The family Cetorhinidae Gill includes one extant genus, Cetorhinus Blainville, and a single living species, the basking shark, C. maximus (Gunnerus). Basking sharks are coastal pelagic to oceanic with circumglobal distribution in boreal to warm-temperate waters of the continental and insular shelves. Second only in size to the whale shark, Rhincodon typus, basking sharks attain a maximum total length of 12 to 15 m (although generally not exceeding 9.8 m), and are planktivorous, feeding by filtering copepods, barnacles, decapod larvae and fish eggs from the water. The first Tertiary records of undisputed cetorhinids are from the middle Eocene of Antarctica, possibly the middle Eocene of Russia, and the late Eocene of Oregon. Eocene cetorhinids are referred to Keasius taylori, and Oligocene through early Miocene basking sharks are traditionally assigned to Keasius parvus. The earliest occurrence of Cetorhinus in the northeastern Pacific is early Miocene, and fossils attributed to this genus are relatively common in middle Miocene through Pleistocene marine sediments of Oregon, California, and Baja California, Mexico. Late Miocene and younger Cetorhinus are conventionally placed in the extant species, C. maximus. Late Miocene fossils of a basking shark from the Coos Conglomerate Member of the Empire Formation, Oregon, were collected in 1972 by students from the University of California, Berkeley. Associated vertebrae and gill rakers compare favorably in size and overall morphology with those of adult Recent C. maximus. Based on correlations of vertebral and gill raker dimensions with the total length for Recent C. maximus, the Empire basking shark is estimated to have been between 4.5 and 5.75 m in total length. Although gill rakers and vertebrae from the Empire Formation compare favorably with those of C. maximus, a definitive identification requires dentition. The occurrence of Cetorhinus cf. C. maximus in the late Miocene of Oregon is consistent with other late Miocene records of this species in California and Chile. C. maximus may range no earlier than late Miocene in the eastern North Pacific.
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