The Chengjiang Lagerstätte and the Cambrian Explosion: New Discoveries from China about the Early Evolution of Life
Loren Babcock (Department of Geological Sciences, Ohio State University, Columbus, Ohio, USA)
12 May 1998
The Chengjiang is emerging as one of the most important lagerstätten on Earth. It dates to the latter stages of the Cambrian Explosion. It is 540-535/536 my old - not a well constrained age. The Chengjiang is one of the few lagerstätten that represents a point within the Cambrian Explosion. The Burgess Shale has widely been heralded as representing the Cambrian Explosion, but it isn’t - there is at least one major extinction between the Cambrian Explosion and the Burgess Shale.
The Cambrian is not the time of the first appearance of organisms, or even metazoans and metaphytes. Life starts at 3.55 by with procaryotes and eucaryotes at 2.1 by. At 1.2 by, one sees the first multicellular algae. Cnidarians and sponges are present before the Cambrian. Ediacaran organisms (570-650 my) are also around before the Cambrian. Evolutionary connections among these pre-Cambrian life forms are unclear. At the Precambrian-Cambrian boundary is a dramatic appearance of most major clades of organisms (>95% of creatures today appeared at the Precambrian-Cambrian boundary). At the PC-C boundayr, there is dramatic evidence that multicellular organisms were active all over the Earth - lots of trace fossils. The boundary is defined by a trace fossil, Phycodes pedum.
Small shelly fossils (SSFs) became abundant after the PC-C boundary. They represent disarticulated sclerites worn by organisms with many types of sclerites. In South China, ~100-150 meters above the PC-C boundary (not sure how much above, though - the PC-C boundary is unclear - lots of faulting and unconformities) is a fairly typical Cambrian assemblage - trilobites (mineralized), brachiopods, sponge spicules, echinoderms (rare), hyoliths, molluscs. A low diversity assemblage. Apart from Burgess Shale and Chengjiang, the Cambrian Explosion would be relatively little known. Lagerstätten are deposits of exceptional preservation - nonmineralized cuticle is common, and gut contents are less common.
The Burgess Shale is famous - ~515-520 my (15-20 my after the Cambrian Explosion) - known from a series of middle Middle Cambrian sites in British Columbia, Canada, in Yoho National Park. Lots of trilobites, brachiopods, non-shell-bearing organisms. Internal soft part bearing organisms are celebrated. The mode of preservation of the Burgess Shale is unknown. There's legs with cuticle on trilobites (like Olenoides). How this leg cuticle got preserved is unknown. Work on Chengjiang helps to understand the mode of preservation. The Burgess Shale has been metamorphosed, destroying the original signature. Why are Burgess Shale-type biotas important? They have nonmineralized creatures (+ internal soft parts/organs) preserved - they tell us about body organization; they preserve for us a record that is more complete than most rocks show. There are 5-6 clades of typical Cambrian creatures. A Burgess Shale-type deposit has >15-20 major clades of creatures. Also, they are important due to being present in an important interval in Earth history. At least, they are at times close to the Cambrian Explosion.
Where are Burgess Shale-type biotas found? ~2 dozen sites in the Cambrian. The Burgess Shale is along the Cordilleran margin of Laurentia. The Cordilleran margin has yielded ~20 or so Burgess Shale-type deposits. Most other Cambrian Burgess Shale-type deposits are from Gondwana. The Chengjiang is on the Yangtze Platform, on the edge of Gondwana. The Cambrian is seeing a fairly dramatic sea level (eustatic) rise. There are 2 good Lower Cambrian lagerstätten - Sirius Passet (Laurentian deposit, now thought to be older than Chengjiang, but less diverse) and Chengjiang.
Chengjiang deposit - a series of Early Cambrian sites (a dozen or so sites) in Yunnan, China. ~535 my at top, ~540 my at bottom (overestimate?). Shelly & non-shelly creatures there; many with soft parts preserved. Chengjiang stratigraphy - has been revised; the lithostratigraphy and allostratigraphy has now been worked out. Lots of unconformities (most previously unrecognized) and faulting make it difficult. Some ages have been revised as well. Have also revised the Proterozoic part of the stratigraphy. Chengjiang deposit - yellow shales, some siltstone/sandstone in Yuanshan Member of Heilinpu Formation. The Chengjiang can be characterized as the entire Yuanshan Member, or as pods/lentils within the Yuanshan Mbr. Preserved soft parts are 1-2 meters above the first trilobites on Earth, and 200 meters above the first SSFs in China. Where that relates to the Avalonian section is unknown: no more than 400 meters above, no less than 200 meters above. The Chengjiang is before the 1st archaeocyathans in the Chinese sections. The Chengjiang is known from a number of localities, but the best is the original locality - Maotianshan (“hat-shaped mountain”). Often very rainy there - difficult to work. The hillside is pretty much cleared off. There are phosphate mines in the Meishucunian in the area. Material weathers very quickly - weathers to mud in the rain before your eyes.
