Shales of the Green River Formation in general have been described by Bradley (1931) and he specifically mentioned those of the fish layer in the Fossil Basin as follows: “Plate 1 shows a thin section of the varved marlstone in the small, unnamed Green River Lake west of Gosiute Lake, where the varves are better developed. Each varve or annual deposit, consists of a layer of microgranular carbonates and a thinner, dark layer of organic matter” (Bradley 1948:645). The X-ray diffraction analyses performed by John Ward Smith of the Laramie Energy Research Center showed that the shales of the “fish layer” consist predominantly of calcite (roughly 60%), aragonite and dolomite (approximately 20%), and quartz (up to 10%).

Although fish are numerous throughout the thickness of this “fish layer,” there are three laminae that contain so many fish that it is almost certain that they represent catastrophic mass mortalities. Two are made up primarily of Priscacara and one consists almost exclusively of Knightia.

X-ray photos show that a rather high percentage of the fish in the shales are not perfectly preserved but have undergone varying amounts of disarticulation. There appears to be an orderly sequence of stages of decomposition—from essentially perfect articulation to total disarticulation. Disarticulation first appears in the most anterior vertebrae. From there it rapidly proceeds anteriorly into the head region and appears to do so posteriorly at a slower rate. In many specimens the head and anterior half of the body are completely disarticulated, while the posterior part of the body shows no disarticulation whatever.

It is assumed that after the dead fish settled to the bottom of the lake, external anaerobic bacteria found access to the “innards” of the fish via the opercular opening. This would account for the first sign of decomposition and disarticulation being just back of the head.

In the blocks of shale covered by X-ray there were 385 fish. For convenience, these were classified into six groups, group I showing no discernable disarticulation and group VI ([Fig. 29]) showing total disarticulation. The number and percentage of fish in each group are as follows:

Group I 223 fish 58%
Group II 38 fish 10%
Group III 14 fish 4%
Group IV 27 fish 7%
Group V 24 fish 6%
Group VI 59 fish 15%

Fig. 29. A partially disarticulated skeleton of a large Priscacara, Group IV.

Because of the predominance of completely articulated fish throughout the quarry and the fact that the fish involved in the mass mortalities show no disarticulation, it seems probable that some connection might exist between the death of the fish and conditions of the lake bottom that would cause their perfect preservation. It would seem that rapid burial might be the most obvious reason for excellent preservation. Thus any factor or combination of factors that would cause rapid precipitation of carbonates and also would cause mortality of fish would satisfy our requirements. One obvious factor that could, at least theoretically, fulfill these requirements would be an annual bloom of blue-green algae that are known to be toxic to fish (Prescott 1948). Such blooms usually occur during the warmest part of the summer when CaCO₃ is least soluble. By extracting CO₂ from the water, these algae are known to cause precipitation of CaCO₃. Thus we have a possible cause for some annual fish kill and perhaps an occasional superbloom that would bring about a catastrophic mass mortality. The highest mortality of fish then might have occurred during late summer algal blooms and at this time also would occur the most rapid precipitation of CaCO₃.

It is not known how much deposition of CaCO₃ would be required to protect a fish from disarticulation. It may be that a very thin layer, especially if mixed with organic ooze, might provide an effective seal to slow decomposition and prevent disarticulation. If sufficient CaCO₃ was precipitated and deposited to cover the fish that died during this period, one might expect perfect preservation. Such fish would fit into group I. Those fish that died just after this period might lie exposed on the lake bottom for most of a year and be subject to disarticulation. If little or no deposition took place during the rest of the year, fish that died just after the period of deposition should be the most completely disarticulated and fit into group VI. During the fall, winter, and spring, fish that died of attritional mortality would be disarticulated according to the length of time they lay on the bottom prior to the next period of deposition.