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Cold-blooded, scaled aquatic vertebrates, fish were the first animals known to develop bones. The first fossil, Anaspis, dates from more than 500 million years ago and is thought to have been armored and jawless. Subsequent jaw formation, 400 million years back or so, may have resulted in the proliferation of fish: Jawless fishes left very few ancestors. Next came the Placoderms, which had both jaws and scales as well as armored plates. Not a single Placoderm survived a mass extinction event about 360 million years ago, but luckily ray-finned fishes and lobe-finned fishes evolved afterward and spawned ancestors we know today. Ray-finned fishes now comprise about 30,000 species throughout freshwater and ocean habitats all over the world.

Globally, 1,851 species of fish — including lampreys and hag fish, sharks, skates, rays and chimaeras, bony fishes, coelacanths, sea urchins and starfish — or 5 percent of described fish species were deemed at risk of extinction by IUCN’s 2010 Red List, 177 of them in the United States.

Arkansas River shiner
Atlantic salmon
Atlantic white marlin
Bluefin tuna
Central California coast steelhead trout
Colorado River cutthroat trout
Delta smelt
Desert pupfish
Devils River minnow
Dwarf seahorse
Gila chub
Headwater chub
Humphead wrasse
Kootenai River white sturgeon
Least chub
Loach minnow
Longfin smelt
Moapa dace
Montana fluvial Arctic grayling
North American green sturgeon
Pacific lamprey
Razorback sucker
Rio Grande cutthroat trout
Roundtail chub
Sacramento splittail
Santa Ana sucker
Smalltooth sawfish
Southern California steelhead trout
Spikedace
Spring pygmy sunfish
Topeka shiner
Unarmored threespine stickleback
Vermilion darter
Virgin River spinedace

:Photo © Paul S. Hamilton