Ecology in deep time

In certain cases the extrapolation into the geological past of conditions and processes relevant to present-day ecology is not automatically valid. Do anatomical designs show improvements through time? How do we treat fossil organisms with no living parallels? What might be the influence of shifts in atmospheric composition? If the end-Cretaceous extinctions are caused by bolide impact, why does ecological trauma persist for hundreds of thousands of years? What was the ecology of the ‘Cambrian explosion’? Has the role of competition been seriously underplayed? Ancient ecosystems may differ from those of today in a variety of unexpected ways.