No abstract

By comparing the Sr isotopic composition of migratory fossil salmon, which lived in the ocean but died in continental regions, to the well established marine Sr isotopic record, the age of the continental deposit could be determined with high accuracy. This approach to marine-continental correlation and dating requires (1) that marine-resident salmon bear a marine S7Sr/86Sr value in their bones or teeth, and (2) that the original S7Sr/86Sr value of fossils is not overprinted by diagenesis. The vertebrae of modern, hatchery-reared salmon exhibit Sr isotopic variations indicative of freshwater to marine migration during bone growth. Modern marine S7Sr/S6Sr values were preserved in growth layers formed later in life.Marine-phase growth layers in the bones and teeth of the late Miocene migratory salmon, Oncorhynchus rastrosus, were subjected to stepwise selective leaching to separate biogenic hydroxyapatite from diagenetic calcium carbonate and recrystallized hydroxyapatite. Although the procedure yielded leachates with Sr/Ca and Ca/P values characteristic of apatite, the leachates had 87Sr/S6Sr values consistently less radiogenic than values for late Miocene seawater (~ 0.7087). The fossils were substantially contaminated by Sr from the hosting clastic sediments. Specimens in continental deposits differed in 87Sr/86Sr value from host sediments by 0.0002 to 0.0200, supporting the conclusion that these salmon were migrants from marine waters. However, because the original Sr isotopic composition of fossil bones and teeth cannot be determined with confidence, archaeological, paleobiological and stratigraphic applications of this technique may be limited.

Two unambiguous discoveries involving rainbow trout require scientific name changes. First, the rainbow trout has been demonstrated to be the same species as the Kamchatka trout. Second, studies of osteology and biochemistry of trout and salmon show that rainbow and cutthroat trout, and their close relatives, the golden, Mexican golden, Gila, and Apache trouts, are more closely related to Pacific salmons (Oncorhynchus) than to brown trout and Atlantic salmon (Salmo). The different names required by these two discoveries will cause some confusion in communications in which the formal classification is used, so we present evidence to acquaint biologists and managers with the rationale for the changes. The species name of the rainbow trout becomes mykiss, an older Latinized indigenous name of the Kamchatka trout. The generic designation of rainbow and cutthroat trout poses a more subjective problem, involving four possibilities: Salmo, Oncorhynchus, Rhabdofario, and Parasalmo. The balance of evidence indicates to us that the generic name for Pacific trouts and salmons should be Oncorhynchus. We suggest recognition of two divergent sister lineages, (1) Atlantic trout and salmon, and (2) Pacific trouts and salmons, as the genera Salmo and Oncorhynchus, respectively. Alternative generic classifications considered include the following: (a) Enlarge Salmo to include all Atlantic and Pacific trouts and salmons. This would be well supported by morphological and biochemical characters, but would fail to emphasize the distinctions between the Pacific and Atlantic groups. (b) Use a separate generic name, Rhabdofario, for rainbow and cutthroat trout, and their inland relatives. This would be valid, but would fail to recognize the gradation between Pacific trouts and Pacific salmons. (c) Continue to assign Pacific trout to the genus Salmo, separate from Oncorhynchus. This would be stable, but at the expense of evolutionary information in the classification—rainbow and cutthroat trout are on the same branch of evolution as the Pacific salmon. To reflect these biological relationships in the classification of trouts and salmons will contribute to better understanding of their life histories and better predictions for their management.

No abstract

The fossil record of Cenozoic floodplain fishes increases from few species in the Paleocene and Eocene to about 5-15 species per locality in the Pliocene and Pleistocene. Modern floodplain habitats usually have more than 5-10 times this many species. The trend could be interpreted as an evolutionary increase, except that there seem to be no ecological or evolutionary reasons to expect ancient floodplains to have fewer species than modern floodplains. The alternate hypothesis is that ecological and fluvial processes destroy most fish bones before they are finally buried. Although floodplain depositional environments trap many fishes, these are subjected to extensive predation and scavenging, thereby reducing the opportunities for bones of small fishes, which make up most of the diversity, to be preserved in the fossil record. Abrasion in bedload probably destroys most small bones that are reworked. Surface collecting methods exaggerate the bias further because fish bones from fluvial rocks are fragmentary, difficult to discover, and difficult to identify. Screen washing for fossils from fine-grained sedimentary lenses should increase the known diversity from floodplain deposits.