PaleoBios

Oncorhynchus rastrosus was a very large, spike-toothed, Pacific Salmon from the mid-Miocene to early Pliocene of the Pacific Northwest (California to Washington). It had two large premaxillary (breeding-fighting) teeth that stuck out laterally from the snout like spikes. It migrated from the Pacific Ocean to inland rivers to spawn, as extant Pacific salmon do today. It was planktivorous, based on numerous, long, over-lapping gill-rakers, and few, small teeth. There are gaps in our knowledge about this interesting salmon. First, one of the localities where many of the paratype specimens were collected (Turlock Lake, California), was not described geologically in the original paper beyond, ‘from the Mehrten Formation’. Here we describe these deposits as cross-bedded sands, gravels and large, rounded cobbles indicative of a fast-flowing, river, which we coin here as the “proto-Tuolumne River,” that periodically overflowed its banks. Paleocurrent directions indicate the river flowed to the southwest, where it joined the San Joaquin River in the Central Valley and flowed southward at this time (~5 Ma), emptying into a marine embayment near Bakersfield. Thus, the Turlock Lake O. rastrosus specimens would have migrated up from this embayment. Second, we investigated whether O. rastrosus developmentally changed before migration upriver to spawn, as extant Pacific salmon do today, by comparing premaxillary teeth from freshwater and coastal marine deposits in California. We found that the largest teeth (with the largest osseous tooth bases) were from freshwater deposits, and that these specimens had the most worn and blunt tooth tips. We propose that this was due to use in territorial defense during spawning and redd (nesting site) construction.