Dr. James G. Acker posted to talk.origins on 25 Nov 1995:
There is a particular class of trilobites called Phacops that shows this exact pattern. The gradual change in the population is found in a single quarry in New Hampshire. The more general fossil finds show a gap in the Phacops line of great (apparent) significance.In short, in most locations, the fossils show a sudden appearance. But the fossils in one small place reveal that the trilobite species evolved there, and then migrated to the other places.