Let’s take Chesapecten as an example: in the lowermost part of the Yorktown Formation, one can find C. jeffersonius, an inflated shell with about 10-14 widely spaced, rounded ribs (based on the ones I have sitting around). In the lower-middle part of the same formation, one finds C. septenarius, with about 5-12 widely spaced, squarer ribs and low-ribcount type C. madisonius with 12-16 lower rounded ribs. In the uppermost part of the Yorktown formation, one only finds high-ribcount type C. madisonius with 20-30 lower rounded ribs.
If one considers these to have been gradually deposited (as pretty much anyone studying them does), then that suggests a transition from one species to another. Whether one agrees with the time period suggested is a different matter, and an issue on which I am slightly less familiar (would have to check which dating method was used).