Knightia Fossil Fish, from Green River Formation, Wyoming, U.S.A. (REF:KF2010)

Knightia is an extinct genus of clupeid clupeiform bony fish that lived in the fresh water lakes and rivers of North America during the Eocene epoch.

Knightia has the distinction of being the vertebrate fossil most often found completely articulated - this is, with its structural parts still attached or aligned as in life. It can be distinguished from its relative Diplomystus by its smaller size, shorter anal fin and slender body.

  

Knightia is frequently found in mass morality layers, in which thousands of individuals have died virtually simultaneously. This clearly suggests shoaling or schooling, but often the cause of death remains unclear - perhaps sudden temperature changes, or water stagnation with falling oxygen content, or rising levels of toxins due to algal blooms. 

It's small size, and common presence in the jaws  or guts of larger fish, suggest that Knightia was near the beginning of the food chains and it self probably fed on plankton.

Its phenomenal abundance in the Green River Formation rocks have led to it being appointed as Wyoming's state fossil.