22–35
Spheroidal with Spheroidal with flattened poles flattened poles 20–36 27–30
Spherical to oval 30–42
Rounded with flattened poles 25
7–8
11
3.9–5.8
–
–
5–8
4–8
–
5–7 Grumousverrucose or echinulate Smooth
9–12 Spinulose
6–11 Baculate
– –
– –
– Granular
– Verrucose to rugose
Vermiculate
Gemmate, verrucose – cerebrelloid or vermiculate China Spain Aptian/Albian Albian
–
7–10 Grumousverrucose or conical spines –
–
–
Spain Barremian
USA, Europe Cenomanian
Portugal Cenomanian
Gomez et al., 2002
Pons, 1979, Kvacˇek, 2000
Pons and Broutin, 1978
China Early Cretaceous Zhou, 1995
Equatorial dimensions of pollen (mm) Width of equatorial band 2.7 of pollen (mm) Number of striate of pollen 7–12 Pollen external sculpture Spinules or echinulate Pollen internal sculpture
–
Location Ages
USA Aptian/Albian
References
Axsmith et al., 2004
China Early Cretaceous Zhou, 1983
Europe Barremian Alvin et al., 1978 Taylor and Alvin, 1984
This paper
Gomez et al., 2002
Male cone Frenelopsis alata F. ologostomata
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Shape of male cone Dimensions of male cone (mm) Number of pollen sac per microsporophyll Pollen shape
Classostrobus arkansensis
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pollen grains together with some membranous residues. On the other hand, it is fortuitous to have clumps of broken pollen grains that exhibited much details of the internal morphology and structure of in situ Classopollis pollen grains. To my knowledge, there are only three species of which the internal structure has been studied. One is in situ pollen grains of Classostrobus comptonensis (Taylor and Alvin, 1984); the two others are dispersed pollen grains Classopollis torosus (Pettitt and Chaloner, 1964) and C. martinottii (Srivastava, 1976). In situ pollen grains have been found in some other male cones, such as Classostrobus arkansensis (Axsmith et al., 2004) and C. cathayanus (Zhou, 1983), but no internal structure of their in situ pollen grains was described. The present study not only confirms the result on dispersed Classopollis pollen grains carried out by Srivastava (1976) but also provides much more detailed information and a new insight into the internal morphology and structure of in situ Classopollis pollen grains. The features such as the size, the number of internal striations and the outer sculpture details are considered to be of systematic significance (Reyre, 1970; Gomez et al., 2002). There is a general morphological similarity between the present pollen grains and those of the other members of Cheirolepidiaceae. It is, however, difficult to attribute the in situ pollen grains of the male cone C. dalatzensis to any described species, either found dispersed or in situ. None of the dispersed Classopollis pollen grains described in detail by Reyre (1970) fully agrees with the pollen grains of the present male cone. On the surface sculpture the present pollen are most like C. rarus and C. noeli, but they are dissimilar in other characters. Of the in situ pollen grains from the male cones Classostrobus and the male cones associated or attached to Pseudofrenelopsis and Frenelopsis there are none that closely resembles the present pollen grains (Table 1). 4.2. The attribution of the male cone and associated vegetative shoot The Classopollis-containing male cone demonstrates that Pseudofrenelopsis dalatzensis is a member of the Cheirolepidiaceae. As mentioned above, the male cone described here is preserved together with a large number of vegetative shoots of Pseudofrenelopsis dalatzensis. The shoot of the male cone and the shoot of Pseudofrenelopsis dalatzensis are very similar to each other in gross morphology. Both have fine, parallel longitudinal ridges and grooves. The internode cuticle of the male cone stalk and cuticle of the microsporophyll of the male cone are very similar to the leaf cuticles of Pseudofrenelopsis dalatzensis. They all have a papillate outer surface and well-developed cutinized hypodermis. The stomata of microsporophylls are smaller in size than those of the leaves, but they are rather similar in shape and distribution. Although a direct connection has not been proved, based on the close association and on the striking similarities in gross morphology and cuticular structure between the microsporophyll and the leaf, and the fertile and vegetative shoots, this new male cone very probably belongs to Pseudofrenelopsis dalatzensis.
