These 50 million year old, Eocene-Era fossil fish come from one
of the world's famous Laggerstatten, the Green River Formation in
Wyoming. A small portion of the fish fossils from Green River exhibits
such fine preservation. The significant extent of soft-tissue preservation
that makes the site famous is evident in this specimen.
Priscacara is perhaps the most popular of the Green River fish fossils.
A member of the Family Priscacaridae, the name Priscacara means
"primitive head". Shaped rather like a sunfish, the genus
sports sturdy, protective dorsal and anal spines. Among the two
species, liops and serrata, serrata is uncommon. The species name
comes from the diagnostic serrated preopercle, seen here in closeup.
Liops is the smaller species, never exceeding 150 mm, whereas serrata
have been found up to 375 mm. The genus went extinct at the end
of the Miocene, and is thought by some to be related to the modern-day
smaller fish is Knightia eocaena, the State Fossil of Wyoming. In
Fossil Lake, these fish reach their maximum size of 25 cm, but averages
roughly half that. Knightia was a schooling fish which is sometimes
found in mass mortality layers confined to a single plane, indicative
of a single event. The one seen here would have served as a prey
species for the Priscacara which just overlies it, showing that
while they were contemporaries, the Knightia fell to the lake bottom
just before it, making them prisoners in time for the past 50 million