Chengjiang

Abstract

Introduction

Locality

The original site discovered in 1984 is at Maotianshan, about 6 km east of the town of Chengjiang, which is itself about 10 km south of the large city of Kunming, in eastern Yunnan Province, China. Since 1984, however, rich occurrences of the same fossil assemblage have been found in many localities where the Yu’Anshan Member of the Heilinpu Formation is exposed. A good selection of maps and photographs of field sites are given by Hou et al. 2004a.

Geological Setting

The Cambrian of eastern Yunnan is part of the Southwest China Platform which, during the Early and mid Cambrian, was part of the South China tectonic plate that lay near the equator, adjacent to Gondwana. “The western part of the Platform is different from the eastern part in terms of both sedimentary facies and biofacies. In the former, the Lower Cambrian starts with dolomites and phosphorites, with an abundance of small skeletal fossils, succeeded by detrital rocks bearing a diversity of trace fossils and, ultimately, an abundance of bradoriid arthropods and redlichid trilobites. In the latter, on the other hand, the Lower Cambrian is mainly composed of detrital rocks intercalated with limestone and its fauna is characterized mainly by protolenid and eodiscid trilobites; a few fossils similar to the Chengjiang Lagerstätte have also been found. The Chengjiang fauna probably lived in a tropical sublittoral sea” (Hou et al. 1999, p. 168).

Age

“In eastern Yunnan ... the uppermost part of what is thought to be Precambrian has few if any fossils. Beginning in the earliest Cambrian, shelly elements first appear, then the skin of many unknown types of animals, clearly different from the trilobites and other fossils found in younger rocks. The shelly elements are now known as ‘small shelly fossils.’ The Lower Cambrian biostratigraphy in this area is based mainly on the succession of small shelly fossils at the base of the Lower Cambrian and on the succession of trilobite assemblages in the sequence above. As many as 3 small skeletal fossil biozones (or assemblages) and 11 trilobite biozones are used to divide the Lower Cambrian in this area and elsewhere on the Southwest China Platform. ... The Abadiella Biozone forms the oldest trilobite zone in the Cambrian of China. It is succeeded by the Eoredlichia Biozone, which contains the soft-bodied Chengjiang fauna. The soft-bodied Chengjiang fossils thus occur ... low down in the Lower Cambrian” (Hou et al. 1999, p. 166).

The Chengjiang fauna occurs about 25 m above the earliest trilobites, Parabadiella, found in this area and on the Yangtze Platform. These are most probably coeval with the late Atdabanian trilobites of the Siberian Platform. However, “correlations within the Lower Cambrian are generally difficult because of the rarity of fossils and the provinciality of the fauna” (Hou et al. 2004b, p. 235).

Stratigraphy

The Chengjiang fauna derives from the Yu’anshan Member of the Heilinpu Formation (formerly long-known as the Qiongzhusi Formation, also spelled “Chiungchussu”) of the Qiongzhusi Stage of the late Early Cambrian (see Brasier 1989, pp. 40-41 for additional reading), located near the western margin of the Southwest China Platform. 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.

“In eastern Yunnan ... the uppermost part of what is thought to be Precambrian has few if any fossils. Beginning in the earliest Cambrian, shelly elements first appear, then the skin of many unknown types of animals, clearly different from the trilobites and other fossils found in younger rocks. The shelly elements are now known as ‘small shelly fossils.’ The Lower Cambrian biostratigraphy in this area is based mainly on the succession of small shelly fossils at the base of the Lower Cambrian and on the succession of trilobite assemblages in the sequence above. As many as 3 small skeletal fossil biozones (or assemblages) and 11 trilobite biozones are used to divide the Lower Cambrian in this area and elsewhere on the Southwest China Platform. ... The Abadiella Biozone forms the oldest trilobite zone in the Cambrian of China. It is succeeded by the Eoredlichia Biozone, which contains the soft-bodied Chengjiang fauna. The soft-bodied Chengjiang fossils thus occur ... low down in the Lower Cambrian” (Hou et al. 1999, p. 166).

The Chengjiang fauna occurs about 25 m above the earliest trilobites, Parabadiella, found in this area and on the Yangtze Platform. These are most probably coeval with the late Atdabanian trilobites of the Siberian Platform.

Taphonomy

“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” (International Subcommission on Cambrian Stratigraphy).

Significance

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). It represents one of the oldest well-preserved animal assemblages.

The soft-bodied fossils include diverse algae, medusiform metazoans, chondrophorines, sponges, chancelloriids, ctenophores, cnidarians, nematomorphs, priapulid worms, hyoliths, possible ectoprocts, inarticulate brachiopods, annelid-like animals, anomalocharidids, onychophorans, a wide variety of other arthropods, including trilobites (though few of these), echinoderms, ‘hemichordates’ and probable early chordates, as well as numerous problematical forms that cannot definitely be assigned to well established taxa. They include virtually all the groups known from the Burgess Shale and other Middle Cambrian localities, thus compressing the available time for the morphological diversification of metazoans, known as the Cambrian Explosion, to just 25 Ma or so.

The fossils occur in rocks of the Qiongzhusi Formation, cropping out widely in eastern Yunnan Province, South China, where they have been known and studied for some time (e.g. Mansuy 1912, Wang 1941, Lu 1941, Ho 1942) and which include stratotypes for the Southwest China Platform. However, it was not until 1984 when Hou Xianguang collected a surprisingly well preserved specimen of Naraoia from Maotianshan, Chengjiang County (fig. 1), that the exceptional soft-body lagerstätte was recognised.

