Zhao et al.—New Jurassic Nilssoniopteris species from Xinjiang of China
A young leaf is preserved (Fig. 9.12). The lamina is ~3.2cm long and 6.5mm wide, with an obtuse to obtuse-rounded apex. Its midrib is 0.7–1mm wide. The secondary veins are thin and conspicuous, arising from
the midrib at almost right angles, except that in the apical laminar part the angles are ~80°. Veins are simple or forked once, twice, or even three times, and the location of bifurcation is not fixed. In addition, a few veins fork and then merge to form a closed loop (Figs. 9.7–9.9, 12.2), or fork again after forming a closed loop (Figs. 9.9, 12.2), or merge with neighboring veins (Figs. 10.1, 12.2), or fork and then merge with neighboring veins (Figs. 9.7, 12.2). The density of veins near the leaf base can be ~18–20 per cm (Fig. 9.3 [lower]) or up to 25 per cm (Fig. 9.11). Above the base, the density of veins is 15–17 per cm near the midrib, becoming denser towards the margins due to their bifurcation; the wider the lamina, the denser the veins. For example, vein density near the leaf margin is ~21 per cm (Fig. 9.3 [upper]), 27 per cm (Figs. 9.6, 9.10, 9.13 [right], 12.2), and ~35 per cm (Fig. 9.2) acropetally. The adaxial cuticle is moderately thick and can be readily
divided into areas along the veins and areas between veins. Those along the veins are 75–100 μm wide, marked by ~3 rows of rectangular or quadrilateral cells (Figs. 10.1, 10.2, 11.1, 11.2). Areas between veins are 360–500 μm wide, with epidermal cells square, rectangular, or more or less isodiametric in shape and regular in arrangement. Cell outlines are clear, with their anticlinal walls sinuous, U-, or even Ω-shaped (Fig. 11.2). Trichome bases are absent from this surface. The abaxial cuticle is thicker than the adaxial one.
Epidermal areas along veins and between veins are distinct from each other. The epidermal areas along veins are ~95 μm wide, composed of 3–4 rows of rectangular or elongated cells. Those between veins are 360–450 μm wide, consisting of irregular epidermal cells with anticlinal walls strongly sinuous, U-, or Ω-shaped (Fig. 11.7–11.12). Trichome bases are also always present along the veins and between veins, densely or sparsely arranged at irregular distances (Figs. 10.3, 10.4, 11.5– 11.10), with a density of ~33 per mm2 (Fig. 12.5). Some bases consist of a single rounded cell (Fig. 11.5, 11.7, 11.10), some of two rectangular or triangle cells (Fig. 11.5, 11.9, 11.19, 11.20), and some of three cells (Fig. 11.6–11.8, 11.21). Little difference is seen between these basal cells and the neighboring epidermal cells on the internal surface under SEM. Generally, the anticlinal walls of these basal cells are straighter and slightly more cutinized than those of the ordinary epidermal cells. Leaves are hypostomatic. Stomata form stomatal bands
(~270–360μm wide) between veins, which are approximately equal to or slightly wider than nonstomatal ones (~200–280μm wide) (Fig. 11.4). Stomata density is ~35–50 per mm2 (Fig. 12.3, 12.4). The stomata are syndetocheilic, nearly rounded (~43μm in diameter) or nearly rectangular (31–40 × 57–65μminsize)
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(Fig. 11.9–11.12). Most of the stomata are vertical or oblique to veins,with a fewparallel to veins (Fig. 12.3, 12.4).Guard cells are sunken below two subsidiary cells. Under SEM, on the internal surface of the cuticle, some of the subsidiary cells show smooth outer edges while some others have toothed outer edges. Sometimes subsidiary cells are laterally surrounded by one or two encircling cells (Fig. 11.10–11.12). Viewed externally, two subsidiary cells, without papillae, are flat and not overly close to each other (Fig. 11.25, 11.26). Epidermal cells on the abaxial surface of midrib are of
several rows of rectangular or rhomboid cells that are long- itudinally arranged, with their anticlinal walls straight (Figs. 10.9, 11.13). Trichome bases are numerously present (Figs. 10.9,
11.15), which is observed fromthe external side to contain two or five cells (Fig. 11.16, 11.17). Stomata are rarely seen on the midrib. So far, only one stoma has been observed (Fig. 11.14).
Etymology.—Specific epithet is taken from the Latin crass, strong, and axis, midrib.
Remarks.—Based on the features of entire-margined leaves, sinuous anticlinal walls of epidermal cells, and the synde- tocheilic stomata, the present specimens should be readily assigned to Nilssoniopteris. A suite of other characteristics dis- tinguishes the current set of specimens from all known species of the genus; they are: (1) macromorphology: the species has broad midrib of the leaf, especially that near the leaf base, with the ratio of midrib width to leaf base width as high as one-third to one-half; (2) venation pattern: besides the simple or forked veins, this species has anastomosing veins to form closed loops or even forwards-backwards turning fork shapes; and (3) cuticular feature: the species has trichome bases of 1–3 cells commonly occurring on the abaxial epidermis. Seward (1911) assigned two specimens from the Middle
Jurassic of the Junggar Basin, Xinjiang to Taeniopteris vittata, although he noticed the differences between those specimens and the type ones of the species. The macromorphological features of Seward’s specimens correspond exactly with those of the present specimens. Although their cuticular features remain unknown, given that this type of our specimens collected from the Middle Jurassic of Xinjiang has their cuticular features confirmed to be bennettitalean, we are confident about including Seward’s specimens in our new species. Among other Jurassic species (Supplementary Data Set),
the one most similar to our new species in leaf macromorphol- ogy and cuticular features is N. vulgaris from the Middle Jurassic of Georgia (Doludenko and Svanidze, 1969). In leaf morphology, leaves of both species are long strap-shaped with cuneate leaf base and the ratio of midrib width to leaf base width is high. In adaxial cuticular features, leaves of both species have epidermal areas along the veins being composed of 2–4 rows
Figure 10. Nilssoniopteris crassiaxis Zhao and Deng, n. sp. from the Xishanyao Formation of the Sandaoling Coal Mine, Xinjiang Uygur Autonomous Region, China. (1, 2) Adaxial cuticle; the cuticles were from the specimen shown in Figurew 9.10; (1) epidermal areas along veins and between veins; notice veins fork once or anastomose (indicated by white arrows); scale bar=500 μm; (2) areas along a vein; scale bar=100 μm; (3–8) abaxial cuticle; (3, 4) the cuticles were from the leaf shown in Figure 9.13 (right); (5) the cuticle was from the specimen shown in Figure 9.2; (6–8) the cuticles were from the specimen shown in Figure 9.10; (3) stomata between veins and trichome bases along veins and between veins; scale bar=500 μm; (4) distribution of stomata and trichome bases; scale bar=200 μm; (5) three trichome bases along veins and one stoma; scale bar=50 μm; (6) three stomata and one trichome base; scale bar=50 μm; (7) several stomata and trichome bases; scale bar=50 μm; (8) stomata between veins and trichome bases along veins and between veins; scale bar=100 μm; (9) abaxial cuticle on the midrib, showing epidermal cells and trichome bases; scale bar=500 μm.
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