|
There have been many popular rockhouding and fossil-bearing
areas on the vast Mojave Desert, but one site in particular used
to consistently attract a great deal of attention. This was the
classic Marble Mountains fossil quarry now situated in the appropriately
named Trilobite Wilderness, San Bernardino County, California.
Here, abundant and well-preserved fossil trilobites could be
found dating from the early Cambrian geologic age, or roughly
518 million years old, some of Earth's most ancient identifiable
animals with hard parts. Although the noted Marble Mountains
fossil quarry is no longer accessible to unauthorized visitors,
due to its inclusion in the recently established Trilobite Wilderness
district, many avid amateur collectors and professional paleontologists
alike still remember with wistful fondness the days when abundant
paleontological remains could be recovered there--days of glorious
exhilaration when fossils of exceptional quality and scientific
importance came to light in the primeval rocks--those ancient,
wonderful trilobites that as a group survived for nearly 300
million years before their eventual extinction just prior to
the rise of the dinosaurs some 245 million years ago. Today,
of course, trilobite hunters who visit the Mojave Desert must
content themselves with fossil localities that occur outside
the boundaries of the federally protected Trilobite Wilderness.
The fossil trilobites in the Marble Mountains occur in
a greenish to rusty-brown, platy-weathering shale called the
Latham Shale, a detrital rock formation dated as lower Cambrian
on the geologic time scale, or roughly 518 million years old.
The Latham was named in 1954 by geologist John C. Hazzard for
its excellent exposures on the western slopes of the Providence
Mountains near an old and famous miner's cabin approximately
40 miles north of the Marble Mountains site--a specific place
within the Providence range that is now off-limits to unauthorized
collectors due to its inclusion in a federally protected wilderness
area (one requires a special Bureau Of Land Management permit
in order to collect legally within a federally administered wilderness
region). Throughout its "type locality," in the region
around the cabin where it was first described in the geologic
literature, the formation is at least 60 feet thick and contains
an abundant fauna of early Cambrian trilobites and brachiopods.
At the classic trilobite quarry in the Marble Mountains, the
Latham Shale averages around 50 feet in thickness and is also
loaded with fossilized carapaces of trilobites, brachiopods,
a siliceous sponge, a soft-bodied coelenterate (perhaps a jelly
fish of some sort), an echinoderm and a mollusk or two. Virtually
all of the trilobite specimens found at the old quarry--and in
other exposures of the Latham outside wilderness boundaries,
as well--were fragmental, although a few extraordinary fortunate
individuals reported that a whole, perfect fossil popped out
at them from the shales.
The main reason there are so few complete, intact trilobite
specimens to be found in the Latham--and at other early Cambrian
sites, for that matter--has to do with the original fragility
of the animal's exoskeleton. In actual life, trilobites possessed
a thin outer covering composed of chitin--a hard, horny substance
protecting the delicate soft-bodied organism within. While this
material can be preserved in the rocks for millions of years,
the problem is that the primitive early Cambrian trilobites--among
the earliest known animals with hard parts--had loosely attached
body segments. Thus, the head, thorax (middle portion) and tail
tended to separate very easily upon the animal's death. Also,
trilobites molted throughout their lives, periodically shedding
their chitinous external covering in much the same way their
modern-day relatives, insects, crabs, scorpions and pill bugs
regularly shed their own exoskeletons during the molting process.
The result was that the trauma of the molting often caused the
already free-moving body segments of the trilobite to disassociate
and break off, to be scattered by the sea currents.
All the trilobites found in the Latham Shale belong to
a single significant family of trilobites called Olenellidae,
or as they are more commonly called, olenellids. These were
marvelously specialized arthropods, well adapted--for a time,
anyway--to their life of burrowing in the muds of the shallow
marine shelves along the margins of the early Cambrian land masses.
Perhaps their most fascinating feature was their set of compound
eyes, consisting of numerous minute calcite crystals in a closely
packed arrangemet. called scientifically a holochroal eye. It's
not known for certain just how clearly the earliest trilobites
were able to focus these eyes, but there is little question that,
at the very least, they were able to distinguish adequately between
predator and prey in their marine habitat. Mysteriously, though,
the olenellids never survived beyond the early Cambrian, roughly
513 million years ago. Why they became extinct has never been
fully explained, although the most logical idea is that, ultimately,
they were ill-equipped to wage a successful struggle for life
in their increasingly competitive environment. The Cambrian Period
had ushered in the Paleozoic Era--the first occurrence of abundant
complex animal life on Earth. It was a time of explosive biological
diversification, and numerous burgeoning plants and animals were
vying for every available ecological niche. Perhaps the olenellids,
generally recognized by paleontologists as the earliest family
of trilobites to appear in the geological record, could not overcome
the inevitable encroachment of other increasingly aggressive
varieties of trilobites into their domain. As a group, though,
trilobites made it past the Cambrian Period and went on to flourish
for close to another 300 million years or so, until at last they
became extinct at the conclusion of the Permian Period, around
245 million years ago.
