Experimental Fish Taphonomy with a Comparison to Fossil Fishes

The abundant fossil fish of the Fossil Butte Member (Green River Formation, southwestern Wyoming) have been an enigma because of the near absence of modern-lake analogues which contain fish remains. The purpose of this study is to compare modern fish taphonomy with fish fossils to better understand the depositional and environmental conditions of Fossil Lake. A literature review of fish taphonomy in modern environments revealed that (1) decay happens quickly, (2) some species and sizes sink preferentially over others, (3) regardless of water temperature, fish do not have a predictable flotation response. Laboratory decay experiments (using various species under controlled conditions of temperature, pressure, salinity, and oxygen concentration) were conducted to further evaluate the fish taphonomic process. Results show that (1) fish tend to disarticulate faster in shallow water than in deep water, (2) flesh can be completely consumed by bacteria in a few weeks leaving bones articulated and skin intact for longer periods of time. Furthermore, (3) oxygen concentration in the water is not a significant factor in the decay process because (4) fish tend to decay from the inside out, probably by anaerobic decay, and (5) decay and flotation rates can be influenced by temperature, size, and salinity. Fossil fish were collected from two marginal and two mid-basin sites from within the Lower Sandwich Bed of the Fossil Butte Member. The bed is tuff bounded, so all of the fish were deposited during the same time in lake history. It was found that the percentage of articulated fish increases towards the basin center. Articulation is better in deeper water because decay gases are often insufficient to explode fish or to refloat them. Experimental evidence indicates that fish preservation in the basin center might be explained by water depths as shallow as 10 m, even in warm water. Well preserved fossil fish should not be used as conclusive evidence for anoxia or cold water; other evidences should be sought. The fish taphonomy of Fossil Basin does not contradict warm shallow lake models for the basin