IMPLICATIONS OF MODERN FISH TAPHONOMY FOR THE PRESERVATION STATES AND DEPOSITIONAL ENVIRONMENTS OF FOSSIL FISH, FOSSIL BUTTE MEMBER, GREEN RIVER FORMATION, SOUTHWESTERN WYOMING

The abundant, well preserved fish of the Fossil Butte Member, Green River Formation, have been an enigma because of the near absence of modern-lake analogues with which to compare it. Various models have been proposed to explain fish preservation in the Green River Formation, but there is considerable disagreement in regards to depositional environments. The literature on fish taphonomy in modern environments reveals that (1) some species sink preferentially over others, (2) many species may either sink or float in cold water (£10° C) or warm water (³15° C), (3) fish that initially sink in cold shallow (£1.5 m) water may refloat after several days, and (4) fish that sink in warm water (shallow or deep) may refloat or remain on the bottom indefinitely.

Laboratory decay experiments (using various species under controlled conditions of temperature, pressure, salinity, and oxygen concentration) were done to further evaluate the fish taphonomic process. Preliminary results show that (1) fish tend to disarticulate faster in shallow water than in deep water because of gas pressure differences in the gut compared to water pressure, (2) flesh can be completely consumed by bacteria in a few weeks leaving bones articulated and skin intact for longer periods of time (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.

The above observations and experiments have implications for the interpretation of Fossil Lake and other deposits with well-preserved fish: (1) The fossil fish record may be biased because some species sink preferentially after death, while others float. (2) In Fossil Lake the percentage of articulated fish increases towards the basin center. At the basin margin (shallow water), poor articulation is explained by some factor(s) that cause fish to "stick" to the bottom allowing decay gases to explode them on the bottom rather than allowing them to refloat. In deeper water, good preservation occurs because decay gases are often insufficient to explode the fish or to refloat them. Fish preservation in the basin center might be explained by water depths as shallow as 5 m, even in warm water. (3) Well-preserved fossil fish should not be used as conclusive evidence for anoxia or cold water; other evidences should be sought.