石炭纪一植物新属Reticalethopteris gen.nov.——对Palaeoweichselia yuani Sze的再研究

RETICALETHOPTERIS, A NEW GENUS OF CARBONIFEROUS PLANTS WITH RESTUDY ON PALAEOWEICHSELIA YUANI SZE

对北祁连山东段和鄂尔多斯西缘新材料的研究表明,甘肃景泰红水堡晚石炭世早期的杂羊齿属(Palaeoweichselia)与西欧的模式标本存在明显差异,遂另建-新属——Reticalethopteris gen.nov..新属与Neuralethopteris是脉延羊齿类植物中分别代表网状叶脉和羽状叶脉的一对姐妹属.

Sixty years ago, while making a study on the Carboniferous fossil plants from Hongshuipu of Jingtai, Gansu, Sze (1933) described a new species of the genus Palaeoweichselia, i.e., P. yuani Sze. Since Palaeoweichselia represents an endemic monotype plant of the Westphalian from the Saar Coalfield in West Europe, the European palaeobotanists (Jongmans, 1942; Gothan 1951, p. 110. footnote, etc.)have almost all along doubted whether this plant actually exists in China, even Sze himself (1953, 1956) had to assign this plant to the genus with considerable reservation. As Li Xing-xue and others (1992) have pointed out, the specimens previously described by Sze have nothing to do with Palaeoweichselia. Instead, it probably represents a new genus allied to Neuralethopteris among the neuropteroid pteridosperms. However, at that time they did not propose a name at generic level for these materials.In recent years, while carrying on field geological work in the east section of the North Qilian Moundains (including Hongshuipu) and the western margin of the Ordos Basin, the authors of this paper also collected some specimens of this type. After a careful study and analysis of related literatures. It is believed that the Chinese materials previously attributed to Palaeoweichselia by Sze might represent a new genus, which is described as follows:Reticalethopteris gen. nov.Pinnate compound leaf at least of two orders; ultimate pinna with a terminal pinnule. Pinnule of Neuralethopterid type, with base either contractedly or decurrently attached to the rachis. Midrib thin, slightly flexuose; lateral veins extending obliquely forward after spreading out from the midrib, often branching 2-3 times to form sparsely anastomosing meshes, with or without rachial veins; intercalated pinnule absent. Reproductive organ unknown.Type species Reticalethopteris yuani (Sze) gen. nov. et comb. nov.Comparison Distinguishable characteristics of the new genus as compared with those of the related genera are shown in Table 1.Occurrence Early stage of the Late Carboniferous (Namurian B-C), North-West China.Reticalethopteris yuani (Sze) comb. nov. (Pl. I-IV)1933 Palaeoweichselia yuani Sze, P. 59-63, pl. 6, figs. 1-12; pl. 7, figs. 1-10.1942 Linopteris muensteri, Jongmans, p. 37.1953 Palaeoweichselia yuani, Sze, p. 47-48, pl. 23, fig. 1; pl. 24, figs. 1-2.1956 Palaeoweichselia yuani, Sze, p. 129.1974 Palaeoweichselia yuani, Gu et Zhi, p. 113-114, pl. 81. figs. 1-3.1991 Palaeoweichselia yuani, Shen et Sun, p. 127-129.Frond unclear in overall morphology, with pinnate compound leaf at least of two orders. Penultimate pinna rachis erect, 3-4mm in diameter, with fine, longitudinal striations on surface. Ultimate pinnae subopposite, forming a relatively large angle with penultimate pinna rachis; ultimate pinna rachis thin, nearly 1mm in diameter, with a terminal pinnule. Pinnule of neuralethopterid type, with base either contracted or decurrent; lateral sides nearly parallel; tip obtuse or subpointed. Midrib thin, flexuosely meandered, dispersed nearly at the tip of pinnule; lateral veins curved outward after being given off from the midrib at an acute angle, branching 2-3 times and intersecting the leaf margin at relatively large angles to form sparsely anastomosing meshes. Decurrent part of the base on the lower side of pinnule with rachial veins. Intercalated pinnule absent.Discussion The Chinese specimens seem to closly resemble Palaeoweichselia defrancei (Brongniart) from the Saar Coalfield, but in the latter, the pinnule is morphologically characterized by the typical Pecopteris, the lateral veins intersect the leaf margin obliquely after branching, and the meshes formed by the secondary laterals or veins become gradually smaller from midrib to leaf margin (sometimes the laterals are not anastomosing to form meshes). Obviously, the Chinese material is different from the genus Palaeoweichselia of West Europe. Moreover, in some Chinese specimens, the ultimate pinnae are often fused into a relatively large apical pinnule, which has never been found in the European P. defrancei. Furthermore, the Chinese species differs also from the European Palaeoweichselia in geological age, the former being found in Nanmrian B-C, while the latter in Westphalian C-D.As Jongmans (1942) pointed out, the specimens described by Sze belong to Linopteris muensteri Eichwar. Howeverd, Sze (1953, p. 48, footnote, etc.) disagreed with Jongmans' opinions; he emphasized: "(Jongmans') opinion is obviously incorrect, because in Linopteris the base of the pinnule appears in heart-shape due to contraction". Besides, the pinnule is much smaller in volume and the anastomosing meshes are also much sparser as compared with those commonly seen in Linopteris and Reticulopteris. Morphologically, the present material is relatively comparable to Alethopteris and Lonchopteris. But in these two genera, the pinnule is generally larger, and the base on the lower side of the pinnule is decurrent with veins arising directly from the rachis. Also, there is a relatively distinct midrib in the pinnule which ranges persistently to the apex. Furthermore, Alethopteris and Lonchopteris differ from our genus also in the typical pinnate veins and the somewhat typical simple reticulated veins respectively.Our new genus is most similar to Neuralethopteris in appearance, especially in such characteristics as the shape and size of the pinnule and the pinnules on the ultimate pinna sometimes fused into a relatively larger terminal, but the genus Neuralethopteris differs from our new genus in the pinnate veins. The authors (Li Xingxue et al., 1992) pointed out: "Among the neuropterid plants, those with pinnate veins and those with reticulated veins always appear in pairs. However, among the neuralethopterid plants, so far no sister genus has been discovered in pair with Neuralethopteris. Providing that the appearance in pair of those elements separately with pinnate veins and reticulated veins is a natural law in the evolution of neuropterid plants, then it can be assumed that the Namurian