Check out Robert Felix's latest book. It's another well researched tome. He does a nice job of spotting the periodic patterns. At every extinction boundary there is a layer of soot, coal, heavy metals and radioactive elements. The living things above the boundary appear fully formed. No incremental evolution. Each boundary also shows a magnetic reversal in the alignment of magnetic lines in the solidified rock. The living things that survive from the previous layer have characteristically low rates of metabolism. They survive well through periods of intense radiation.
Not to spoil the plot, but we're due for another reversal. Soon.