There are some other Cheirolepidiaceous vegetative shoots recorded from the Dalazi Formation: F. elegans (Chow and Tsao, 1977), Pseudofrenelopsis intermedia (i.e. Suturovagina intermedia Zhou, 1983). They are clearly distinguished in gross morphology and cuticles features and cannot be confused with Pseudofrenelopsis dalatzensis. Moreover, they are not found in association with the present Classopollis-containing cone in the collection. 4.3. The ontogenic stage of the male cone The morphology and organization of the male cone suggest that C. dalatzensis is probably immature. The microsporophylls are tightly oppressed and after maceration most of the grains remain in massive clumps. Associated with them, abundant membranous debris are found, which may represent fragments of the fragile inner linings of pollen sacs (tapetum) as they lack any preserved cell structure. The morphology and structure of the pollen grains also indicate that they are underdeveloped. The pollen grains found in the cone are up to 21 mm in diameter, but Classopollis pollen grains found dispersed in palynological preparation from the sediment are usually 28 mm in diameter (Fig. 6). Most dispersed species (normally mature) of Classopollis described in detail by Reyre (1970) are larger than the present pollen. The pollen wall (sexine and nexine) is usually less than 1 mm thick in the described pollen grains of C. dalatzensis, but is 1.5–2 mm in the mature pollen of Classopollis reported by Srivastava (1976), Taylor and Alvin (1984), and 2.6–2.8 mm reported by Pettitt and Chaloner (1964). Srivastava (1976) has reported that the nexine layer was absent in Classopollis. On the contrary, Taylor and Alvin (1984) has reported that the nexine layer is well developed in the in situ pollen grains of Classopollis comptonensis; the nexine lamellae of pollen grains in immature cones are less developed than in those of the mature cones; in some pollen grains the nexine region is occupied by amorphous or granular materials. In the present material, the inner nonsculptured lamellate layer (nexine) is very thin in a few grains. It is not known whether the nexine was not fully developed due to immaturity or if it was destroyed during preservation or maceration. In respect to their small size and thin pollen wall, it seems likely that the pollen grain is underdeveloped. As in some other gymnosperms, the nexine usually develops at a relatively late stage after sexine development is well advanced (Audran, 1981). Studies on the wall structure of Classopollis by transmission and scanning electron microscopy showed that in mature pollen grains baculate layers were well developed and the bacula could reach 0.6–0.8 mm height (Courtinat, 1980; Pettitt and Chaloner, 1964; Srivastava, 1976; Taylor and Alvin, 1984; Zavialova, 2003). Although some sections of pollen grains from C. dalatzensis show that the sexine consists similarly of the tectate layer and the baculate layer (Fig. 4(e)), the baculate layer indeed did not develop well in most pollen grains. In some case, baculae in the inner surface of pollen grain sexine were replaced by low, raised lumps separated by shallow groove
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Fig. 6. Classopollis spp., associated with C. dalatzensis sp. nov. (SEM). a: distal view of a pollen grain. Scale bar 20 mm; b: proximal view of a pollen grain, showing strand-like elements in the triradiate slit. Scale bar 20 mm.
(Fig. 5(d)) which make the inner surface cerebrelloid. This may also suggest that the pollen grain is immature and the baculate layer was at the phase of forming. Taylor and Alvin (1984) found orbicules often adhering to pollen grains and incorporating or associating membranes of maceration. Other previous studies also show that orbicules were often found to adhere to pollen grains (Barale et al., 1988; Gomez et al., 2002; Axsmith et al., 2004). But in the present study no orbicule has been observed. This also indicates that the male cone was probably immature. The sculpture on the outer surface of pollen grains of C. dalatzensis (Fig. 5(e)) is very similar to some dispersed pollen grain (Fig. 6), which may suggest that the outer sculpture layer had formed before pollen grains were dispersed. Acknowledgement My special thanks are extended to Professor Zhou Z.Y. who gave me enormous help in the present study: much valuable advice on technical methods, numerous discussions and critical comment on the draft. I thank Drs Axsmith B.J., Kvacˇek J. and Wang Y.D. who kindly sent me some reprints concerning Classostrobus and Frenelopsis. The study was financially supported by the National Natural Science Foundation of China (no. 40572010). References Alvin, K.L., Spicer, R.A., Watson, J., 1978. A Classopollis-containing male cone associated with Pseudofrenelopsis. Palaeontology 21, 847–856. Alvin, K.L., Spicer, R.A., Watson, J., 1994. A new coniferous male cone from the English Wealden and a discussion of pollination in the Cheirolepidiaceae. Palaeontology 37, 173–180. Audran, J.C., 1981. Pollen and tapetum development in Ceratozamia mexiana (Cycadaceae): sporal origin of the exinic sporopollenin in cycads. Review of Palaeobotany and Palynology 33, 315–346.
Axsmith, B.J., Krings, M., Waselkov, K., 2004. Conifer pollen cones from the Cretaceous of Arkansas: Implications for diversity and reproduction in the Cheirolepidiaceae. Journal of Paleontology 78, 402–409. Barale, G., Fernandez-Marron, T., Alvarez-Ramis, C., 1988. Étude de cônes mâles de Frenelopsis schenke emend. Reymanówna et Watson, 1976 (Cheirolepidiaceae) du Crétacé supérieur de Torrelaguna (Province de Madrid, Espagne). Geobios 21, 187–199. Chow, T.Y., Tsao, C.Y., 1977. On eight new species of conifers from the Cretaceous of East China with reference to their taxonomic position and phylogenetic relationship. Acta Palaeontologica Sinica 16, 165–181 (in Chinese with English abstract). Courtinat, B., 1980. Structure d’adhérence des grains de pollen en tétrade du genre Classopollis Pflug, 1953, de l’Hettangien de Saint-Fromont, Manche (France). Geobios 13, 209–229. Deng, S.H., Yang, X.J., Lu, Y.Z., 2005. Pseudofrenelopsis (Cheirolepidiaceae) from the Lower Cretaceous of Jiuquan, Gansu, northwestern China. Acta Palaeontologica Sinica 44, 505–516. Gomez, B., Martín-Closas, C., Barale, G., Solé de Porta, N., Thévenard, F., Guignard, G., 2002. Frenelopsis (Coniferales: Cheirolepidiaceae) and related male organ genera from the Lower Cretaceous of Spain. Palaeontology 45, 997–1036. Kvacˇek, J., 2000. Frenelopsis alata and its microsporangiate and ovuliferous reproductive structures from the Cenomanian of Bohemia (Czech Republic, Central Europe). Review of Palaeobotany and Palynology 112, 51–78. Pettitt, J.M., Chaloner, W.G., 1964. The ultrastructure of the Mesozoic pollen Classopollis. Pollen et Spores 6, 611–620. Pons, D., 1979. Les organes reproducteurs de Frenelopsis alata (K. Feistm) Knobloch, Cheirolepidiaceae du Cénomanien de l’Anjou, France. 104e Congrès national des Sociétés savantes, Bordeaux, Sciences I, 209–231. Pons, D., Broutin, J., 1978. Les organes reproducteurs de Frenelopsis ologostomata, (Crétacé, Portugal). 103e Congrès national des Sociétés savantes, Nancy II, 139–159. Reyre, Y., 1970. Stereoscan observations on the pollen genus Classopollis Pflug, 1953. Palaeontology 13, 303–322. Rothwell, G.W., Mapes, G., Hilton, J., Hollingworth, N.T., 2007. Pollen cone anatomy of Classostrobus crossii sp. nov. (Cheirolepidiaceae). Coal Geology 69, 55–67. Srivastava, S.K., 1976. The fossil pollen genus Classopollis. Lethaia 9, 437–457. Tao, J.R., 2000. Fossil plants of the Early Cretaceous in Northeast China. In: Tao, J.R. (Ed.), The evolution of the Late Cretaceous–Cenozoic floras in China. Science Press, Beijing, pp. 6–11 (in Chinese).
