A Mid-Pliocene Freshwater Fish Fauna From Sonoma Mountain, Sonoma County, California

Purpose of the study:\ud The purpose of this investigation is the description of fossil material from a\ud Mid-Pliocene volcanic ash deposit from Pressley Road on Sonoma Mountain. This fossil fish assemblage will be compared to Pliocene fish assemblages in western states and Pliocene and Pleistocene fish assemblages and Recent fish communities in Northern California in an attempt to understand distribution of freshwater fish species in Western North America.\ud Procedure:\ud Fossils were collected by random sample, cleaned, and labeled. Fish elements were identified by comparison with illustrations of fossil fish elements and with skeletal elements of Recent fishes. Plant fossils were identified by comparison with illustrations of fossil flora of Northern California and the Recent flora of Sonoma Mountain.\ud Findings:\ud Six genera of fish were identified representing six families: Oncorhynchus\ud sp. (Salmonidae), Gasterosteus cf. G. aculeatus (Gasterosteidae),\ud Hysterocarpus cf. H. traskii (Embiotocidae), Mylopharodon cf. M.\ud conocephalus (Cyprinidae), Catostomus cf. C. occidentalis, (Catostomidae),\ud and Lepomis cf. L. cyanellus (Centrarchidae). The latter three taxa were exclusively freshwater in origin, suggesting a connection to an interior\ud drainage in the past. The centrarchid, Lepomis cf. L. cyanellus, was the first\ud occurrence of this genus in the Pliocene fossil record of California. Conclusions:\ud The freshwater fish fauna from Sonoma Mountain is similar to other\ud Mid-to Late Pliocene fossil assemblages and to Recent fish communities in\ud Northern California. The primary freshwater genera from the Pressley Road\ud assemblage show connections to the Pliocene fossil fish fauna in the\ud Mississippi drainage and the Great Basin drainages. The extensive geological\ud changes throughout the Pliocene have greatly altered the drainage systems in\ud Western North America, making it difficult to find faunal connections