cluding Anomalocaris. It’s a shame that there is not a Burgess Shale–like deposit in the earliest Ordovician rocks so that we could really take a census of how many soft-part animals known from the Cambrianaged Burgess Shale made it through this biotic crisis. It may be that the end-Cambrian event was every bit as destructive as any of the Big Five—but that it has remained underestimated because fewer animals at the time had skeletons and were thus less likely to have been preserved. Whatever its destructiveness, one thing seems clear: the extinction appears to coincide with a rapid drop in oxygen, and this drop may be related to fundamental changes in the carbon cycle. As noted in Chapter 2, sudden drops in oxygen were mass extinction instigators. But the extinction was also an instigator of future diversity, if the relationship noted in Chapter 2 about low oxygen stimulating new species formation is correct.

It would be fascinating to compare the respiratory structures in those animals going extinct at the end of the Cambrian to those that survived. My suspicion is that the survivors were more efficient respirers than those that died out. This is research for the future, however.

The oxygen drop at the end of the Cambrian was short lived. There was a rebound in atmospheric oxygen levels, and with the rebound came a higher diversity of life in the succeeding Ordovician period, coming, perhaps, from a variety of new body plans evolved in the crisis of low oxygen at the end of the Cambrian. This Ordovician expansion of life is the subject of the next chapter.