NEW OBSERVATIONS ON DISCOIDAL FOSSILS FROM THE UPPER SINIAN XINGMINCUN FORMATION, LIAONING PROVINCE, NORTHEAST CHINA

Several types of discoidal structures, from the mudstone of the late Neoproterozoic Xingmincun Formation in Dalian area, southern Liaoning Province, Northeast China, was originally compared with the worldwide Ediacaran "medusoid forms" on the basis of their seeming resemblances, i.e. discoidal shape and the occurrence of supposed concentric rings. More recently, these Chinese fossils have been considered as primitive forms of the Ediacara-type metazoans for the researcher claimed that "the body configuration and living style appeared more like a unicellular organism". Most recent collecting has led to an ingathering of more than 300 specimens of discoidal fossils. This new material provides much more complete information on morphology. Preliminary observation on some of the beautifully preserved specimens indicates that the Chinese specimens are significantly different from the typical Ediacaran discoidal remains in many aspects. First, the Chinese specimens occurred in dark green (weathering to yellowish green) mudstones rather than in sandstone as the typical Ediacaran fossils did. Fossils were strongly compressed to a thin layer of dark gray (weathering to yellow) film, about 0.2 mm in thickness, rather than casts or moulds. Second, the Chinese discoids are relatively small, ranging from 5 mm to 40 mm in diameter, much smaller than the Ediacaran discoidal impressions, which may reach to half meter in diameter. Third, there is firm evidence that at least some Ediacaran discoidal fossils are holdfasts of frondose objects. However, neither have frondose remains been found in the several fossil localities, nor did a stem-like structure occur in any of our several hundred specimens. Fourth, there lacks specimen with radial structures, which are very common in typical Ediacaran assemblages. Finally, also most distinguishably, our specimens show helical rings rather than concentric rings as previously thought. The larger specimen has more cycles than the smaller, which may give some indication of the growth pattern of this enigmatic organism. Assigning these fossils to any known taxon seems to be difficulty. However, it is clear that they provide a different and complementary perspective on the late Neoproterozoic biodiversity.