The earliest fish that can definitely be identified as a salmon is Eosalmo
driftwoodensis. It lived during a time span of 50-40 million years ago, which
was during the Eocene period. Fossils of this fish have been found in British
Columbia, Canada and Washington state in the U.S. The adult length was up to 30
cm.
An article titled Angling for Fossil Salmon was published by the Geological
Survey of Canada. It is part of a series called Past Lives: Chronicles of
Canadian Paleontology. This excerpt brings up some important points about the
paleontology of salmon:

"Mark Wilson believes that Eosalmo, a fish as long as 30 cm., lived in lakes
and rivers draining into the Pacific Ocean. Many of the salmonid species alive
today in the same region of British Columbia migrate to the sea at a young age,
and then return to fresh water as adults to spawn. However, the presence of an
almost complete age range of this species in the ancient lake sediments near
Smithers, including juveniles as small as 15 cm., suggests that this species did
not migrate to the sea. If the young fishes went to the sea and only returned as
much larger adults, the lake sediments should lack small and medium sized
specimens of this fish. These fossil fish suggest that the ancestors of the
salmon did not migrate to the sea and this habit evolved relatively recently.
Wilson also pointed out that Eosalmo driftwoodensis is an almost perfect
morphological intermediate ("missing link") between the two subfamilies of the
Salmonidae - the Salmoninae (salmon, trout, and char) and
the Thymallinae (grayling). This observation indicates that salmon evolved from
a more grayling-like form."
[Note - Mark Wilson is a professor at the University of Alberta.]

From 15-10 million years ago (a segment of the Miocene), a giant salmon
existed. This species is called Smilodonichthys rastrosus and it was about 10
feet long and weighed 500 pounds. Smilodonichthys had fang-like teeth.
Modern salmon such as the genus Oncorhynchus had emerged by 2 million years
ago, which was late Pliocene. Many Pleistocene fossils of Oncorhynchus have
been found in a formation that evolved from Pleistocene clay near Kamloops Lake
in British Columbia, Canada. Radiocarbon dating shows that these fossils are
15-18 thousand years old. Analysis of the bone collagen indicates that these
salmon did not eat food that came from the ocean. The lake in which they lived
apparently did not have an outlet to the Pacific.

Neal Robbins










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