Brief oxygenation events in locally anoxic oceans during the Cambrian solves the animal breathing paradox
"Oxygen is a prerequisite for all large and motile animals. It is a puzzling paradox that fossils of benthic animals are often found in black shales with geochemical evidence for deposition in marine environments with anoxic and sulfidic bottom waters. It is debated whether the geochemical proxies are unreliable, affected by diagenesis, or whether the fossils are transported from afar or perhaps were not benthic. [...]"
Source: Scientific Reports
Authors: Tais W. Dahl et al.