LIFE × EVOLUTION

The Window on Soft Bodies

~518 to 500 million years ago

Phanerozoic · Paleozoic · Cambrian

For a few tens of millions of years, muds at Chengjiang, in the Canadian Rockies and at a handful of other sites preserved gills, guts, eyes and nerve cords as films of carbon. What stopped the decay was a shortage of the chemicals bacteria need to rot a body: little sulfate in the seawater, little oxygen at the bottom, and a crust of carbonate cement sealing each bed almost as soon as it was laid down.

Why it matters

Almost everything known about Cambrian animals without shells comes from these few deposits. They are the reason we know the seas held more than skeletons, and the reason we cannot say how typical what they show us was. The window closes through the early Palaeozoic, as the oceans and their sediments take on more oxygen and more sulfur.

Dating & uncertainty

The window is a property of the rock, not of the animals: it opens where seafloor chemistry stopped decay, and closes through the early Palaeozoic as oceans and sediments became more oxygenated. Whether the sealing mechanism is early carbonate cement starving the mud of sulfate, as proposed, or something else, is disputed in print.

Sources

1. Gaines et al. (2012), PNAS · Mechanism for Burgess Shale-type preservation
2. Butterfield (2012), PNAS · Does cement-induced sulfate limitation account for Burgess Shale-type preservation?
3. Sperling et al. (2018), Emerging Topics in Life Sciences · On the edge of exceptional preservation: insights into the role of redox state in Burgess Shale-type taphonomic windows
4. Lei et al. (2024), Geology · Unveiling an ignored taphonomic window in the early Cambrian Chengjiang Biota

Earth history as one day

21:15:422 h 44 min before midnight

if Earth’s 4.54 billion years were compressed into a single day