This is the "Fossil Friday" post #4. Expect this to be a somewhat regular feature of the website. We will post any fossil pictures you send in to [email protected]. Please include a short description or story. Check the #FossilFriday Twitter hash tag for contributions from around the world!
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This week we visit the Green River Formation and its enormously diverse faunal and floral fossil. The Green River Formation dates to the Eocene, about 53.5 to 48.5 million years ago. The formation is composed of very fine layers, a dark layer during the growing season and a light-hue inorganic layer in the dry season. Each pair of layers is called a varve and represents one year.
The sedimentary layers were formed in a large area named for the Green River, a tributary of the Colorado River. The three separate basins lie around the Uinta Mountains (north, east, and south) of northeastern Utah:
- an area in northwestern Colorado east of the Uintas
- a larger area in the southwest corner of Wyoming just north of the Uintas known as Lake Gosiute
- the largest area, in northeastern Utah and western Colorado south of the Uintas, known as Lake Uinta
Fossil Butte National Monument in Lincoln County, Wyoming is in a part of the formation known as Fossil Lake because of its abundance of exceptionally well preserved fish fossils.
The following fossils were contributed by ESCONI member Keith Robitschek. All of these specimens were self collected and prepared. They all have remarkable preservation and detail. Thank you, Keith!
Green River Fossil Lake Aquarium
The fish aquarium below contains mostly Knightia, both Knightia eocaena and Knightia alta. Hidden within this school is a lone Amphiplaga brachyptera.
Knightia alta
Knightia is a representative of the herring family of fishes. They are the most common of the Green River fish forming immense schools. They can be as large as 10 inches but in most cases are found to range from 2 ½ inches to 4 inches.
Amphiplaga brachyptera
Amphiplaga is in the trout-perch family. They only have a superficial resemblance to trout and perch but are not related to either group. They have never been found in mass mortally, indicating that they were solitary fish and didn’t gather in schools. They are a small fish with a maximum length of about 5 ½ inches. Most common size is around three inches but are relatively rare to find.
The fish aquarium below contains mostly Diplomystus dentus but also contains Mioplosus labracoides, Priscacara liops and Phareodus encaustus.
Diplomystus dentus
Diplomystus (Diplo) belonged to a primitive form of herring-like fish and is one of the most common fish. It is also one of the most common of the large fish. They can be as large as 24 inches. A schooling fish that commonly range from three to seven inches in length.
Mioplosus labracoides
Mioplosus (Mio) belong with the perches and basses. Classifying this species is difficult since there are over 10,000 living species of Perciformes, the largest order of vertebrate animals. They can be as large are 20 inches long. Well known for being preserved with another fish in its mouth. This example contains the remains of a fish in its stomach.
Priscacara liops
Priscacara liops (Priscy) belong with the perches and basses. It contains tooth pads within its throat indicating that they ate arthropods and small mollusks. They are a schooling fish much like today’s Bluegill. They are frequently found to be a little less than 6 inches long.
