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.

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Q959.842 P534.51