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Taylor, T.N., Alvin, K.L., 1984. Ultrastructure and development of Mesozoic pollen: Classopollis. American Journal of Botany 71, 575–587. Watson, J., 1988. The Cheirolepidiaceae. In: Beck, C.B. (Ed.), Origin and evolution of gymnosperms. Columbia University Press, New York, pp. 382–447. Zavialova, N.E., 2003. On the ultrastructure of Classopollis exine: a tetrad from the Jurassic of Siberia. Acta Palaeontologica Sinica 42, 1–7. Zhang, C.B., 1986. The middle-late Early Cretaceous strata in Yanji Basin, Jilin Province. Journal of Changchun College Geology 2, 15–28 (in Chinese with English abstract).
Zhang, W., Zhang, Z.C., Zheng, S.L., 1980. Spermatophyta. In: Shenyang Institute of Geology and Mineral Resources, Chinese Academy of Geological Sciences (Ed.), Palaeontological Atlas of Northeast China, Mesozoic and Cenozoic. Geology Publish House, Beijing, pp. 263–307 (in Chinese). Zhou, Z.Y., 1983. A heterophyllous cheirolepidiaceous conifer from the Cretaceous of Eastern China. Palaeontology 26, 789–811. Zhou, Z.Y., 1995. On some Cretaceous Pseudofrenelopsis with a brief review of cheirolepidiaceous conifers in China. Review of Palaeobotany and Palynology 84, 419–438.
Geobios 41 (2008) 689–698 http://france.elsevier.com/direct/GEOBIO/
Article original
A male cone of Pseudofrenelopsis dalatzensis with in situ pollen grains from the Lower Cretaceous of Northeast China§ Un cône mâle de Pseudofrenelopsis dalatzensis avec des grains de pollens in situ du Crétacé inférieur (NE, Chine) Xiao-Ju Yang Nanjing Institute of Geology and Palaeontology, Palaeobotany and Palynology, Chinese Academy of Sciences, 39, East Beijing Road, 210008 Nanjing, Jiangsu Province, China Received 12 March 2007; accepted 29 June 2008 Available online 17 July 2008
Abstract A Classopollis-containing male cone found in close association with Pseudofrenelopsis dalatzensis from the Lower Cretaceous Dalazi Formation of Yanji Basin, eastern Jilin, China was studied using scanning electron microscopy and described as Classostrobus dalatzensis sp. nov. The small oval cone, borne on the top of a stalk (fertile short shoot), consists of helically arranged microsporophylls, each with a rhomboid and expanded distal head. The short shoot is divided into indistinct nodes and internodes with fine longitudinal striations. Cuticles of the microsporophyll and the internode of the short shoot are generally similar to Pseudofrenelopsis dalatzensis internode cuticles, all of them having papillate outer surfaces. Stomata are similar in structure and arrangement, with papillae of subsidiary cells overhanging the stomatal pit. Epidermal cells are rectangular or isodiametrical, each with a robust papilla on the periclinal wall. Hypodermal cells are present between the stomatal files. The internal structure of the in situ Classopollis pollen grains was studied in detail. Pollen grain is small, with thin sexine, poorly developed nexine and may be immature. This is the first record of in situ pollen grains Classopollis in China of which the internal structure is known in detail. # 2008 Elsevier Masson SAS. All rights reserved. Résumé Un cône mâle contenant un pollen du type Classopollis et trouvé en association étroite avec le feuillage Pseudofrenelopsis dalatzensis dans la formation Dalazi du bassin de Yanji (Crétacé inférieur, Jilin oriental, Chine) a été étudié au microscope électronique à balayage et décrit comme Classostrobus dalatzensis sp. nov. Ce petit cône ovale, porté au sommet d’un pédoncule (brachyblaste fertile), est constitué de microsporophylles insérées hélicoïdalement, chacune portant à leur extrémité une expansion rhomboïdale. Le pédoncule, indistinctement divisé en nœuds et entrenœuds, est strié longitudinalement. La cuticule des microsporophylles et des entre-nœuds des pédoncules est généralement similaire à celle des entre-nœuds de Pseudofrenelopsis dalatzensis, toutes les deux montrant une surface externe papilleuse. La structure et la disposition des stomates sont similaires, avec des papilles des cellules subsidiaires surplombant l’ostiole. Les cellules épidermiques sont rectangulaires ou isodiamétriques, chacune avec une papille robuste sur la paroi anticline. Un hypoderme est présent entre les rangées stomatiques. La structure interne des grains de pollen Classopollis trouvés in situ a été étudiée en détail. Le grain de pollen est petit, avec une sexine fine, une nexine peu développée et probablement immature. Il s’agit de la première donnée pour la Chine de grains de pollen de Classopollis trouvés in situ dont la structure interne est connue en détail. # 2008 Elsevier Masson SAS. All rights reserved. Keywords: Classopollis; Pseudofrenelopsis; Cheirolepidiaceae; Cretaceous; China Mots clés : Classopollis ; Pseudofrenelopsis ; Cheirolépidiacées ; Crétacé ; Chine
§
Corresponding editor: Marc Philippe. E-mail address: [email protected].
