Hiemalora From The Ediacaran Dengying Formation In The Yangtze Gorges Area,South China

The Ediacaran biota is a globally distributed fossil assemblage in the terminal Neoproterozoic,it reprensents hitherto the oldest macroscopic multicellular soft-bodied fossil assemblage.Paleonotological and paleoecological studies of the Ediacaran biota can help us to understand the features of the Earth biosphere on the eve of the Cambrian exposion.Among the disc-shaped fossils,Hiemalora occurs abundantly in the Shibantan biota discovered from the Ediacaran Dengying Formation(551—541 Ma)in the Yangtze Gorges area,Hubei Province,South China.It consists of a central disc and surrounding tentacle-like structures.Based on morphological features and quantitative analysis of ratios between disc diameter and length of the longest tentacle of each specimen,four species are recognized in Hiemalora specimens discovered from the Dengying Formation,they include Hiemalora stellaris,Hiemalora pleiomorphus,Hiemalora sp.1 and Hiemalora sp.2.Statistics of the specimen numbers of Wutubus,Aspidella,Hiemalora,frond and a group including all other Ediacara-type fossils in the Shibantan biota indicate that Hiemalora is one of the dominant components of the Shibantan biota.One specimen discovered from the bottom of the Shibantan Member shows that a Hiemalora is attached to a frond-type structure,which resembles those of Charniodiscus spinosus.The overall morphology of this fossil resembles that of Primocandelabrum hiemaloranum found in Newfoundland area of Canada,indicating that Hiemalora can be attached to different kinds of fronds as a holdfast.Taphonomic analysis suggests that Hiemalora may live in the sediments below the microbial mats before it is buried and preserved as a mold.Meanwhile,the stem and frond live in water column before they are buried on the surface of the microbial mats and preserved as an impression.Two different growth strategies of Hiemalora are proposed,one is mainly depended on the increase of disc size,and the other is on the extension of the tentacle-like structure.Statistic analysis of the size of discs and fronds shows a well positive correlation between the sizes of discs and fronds,suggesting that the overall height of the organism can be inferred through the measured disc diameter.The fixing strength of Hiemalora is enhanced through the size increase to support larger fronds.