Stewart Valley

In 1959, during field studies for the UC Berkeley Department of
Paleontology, John Mawby, Donald Savage, and S. David Webb, stumbled upon
Stewart Valley. Little did they know that they had discovered the most
diverse assemblage of fossils from the Miocene era in North America.(1)
Located in mid-west Nevada at approximately 380 N latitude by 118 W
latitude (2), the valley is flanked by the Gabbs Valley Range and the
Cedar Mountains. At the time they were not aware of the significance of
this unique paleontological area. For five years, the only research
conducted in Stewart Valley concerned paleomammals. In 1964 the potential
of the locality to provide the most complete record of Miocene life in
North America was recognized. Under the incentive of Harvey Scudder from
the California Academy of Sciences, the area became acknowledged as a
national treasure. Local residents helped to make the United States
Congress aware of the situation. Congress then issued an order setting a
64 square kilometer portion of the valley aside as an Area of Critical
Environmental Concern. Now Stewart Valley is protected for science. This
new level of protection works to preserve the paleontological heritage of
Stewart Valley by prohibiting any harmful alteration of the site. United
States law normally allows a citizen to remove fossils of plants, fish,
and rocks without a permit. With the congressional order this was no
longer the case for the valley. To understand the totality of Stewart
Valley, we must understand what the Miocene epoch was like and how the
area came to be a zone of fossilization.

The Miocene epoch began 23.8 million years ago during the Cenozoic Era.
The next eighteen million years were a time of great intercontinental
migration. Tectonic plates were separating in certain parts of the world,
while others were joining. As a result, temperatures rose compared to
those of the Oligocene epoch. In the middle of the Miocene, the warming
slowed down, and eventually a period of cooling ensued as the global
climate changed in response to Antarctic glaciation. For Stewart Valley,
the epoch began with a mean average temperature of 10 degrees Celsius and
an annual rainfall of 35-40 inches, as suggested by the discovery of
fossils of deciduous trees such as the oak. The climate became more
equable, and vegetation systems likewise evolved. The gradual change in
plant fossil indicates a decrease in the mean annual temperature of
approximately 2-3˚Celsius which was accompanied by a reduction in
precipitation about 10 inches(3). The transition in climate was matched
by morphological evolution in animals. Temperate ecosystems also
rediversified as tropical ecosystems retreated. The northern coniferous
forests expanded, but the cooling temperatures and interior drying led
the way for the appearance of kelp forests and grasslands. Grasslands,
much more open than forests, soon came to dominate the landscape along
with deserts. Angiosperms continued to radiate through newly formed
ecosystems.

As angiosperm radiation continued so too did the radiation of mammals
especially, as well as other animals. Empty adaptive zones became
present, and appropriate evolutionary novelties arose. Many adaptive
radiations occur after mass extinctions, however this is not a direct
consequence of the extinctions. Generally the environmental changes
responsible for the mass extinctions create the new adaptive zones that
get filled by new species. A mass extinction did not occur directly prior
to the Miocene. Our climate today is not drastically different than the
climate that evolved during the Miocene. This fact accounts for the
similarity of Miocene fossils with organisms present today.

Volcanic activity contemporaneous with tectonic faulting and warping,
created a northwest-southeast drainage system that converged upon Stewart
Valley. The drainage system eventually formed a lake sometime during the
Miocene epoch. The lake was approximately 4-5 miles wide east to west and
8-10 miles long covering what is thought to have been a plateau higher
than the present day elevation. A CLAMP (Climate-Leaf Analysis
Multivariate Program) analysis involves looking at,

    "gross physical aspects of leaves, including outlines, shapes, and
    sizes that can be readily observed on fossilized leaves…we used
    canonical correspondence analysis….to rank samples
    simultaneously relative to several environmental factors (such as
    temperature and precipitation values)…Because leaf physiognomy
    character states have typically non-linear relations to environmental
    parameters"(4).

Doing a CLAMP analysis on the plant fossil specimens from Stewart Valley
has led researchers to believe that Stewart Valley was at a higher
altitude during the Miocene epoch than it is now. Due to the spreading of
the tectonic plates, the entirety of Nevada has been sinking, taking
Stewart Valley along with it.

As organisms from the surrounding plateau fell into the lake or were
carried in by the drainage in to the lake, they sank to the anaerobic
depths of the body of water. Because of the absence of life at the bottom
of the lake, there was no microbial decomposition of the specimens. Silt
then settled to the bottom of the lake and trapped the soon to be
fossils. The fine grain of the silt sediment at this site also provided
for conditions that would preserve the fossils well. The continued
deposition bore down on the specimens, compressing them in layers of
silt. This leads to the formation of compressions and impressions.
Compressions were largely formed when specimens fell into the lake
followed by rapid burial.

A new theory dubbed "The Ziplock Hypothesis" states that the mucus trails
left by the diatoms present in the water may have helped to seal the
fossiliferous material as if it was being preserved in a ziplock bag.
Photographs taken by Scanning Electron Microscopes show traces of what is
believed to be the mucus from the diatoms surrounding the fossilized
specimen.

