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Pohl 2008

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http://john-pohl.suite101.com/the-evolution-of-salmon-part-2-a51343

Early Lineage

{1}55 million years ago, a new type of fish had risen to dominance in the coasts, lakes and streams of western North America. These were the teleosts, fishes whose skeletons are made of cartilage rather than bone.

{2}Teleosts today include almost all jawed fishes. Like previously dominant forms, they gained the upper fin over their contemporaries through improvements in body form and physiology. Theirs was a more efficient respiratory system, while their shape and musculature—changed relative to earlier fishes—allowed for more rapid and complex movements. These changes enhanced the teleost ability to survive in a world of “hunt and be hunted.”
Freshwater or Salt?

The early ancestors of salmon were teleosts, and the {3}fossil record suggests that they originated in freshwater. The first known salmonid ancestor, Eosalmo driftwoodensis, looked much like today’s grayling and lived in primeval lakes in western Canada approximately 40 to 50 million years ago. The fossil record has filled in marginally since then, with additional ancient trout, graylings, and whitefishes showing up in freshwater sediments.

{4}This freshwater origination is significant, especially when coupled with the fact that primitive salmonids, like graylings and whitefishes that are evolutionarily older than salmon yet still exist today, also live in freshwater (though some graylings do venture into estuaries.) If both extinct members of the family and older existing salmonids dwell primarily in freshwater, it seems likely that the early salmon, which evolved from these ancestors, first lived out their lives in lakes and rivers prior to developing anadromy.
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