The
Green River Formation in the United States is one of the world’s most
famous Lagerstätte fossil sites. Its
faunal assemblage contains fossils spanning the entire Tree of Life, across
plants, invertebrates
and vertebrates (including reptiles, mammals and even primates), and even Stromatolites.
Despite the diversity, Green River is best known for its exquisite fish fossils.
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 southwest
Wyoming, western Colorado and eastern Utah is one of them. The formation is
one of the largest lacustrine (i.e., lake) sedimentary accumulations in the
world, with an average thickness of some 2000 feet. The vast deposits were
laid down between about 48 and 54 million years ago, during the Eocene Epoch
of the Cenozoic Era.
The
Green River Formation is actually a heterogeneous complex of lakes that differed
in their ecological and geological characteristics,
geological timeframes
and thus fauna and flora of their time. The complex comprises three primary
lakes formed as a consequence of drainage from tectonic highlands involved
in the uplift of the Rocky Mountains from the late Cretaceous through the
Paleocene periods some 80 to 55 million years ago. Fossil lake, centered
in Southwest
Wyoming, is the smallest and appeared briefly during the early Eocene. The
Lake Gosiute deposits span the period from Lower to Middle Eocene, and the
largest deposit from Lake Uinta that ranges across the Utah-Colorado border,
spans most of the Eocene Epoch.
During
the Eocene, based on the fossil record, the region was sub-tropical to temperate.
Some 60 vertebrate taxa have been
described from the formation,
as well as abundant invertebrates and plants. Green river has been noted
for its well-preserved fish since mid-way through the 19th century. 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.
The
majority of fish fossils are taken from the Fossil Lake area from two layers:
1) the
so-called 18-inch layer; and 2) the spilt fish layer. The
best preserved
fish come from the 18-inch layer. Because the sediment is highly laminated,
the fish can often be removed nearly whole. This layer, in the area near
Fossil Butte, does indeed average about 18 inches in thickness, corresponding
to about
some 4000 years of sedimentary deposition. The composition of the limestone
indicates that the layer was formed in deep water far from shore. By
contrast, the so-called split-fish layer is not laminated, making removal
and preparation
of the best fish fossils more difficult. The layer is about six feet
thick, and the fauna indicates water that was better circulated than that
associated
with the 18-inch layer. Some 19 genera of Eocene fish come from the Green
River formation as shown in the table below..
Green
River Formation Fossil Fish
Genus |
Common
Name |
Order |
Family |
Number
of species |
Relative
abundance |
Example |
Heliobatis |
skate
/ stingray |
Rajiformes |
Dasyatidae |
1 |
Rare |
|
Lepisosteus |
gar
fish |
Lepisosteiformes |
Lepiosteidae |
3 |
Rare |
|
Amia |
bowfin |
Amiiformes |
Ammiidae |
2 |
Extremely
rare |
|
Priscacara
serrata |
- |
Perciformes |
Priscacaridae |
2 |
Common |
|
Phareodus |
- |
Osteoglossiformes |
Osteoglossidae |
2 |
Uncommon |
|
Knightia
alta |
herring |
Clupeiformes |
Clupeidae |
2 |
Extremely
common |
|
Diplomystus |
herring |
Ellimmichthyiformes |
Ellimmichthyidae |
1 |
Very
common |
|
Notogoneus |
- |
Gonorynchiformes |
Gonorynchidae |
1 |
Rare |
- |
Amphiplaga |
trout |
Percopsiformes |
Percopsidae |
1 |
Rare |
|
Mioplosus |
perch |
Perciformes |
Percidae |
2 |
Uncommon |
|
Gosiutichthys
parvus |
herring |
Clupeiformes |
Clupeidae |
1 |
Uncommon
to Rare |
|
Asineops |
- |
Incertaesedis |
Asineopidae |
1 |
Rare |
- |
Eohiodon |
mooneye |
Osteoglossiformes |
Hiodontidae |
1 |
Extremely
rare |
- |
Amyzon |
sucker |
Cypriniformes |
Catostomidae |
1 |
Rare |
- |
Astephus |
catfish |
Siluriformes |
Ictaluridae |
1 |
Very
rare |
- |
Hypsidoris |
catfish |
Siluriformes |
Ictaluridae |
1 |
Extremely
rare |
- |
Erismatopterus |
trout |
Percopsiformes |
Percopsidae |
1 |
Rare |
- |
Crossopholis |
paddlefish |
Acipenseriformes |
Polydontidae |
1 |
Extremely
rare |
- |