0016-6995/$ – see front matter # 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.geobios.2008.03.001
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1. Introduction The Cheirolepidiaceae is a Mesozoic conifer family that was recorded from the Late Triassic to the Late Cretaceous around the world. Since the 1970, information about this group, its biology, structure and palaeoecology, have increased based on both palynological studies and megafossil investigations. One of the distinctive and reliable characters for this family is the possession of distinctive and unusual pollen grains of the genus Classopollis Pflug (Watson, 1988). A discovery of a permineralized fossil cone Classostrobus crossii Rothwell et al. from the Middle Jurassic of southern England provides the newly evidence of male cones contained Classopollis (Rothwell et al., 2007). In the Early Cretaceous, frenelopsid forms reached the peak of radiation more than 20 species have been recorded around the world based on the numerous limited and/or ambiguous materials of vegetative shoots. Although vegetative shoots are of rather frequent occurrence, male cones with in situ pollen grains are rare. Among the small number of male cones attributed to Frenelopsis and Pseudofrenelopsis, Classostrobus arkansensis (Axsmith et al., 2004), C. cathayanus (Zhou, 1983), C. comptonensis (Alvin et al., 1978, 1994), C. turolensis and C. ugnaensis (Gomez et al., 2002), and male cone of Frenelopsis alata (Kvacˇek, 2000) are well preserved. In China, only two forms of Classopollis-containing cones have been found: Classostrobus cathayanus (Zhou, 1983) and the male cone attributed to Pseudofrenelopsis papillosa (Zhou, 1995). In the Yanji area, eastern Jilin Province, several cheirolepidiaceous conifers were reported (Chow and Tsao, 1977; Zhang et al., 1980; Zhang, 1986; Zhou, 1995), but they are all fragmentary vegetative shoots. During a recent field trip to the same area, the present author collected a Classopollis-bearing cone associated with numerous Pseudofrenelopsis dalatzensis (Zhou, 1995) remains from the type locality of the Dalazi (Dalatze) Formation, Zhixin, Longjing county. Pseudofrenelopsis dalatzensis was described based on a few fragmentary vegetative shoots from the Early Cretaceous Dalazi Formation (Chow and Tsao, 1977; Zhou, 1995) and recently was found in the Lower Cretaceous Xiagou Formation of Jiuquan Basin, Gansu, northwestern China (Deng et al., 2005), but its reproductive organs have never been reported. This article presents new data regarding the male cone with in situ pollen grains, and cuticles of the microsporophyll and associated vegetative shoots. One of the most interesting and fortuitous results of this research is that some clumps of broken pollen grains reveal detailed internal structure of the in situ Classopollis pollen grains.
Fig. 1. Sketch map, showing the location of Longjing, Jilin Province, Northeast China.
oil shale beds and yields abundant animal and plant fossils (Zhang, 1986; Tao, 2000). Pseudofrenelopsis dalatzensis was the most common plant collected from the Dalazi Formation, especially in some dark gray-yellow fine-grained siltstones. The male cone is a compression with a short stalk. Pieces of cuticles and pollen masses were removed from the specimens, first treated with hydrofluoric acid (HF) for 12 hours, followed by maceration with Schulze’s solution (nitric acid and potassium chlorate). The time for oxidation depended upon the degree of coalification of the compressed specimen, usually about three to five hours for cuticle specimens and five to eight hours or more for pollen mass. After the specimens became yellow and translucent, they were rinsed with water and were treated for 0.5 to one minute with 5% ammonia and rinsed
2. Material and methods The material was collected from the middle part of the upper Lower Cretaceous Dalazi Formation, well exposed along a road near Zhixin town, Longjing county, Jilin province, northeastern China (Fig. 1). The Dalazi Formation, which is one of early angiosperm-bearing deposits in China, consists of gray-yellow conglomerates, sandstones and black shales, intercalated with
Fig. 2. a: C. dalatzensis sp. nov., holotype, PB20208, showing male cone borne on a short shoot and spirally arranged microsporophylls. Scale bar: 2 m; b: vegetative shoot of Pseudofrenelopsis dalatzensis, showing spirally arranged leaves and distinct longitudinal ridges and grooves, PB20207. Scale bar 4 mm.
