LIFE × EVOLUTION

Cambrian Lagerstätten and Soft Bodies

~518 to 500 million years ago

Phanerozoic · Paleozoic · Cambrian

Muds at Chengjiang, in the Canadian Rockies and at a few other sites preserved gills, guts, eyes and nerve cords as films of carbon for a few tens of millions of years, from about 518 million years ago.

A Lagerstätte is a deposit in which fossils are exceptionally well preserved. 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, mostly as films of carbon. Decay appears to have stopped because bacteria lacked the chemistry they require to break a body down. The seawater contained little sulfate, a dissolved compound of sulfur and oxygen. Mud without oxygen is called anoxic. In anoxic mud the principal means of decay is sulfate reduction, a reaction in which bacteria use sulfate in place of oxygen, so scarce sulfate limits that reaction. There was also little oxygen at the sea bottom. A crust of carbonate cement, a mineral that binds the grains, sealed each bed of mud almost as soon as it was laid down.

Why it matters

Almost everything known about Cambrian animals without shells comes from the few Lagerstätten of that period. These deposits establish that the seas contained soft-bodied animals as well as animals with skeletons. It is not possible to tell how representative the animals they show are of all those that lived.

Dating & uncertainty

Preservation of soft parts as carbon films depends on the rock and not on the animals. It occurs where sea floor chemistry halted decay, and the interval during which it occurs is called a window. The window closes through the early Palaeozoic, the first part of the era that opens with the Cambrian, probably as oceans and sediments become richer in oxygen and sulfur. What sealed the mud remains disputed: an early carbonate cement that deprived the mud of sulfate, or some other mechanism.

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