Chengjiang

From Palaeos

Cambrian

Chengjiang

Lower Cambrian soft-tissue fossils from Chengjiang, near the city of Kunming in Yunnan Province, China, preserve a diverse biota dated approximately 515 to 520 Ma, some 25 Ma after the beginning of the Cambrian and pre-dating the Middle Cambrian Burgess Shale by perhaps 10 to 15 Ma (Martin et al. 2000).

Geological Setting

The fauna derives from the long-known Qiongzhusi Formation (syn. "Chiungchussu") of the Qiongzhusi Stage of the late Early Cambrian.

The sediments are finely laminated mudstones, formed in an outer shelf detrital belt, in quiet water environments. The frequency of macrobenthos suggests that the water was relatively well oxygenated, although bioturbation is sparse so that possibly only the uppermost layer at the sediment surface was oxygenated.

The preservation of non-mineralized organismal parts probably resulted from repeated rapid burial events that prevented the bodies from destruction by currents, bioturbation and other biotic activities such as by scavengers and carnivores. Burial is also thought responsible for absence of sulphate reduction which apparently was low so that the soft-parts were preserved in a number of different layers in the formation. Well developed anoxic conditions are reflected by several layers with carbon-rich deposits in the Qiongzhusi Formation. These conditions probably led to frequent mass mortalities which may have played a significant role in the soft-part preservation.

Biota

The Chengjiang biota occurs about 25 m above the earliest trilobites (genus Parabadiella) found in this area and on the Yangtze Platform (which are most probably coeval to late Atdabanian trilobites of the Siberian Platform).

Perhaps the most surprising aspect of the Chengjiang biota is the wealth of taxa from so many different groups. The fossil assemblages includes diverse algae, medusiform metazoans, chondrophorines, sponges, chancelloriids, sea anemones, priapulid worms, hyoliths, possible ectoprocts, inarticulate brachiopods, annelid-like animals, lobopodians, trilobites and non-trilobitic arthropods, hemichordates and probable earliest chordates as well as taxa that cannot definitely be assigned to any well established groups.

It is not only the diversity and early appearance in the fossil record which makes the Chengjiang assemblage fabulous, but also the fine preservation which offers the opportunity to learn more about the morphology of these early creatures. One of the most outstanding examples might be Microdictyon, the isolated sclerites of which were known from numerous localities on various continents, but none of the specialists had any idea how this creature could have been organized. The discovery of these net-like scales of Microdictyon on a worm-like animal resolved the question.

References

Grotzinger, J.P.; Bowring, Samuel A.; Saylor, Beverly Z.; Kaufman, Alan J. (1995): Biostratigraphic and Geochronologic Constraints on Early Animal Evolution. Science, 270: 598-604.

Li, Chia-Wei; Chen, Jun-Yuan; Hua, Tzu-En (1998): Precambrian Sponges with Cellular Structures. Science v. 279, issue of 6 February 1998, pp. 879 - 882.

Martin, M.W.; Grazhdankin, D.V.; Bowring, S.A.; Evans, D.A.D.; Fedonkin, M.A.; Kirschvink, J.L. (2000): Age of Neoproterozoic Bilaterian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution. Science v.288: 841-845.

Shu, D-G.; Luo, H-L.; Conway Morris, S.; Zhang, X-L.; Hu, S-X.; Chen, L.; Han, J.; Zhu, M.; Li, Y.; Chen, L-Z. (1999): Lower Cambrian Vertebrates from South China. Nature 402: 42-46.

Taylor, Rod S. (1999): 'Waptiid' Arthropods and the Significance of Bivalved Carapaces in the Lower Cambrian. Palaeontological Association 44th Annual Meeting, University of Edinburgh, 17-20 December 1999 (Oral Presentation)