The living members of the Argopecten gibbus stock include the bay and calico scallops, Argopecten irradians (Lamarck) and A. gibbus (Linné), both common in the western Atlantic and Gulf of Mexico; the less common A. nucleus (Born) of the Caribbean, southern Gulf of Mexico, Antilles, and southeastern Florida; and the common A. circularis (Sowerby) and A. purpuratus (Lamarck) of the eastern Pacific. The fossil members of the stock include the ancestors of these living species together with Argopecten eboreus (Conrad), an extinct species or species-group not ancestral to any of the later taxa. This study seeks to determine evolutionary relationships within the Argopecten gibbus stock by working back through the fossil record from a model of the morphological and ecological relationships of living species and subspecies. Biologically, the study is limited to an analysis of the morphology and ecology of the living taxa deduced from population samples. Paleontologically, it is limited to an analysis of morphological variation among samples of fossil populations collected from upper Cenozoic strata (Alum Bluff Group of the middle Miocene through the Pleistocene) exposed on the Atlantic and Gulf Coastal Plains of the United States. The time span investigated is about 18 million years, according to the latest published scale of absolute time.
Differences between samples were studied and evaluated by means of morphometric data consisting of 70 measurements and form ratios of the outline, ligamenture, and musculature of each valve. Using an electronic digital computer, data were subjected to univariate and bivariate analyses, and samples were compared using machine-plotted, bivariate scatter diagrams, reduced major axes, and other graphical techniques. Data from right and left valves were treated separately, except that they were recombined in the study of characters that differ between valves, thereby furnishing new information on intervalve features.
The postulated phylogeny shows a poorly known species, Argopecten species b, in the early middle Miocene (Oak Grove Sand), that is apparently very near the origin of the stock. This species evolved phyletically through A. nicholsi (Gardner) of the Shoal River Formation and A. choctawhatcheensis (Mansfield) of the Arca Faunizone into A. comparilis (Tuomey & Holmes) of the upper Miocene (Tamiami, Pinecrest, Duplin, and Yorktown Formations). A. comparilis was apparently broadly adapted and widely distributed, living in bays, sounds, and open marine waters in the western Atlantic, Gulf of Mexico, and Caribbean and probably extending through seaway passages to the Pacific, where it gave rise phyletically to A. circularis. By the end of the Miocene, on the eastern side of the Americas, this variable species had split, giving rise to a primitive bay scallop, A. anteamplicostatus (Mansfield), that, like the living bay scallop (its phyletic descendant), was probably ecologically restricted to the semienclosed waters of bays and sounds, and to another species, A. vicenarius (Conrad), probably restricted to open marine waters like the living calico scallop. The primitive bay scallop was apparently unable to reach the Pacific, but the open-marine species seems to have given rise to both the Pacific A. purpuratus and the Atlantic calico scallop, A. gibbus. The living Pacific A. circularis is morphologically primitive in that it resembles the Miocene species A. comparilis more than it does any of the later species on the eastern side of the Americas and is ecologically primitive in that it is broadly adapted and able to live both in bays and sounds and in open marine waters. During the Pleistocene, A. nucleus, a tropical bay scallop, is inferred to have split from A. gibbus and to have become morphologically convergent on the true bay scallop, A. irradians. A. eboreus, a common scallop on the eastern side of the Americas in the Miocene and Pliocene, represents a highly variable yet morphologically persistent lineage that neither split nor gave rise phyletically to other species and that became extinct during the early Pleistocene. In certain features of morphology, the A. gibbus lineage is convergent on the A. eboreus lineage, indicating that the extinct species may also have been restricted to open marine waters.
On the basis of the materials analyzed thus far, the evolution (both phyletic change and splitting) of the stock has been faster on the Atlantic side of the Americas than on the Pacific side, with the living Pacific species resembling late Miocene and early Pliocene Atlantic species. Because barrier islands seem to have played a key role in speciation within the stock, it would appear that evolutionary differences may have been caused by the active coastal tectonism of the Pacific side destroying such island barriers before genetic differences between inshore and offshore scallop populations could arise.
With regard to nomenclature, the name Argopecten is shown to be a senior synonym of Plagioctenium; the generic name Aequipecten is rejected for American species related to Argopecten gibbus; and it is concluded that the generic name Chlamys, sensu lato, is better applied as the subfamily name Chlamydinae. The species name Argopecten vicenarius (Conrad), unused since 1898, is reinstated as the only available name for an important taxon occurring in the Caloosahatchee Marl of Florida and the Waccamaw Formation of the Carolinas.