My Favorite Fossil: Oncorhynchus lacustris

In southern Idaho, the Snake River drains westward, forming an arc that dips first to the south and then swings northwest near the city of Boise. West of Boise, it turns to the north, to form the boundary between Idaho and Oregon. Along the flanks of the western portion of the Snake River Valley, south and southwest of Boise, are bluffs formed of pale sediments. These sediments are thinly-bedded, are several hundred feet deep, and cover much of the surface of the southwest corner of Idaho. The pale sediments contain many thousands of preserved fish bones, plus other fossils of freshwater organisms like clams, snails, and diatoms. Collectively, they give us a record of the history of an ancient freshwater lake.

Volcanic rocks are intercalated with these lake beds, the results of eruptions both violent and peaceful. The volcanic rocks can be dated radiometrically and these provide a chronology for the lake beds. Altogether, the lake, approximating the size of current Lake Erie but deeper, occupied its basin between about 9 million years ago and 2 million years ago. The ancient lake has been christened “Lake Idaho” by geologists. The sediments of Lake Idaho have been under investigation by scientists from the University of Michigan, Boise State University, and Idaho State University for several decades. It has been my privilege to be associated at intervals with paleontological efforts coordinated by Prof. Gerald Smith of the Univ. of Michigan, which has been the focal institution for the study of western North American fish evolution and biogeography occurring during the Neogene Period of geological time—in other words, over the past 25 million years or so.

Among the 30 or so fish species which can be identified from Lake Idaho are those belonging to several species of trouts, salmon, and their cousins, all belonging to the family Salmonidae. My favorite fossil is one of these: the large extinct trout, Oncorhynchus lacustris. The accompanying figure is a skull of O. lacustris which I collected in 1987, my first summer to participate in paleontological surveys of this region. The first fossils of O. lacustris were brought to light by the great Geological Survey of the Fortieth Parallel, which ran from 1867 until 1872 under the scientific direction of the geologist Clarence King. The anterior portion (snout) of a large, well-toothed fish was forwarded to the famous vertebrate paleontologist Edward Drinker Cope, who recognized it as an ancient trout and christened it Rhabdofario lacustris.

As part of my own researches on fossil salmonids, conducted with Gerald Smith, I recognized that R. lacustris was really quite similar to our modern Steelhead Trout and served as one of the top predators of the ancient Lake Idaho. The reclassification of R. lacustris as O. lacustris actually spurred us on to an extensive anatomical examination of our western North American trouts, revealing that they were more closely related to Pacific salmon than to Atlantic trouts, which was the prevalent view for most of the 20th Century. Oncorhynchus lacustris is a beautiful fossil and in life must have been as beautiful as a modern Rainbow or Steelhead Trout.

The different identified species of fossil salmonid fishes from Lake Idaho and elsewhere in western N. America shed light on the genealogy and ecological roles of western trouts and salmons, genus Oncorhynchus. For example, we have identified fossils of ancient Cutthroat Trouts in sedimentary deposits from many localities throughout the intermountain West, but not from the extensive deposits of Lake Idaho. It appears that in the past, just as today, Cutthroats and Rainbows did not co-exist for long periods in the same habitat. Many other aspects of the history of the tribe of salmonid fishes, such as the history of migratory behaviors, are being revealed by these wonderful fossil forms. And they help to inform our developing picture of the history of western North America and its aquatic ecosystems during the past few tens of millions of years. Wow!