Depositional environment of the Chengjiang - originally based on the Burgess Shale - thought to be an anoxic basin adjacent to the base of the Cathedral Reef, where fossils were washed down by slumps, where they were stunned, buried, and preserved. This model has been applied to all other Cambrian lagerstätten. However, the Burgess Shale model jives with its geologic evidence. It doesn’t really apply elsewhere. The model has been applied to Chengjiang. Interpretation - creatures living in shallow, nearshore setting & washed downslope into a deeper anoxic area like Burgess Shale. So, inferring turbidites.
Matrix of Chengjiang samples - not black shale. A black shale does exist adjacent to Yuanshan - lower in the Heilinpu Formation - no exceptionally preserved fossils there - only a few pelagic trilobites. So, the black shale model doesn’t work with Chengjiang. Chengjiang has pinstripe bedding - like tidal rhythmites - combine that with a setting in a shallow marine platform, with occasional evidence of fluvial/flood deposits entering the main basin. Some bacteria look like they were desiccated and rewashed out. Deposited in a tidally influenced setting, it is concluded - lagoonal, estuarine - this explains the strange faunal variation from 1 locale to the next. A couple of supratidal areas? Areas of fluctuating salinity - why they were preserved - get hard part remains like trilobites + nonmineralized organisms (including appendages).
Trilobites are preserved as internal molds of once calcitic skeletons. Other soft bodied creatures are whole body fossils - have been phosphatized (fluorapatite). So, Cambrian (& Pennsylvanian) lagerstätten have preservation due to thin phosphatic deposition (7-10 days to 3 weeks after death). Get phosphatization if salinity fluctuations knock out bacterial biodegraders and scavengers. Saltwater creatures decay slowly in freshwater and vice versa. Saltwater creatures decay fast in saltwater and vice versa.
Chengjiang creatures - some are Proterozoic holdovers (like spiral bluegreen bacteria). The overall composition of creatures at Chengjiang - censusing >3000 specimens - arthropods dominate the biota. The next most abundant (~30%) are bacteria & algae remains, though it is difficult to quantify broken thalli and clumps of organisms. Everything else (including “typical” Cambrian creatures) is <3% of the Chengjiang biota. Arthropods dominated living Chengjiang communities, just like today. Arthropods today are 90-95% of all animals on Earth (in terms of species). New arthropods are constantly being described from Chengjiang. Most organisms (including hard-part fossils) are not abundant.
Mineralized (<3% of biota) vs. nonmineralized (>97% of biota) creatures - the nonmineralized creatures stand almost no chance of being preserved under normal conditions. So, 97% of a Cambrian community doesnt’ get preserved normally.
Holdover from Proterozoic - there is one type of frond-like/seapen-like fossil similar to Charniodiscus. If it is related to Charniodiscus, it is clear that there were Ediacaran holdovers. The Chengjiang frond-like fossil was found near the top of the Chengjiang zone - one specimen was found lying atop a trilobite sclerite.
Sponges are common - clearly holdovers from the Proterozoic. Proterozoic sponges are known.
One mollusc specimen - the earliest mollusc on Earth.
Nonmineralized animals - worms (priapulids with everted heads & have sediment-filled guts), arthropods. Arthropods include large bivalved things, naraoiids (a bit strange; have walking appendages, gills, gut tract, attachment points for muscles for legs), earliest chelicerate (possibly) - a primitive scorpion? No definitive chelicerates are known before in the Cambrian. In the Burgess Shale, there is one chelicerate, but it is a bit problematic - that one lacks chelicerae. The Chengjiang has a chelicerae-bearing arthropod. Chengjiang also has an early chordate, according to some (Yunnanozoon) - it is an early protochordate according to some researchers, but a hemichordate according to others. There are also strange things - Eldonia pineapple rings - strange. Also anomalocaridids - large, with ferocious mouth parts - 3 meters long.
Chengjiang tells us: the Cambrian Explosion was very rapid - more rapid than previously thought. Major body plans appeared early. There was considerable body plan experimentation (most went extinct). Predation was an important factor. Complex nervous systems developed early.
Sea pen interpretation - the problem is we never see soft parts. We never see a single tentacle from these, despite fossil hydroids having them in the Mazon Creek Lagerstätte. Ediacaran holdovers in the Chengjiang are in the bases of tempestites, not in the shales.
The Chengjiang stuff is all obviously transported (parautochthonous). We don’t know whether they were alive or dead when transported.
There are laterally adjacent fluvial deposits. Some sandstones near the top of these sections are channels.
Often a kneejerk reaction: lagerstätten imply anoxic basins. But, this doesn’t work at Chengjiang.