Plant debris and ash also fell into the lake, creating a volcanic slurry
which then gelled into a silicified deposit. Plants, animals, insects,
and almost every type of life are represented in the Stewart Valley
collection. Because of the fine grain of the sediment, paper-thin shales
reveal great detail in the plant morphology. Some specimens even show the
veination of the leaves, and some show the damage done to leaves by
insects. A great wealth of animal fossils has also been preserved in
Stewart Valley.

There are two horizons of fossil plants in Stewart Valley, separated by
about 15 meters of rock. The Law of Superposition states that in unturned
sediment the older layers of rock are deeper. The older paleoflora, found
in the Gilbert Andesite formation, dates to about 16 million years ago;
while the younger horizon, found in the Savage Canyon formation, is about
15 million years old. There are certain common plants that definitely
grew nearby the lake and left their fossil signature: cedar,
serviceberry, cherry, rose, willows, and poplar trees. As a million years
went by in the middle of the Miocene many more common plants appeared.
Fossils of fir, spruce, hemlock, walnut, red oak, live oak, and maples
have been found in the younger sediment layer. Fossils found in the
deeper horizon were deposited in the deeper parts of the lake basin, and
represent many of the woody plants living in the area. The upper horizon
contains fossils of plants that lived at or near the borders of the
shallow, swampy area of the lake. All of these vegetative samples, along
with current knowledge of the metabolic activity of these samples,
indicate the cooling and drying components of the climate change.
Specifically, the compositional change in taxonomy between the two
horizons of paleoflora show a twenty-five cm reduction in annual
precipitation in the Stewart Valley area. The temperature also must have
dropped a few degrees Celsius as explained earlier in the paper. These
climate changes were most drastic in summer time.

The abundance of information gathered regarding pollen and spores was
obtained from the fossils of the younger paleoflora of the valley.
Although dicotyledonous and cone-bearing plants were present in both
horizons of paleoflora, the dominant source of pollen varies between
fossil layers. Seventy-five percent of the pollen samples originate from
dicotyledonous plants such as oak, elms, and walnuts, and the other
twenty-five percent. The younger fossil layers show a transitional
increase in conebearing plant. Soon the 75% dicot pollen dominance was
replaced by a 75% cone-bearing pollen dominance from pine, spruce, and
fir. These changes are direct evidence for the progressive drying and
cooling of the surrounding environment.

One of the more abundant species of fossil found at Stewart Valley site
is the Lynothamnus floribundus which is more commonly known as the
Channel Ironwood. Before the discovery of numerous fossils in Stewart
Valley, the Channel Ironwood was presumed to be neoendemic to the Channel
Islands because it currently grows nowhere else in the wild. However, the
discovery of the species in Stewart Valley, and subsequently in Oregon,
mainland California, and all over the western United States discounted
this idea. Fossils of plant species such as Lynothamnus floribundus have
helped to determine the paleoclimate of Stewart Valley during the Miocene
era.

By taking note of the climates that the closest living relatives of the
plants represented in the fossil record, a paleobotanist can help
determine what the climate was probably like sixteen million years ago.
This approach is relatively accurate because plants and other organisms
generally adapt to the same climatic conditions in similar ways. Thus if
modern plants have plants have adaptations similar to those found in
Stewart Valley, the Miocene climate of Stewart Valley most likely was
somewhat similar to the climates of the currently existing plants of that
type. This reasoning has been used to find fault with the original theory
that Stewart Valley began as a low spot which has been rising due to
tectonic activity because many of the plant species found in Stewart
Valley are known to live at much higher elevations than previously
thought to have existed there at that time.

In addition to the many plant fossils, the Stewart Valley fossil
collection contains a diverse assembly of insect fossils that have been
useful in learning about the insect fauna of the Middle Miocene. As a
general rule, the insects living around Stewart Valley Lake in the Middle
Miocene are quite similar to species extant today. Thus far, the insects
found in Stewart Valley have belonged to the same major taxa that are
around today‹no new insect taxa have been discovered at the Stewart
Valley site. The closest relatives of the species of insect found in
Stewart Valley are in Asia thus suggesting a connection between Asia and
North America at some point. Some species of insect found at the Stewart
Valley site include dragonflies, crickets, termites, aphids, butterflies,
mosquitoes, fruit flies, bees, yellow jackets, and ants. There are also
several spiders.

Rapid preservation is the reason behind the relative lack of
disintegration of the insect fossils. A very high or very low
temperature, or a highly acidic or basic, saline water content prevents
normal microbial action on fossils. The abundance and detail of the
specimens attests to the high quality of their preservation, which took
place in the volcanic ash. The fact that the insect fossils have remained
intact suggests that little or no horizontal shift of the specimens took
place other than wind-rifting on the surface of the lake, but waves were
not present.

