The Jinxian Biota revisited: taphonomy and body plan of the Neoproterozoic discoid fossils from the southern Liaodong Peninsula, North China

摘要

The fossil record indicates that complex multicellular organisms started to become dominant in the second half of the Neoproterozoic. However, many macroscopic fossils of this period are not yet well understood. As one example, the Jinxian Biota includes some affinity-unresolved, millimeter-to centimeter-sized discoid fossils of probable pre-Ediacaran age from the shales of the Xingmincun Formation, in southern Liaodong Peninsula, China. This paper presents new observations of these fossils based on new material. Three types of preservation were identified and analyzed. The organisms were probably transported by turbidity currents, rapidly buried in fine-grained deposits and then compacted to yield thin films. Pyrite-and carbonate-related mineralization may have been involved in their early diagenesis, but local-controlled late diagenesis altered the fossil-related mineral composition to that observed today. The concentric annular relief on the fossil surfaces exhibits a "half convex, half concave'' pattern, which is interpreted to reflect the differentiated mechanical nature between adjacent annuli. New specimens have been found that support the existence of programmed fission and budding. In addition, another group of previously ignored discoid fossils are first described here. With the same preservation as the "normal'' discs, these fossils lack any concentric relief and always occur in clusters. The relationship between the two types of discs remains unclear. Even with all of the new information, it remains impossible to indubitably correlate the Jinxian Biota to any known taxonomic group. However, it is quite probable that these fossils represent a group of eukaryotic organisms.