Knightia Mortality Plate
Knightia Mortality Plate
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Knightia Mortality Plate

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Knightia eocaena mortality layer, unrestored specimen
Eocene (52.3 Million Years Ago) 
Green River Formation, Wyoming, USA 
In Stone Fossils Private Quarry 

No. of Knightia: 4

Plate size approx: 5.25" x 7.25" 

Fish Size: 1.75"- 2"

Knightia eocaena & Knightia alta

Order Clupeiformes, Family Clupeidae
The Clupeidae family has an estimated 50 modern freshwater species. Clupeidae species can lay as many as 200,000 eggs at once, allowing species to multiply quickly. Modern species are also prone to mass die offs.
K. eocaena is:

  • The most common fish found from Fossil Lake.
  • The most commonly found articulated vertebrate fossil in the world.
  • The Wyoming state fossil.

Both K. eocaena and K. alta are frequently discovered in mass mortality plates, some containing up to 100 fish per square meter. This suggests that, like their modern relatives, the Knightia species were sensitive to environmental changes.
K. eocaena specimens of all sizes are found in mass mortality plates, indicating this species schooled as both a juvenile and adult. The average adult size of K. eocaena specimens is about 6 inches, though specimens as large as 10 inches have been found.


Mass mortality plates represent a catastrophic "event" in Fossil Lake. These fossils can represent one of several "events."

  • Change in water temperature - A change of 1-3 degrees F could kill fish that require a specific water temperature to survive.
  • Change in lake water pH - Several volcanoes existed to the north of Fossil Lake. A volcanic eruption, resulting in volcanic ash entering the lake, could cause a sudden change in the lake water's pH. This could mean catastrophe to lake life.
  • Under-filled state of Fossil Lake - If the lake entered an under-filled state due to an increased period of evaporation, small pools of water could form. Fish trapped in pools would die due to starvation and suffocation.
  • Blue-green algal bloom - As bacteria dies, it consumes nearby oxygen from the water. This lack of oxygen rich water often cause fish to suffocate.
  • Overturn of lake water - If saltier water near the bottom of Fossil Lake rose to the surface, freshwater fish would die in the mixed salt and freshwater.