Evolution What the Fossils Say and Why it Matters

194 Evolution? The Fossils Say YES!

FIGURE 8.8. Evolution of the scallop Chesapecten during the Miocene and Pliocene, as preserved along the
Calvert Cliffs in Chesapeake Bay. (After Miyazaki and Mickevich 1982: plate 1; originally based on Ward and
Blackwelder 1975; used by permission of Plenum Publishing)

YORKTOWN Fm.

UPPER

C. SEPTENARIUS C. MADISONIUS

YORKTOWN Fm.

LOWER

C. JEFFERSONIUS

C. MIDDLESEXENSIS

C. SANTAMARIA

C. NEFRENS

C. COCCYMELUS

EASTOVER Fm.

ST. MARYS Fm.

MARYLAND

CHOPTANK Fm.

ZONES 16 - 20

CALVERT Fm.

ZONE 14

CALVERT Fm.

ZONE 10

MIDDLE

MIOCENE

UPPER MIOCENE

LOWER PLIOCENE

the Late Cretaceous. Originally, they were the subject of a classic study by Rowe (1899), who
thought that they provided a clear case of gradual transformation from smaller shells with
primitive characteristics (derived from the ancestral form Epiaster) to shells that are broader,
with the tallest and broadest parts shifted forward. The groove in the front deepens and fills
with tiny bumps, and the mouth shifts forward with a more prominent lip. The area for the