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 boundary, 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.