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again. Cuticles of the stalk of cone and microsporophylls were observed with Nikon Eclipse E600 light microscopy and photographed with Nikon Coolpix 950 in the Department of Palaeobotany and Palynology of Nanjing Institute of Geology
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and Palaeontology, and then, together with cuticles of the vegetative shoot and pollen masses, were mounted respectively on stubs using double-sided adhesive tape, air dried and coated with gold, then observed and photographed with the LEO-1530
Fig. 3. a, b: cuticles from the cone stalk of C. dalatzensis sp. nov. (SEM); a: outer view of internode cuticle, showing papillae and holes that spread all over the outer surface. Scale bar 20 mm; b: inner view of internode cuticle, showing the well-defined stomatal and nonstomatal files. Note the presence of cutinized hypodermis in the nonstomatal files. Scale bar 50 mm; c, d: cuticles from the microsporophyll of C. dalatzensis sp. nov. (SEM); c: outer view of cuticle, showing papillose surface. Note the stoma (arrows). Scale bar 50 mm; d: inner view of cuticle, showing the stomatal and nonstomatal files. Note the stoma (arrows). Scale bar 50 mm; e, f: cuticles from the vegetative shoot of Pseudofrenelopsis dalatzensis (SEM); e: outer view of the abaxial cuticle, showing the papillate outer surface and the distinct stomatal files. Note each epidermal cell bears a papilla. Scale bar 10 mm; f: inner view of abaxial cuticle, showing the well-defined stomatal and nonstomatal files. Note the cutinized hypodermis consisting of elongate cells, and the thick anticlinal wall of polygonal or isodiametrical epidermal cells. Scale bar 10 mm.
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scanning electron microscope in the State Key Laboratory of Palaeobiology and Stratigraphy, Chinese Academy of Sciences, Nanjing, China. A vegetative shoot of Pseudofrenelopsis dalatzensis was macerated in order to compare with the cuticles of the stalk of the male cone and to verify the relationship between them (Figs. 2(b) and 3(e, f)). A detailed study of vegetative shoots will be the subject of a future contribution. All the specimens and stubs are housed in the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. 3. Systematic palaeontology Genus Classostrobus Alvin et al., 1978 Type species: Classostrobus comptonensis Alvin et al., 1978. Classostrobus dalatzensis sp. nov. Etymology: The specific epithet ‘‘dalatzensis’’ is coined after the type locality name and refers to the vegetative shoots of Pseudofrenelopsis dalatzensis with which the male cone is associated. Holotype: Specimen PB 20208. Repository: All described specimens are housed in the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China. Type locality: Dalazi, Zhixin, Longjing County, Jilin Province, Northeast China. Stratigraphic horizon: Dalazi Formation. Age: Early Cretaceous (Aptian-Albian). Diagnosis: Male cone borne terminally at the top of a stalk with fine longitudinal ridges and grooves on its surface. Microsporophylls helically arranged expanded distal head rhomboidal with an acuminate to acute apex. Microsporophyll cuticle similar to that of cone stalk. Each epidermal cell bearing a papilla on the outer surface of the periclinal wall. Stomatal files well-defined. Stomata elliptical to rounded, transversely or obliquely oriented. Guard cells slightly sunken, surrounded by five to six papillate subsidiary cells. Epidermal cells rectangular, polygonal or isodiametric. Cutinized hypodermal cells present between stomatal files, longitudinally elongated. In situ pollen of the Classopollis type, small, with thickened equatorial band bearing six to 11 striae. External sculpture of pollen baculate, internal sculpture verrucose, granulate or cerebrelloid. Description: General morphology. The Classopollis-containing male cone (Fig. 2(a)) is preserved isolated, borne at the top of a stalk (fertile short shoot). Stalk consists of distinct nodes and internodes about 11 mm long, 2.4 mm wide at the lower portion and 3.8 mm wide at the top. Outer surface of the stalk is covered with parallel, fine longitudinal ridges and grooves which represent the stomatal and nonstomatal files respectively. Male cone is ovoid in shape, 10 mm wide at the widest part and 14 mm long (not complete) with a central axis bearing numerous, spirally arranged, imbricate microsporophylls. Microsporophylls arise from the axis at wide angles
(sometimes at about 908). The microsporophyll consists of a proximal stalk (up to 4 mm long) and a distal rhomboidal head. The apex of the head is acuminate to acute and incurved. Fragments of pollen sacs containing Classopollis pollen grains have been found in maceration, but there is no definite information about their number, size and their attachment on the microsporophyll. Cuticle of the stalk of the male cone. Cuticle from the stalk of the male cone is thick. Outer surface is covered with numerous tiny holes and a robust papilla, about 12–16 mm in size, is almost invariably present on the outer surface of each epidermal cell (Fig. 3(a)). Stomatal files are well defined; stomata are rounded to elliptical, commonly about 35–40 mm in diameter and transversely or obliquely oriented; guard cells are slightly sunken, the outline of subsidiary cells is indistinct (Fig. 3(b)). Epidermal cells in stomatal files are polygonal or isodiametric, about 20–30 mm in diameter with about 