PRESERVATION OF A DIVERSE ICHTHYOFAUNA IN MIOCENE AND PLIOCENE LAKES ON THE WESTERN SNAKE RIVER PLAIN, IDAHO AND OREGON

The Western Snake River Plain (WSRP) is a Neogene rift that hosted a series of lakes. Lacustrine sediments in the WSRP are excellently exposed at many localities south of the Snake River. They are primarily friable silts and sands and interbedded volcanics, with abundant fossils, of the Poison Creek Fm (11.3 to 8.9 Ma), Chalk Hills Fm (ca 8.5 to a minimum of 6.4 Ma), and Glenns Ferry Fm (4.3 to 1.8 Ma), each separated by unconformities. Interbedded basalts and tephras provide an excellent radiogenic stratigraphic context for understanding evolutionary diversification. The Poison Creek Fm remains poorly understood; but the Chalk Hills Fm and Glenns Ferry Fm have been under investigation for over a century. Sedimentological and biological data (limestones, diatoms, ostracods, mollusks, and fishes) reveal the lakes to have been well-oxygenated and slightly alkaline. The Glenns Ferry Lake was deep and oligotrophic. Tens of thousands of individual fish bones from these lakes and many intact crania have been collected since 1870. Most fish bones here are disarticulated, excellently preserved, free of matrix and without distortion. Collectively the assemblage provides large samples of three-dimensional elements, which can be compared with those of extant and fossil taxa. The ichthyofaunas of the Chalk Hills (25 sp.) and Glenns Ferry (32 sp.) lakes include representatives of salmonids, minnows, suckers, catfish, sunfish, and sculpins. These assemblages are the most diverse faunas from any Neogene site in North America and include the largest diversity of fossil salmonids anywhere; they provide benchmark data for historical biogeography of the western North American fish fauna. Comparison with modern analogs permits interpretations of niche partitioning and speciation processes. The presence of migratory and lake-locked salmonids suggest intermittent connection to the sea, at the same time that salmonid remains first appear in coastal marine and tributary sediments of California, Washington, and Oregon. WSRP Neogene taxa are most closely related to modern fishes of the Sacramento and Klamath drainages, while ichthyofaunas from the Ringold Fm, adjacent to the Columbia River in central Washington, imply regional connections before and during the latter half of the Glenns Ferry deposition.