一种特殊的晚二叠世茎化石——Guizhouoxylon dahebianense gen. et sp. nov.

A NEW SPECIAL PETRIFIED STEM, GUIZHOUOXYLON DAHE. BIANENSE GEN. ET SP. NOV., FROM UPPER PERMIAN IN SHUICHENG DISTRICT, GUIZHOU, CHINA

描述了贵州水城大河边煤矿晚二叠世龙潭组的植物茎干化石Guizhouoxylon dahebianense gen. et sp. nov., 茎具清楚生长轮,髓部较大,初生木质部内始式,原生木质部由环纹管胞、稀螺纹管胞组成,后生木质部包括有密螺纹管胞和密螺纹至梯纹增厚的导管组成,导管近端部的穿孔板椭圆形,梯纹至网纹增厚。次生木质部密材型,射线单列,一般低矮,管胞自内向外由梯状纹孔变为不规则拥挤分布的具缘纹孔,偶而可见木薄壁细胞。根据后生木质部中导管的穿孔板类型与低等被子植物Platanus acerifolia Willd相似,也讨论了被子植物的起源问题。

A new special petrified stem, Guizhouoxylon dahebianense gen. et sp. nov., was preserved together with a foliage fossil of Gigantonoclea guizhouensis Gu et Zhi which has recently been anatomized by the writers. This stem had been yielded from the Longtan Formation, lower part of Upper Permian in Dahebian Coalfield, Shuicheng District, Guizhou, SW China. The new genus and species is characterized by a series of transitional bordered pits from scalariform to crowded multiseriate subisometric ones on the same tracheids both in the metaxylem and in the early wood of the secondary xylem, but with only one vertical row of circular bordered pits on the latter wood tracheids of the secondary xylem. Another interesting character is a special structure, llke the scalariform-reticulate perforation plate, occurring on the end wall of some tracheary elements in the metaxylem. The special structures suggest that the species differs from any group of gymnosperms, but resembles some primitive angiosperms to some extent. The new species is dealt with as a special plant of incertae sedis in this paper. DESCRIPTION OF NEW GENUS AND SPECIES Guizhouoxylon gen. nov. Type species Guizhouoxylon dahebianense gen. et sp. nov. Diagnosis Eustele. pith, bark, primary and secondary xylem preserved. Pycnoxylic wood with weak growth rings. Pith non-septate, elliptic or three-lobed, heterocellular with sclerotic, secretary, and parenchyma cells arranged in vertical rows. Primary xylem endarch. Tracheids of protoxylem with annular, helical, dense helical, scalariform-reticulate thickenings. Tracheids of metaxylem and early wood of secondary xylem with a series of transitional pitting stages from scalariform to multiseriate crowded subisometric bordered pits on radial walls. Rays tmiseriate, 1—11 (usually 1—4) ceils high; cross-field with 3--8 scalariform, elliptic, or oval simple pits. Traces of leaves and branches in sparse spiral arrangement. Bark thin; cortex and periderm narrow. Distribution: Guizhou, China; Late Permian. Guizhouoxylon dahebianense gen. et sp. nov. (P1. Ⅰ, figs. 1—9; P1. Ⅱ, figs. 1—10; P1. Ⅲ, figs. 1—10; P1. IV, figs. 1—10) Stem about 14cm in length, and 3.2 cm in diameter. Pith non-septate, elliptic (5×7mm) to three-lobed caused by development of leaf or branch traces, heterocellular with sclerotic cells (about 50μm in three-dimension diameters, mostly assembled in giant nests near the margin especially in leaf and branch gaps), secretary ceils (50—60μm long and 30—60 μm in diameter, with black contents), and parenchyma as well as giant elliptic (250×550 μm) or commissural cells; all of these ceils arranged in vertical rows. Several approximate rows of secretory cells looking like a secretory canal with a length of more than 3000 μm but less than 200 μm in total diameter. Primary xylem endarch. Protoxylem tracheids 10—40 μm in diameter, with annular, helical, dense helical, and scalariform-reticulate thickenings. Metaxylem tracheids rectan,gular in cross field and about 30×50 μm in dameter, with a series of transitional pitting stages from scalariform to crowded multiseriate, elliptic or oval bordered pits; all of the types possibly occurring on a single tracheid, but major ones being scalariform pits. Some special structures like the scalariform-reticulate perforation plate occurring on the