1.3–2.5 mm thick anticlinal wall (Fig. 3(b)). Hypodermal cells between stomatal files are elongated and longitudinally arranged, about 7–25 mm 15–42 mm in size (Fig. 3(b)). There are no stomata in the marginal area that consist entirely of longitudinal elongated cells. Cuticle of the microsporophyll of the male cone. Cuticle of the microsporophyll is thick with cutinized hypodermis. The outer surface is covered with numerous tiny holes (Fig. 3(c)). Each epidermal cell bears one rounded papilla 9–13 mm in diameter on the outer periclinal wall, the aperture of stoma is surrounded by papillae of the subsidiary cells (Fig. 3(c)). Hypodermal cells between stomatal files are elongate, 3–8 mm wide and arranged in regular longitudinal files (Fig. 3(d)). Stomata are elliptical, typically 22 mm 30 mm in size (Fig. 3(d)). Outlines of subsidiary cells surrounding the two guard cells are indistinct (Fig. 3(d)). Guard cells are semilunar in outline, 15 mm long and 2.8 mm wide at the widest part, forming a transversely or obliquely oriented aperture. Outlines of epidermal cells are also not well marked, especially in stomatal rows (Fig. 3(d)). Pollen grains. Pollen grains are of the Classopollis type, more or less spherical, about 16–21 mm in diameter (mean 18 mm, 50 grains measured). Equatorial band is distinct (Figs. 4(b–f), 5(b, d, f)) and about 3.9–5.8 mm wide. In the inside view the band consists of eight to 11 transverse flanges and grooves. These flanges and grooves form the characteristic parallel equatorial striae of the pollen grains. The flanges are massive with a smooth or slightly wavy edge. In some case, the flange edges are uneven with flutes and papilla-like teeth (Fig. 4(d)). Occasionally, the flutes of adjacent flanges may form a common flute perpendicular or oblique to the equatorial band (Fig. 5(f)). Subequatorial circular furrow (rimula), situated at the distal edge of an equatorial band, is about 2–4 mm in width (Fig. 5(b)). At the distal pole, the cryptopore is typically 3.2 mm in diameter (Fig. 4(f)). At the proximal pole, the triradiate laesurae are typically 3.2 mm long (Fig. 5(a)). Wall of pollen grains consists of two layers: nexine and sexine. The nexine is a nonsculptured layer that does not exist in most pollen grains. In fact, the nexine layer can occasionally be observed in a few pollen grains. The nexine is very thin, up to 143 nm in thickness and roughly follows the inner contour of
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Fig. 4. In situ pollen grain of C. dalatzensis sp. nov. (SEM). a: a pollen mass. Note pollen was broken and in small size. Scale bar 10 mm; b: a clump of broken pollen grains, showing internal morphology. Scale bar 2 mm: c: three fractured pollen grains, showing internal morphology. Scale bar 2 mm; d: a fractured pollen grain, showing portion of the striate band. Note the uneven edge of flanges with papilla-like teeth (arrows). Scale bar 1 mm; e: one fractured pollen grain. Note the striae of pollen grain, and the tectate layer and baculate layer of the sexine (arrows). cp: cryptopore. Scale bar 1 mm; f: inner view of distal pole showing the cryptopore (cp). Scale bar 3 mm.
the sexine (Fig. 5(c)). The sexine consists of a tectate layer and a baculate layer (Fig. 4(e)). The tectate layer is homogeneous enveloping the entire pollen grain, thicker at the equatorial area, usually about 0.5 mm at the grooves and 1.4 mm at the flanges (Fig. 5(d, f)), thinner at the cryptopore, subequatorial circular furrow and at the trifid tetrad scar about 0.14 mm. The baculate layer is absent on the cryptopore, the trifid tetrad scar and the subequatorial canal.
Surface sculpture of the pollen grains (the external sculpture of sexine) consists of bacula with a density of about 78/mm2. The bacula are very tiny, usually 0.08–0.15 nm high and no more than 0.1 mm in diameter (Fig. 5(e)). The infratectal sculptural elements may be absent or reduced. The various arrangements and fusion of these sculptural elements create various patterns of sculpture: gemmate (Fig. 5(a, b)), verrucose or verrucate (Fig. 5(e)), cerebrelloid or vermiculate (Fig. 5(d)).
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Fig. 5. In situ pollen grains of C. dalatzensis sp. nov. (SEM). a: inner view of proximal pole, showing the trifid tetrad scars. Scale bar 3 mm; b: a fractured piece of pollen grain, showing the striae at the equatorial band and the subequatorial circular furrow (arrow). Scale bar 2 mm; c: a fractured piece of pollen grain, showing the nexine (arrow). Note the nonsculptured nexine roughly follows the inner contour of the sexine. Scale bar 2 mm; d: a fractured piece of pollen grain, showing the cerebrolloid or vermiculate sculpture on the inner surface. Scale bar 1 mm; e: detail of pollen surface sculpture, showing the bacula of the outer surface (center) and the verrucose of the inner surface (upper). Scale bar 1 mm; f: fractured piece of inner surface, showing portion of striate band. Note the flutes (arrows) on flanges formed a common flute perpendicular or oblique to equatorial band. Scale bar 1 mm.
4. Discussion 4.1. A general comparison and discussion Like many other male cones of fossil plants (Table 1), the male cone C. dalatzensis associated with Pseudofrenelopsis dalatzensis was not completely known. The specimen is compressed oblique to the bedding plane. The generic identification appears, however, secure as characteristic Classopollis pollen grains have been found in situ.
