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Taphonomy

Fossils are found in thin muddy beds 1-2 mm thick. Soft-parts are preserved as aluminosilicate films, biomineralised parts are decalcified and organic-walled fossils have been replaced by iron-rich clay-minerals or hematite. Weathering of fossils is common and they appear as red (possibly oxidised pyrite) on a yellow matrix (e.g. Maotianshania). Compression has occurred, with some fossils appearing wrinkled and in other cases appendages are visible through the outer covering. Fossils tend to align to bedding so that a dorsal or ventral view is most common (appendages typically penetrate beds).

Excellent preservation is attributed to short transport and rapid burial in an 'undisturbed' environment. Some brachiopods and priapulid worms are found in situ, buried in life position. Transport of live or freshly killed animals is also cited, death being caused by asphyxia. A paucity of mobile swimmers indicate they were probably able to escape the sediment flow. Incursion of anoxic waters likely played a part, either in causing asphyxia or aiding preservation after death from scavengers and decay. It has been hypothesised that the high number of arthropods present indicate an accumulation of carcasses in anoxic bottom waters prior to sediment flow. Preservation has also been attributed to the blanket-like protection of microbial mats. Lack of pyrite indicates a non-sulfate reducing environment. Therefore oxygen was present, but low and hence another mechanism such as fluctuating salinity may result in the lack of scavengers. However, a complete taphonomic study is still lacking for the Chengjiang fauna.

Ref: Bottjer et al, 2002.

Palaeoecology

Likewise, a complete palaeoecological study has not yet been undertaken. Like Burgess, the Chengjiang fauna is dominated by arthropods and algae, with >97% of fauna representing non-mineralised forms (that would otherwise be lost to the fossil record). Chengjiang also contains a significant number of lobopods. However, it is already clear that Chengjiang's inhabitants occupied a wide variety of lifestyles, including:

The base of the food chain was probably occupied by phyto- and zooplankton, similar to modern oceans. Infaunal forms include brachiopods as well as the carnivorous priapulids. Epifaunal forms include suspension feeders such as Dinomischus and Heliomedusa as well as deposit feeders such as Naraoia. Coprolite evidence show that bradoriids were the main source of food for Chengjiang's carnivores, which include Anomalocaris and Luolishania. Isoxys represents the swimming pelagic form, as do many of the chordates. Scavenging is also evident from animals found attached to specimens of Eldonia and Rotadiscus.

Chengjiang shows a mixing between transported and in situ faunas. Although this makes separating communities complicated the important point to note is that Chengjiang represents evidence of a diverse and quite advanced marine community. Tiering (or 'stratifcation') is notable, probably extending some 30 cm above the sea bed, and several centimetres below it.

Ref: Bottjer et al, 2002.

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