Paleoecology is the study of ancient biotic communities and their
relationship to the abiotic environment. The conclusions of paleoecology
are reached by studying in detail the fossils and sediments and whatever
relationships exist between them. Its ultimate aim is to build a picture
of the climate, flora, fauna, and topographic setting of an area. For
this reason it borrows heavily from paleontology, paleobotany,
sedimentology, and climatology.
From studying modern environments, it is observed that each has its own
type of sediment. When certain sediments are found in the rock record,
it is generally assumed that they represent environments similar to
modern ones that produce similar deposits. For example, limestone is
forming today in warm, shallow, well-lighted, well-aerated water. When
limestone is found in a rock sequence, it is usually assumed that the
same or similar environmental conditions occurred in that area in the
past. Fossils can further refine the interpretation.
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.
[Illustration: 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.
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%
[Illustration: 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.
If the foregoing is true, one should be able to determine the
approximate time of year fish died by the degree of disarticulation. One
might assume that blooms of blue-green algae and hence precipitation of
CaCO₃ would take place sometime during August and/or September. Thus we
should expect the most fish and those most perfectly preserved to have
died during this time period (group I). Those fish that died in October
and/or November should be the most completely disarticulated (group VI),
and those that died in June or July should show only a slight degree of
disarticulation (group II). The distribution of the stages of
disarticulation seems to fit almost exactly the pattern that one would
expect if this interpretation is correct.
Gunter (1947) has shown that annual periods of excess salinity in Texas
lagoons cause an annual increase in the death of fish and occasionally a
catastrophic mass mortality. Because Lake Gosiute is known to have been
saline, it might be assumed that somewhat similar chemical conditions
prevailed in the Fossil Lake. It is not known definitely that the two
lakes were ever connected but if they were, it was most probably a
narrow connection near the southern end of Fossil Lake and probably a
rather temporary connection.
That a rather long period of aridity occurred in the general region is
demonstrated by various depositional features, primary structures, and
salt deposition in Gosiute Lake in a part of the Green River Formation.
These deposits appear to have settled down during Lostcabinian (late to
early Eocene) times. Because the Wasatch Formation immediately
underlying the Fossil Butte Member of the Green River Formation in the
Fossil Syncline Basin is Lysitean (mid to early Eocene), it is probable
that the fish deposits are of Lostcabinian age. Thus it may well be that
the long period of aridity occurred during the deposition of the fish
beds. While the Fossil Lake probably never reached the high degree of
salinity present in Lake Gosiute, it was probably sufficiently saline
that periods of excessive evaporation could increase the salinity enough
to contribute to the mortality of fish and occasionally cause a
catastrophic mass mortality such as those described by Gunter. The
presence of aragonite and dolomite in the shales suggests that at least
some of the carbonate deposition might well have been because of
excessive evaporation and high concentrations of carbonates (Smith 1974,
pers. comm.).
At the University of Wyoming, an attempt is being made to interpret
scales of fish from the quarries. In a number of specimens annuli can be
observed and circuli counted. Although removal of scales for study is
extremely difficult, a number has been removed and photographed with a
scanning electron microscope. In Lake George, Florida, black crappie
develop annuli during January, February, March, and April (Huish 1954).
Climatic conditions in north central Florida may be similar to those
that existed in western Wyoming during the early Eocene. Thus annuli may
have developed at the same time of year. By counting the number of
circuli between the last annulus and the edge of the scale, it should be
possible to determine the approximate time of death of a fossil fish. If
this should correlate with the degree of disarticulation, a check on
this interpretation should be possible. Sufficient data are not yet
available, however, for the results to be conclusive.
It is not intended to suggest that the conditions outlined above could
account for all of the fish concentrations in the Green River Shales. In
certain shales in the Green River Basin, for example, the concentrations
of _Knightia_ in nonvarved shales appear to have been deposited in quite
shallow water. These fish are extremely well preserved but were
obviously laid down under conditions quite different from those at
Fossil Butte. Much study of these occurrences will be necessary before
interpretations are possible.
The story, as told here, should make it clear that the earth is
ever-changing. What was once a beautiful, deep lake in an area of lush
tropical forests is now a dry, sagebrush desert. What was once an area
in which sediments were accumulating is now an area of erosion,
occasioned by a broad uplift of the region near the end of Tertiary
time.
It is hoped that this brief story of the geologic history of Fossil
Butte National Monument will give the reader some appreciation of the
geologic complexity not only of the monument itself but of the
surrounding area as well. Certainly such knowledge will add to the
enjoyment of a visit to the monument.
It may seem that geologists have all of the answers. This, however, is
not so. Interpretations are made on the basis of available evidence.
Each time a geologist studies an area more geologic data and more
evidence become available and our interpretations become a little more
accurate. It will be many years before all details of this fascinating
history will be known.