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SB-001
Diplomystus
Fossil herring relative
- Locality
- Kemmerer, Wyoming
- Formation
- Green River Formation
- Age
- About 52 million years old
- Period
- Eocene
- Acquired
- March 2024
The Fish
Diplomystus is an extinct genus of ray-finned fishes, named by Edward Drinker Cope in 1877; its type species is Diplomystus dentatus. It belongs to the Ellimmichthyiformes, distantly related to modern herrings, anchovies and sardines. Four species are currently recognised. D. dentatus is found up to about 65 cm and is the second most common fossil fish at Fossil Lake, after Knightia. Its mouth turns upward, which the Park Service reads as feeding at the lake surface, and it preyed on smaller fishes. Many specimens are preserved with a Knightia lodged in the mouth.
Fossil Lake: The Green River Formation
The Green River Formation is the sediment record of three Eocene lakes in what are now Wyoming, Utah and Colorado. Fossil Lake, in Fossil Basin near Kemmerer, was the smallest, about twice as long as it was wide and never more than about 100 feet deep. Diplomystus has been found in all three lakes, but it is common only at Fossil Lake.
The Fossil Butte Member, deposited when the lake was deepest and widest, is the most productive fossil-bearing unit. Its "split fish layer" is about 1.8 m (6 ft) of laminated limestone that splits along its bedding to expose fossils, and it represents some 4,000 years of deposition. Ash beds have preliminary uranium–lead dates of about 52.9 to 52.5 million years ago.
A layer of salty water on the bottom kept scavengers out, and a mat of cyanobacteria held carcasses in place as they decayed. Fish preserved this way stay articulated. Fossil Butte National Monument was established on October 23, 1972, and protects about 13 square miles of the old lake basin.
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The Eocene World
The Eocene Epoch began about 56 million years ago and ended about 33.9 million years ago. Its name comes from the Greek ēōs, "dawn," and kainos, "new." Charles Lyell introduced the term in 1833, in the third volume of his Principles of Geology.
Fossil Lake formed during the Early Eocene Climatic Optimum, the longest sustained warm interval of the Cenozoic, which lasted from about 53 to 50 million years ago. The National Park Service places the lake near the height of that warmth, around 52 million years ago. Although Fossil Basin lay in a continental interior at mid-latitude, the climate there was warm-temperate, comparable to the Gulf Coast of the United States today. The lake's surroundings supported palms, and crocodilians lived in and around the water.
The warmth did not last. Global cooling began around 49 million years ago and continued to the Eocene–Oligocene transition near 33.9 million years ago, when Antarctic ice sheets expanded. The lake's upper beds, the Angelo Member, record a lake that was shrinking, shallowing and growing saltier as evaporation outpaced the water supply. The basin eventually filled with sediment, and the area has since changed from a forested lake to the sagebrush grassland seen today.
Provenance
The slab holds a single fish lying on its side in pale grey limestone, with the head at the lower right and the tail at the upper left. The skeleton is preserved as a dark brown carbon film. The backbone, ribs, dorsal and anal fins and a deeply forked tail can be made out. Measured against the ruler in the dealer photo, the fish is roughly 15 cm (6 inches) from snout to tail tip, which is small for its species.
It was acquired in March 2024 from The Stones & Bones Collection, a fossil dealer. The catalog records its locality as Kemmerer, Wyoming. No more specific quarry or layer is recorded, so the age given on the label is that of the Fossil Lake sequence as a whole, about 52 million years. The photograph below is the dealer's image of the slab; the 3D model on this page is a scan of the same piece.