It is difficult to determine the number of pollen sacs per microsporophyll in compressed specimens. According to Watson (1988), the pollen sac number by the different Cheirolepidiaceae species varies from two to eight. No well-preserved pollen sacs were found in the present species. Some pollen masses (not figured) obtained by oxidation of the coalified compression are cylindrical in shape. They are generally similar to other intact pollen masses so far obtained in a single sac (Watson, 1988; Barale et al., 1988). It is unfortunate that when treated with ammonia these pollen masses broke and left clumps of broken
Table 1 Comparison of some male cone of or associated with Frenelopsis and Pseudofrenelopsis
Associated
C. cathayanus
C. comptonensis
C. dalatzensis
C. turolensis
C. ugnaensis
Pseudofrenelopsis parceramosa Round to ovoid 6–14 9–20
Pseudofrenelopsis intermedia Spherical 20 23
Pseudofrenelopsis parceramosa Spherical 4–5
Pseudofrenelopsis dalatzensis Ovoid 10 14
Frenelopsis turolensis Oval to elliptical 4.5–4.9 2.8–3.6
Frenelopsis ugnaensis — 3.5 2.5
Elliptical 6–12 4–8
Pseudofrenelopsis papillosa Oval to elliptical Ellipsoidal 11–13 7–10 12 15
2?
–
3?
4?
–
–
–
–
–
Spherical to oval 12.5–37.5
Spheroidal with flattened poles 26–41
Spheroidal with flattened poles 30–40
Spheroidal with flattened poles 16–20
Spheroidal with fattened poles 22–35
Spheroidal with Spheroidal with flattened poles flattened poles 20–36 27–30
Spherical to oval 30–42
Rounded with flattened poles 25
7–8
11
3.9–5.8
–
–
5–8
4–8
–
5–7 Grumousverrucose or echinulate Smooth
9–12 Spinulose
6–11 Baculate
– –
– –
– Granular
– Verrucose to rugose
Vermiculate
Gemmate, verrucose – cerebrelloid or vermiculate China Spain Aptian/Albian Albian
–
7–10 Grumousverrucose or conical spines –
–
–
Spain Barremian
USA, Europe Cenomanian
Portugal Cenomanian
Gomez et al., 2002
Pons, 1979, Kvacˇek, 2000
Pons and Broutin, 1978
China Early Cretaceous Zhou, 1995
Equatorial dimensions of pollen (mm) Width of equatorial band 2.7 of pollen (mm) Number of striate of pollen 7–12 Pollen external sculpture Spinules or echinulate Pollen internal sculpture
–
Location Ages
USA Aptian/Albian
References
Axsmith et al., 2004
China Early Cretaceous Zhou, 1983
Europe Barremian Alvin et al., 1978 Taylor and Alvin, 1984
This paper
Gomez et al., 2002
Male cone Frenelopsis alata F. ologostomata
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Shape of male cone Dimensions of male cone (mm) Number of pollen sac per microsporophyll Pollen shape
Classostrobus arkansensis
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pollen grains together with some membranous residues. On the other hand, it is fortuitous to have clumps of broken pollen grains that exhibited much details of the internal morphology and structure of in situ Classopollis pollen grains. To my knowledge, there are only three species of which the internal structure has been studied. One is in situ pollen grains of Classostrobus comptonensis (Taylor and Alvin, 1984); the two others are dispersed pollen grains Classopollis torosus (Pettitt and Chaloner, 1964) and C. martinottii (Srivastava, 1976). In situ pollen grains have been found in some other male cones, such as Classostrobus arkansensis (Axsmith et al., 2004) and C. cathayanus (Zhou, 1983), but no internal structure of their in situ pollen grains was described. The present study not only confirms the result on dispersed Classopollis pollen grains carried out by Srivastava (1976) but also provides much more detailed information and a new insight into the internal morphology and structure of in situ Classopollis pollen grains. The features such as the size, the number of internal striations and the outer sculpture details are considered to be of systematic significance (Reyre, 1970; Gomez et al., 2002). There is a general morphological similarity between the present pollen grains and those of the other members of Cheirolepidiaceae. It is, however, difficult to attribute the in situ pollen grains of the male cone C. dalatzensis to any described species, either found dispersed or in situ. None of the dispersed Classopollis pollen grains described in detail by Reyre (1970) fully agrees with the pollen grains of the present male cone. On the surface sculpture the present pollen are most like C. rarus and C. noeli, but they are dissimilar in other characters. Of the in situ pollen grains from the male cones Classostrobus and the male cones associated or attached to Pseudofrenelopsis and Frenelopsis there are none that closely resembles the present pollen grains (Table 1). 4.2. The attribution of the male cone and associated vegetative shoot The Classopollis-containing male cone demonstrates that Pseudofrenelopsis dalatzensis is a member of the Cheirolepidiaceae. As mentioned above, the male cone described here is preserved together with a large number of vegetative shoots of Pseudofrenelopsis dalatzensis. The shoot of the male cone and the shoot of Pseudofrenelopsis dalatzensis are very similar to each other in gross morphology. Both have fine, parallel longitudinal ridges and grooves. The internode cuticle of the male cone stalk and cuticle of the microsporophyll of the male cone are very similar to the leaf cuticles of Pseudofrenelopsis dalatzensis. They all have a papillate outer surface and well-developed cutinized hypodermis. The stomata of microsporophylls are smaller in size than those of the leaves, but they are rather similar in shape and distribution. Although a direct connection has not been proved, based on the close association and on the striking similarities in gross morphology and cuticular structure between the microsporophyll and the leaf, and the fertile and vegetative shoots, this new male cone very probably belongs to Pseudofrenelopsis dalatzensis.
