The fossils of the Cambrian era reveal the basic body plans of just about any organism in existence today. They also reveal a great deal of the body types that did not survive. Some of the alien looking creatures in the Burgess Shale either never survived a mass extinction or else have simply mutated into something a little more recognizable.

A far as questions go, mine mainly follow the pattern of lost possibilities and how certain adaptations helped some survive while others were too well adapted to their environment to cope with the changes that caused mass extinctions.