Fossil treasure house for exploring the Cambrian explosion

Abstract

Since the 1980s, fossil assemblages of a similar composition and preservation state to the Burgess Shale fauna have been found in Cambrian shales in other parts of Laurentia, for example the Sirius Passet fauna in North Greenland, and in other Cambrian continents, for example the Chengjiang fauna in South China. Such fossil assemblages, termed as Burgess Shale-type fossil Lagerstatten, have long served as fossil treasure houses to explore the Cambrian explosion. In addition to the Chengjiang fauna, more than ten Burgess-Shale-type Lagerstatten are now known from the early to middle Cambrian shales of South China in a succession of stratigraphical horizons [3]. Comparison of such fossiliferous Lagerstatten in a chronological framework enables us to trace the evolutionary changes of biological communities in mud-dominated marine environments. Moreover, comparative studies between a single lineage from different horizons may reveal evolutionary trends in lineages through time. The Xiazhuang Assemblage discovered by Zeng et al. [4] is the latest addition of such wonderful fossil treasure houses. A brief survey shows that the Xiazhaung Assemblage generally resembles the Chengjiang fauna in faunal composition, yielding predominantly soft-bodied fossils, including arthropods, brachiopods, priapulids, lobopods and some problematic taxa, with arthropods being the most dominant group. Specimens illustrated by Zeng et al. [4] comprise a variety of species that are similar or identical to those from the Chengjiang fauna. However, some of the most common species, e.g. Kunmingella douvillei, have not been found in the Xiazhuang Assemblage. The current fossil collection of the Xiazhaung assemblages is rather limited in both number of specimens and species, largely due to constrained excavation. In contrast, 20 years of collecting the Chengjiang fauna at many sites across Eastern Yunnan has yielded more than 200 species and hundreds of thousands of specimens. It is therefore worthwhile to increase the intensity of fossil collection at different localities and sites.