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KNIGHTIA FISH FOSSILS
Knightia, a small species of freshwater herring, lived and died in the Eocene Epoch, approximately 50 million years ago. We know this because their fossils have been found in great number in the limestone strata of the Green River Formation in southwestern Wyoming. Fossil evidence suggests that they traveled in large schools and ate primarily algae, plankton, and insects; in turn they were an important part of the diets of Diplomystus, Mioplosus, and other larger prehistoric fish. It is not uncommon to find a Knightia in the stomach (or even the mouth) of a Diplomystus. The remains of many other kinds of fish, mollusks, insects, and plants have been found alongside them, suggesting a subtropical (think Florida!) environment. The Knightia are the most common fossils, however.
What killed the Knightia in such large numbers? Paleontologists speculate that rapid changes in temperature or changes in lake chemistry may have been the cause. Modern herrings are very sensitive to drastic changes in their environment, particularly temperature fluctuation.
Whatever the reason, large quantities of Knightia were preserved by being quickly buried in sediments. The best-preserved specimens died in summer, when oxygen levels were lowest due both to thermal stratification and algal blooms. A lack of oxygen both slowed decay and deterred scavengers, as well as increasing precipitation of the calcium carbonate sediment that became the limestone in which the fossils are now embedded.
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