Fossil treasure house for exploring the Cambrian explosion

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.


GEOSCIENCES Special Topic: Paleontology in China
Fossil treasure house for exploring the Cambrian explosion

Xingliang Zhang
The abrupt appearance of animal phyla in the fossil record at the beginning of the Cambrian period, known as the Cambrian explosion [1], is the most dramatic chapter in the annals of animal evolutionary history. As early as 1909, the great paleontologist Charles D. Walcott discovered the Burgess Shale fauna in the Middle Cambrian Burgess Shale of British Columbia, Canada. This discovery was thought to closely represent a Cambrian community because, in addition to the hard skeletons normally preserved, soft tissues were also available and represented in fine detail [2].
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 Lagerstätten, have long served as fossil treasure houses to explore the Cambrian explosion. In addition to the Chengjiang fauna, more than 10 Burgess Shale-type Lagerstätten are now known from the early to middle Cambrian shales of South China in a succession of stratigraphical horizons [3]. Comparison of such fossiliferous Lagerstätten 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 softbodied 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.
The stratigraphical horizon of the Xiazhuang assemblage is one trilobite zone above the Chengjiang fauna and three trilobite zones below the Guanshan fauna [4]. It can be correlated into the upper part of the Cambrian Stage 3, within the largest phase of the Cambrian explosion [5]. The fossils are excellently preserved, revealing the anatomy of soft parts and containing valuable phylogenetic information. The overall preservation quality is on par with that of the Chengjiang and Burgess Shale faunas (Fig. 1).
Thus far, 20 of the 38 living animal phyla made their first appearances during the Cambrian explosion, including 12 non-skeletonized phyla that have only been known from exceptionally preserved faunas. The remaining 18 living phyla either appeared in later geological periods or were absent in the fossil record, probably because they lacked biomineralized body parts (except for the Bryozoa) [5]. Therefore, it is very likely that these phyla did appear in the early Cambrian but are yet to be found from the Xiazhuang assemblage and other Burgess Shale-type Lagerstätten.