内蒙古早三叠世的Scalaroxylon及其演化意义

描述产于内蒙古下三叠统老龙头组木化石梯纹木属的一个新种ScalaroxylonjalaidqienseZhangetZhengsp.nov.及一个新联合种S.multiforium(ZhengetZhang)ZhangetZhengcomb.nov.。Scalaroxylon原产于德国上三叠统考依波层,当前报道的产自内蒙古的新材料是该类型化石在中国的首次发现。该属以次生木质部为代表,生长轮不存在,它的主要特征是管胞径向壁纹孔全为梯纹纹孔;木射线1至多列,异形;射线细胞的水平壁无孔,端壁通常具孔,弦向壁多数具孔。根据次生木质部管胞的纹孔类型和木射线构造特征对该属在植物系统演化上的意义进行了初步探讨,认为它代表了种子蕨类到苏铁类之间的中间环节。

EARLY TRIASSIC SCALAROXYLON IN NEI MONGOL AND ITS EVOLUTIONARY SIGNIFICANCE

Two new taxa ascribed to Scalaroxylon jalaidqiense Zhang et Zheng sp. nov. and S. multiforium (Zheng et Zhang) Zhang et Zheng comb. nov. were described in this paper from the Lower Triassic Laolongtou Formation of Defatun district, Jalaid Qi, Nei Mongol, China. The genus Scalaroxylon was found originally from the Upper Triassic (Keuper) of Franconia, Southern Germany. The new material documented represents the first record of this fossil wood genus in China. This genus is characterized by the secondary xylem with no growth rings. The pittings on radial walls of tracheids are of primitive scalariform type. The xylem rays are of a heterogeneous type. Both vertical and transverse ray cells are present. The horizontal and end walls of the ray cells have occasional pits, while their tangential walls are mostly pitted. This kind of special structure suggests that the genus Scalaroxylon differs from Bennettitales and primitive an- giosperms, but resembles some Late Paleozoic Pteridospermae (e. g. Medullosa) and living Cycadales (e. g. some species of Zamia). Thus, the Scalaroxylon may represent an intermediate form between the Pteridospermae and Cycadales.