Giant pectinids--taxa that regularly attain more than 90 mm in diameter--appear at or near the base of the "Vaqueros Stage" and coeval units of the Pacific coast megainvertebrate chronology. Similar giant forms appear in lower Miocene strata in Europe (Aquitanian Stage). Recognition of this evolutionary event in these widely separated provincial sequences, together with diversity trends in the Pectinidae as a whole and certain foraminiferal correlations, argues for placement of the Paleogene/Neogene boundary at the base of the "Vaqueros Stage" of the Pacific coast sequence.

The early Miocene of Pacific coast basins is characterized by abundance and taxonomic diversity of giant pectinids (Amussiopecten, Lyropecten, Macrochlamis, and Vertipecten). Of these, only Vertipecten is present in underlying Oligocene strata, and the few Oligocene species are only of medium size. Taxonomic diversity was greatest during the middle Miocene (Amusium, Amussiopecten, Lyropecten, Patinopecten, and Vertipecten), but decreased sharply during the late Miocene (Fortipecten, Lyropecten, and Patinopecten) and Pliocene (Leopecten, Lyropecten, and Patinopecten).

The giant pectinids can be separated into a southern group (Amusium, Amussiopecten, Leopecten, Lyropecten, Macrochlamis, and Nodipecten) and a northern group (Patinopecten s.s., Pationopecten [Lituyapecten], and Fortipecten). The survivors of these two groups, Nodipecten and Patinopecten, have mutually exclusive modern geographic ranges and are restricted, respectively, to warm-water and temperate- to cool-water molluscan provinces along the Pacific coast.

Taxonomic diversity trends and migrational patterns of these pectinids reflect climatic amelioration during the early and middle Miocene, followed by cooling during the late Miocene and Pliocene.

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