There are some other Cheirolepidiaceous vegetative shoots recorded from the Dalazi Formation: F. elegans (Chow and Tsao, 1977), Pseudofrenelopsis intermedia (i.e. Suturovagina intermedia Zhou, 1983). They are clearly distinguished in gross morphology and cuticles features and cannot be confused with Pseudofrenelopsis dalatzensis. Moreover, they are not found in association with the present Classopollis-containing cone in the collection. 4.3. The ontogenic stage of the male cone The morphology and organization of the male cone suggest that C. dalatzensis is probably immature. The microsporophylls are tightly oppressed and after maceration most of the grains remain in massive clumps. Associated with them, abundant membranous debris are found, which may represent fragments of the fragile inner linings of pollen sacs (tapetum) as they lack any preserved cell structure. The morphology and structure of the pollen grains also indicate that they are underdeveloped. The pollen grains found in the cone are up to 21 mm in diameter, but Classopollis pollen grains found dispersed in palynological preparation from the sediment are usually 28 mm in diameter (Fig. 6). Most dispersed species (normally mature) of Classopollis described in detail by Reyre (1970) are larger than the present pollen. The pollen wall (sexine and nexine) is usually less than 1 mm thick in the described pollen grains of C. dalatzensis, but is 1.5–2 mm in the mature pollen of Classopollis reported by Srivastava (1976), Taylor and Alvin (1984), and 2.6–2.8 mm reported by Pettitt and Chaloner (1964). Srivastava (1976) has reported that the nexine layer was absent in Classopollis. On the contrary, Taylor and Alvin (1984) has reported that the nexine layer is well developed in the in situ pollen grains of Classopollis comptonensis; the nexine lamellae of pollen grains in immature cones are less developed than in those of the mature cones; in some pollen grains the nexine region is occupied by amorphous or granular materials. In the present material, the inner nonsculptured lamellate layer (nexine) is very thin in a few grains. It is not known whether the nexine was not fully developed due to immaturity or if it was destroyed during preservation or maceration. In respect to their small size and thin pollen wall, it seems likely that the pollen grain is underdeveloped. As in some other gymnosperms, the nexine usually develops at a relatively late stage after sexine development is well advanced (Audran, 1981). Studies on the wall structure of Classopollis by transmission and scanning electron microscopy showed that in mature pollen grains baculate layers were well developed and the bacula could reach 0.6–0.8 mm height (Courtinat, 1980; Pettitt and Chaloner, 1964; Srivastava, 1976; Taylor and Alvin, 1984; Zavialova, 2003). Although some sections of pollen grains from C. dalatzensis show that the sexine consists similarly of the tectate layer and the baculate layer (Fig. 4(e)), the baculate layer indeed did not develop well in most pollen grains. In some case, baculae in the inner surface of pollen grain sexine were replaced by low, raised lumps separated by shallow groove
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Fig. 6. Classopollis spp., associated with C. dalatzensis sp. nov. (SEM). a: distal view of a pollen grain. Scale bar 20 mm; b: proximal view of a pollen grain, showing strand-like elements in the triradiate slit. Scale bar 20 mm.
(Fig. 5(d)) which make the inner surface cerebrelloid. This may also suggest that the pollen grain is immature and the baculate layer was at the phase of forming. Taylor and Alvin (1984) found orbicules often adhering to pollen grains and incorporating or associating membranes of maceration. Other previous studies also show that orbicules were often found to adhere to pollen grains (Barale et al., 1988; Gomez et al., 2002; Axsmith et al., 2004). But in the present study no orbicule has been observed. This also indicates that the male cone was probably immature. The sculpture on the outer surface of pollen grains of C. dalatzensis (Fig. 5(e)) is very similar to some dispersed pollen grain (Fig. 6), which may suggest that the outer sculpture layer had formed before pollen grains were dispersed. Acknowledgement My special thanks are extended to Professor Zhou Z.Y. who gave me enormous help in the present study: much valuable advice on technical methods, numerous discussions and critical comment on the draft. I thank Drs Axsmith B.J., Kvacˇek J. and Wang Y.D. who kindly sent me some reprints concerning Classostrobus and Frenelopsis. The study was financially supported by the National Natural Science Foundation of China (no. 40572010). References Alvin, K.L., Spicer, R.A., Watson, J., 1978. A Classopollis-containing male cone associated with Pseudofrenelopsis. Palaeontology 21, 847–856. Alvin, K.L., Spicer, R.A., Watson, J., 1994. A new coniferous male cone from the English Wealden and a discussion of pollination in the Cheirolepidiaceae. Palaeontology 37, 173–180. Audran, J.C., 1981. Pollen and tapetum development in Ceratozamia mexiana (Cycadaceae): sporal origin of the exinic sporopollenin in cycads. Review of Palaeobotany and Palynology 33, 315–346.
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