Knightia

Knightia has the distinction of being the vertebrate fossil most often found completely articulated - this is, with it's structral parts still attached or aligned as in life. It can be distinguised from it's relative  Diplomystus by it's smaller size, shorter anal fin and slender body.
Knightia is frequently found in mass morality layers, in which thousands of individuals have died virtually 
simulateously. 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.
It's phenomenal abundance in the Green River Formation rocks have led to it being appointed as Wyoming's state fossil.