Punctuated Equilibrium and the Validation of Macroevolutionary Theory 771
9 - 3B. Relative timings of punctuational events throughout Williamson's entire series. From
Williamson, 1981.absolute dates for the individual shells on a single bedding plane. (Admittedly, this
technique cannot be generally applied—especially to sediments of appreciable age,
where errors of measurement for any method of dating must greatly exceed the full
span of the bedding plane. But this method can be used for late Pleistocene and
Holocene samples.)
On a single mud flat (a modern "bedding plane," if you will) on the island of
Great Inagua, we found a complete morphological transition between the extinct
fossil pulmonate species Cerion excelsior and the modern species Cerion
rubicundum. Many lines of evidence indicate that this transition occurred by
hybridization, as C. rubicundum migrated to an island previously inhabited only by
C. excelsior among large species of Cerion. Ordinarily, we would find such a
complete morphological transition on single bedding