Studies on Annalepis from Middle Triassic along the Yangtze river and its bearing on the origin of Isoëtes

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 affinities 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. Isoëtales represents a distinct evolutionary sequence and is inferred as being evolved probably from Palaeozoic Sigillaria . 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 Isoëtes 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.