SYSTEMATICS AND BIOGEOGRAPHIC HISTORY OF THE VENERID GENUS SECURELLA IN THE NORTH PACIFIC

A biogeographic, biostratigraphic and phylogenetic analysis of the Genus Securella (Parker) reveals its relationship to other members of the venerid subfamily Chioninae and to oceanographic patterns in the north Pacific during the Neogene. The earliest record of Securella is in the Oligocene of central California, suggesting an origination in the eastern Pacific. During the Miocene the genus spread from California, north to Alaska and west to Russia. In the Pliocene it spread equatorward, eventually becoming extinct in the north Pacific and surviving only in southern California and warm water refugia in Japan. Today there is a single extant Japanese species. The spatial and temporal history correlates well with the Northern Hemisphere warming and cooling trends during the Miocene and Pliocene respectively.

A phylogenetic analysis involving two tropical and three cold-water outgroup genera produced fourteen equally parsimonious trees of length 50. These trees give evidence to Securella as a paraphyletic group, with Mercenaria and Humilaria nested within the clade. The analyses support the reassignment of S. staleyi (Gabb) and S. montesanoensis (Weaver) to the genus Protothaca. They also indicate that Mercenaria is very closely related to Securella, reflecting the possibility that they are sister-taxa. The analysis supports the genus Chionopsis as the sister-group to both Securella and Mercenaria. This leads to an interesting scenario during the Oligocene: Chionopsis being located in both the tropical Atlantic and Pacific, while Mercenaria is restricted to the western Atlantic and Securella to the eastern Pacific. This scenerio supports the idea of Securella originating on the eastern side of the Pacific, and migrating in correlation with the Middle Miocene warming, north and west, to Russia and Japan. If this is true, it could have major implications for further study of this global warming trend.