The mollusk deposits found in the lacrustine strata of lower Stewart
Valley give a lot of information about the aquatic habitat in Stewart
Valley Lake. Stewart Valley had both a diverse and remarkably
concentrated population of mollusks in the lake beds. The collection
includes bivalves and gastropods which are small members of the arthropod
phylum. The mollusk fossils come from a layer of the lacrustine sequence
evaluated to represent life about fifteen million years ago. It appears
that the Stewart Valley clams and snails are ancestral to many of the
molluscan faunas throughout the area. Because freshwater mollusks are
sensitive to their environment, the morphological adaptations of the
shells can attest to temperature, energy levels, and water chemistry in
the lake. When mollusks of a variety of genera are brought together, it
is very likely that the conditions that support these organisms were
present in the place where they were found.

Stewart Valley also boasts an extremely complete vertebrate fossil
record. The mammal population, like many of the other populations, was
extraordinarily diverse. The woodlands and savannah surrounding the lake
were ideal conditions for a plethora of animals. The discoveries of
horse, camel, hare, prongbuck, long-legged rhino, and rodent fossils
suggest the existence of plenty of open country for foraging. Predatory
and scavenging carnivores are also known to have roamed over the entire
area. Bones and other remnants of these vertebrates were buried in the
banks of the floodplains around the lake. The abundance of volcanic
clasts present in these sediments supports the idea that a source of
volcanic terrain existed not far from the lake. Beavers and Plesiorex, a
shrew-like animal that fed on insects, are the most abundant fossil in
the Stewart Spring district of Stewart Valley. The community living near
the spring is thought to have consisted of bony fish, amphibians,
chelydrid turtles, anseriform birds, beavers, other rodents, mastodonts,
short-legged rhinos, and various insectivores.

Unlike the mammal and insect diversity known in Stewart Valley, there are
comparatively few known fish species have been found in the lake and
river deposits. These include a kind of salmon or trout and a type of
minnow. The salmonid fossils are much less abundant than the cyprinids
(minnow) and are found mainly in the upper stratigraphic sections of the
lacrustine deposits. The genus and species of the fish fossils found in
the fluvial region of Stewart Valley have not yet been identified because
of their rarity. In contrast to the lack of bony fish diversity, an
amazing number of feathers have been retrieved from the paper shales,
but, attempts at identifying the feathers was unsuccessful due to the
fact that the feathers were mainly the under feathers used for insulation
common to many kinds of birds. However, a number of bird species have
been cataloged such as the golden eagle, the burrowing owl, the turkey
vulture, and the pinyon jay.

Because of the wealth of fossils to be discovered at the Stewart Valley
site, its preservation is of the utmost importance. Researchers from
several acclaimed universities, such as Diane Erwin and Howard Schorn of
the University of California at Berkeley and Neil O’Brien from the
State University of New York College at Potsdam continue to utilize the
fossil bed of Stewart Valley for their studies. Diane Erwin and Howard
Schorn are currently working with Lyonothamnus. Neil O’Brien is
using specimens recovered from Stewart Valley to further his hypothesis
on the influence of diatomaceous mucus in the preservation of fossils.
Research like this would not be possible if the potential of Stewart
Valley had not been recognized early on and labeled as an area of
critical environmental concern thereby setting it aside for the
betterment of humanity.

    References

    Bertram, K; Pigg, K; Schorn, H; Erwin, D. 1997. Middle Miocene flora
    from the Virgin Valley, northwestern Nevada, USA. American Journal of
    Botany. 84: 374

    Erwin, Diane M.; Schorn. Howard E. 1997. Channel ironwood
    (Lyonothamnus Spiraeoideae, Rosaceae) in the Miocene of Nevada.
    American Journal of Botany. 84(6): 384

    Erwin, Diane; Schorn, Howard. 2000. Revision of Lyonothamnus A. Gray
    (Rosaceae) From the Neogene of Western North America. International
    Journal of Plant Sciences. 161(1):179-193.

    Schorn, Howard; Scudder, Harvey; Savage, Donald; Firby, James., 1989:
    General Stratigraphy and Paleontology of the Miocene Continental
    Sequence in Stewart Valley, Mineral County, Nevada, USA. Proceeding
    of Internation Symposium on Pacific Neogene Continental and Marine
    Events. p.157-173.

    Scudder, H.I;. Schorn, H; Savage, D.; Firby, J., Lugaski, T.; Mawby,
    J.E.. 1986. Report of a Paleontological Inventory of the Stewart
    Valley Fossil Area (Proposed as an Area of Critical Environmental
    Concern). Bureau of Land Management, Nevada State Office, US. Dept.
    of the Interior, Reno. NV.

    Wolfe, J.A., 1964: Miocene floras from Fingerrock Wash., southwestern
    Nevada. U.S. Geological Survey Prof. Paper 454-N.

    Wolfe, J.A.; Schorn, H.E.; Forest, C.E.; Molnar, P., 1997.
    Paleobotanical Evidence for High Altitudes in Nevada during the
    Miocene. Science. 276:1672-1675.