slope'wall near the end of some tracheary elements of the metaxylem. Secondary xylem in early wood wider; rectangular tracheids (25×40 μm in radial diameter and about 35 μm in tangential diameter) also with tranditional-type pits including a few scalariform pits and most multiseriate oval or ellipical pits which are crowded together more or less like the Araucarioxylon type. Scalariform and multiseriat bordered pits also visible on the radial wall of a single tracheid element or the tangential wall of a tracheid near the cambium. Later wood narrower, only about 2 or 3 cells deep, with tracheids about 15 μm in radial diameter, and with only one vertical row of circular bordered pits on the radial wall. Some secretory sacs scattered in the secondary xylem, especially near the wounded parts, occasionally with several secretary tracheids caused by injury. Rays uniseriate, only 1—11 (usually 1—4) ceils high; ray's parenchyma cells 40—70 μm long, 15—35 μm high, and 10—13 μm wide. Cross-field with 3 scalariform or 4—6 elliptic or 6—8 oval simple pits, which are 10—20 μm long and 7—9 μm wide. Some rays connected with the end of some tracheids in tangential section. Bark about 1.5 mm thick, consisting of phloen (650 μm wide), cortex (about 400 μm wide, mixed with fibriform cells among the parenchyma ceils), and periderm (about 150—350 μm wide) is well as some segments of parenchyma and outer periderm. Leaf or branch traces developed in sparse spiral arrangement, with a total number of less than 10 in the 14cm-long stem. Horizon and Locality Longtan Formation, Upper Permian; Dahebian Coalfield, Shuicheng, Guizhou, China. COMPARISON AND DISCUSSION It is significant to make a comparison with other groups. The appearance of transitional pitting stages from scalariform to crowded multiseriate elliptic, oval bordered pits on a single tracheid can be found from metaxylem to the early woods of all secondary xylem of Guizhouoxylon dahebianense. Since 'there is a total absence of scalariform pitting in conifers, Ginkgo, and the Gnetales, but that is present in the secondary exlem of cycads, cycadeoids, and angiosperms. Cordaiteans and some pteridosperms have both metaxylem and first-formed secondary xylem composed of scalariform tracheids' (Stewart, 1983, p. 369), G. dahebianense is obviously different from those pycnoxylic gymnospermous groups whether in extinct or in living ones. It differs from such cycadeoids as Sahnioxylon (Homoxylon) rajmahalense and Bucklandia sahnii, and from cycads, all of which have large pith, thicker cortex, and narrower manoxyiic secondary wood, although the two cycadeoid species have similar transitional pitting tracheids in their secondary xylem to those of the present species. G. dahebianense seems more similar to some pri.mitive angiosperms with vesselless wood, such as Drimys, Tetracentron and Trochodendron, not only in the massive pycnoxylic wood but also in the same type of pitting whether in early wood or in later wood, except for the much lower and narrower rays and the more crowded pits of early woody tracheid than those of angiosperms. On the other hand, the scalariform-reticulate structure similar to a perforation plate is important for comparing the new species with angiosperms, but we can not be sure that it is a true perforation plate. Perhaps, there was a remote relationship between G. dahebianense and some primitive angiosperms. In short, G. dahebianense is a special plant which differs from any extinct and living gymnospermous or angiospermous group although it seems much closer to primitive angiosperms. We can not determine whether there is any possibility of a higher toxonomic candidate in which the stem should be sorted. Therefore, we temporally suggested G. dahebianense as a special typical plant of incertae sedis.