Miocene salmon (Oncorhynchus) from Western North America: Gill Raker evolution correlated with plankton productivity in the Eastern Pacific

Miocene salmon were more specialized than modern salmon in their feeding apparatus. Computed tomography scans of two late Miocene Pacific salmon reveal unusually long, closely-spaced, and numerous gill rakers (plankton straining structures in the pharynx). Middle Miocene Oncorhynchus rastrosus also has more numerous and more finely spaced gill rakers than living species. The fossils described here, including one new species, appear to be related to modern Sockeye Salmon and Chum Salmon, differing most in the numbers and morphology of gill rakers. These structural adaptations to plankton feeding in a group of fish otherwise adapted to fish predation are concordant with oceanographic evidence for remarkably high plankton productivity in the North Pacific Ocean during the middle Miocene.