J Integr Plant Biol. ›› 1998, Vol. 40 ›› Issue (8): -.

• Research Articles •    

Studies on Annalepis from Middle Triassic Along the Yangtze River and Its Bearing on the Origin of Iso?tes

MENG Fan-Song   

Abstract: Specimens of Annalepis have been found from the Middle Triassic Series in the Yangtze River valley and described, including a new species. Annalepis is similar to the living plant, Isoëtes , in many aspects, such as the herb with a corm, more or less clustering sporophylls, presence of a ligule, monolete suture microspores of Aratrisporites -type and trilete megaspores, etc. The similarities of features between the two plants indicate their close affmities and that A. brevicystis might be the ancestry of Isoëtes . Since there is a ligule on the sporophyll of Pleuromeia, the Pleuromeiaceae represented by Pleuromeia could be allied to the Isoëtaceae, and both families should belong to Isoëtales. Isëtales represents a distinct evolutionary sequence and is inferred as being evolved probably from Palaeozoic SigiUaria . In Mesozoic age, two evolutionary lines are proposed: one as a line from Pleuromeia to Nathorstiana in relation to wood-monopolar rhizophore; the other as a line from Annalenis to lsoOtes related to herb-bipolar corm. Annalepis appeared almost simultaneously in the Yangtze River valley in early Middle Triassic, but the distribution of Annalepis was concentrated from east to west during middle-late Middle Triassic, in relation to the marine regression which occurred cyclically from east to west on the Yangtze Platform during that time.

Key words: Yangtze River valley, Middle Triassic, Annalepis, lsoë, tes, Origin, Evolution

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