As can be interpreted from foregoing discussions, the sediments deposited in the Fossil Basin vary from stream and flood-plain fluvial to lacustrine. At times, material eroded from surrounding uplands was carried by streams throughout the basin. These rocks are exemplified by the Evanston and parts of the Wasatch and Fowkes formations. When the lake appeared, lacustrine sediments, marlstone, and shale were deposited in the lake, while around the periphery of the basin, fluvial sediments continued to accumulate.
Plants found in the lake sediments of Fossil Basin tell us much about the climatic conditions that prevailed during the Eocene. The flora is quite similar to that now existing in the southeastern United States, reflecting a warm and humid climate. One of the striking examples is that of huge palm fronds that occasionally are found in the fish quarries at Fossil Butte. These seem to be fronds that blew from the trees into the lake. Soon they became water-logged and settled to the bottom to be preserved.
The land animals indicate that a rather wide range of ecologic niches existed over the basin before the lake came into being, around the lake during its presence, and again all over the basin after the lake disappeared. Many of the smaller mammals, some of the rodents, and most of the primates were almost certainly arboreal. The large mammals such as uintatheres, pantodonts, and tapiroids may have been stream-side or marsh dwellers. Probably inhabiting the forest floor and feeding on low bushes and undergrowth were such forms as the condylarths, horses, artiodactyls, and some of the rodents. Feeding on the rodents and other smaller mammals were the creodonts and miacids. Tiny shrew-like forms scampered about the undergrowth and fed upon worms and insects.
Bradley (1963) has estimated possible temperature and precipitation levels in southwestern Wyoming during the Eocene. Bradley’s modern analogues were the Gulf Coast and Great Lakes regions. From a series of calculations, an average annual temperature of 65°F is postulated. This could have fluctuated greatly in the inland setting of Fossil Basin. Precipitation amounts were possibly on the order of 30-43 inches annually. The amount of annual evaporation was also possibly in the range of 30-43 inches.
It is believed that Fossil Lake was thermally stratified—that is, with colder, denser water at depth (hypolimnion) and warmer, less dense water (epilimnion) nearer the surface. The deeper waters probably would have been devoid of oxygen, hence essentially uninhabited other than by anaerobic bacteria that survive without oxygen. If the bottom had been oxygenated, many types of life would have burrowed into the sediment thus destroying the delicate varves ([Fig. 28]). The lake was probably deep enough that wind and wave action did not roil the bottom sediments.
Fig. 28. A section of shales from the fish beds showing thin laminae that are interpreted as varves. Enlarged six times.
There have been many attempts to interpret the taphonomy (see glossary) of concentrations of fish at Fossil Butte. Bradley (1948) interpreted the cause of death and the reason for their preservation as follows:
In this basin (Fossil, Wyo.) hundreds of thousands of beautifully preserved fish are entombed in the varved sediments. Even the delicate fin and tail rays and other bones originally held in place only by tissue are virtually undisturbed, and even the scales are in place almost completely undisturbed. It seems to me that the picture of this lake as a thermally stratified water body provides nearly all the necessary information to account for the excellent preservation of these fish. Only in the stagnant hypolimnion could they have escaped being torn to pieces by scavengers or distorted by bottom feeders. It is significant that all the well preserved fish are in varved sediments. Those in non-varved sediments are a disordered mass of broken and chewed up bones.
The only part of the story lacking now is how the fish died and got into the hypolimnion. Limnology offers two possible explanations. Sometimes when the surface of a lake gets excessively warm, fish will plunge into deep water and might thus penetrate the hypolimnion, be overcome by hydrogen sulphide, and also have the gas in their swim bladders chilled so that they sank at once to the bottom. Once there, only anaerobic bacteria would attack them. The other hypothesis is that the thermally stratified lake was suddenly chilled so that it overturned more rapidly than the hydrogen sulphide could be oxydized and so killed off large numbers of fish. This seems a little more probable as the fossil fish are of all ages and sizes.
The fish in these quarries have been collected since the 1870s but mainly by commercial collectors. Most museum collections were purchased from these commercial collectors, hence they consist almost entirely of perfectly preserved fish, as poorly preserved specimens would be discarded by the collectors as of no monetary value. The result has been that most people, including Bradley, were misled into believing all of the fish in the varved sediments of the quarry were perfectly preserved. As nearly as can be determined, the first attempt to systematically collect and study this concentration of fish was that by paleontologists from the University of Wyoming (McGrew 1974). This work threw much new light on the occurrence and made available much new data that permit new interpretations.