“The discoveries in 1984 of the Lower Cambrian soft-bodied faunas of Sirius Passet in North Greenland and of Chengjiang in southwest China has had a much greater impact on the significance of the Burgess Shale fauna – it is no longer the sole major source of our knowledge of Cambrian animal life. The older, Sirius Passet fauna includes animals that look significantly more primitive than those in the Burgess Shale,and seems to have few animals, if any, in common. In contrast, the Chengjiang fauna has so many animals in common with the Burgess Shale that it seems to be its geographic extension. This demonstrates that the Cambrian “evolutionary explosion” was over by the time of the Chengjiang fauna. It shows further, that the Burgess Shale/Chengjiang fauna was evolutionarily very stable, and changed little over the 15 to 20 million year interval between the two faunas. Thus the pattern for animal evolution was set in the early and middle Cambrian: short periods of rapid, significant change, exemplified by the Cambrian explosion, alternating with long periods of little change, exemplified by the Burgess Shale/Changjiang fauna” (Collins: “The Geology and Biology of the Middle Cambrian Burgess Shale, Canada’s Most Important Fossil Fauna”).

Taxa

The Sirius Passet (north east Greenland) fauna is approximately the same age as the Chengjiang fauna, but the two assemblages are not closely similar. Although it is significantly older, the Chengjiang fauna is most easily compared to the Burgess Shale fauna, which does have many families and several genera – though no species – in common. Both Chengjiang and the Burgess Shale are dominated by sponges and arthropods; the two groups together comprising about 60% of each fauna.

“The Chengjiang fauna is basically a benthic fauna. Most species lived directly on the soft, level bottom. A few burrowed in the silty sediment, for instance linguloid brachiopods and presumeably some worms. We can not state for certain that there were any animals that were permanent swimmers, although it is quite clear that many of the arthropods could swim, many of them perhaps very well. At the same time we realize from the mud-filled guts of several arthropods that in many cases their appendages had not yet evolved into the efficient mouthparts they are in modern arthropods. They fed by simply swallowing mud, and this could of course only be done on the bottom. Similar mud-eating habits were common among the hyoliths, a group of extinct animals which had a cone-shaped shell and an operculum to close the open end.

“However, some of the arthropods were carnivores, feeding on small arthropods, worms and the like. Nematomorph and pripulid worms and anomalocaridids were also carnivores, who certainly often swallowed their prey in one piece. If the supposed cnidarians really were cnidarians, they would also have been carnivores. Many small arthropods could have fed from carcasses, for instance the ostracode-like bradoriids. Many animals no doubt strained their food directly from the water. This must have been the case with sponges, brachiopods, eldoniids and echinoderms.

“In summary, we can say that the community is a Burgess Shale-type level-bottom fauna with large proportions of mud-eaters, filter-feeders and carnivores, and with a dominance of arthropods, worms and sponges” (Hou et al. 1999, p. 169-170).

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, ctenophores, cnidarians, nematomorphs, priapulid worms, hyoliths, possible ectoprocts, inarticulate brachiopods, annelid-like animals, anomalocharidids, onychophorans, a wide variety of other arthropods, including trilobites (though few of these), echinoderms, ‘hemichordates’ and probable early chordates, as well as numerous problematical taxa.

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 (see Ramsköld & Hou 1991).

Systematics

A selection of Chengjiang fossil taxa are listed here. It is far from comprehensive, or even representative; longer lists are available elsewhere although none are complete for any longer than the time from one field season to the next, or until the next taxonomic paper is published. In general, I have given preference to the most common forms, and also to those which are also known from elsewhere, usually the Burgess Shale, or which shed light on interesting evolutionary events in main lineages (such as the trilobites).

Sponges, predominantly desmosponges, are a common and diverse component of the Chengjiang fauna.

Family Choiidae

Genus Choia Walcott 1920

Choia xiaolantianensis Hou et al. 1999

Relatively common; resembling a flat cone with radiating spicules. Although this species is known only from Chengjiang, the genus was originally described from the Burgess Shale.

Family Leptomitidae

Genus Leptomitus Walcott 1886

Leptomitus teretiusculus Chen et al. 1989b

Superphylum Gnathopoda Lankester 1877

Luolishania longicruris Hou & Chen 1989b

A rare lobopodian of ~15 mm overall length. The head is rounded and slightly elongate; trunk slender and tapering posteriorly; 16 pairs of long annulated legs (after Hou et al. 2004a).

Paucipodia inermis Chen et al. 1995 emend. Hou et al. 2004b

Diagnosis: Currently the same as for the genus, as the latter is monotypic. It will have to change if/when another Paucipodia species is described.

Description: This animal is one of the largest Cambrian xenusians. Hou et al. (2004b) report a complete specimen as 106.9 mm long and up to 4.3 mm wide.

Discussion: Paucipodia inermis was described by Chen et al. (1995) based on four incomplete specimens from the Chengjiang fauna locality at Maotianshan. The emended diagnosis by Hou et al. (2004b) is based on a further 20 specimens collected from Anshan section near the village of Mafang.

Phylum Arthropoda von Siebold 1848

	1848	Arthropoda von Siebold
	1938	Lobopodia Snodgrass
	1995	Panarthropoda Nielsen
	1997	Lobopodia Snodgrass 1938; Budd
	2001	Panarthropoda Nielsen; Nielsen, p. 194-197
	2001a	Arthropoda, Budd

Like the other lagerstätten of similar age, many of the Chengjiang arthropods are problematica; their relationships to higher taxa and better known lineages are poorly understood. Pre-eminent among the more “conventional” forms are the trilobites. The Chengjiang fauna is not much younger than the first known tribiltes of the region, and they are among the earliest known trilobites from anywhere. Most common are the redlichioids of the genera Eoredliichia (also known from Australia) and Yunnanocephalus. A third redlichioid, Kuanyangia, is relatively rare.

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