This 50 million year old, Eocene-Era fossil fish comes 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.
specimen comes from the so-called 18 inch layer noted for
the fish coming out complete on one side of the matirx and
exquisite soft-tissue preservation. The preservation here
is particularly superb.
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
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
the Green River Formation: Class Actinopterygii, the ray-finned
bony fishes, comprise almost half of all known species of vertebrates,
some 20,000 extant species. There are numerous locations worldwide
that are noted for wondrous preservation of bony fishes, and the
Green River formation that covers some 25,000 square miles of SW
Wyoming, west Colorado and east Utah is one of the premier examples.
The formation is one of the largest lacustrine sedimentary accumulations
in the world, and spans the period from 40 to 50 million years ago
during the Eocene Epoch.
the Eocene, based on the fossil record, the region was sub-tropical
to temperate. Some 60 vertebrate taxa have been described from the
formation, including crocodiles, boa constrictors, and birds, as
well as abundant invertebrates and plants. The unusually excellent
preservation of the Green River fish fossils is usually attributed
to a combination of two factors: 1) a cold period during the Eocene
that would have caused dead fish to sink faster due to a less inflated
swim bladder; and 2) the great depth of the lakes and the consequent
anoxic conditions that would have often prevented scavengers from
disturbing the carcasses.