Ediacaran fossils — the earliest complex macroscopic organisms, 635 to 538 million years old — are preserved with extraordinary fidelity. Soft-bodied organisms that should have decayed without trace are instead imprinted in fine detail in sandstone and siltstone. The traditional explanation was that Ediacaran organisms had tough, resistant integuments — something about their biology made them harder to destroy than modern soft-bodied organisms.
The preservation is environmental, not biological. Ediacaran ocean chemistry favored rapid cementation of clay minerals around organic remains. When an organism died and was buried in sediment, the pore water chemistry — high silica, low sulfate, specific pH and redox conditions — promoted the formation of authigenic clay coatings on and within the organic tissue within days to weeks. The clay replaced the organism while its shape was still intact.
Modern experiments demonstrate the mechanism. Soft organic material buried in sediment under Ediacaran-like water chemistry develops clay mineral coatings that preserve three-dimensional morphology. The same material under Phanerozoic-like water chemistry decays to nothing. The experiment controls for biology — the substrate is identical. The difference is water chemistry.
This means the Ediacaran fossil record is a window opened by geochemistry, not by biology. The organisms were not tougher than their Phanerozoic descendants. The ocean was more willing to cast them in stone. When ocean chemistry shifted — sulfate increased, silica decreased, the early Cambrian transition remade the water column — the preservation window closed. The Ediacaran fauna didn't die out because they were replaced; they disappeared from the record because the conditions that preserved them ended.
The absence of Ediacaran-type preservation after the Cambrian is not evidence that Ediacaran-type organisms went extinct. It is evidence that the coffin stopped forming.