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The Chengjiang Fossil Site - Yunnan Province, southwest China

The Chengjiang Fossil Site is one of the earliest deposits containing organisms that are very well preserved, dating from early in the Cambrian radiation (Cambrian Explosion) of marine creatures. The Chengjiang Beds are comprised of grey marine mudstones, that is mainly composed of fine-grained sediment, that is several hundred metres deep contains fossils in a number different levels. In this deposit the soft tissue of many of the fossils has been preserved in exquisite detail, some of the soft tissue being replaced by phosphate while others are replaced by pyrite, some surviving in the form of thin films of organic material. When the grey sediment is exposed to air it weathers to a light grey or yellow colour, the fossils sometimes also being grey, though some are reddish, with their internal organs being picked out in shades of grey, brown and black.

At the time of its discovery in 1984 (it was known of since 1912) it was originally believed to correspond to the Burgess Shale period, that was already well known, dating to the Middle Cambrian, that has also produced many arthropods and Pikiai, a putative chordate, all exceptionally well preserved. It was later found to be older than the Burgess Shale, dating to about 525-520 Ma in the middle Early Cambrian.

More than 10,000 specimens have been produced by the rich Chengjiang site, the fauna being mostly arthropods, such as trilobites and trilobite-like forms, sponges, brachiopods and worms, as well as other groups that include what are believed to possibly be basal deuterostomes, such as vetulicolians and yunnanozoons, as well as what are suggested to be the first fishes (Shu, 2003), altogether 90 or more species. Some animals present appear to be unique to this site, though there are also some arthropods that resemble those of the Burgess Shale. Most are believed to have been benthic filter-feeders on the sea floor, though there were also a few floaters and swimmers, some of the larger arthropods being clearly predators that are assumed to have fed on the smaller benthic forms.

Exposures²

At Maotianshan near Kunming in Yunnan Province in southern China, the best exposure of the Chengjiang Fossil-Lagerstãtte is found that also yielded many soft-bodied fossils that are similar to those from the Burgess Shale (Chen & Zhou, 1997; Hou et al., 2004). This site is from the Lower Cambrian, Atdabnian Stage, making it similar in age to the Sirius Passet assemblage. Many of the Burgess Shale animals are also known from the Chengjiang site, among which are Hallucigenia and Anomalocaris, as well as new genera from China, of arthropods, worms, sponges and brachiopods, etc. As the Southern China Craton was thousands of kilometres from the continent of Laurentia, comprising North America and Greenland, it is considered remarkable that the faunas of the Burgess Shale and the Chengjiang site are so similar. Shu et al., 2001, proposed the new phylum Vetulicolia, that included arthropod-like metazoans hat were segmented, with obvious gill slits, suggesting a deuterostome affinity. Shu et al., 1999, reported a surprising faunal element, an agnathan fish that before that time had been known from the Lower Ordovician, indicting that first vertebrates arose in the Cambrian Explosion.

The fossil preservation at this site has been described as spectacular, with reddish-purple impressions on shale that was orange, appearing to result from rapid burial by catastrophic turbidity flows. It has been suggested that the animals in the Chengjiang deposit where probably buried at the site where they lived that was adjacent to a delta front by the sudden deposition of the sediment. The Maotianshan Shale Member, that is 50 m (160 ft thick) of the Yu'anshan Formation is composed of graded layers of mudstone that record sedimentation events that are short and episodic.

Sources & Further reading

1. Benton, Michael J., 2005, Vertebrate Palaeontology, 3 rd ed., Blackwell Publishing.
2. Selden, Paul, Nudds, John, 2004, Evolution of Fossil Ecosystems, Manson Publishing

Links

1. Preservation, Taphonomy and Palaeoecology of the Chengjiang Biota

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