Roughly 280 million years earlier, though, trilobites flourished,
and their remains have been found on practically every continent
on Earth. In the western United States--in California and Nevada,
specifically--most well-known trilobite localities occur in early
Cambrian through middle Ordovician-age rock deposits. And, of
course, one of the most famous places in California to find early
Cambrian trilobites was at the prime fossil quarry in the Marble
Mountains. There, the oldest geologic rock formation exposed
is the early Cambrian Zabriskie Quartzite. This is a massive,
resistant accumulation of heat and pressure-altered sandstone,
mostly unfossiliferous except for a stray olenellid or two, plus
distinctive vertical tubes that paleontologists call Skolithos.
These represent the feeding burrows and living chambers of
some kind of ancient, extinct suspension feeder. Sitting directly
atop the Zabriskie--in bold, dramatic stratigraphic contact--is
the fabulous early Cambrian Latham Shale, the specific rock deposit
which contained a profusion of trilobite remains at the fossil
quarry in the Marble Mountains; it's approximately 50 feet thick
here, consisting of rusty-red to greenish-colored shale that
typically weathers to recessive slopes, often masked by eroding
debris from the overlying, younger rock formations in the local
stratigraphic section.
All told, roughly 21 different species of fossils--from
tracks and trails of soft-bodied organisms (who left no other
evidence of their existence)--to a siliceous sponge have been
identified from the Latham Shale of the Marble Mountains. In
addition to trilobites, the fossil faunal list includes a coelenterate
(possible jelly fish), three species of brachiopods, two kinds
of mollusks, an annelid (worm), an echinoderm, anomalocaris fragments
(this was the largest predator of the early Cambrian seas--olenellids
likely ducked whenever they saw this monster lurking about...),
and Girvanella nodules (precipitated by a species of cyanobacteria,
blue-green algae).
But by far the most-common specimens found are trilobites,
whose remains in the shales typically consist of a lone head
shield called a cephalon (although thoracic and tail segments
are sometimes encountered, as well). While this fragmental preservation
might appear meager and nondiagnostic, a single isolated cephalon
is quite enough to accurately identify the genus-species of the
animal from which it originated. The seven most-abundant species
of trilobites found in the Latham Shales are (in no particular
order of dominance in the fauna): (1) Bristolia bristolensis;
(2) Bristolia insolens; (3) Olenellus nevadensis;
(4) Olenellus mohavensis; (5) Olenellus fremonti;
(6) Olenellus clarki; and (7) Olenellus gilberti.
Additional trilobites identified from the Latham Shale include
Bristolia anteros, Peachella iddingsi and two new
species--one an olenellid, the other a ptychopariid.
After the prolific trilobite remains, probably the second-most
commonly found fossil specimen at the Latham Shale quarry was
an unusual variety of brachiopod, referred to scientifically
as Paterina prospectensis. This is one of the oldest species
of brachiopods ever found, a significant find, indeed, but it
was often overlooked by collectors in their single-minded eagerness
to uncover trilobites. When spotted on the shales it closely
resembles a tiny phonograph record, a flattened, circular to
oval specimen slightly less than a half-inch in diameter.
Lying directly above the trilobite-rich Latham Shale is
a gray to dark blue ledge-forming limestone approximately 140
feet thick. This is the slightly younger lower Cambrian Chambless
Limestone, within whose dense, rather massive strata occur prolific
remains of a fossil algae. Many collectors at the trilobite quarry
often developed quite a fascination and fondness for these extinct
plant remains, taking time out from their intense trilobite-searching
to look for the dark oval structures, roughly a half-inch to
two inches in diameter, embedded in the lighter-colored grayish-blue
limestones. For decades, paleontologists have called these curious
algal bodies Girvanella, a catchall genus used to describe
any nonspecified cyanobacterial remains present in rocks of Cambrian
through Ordovician (475 million years ago) age in the Mojave
Desert and neighboring Great Basin. Although very little is know
about the possible life history of Girvanella, it is believed
to have been a species of blue-green algae which, based on its
exclusive occurrence in silt-free limestone, apparently preferred
to propagate in warm, shallow, clear sea waters. In addition
to Girvanella cyanobacteria bodies, the Chambless
Limestone also yields two species of brachiopods, a mollusk,
seven kinds of trilobites and an echinoderm.
For all intents and purposes, the Chambless Limestones
marks the uppermost, or youngest exposed horizon of the lower
Cambrian system in the vicinity of the trilobite quarry. Above
it occur shales, sandstones, quartzites and limestones of the
predominately middle Cambrian Cadiz Formation, a sparsely fossiliferous
unit approximately 75 feet thick. Some five species of trilobites
have been recovered from it, in addition to three kinds of brachiopods.
And above the Cadiz Formation, capping the entire sequence of
Cambrian formations in the Marble Mountains, is the Upper Cambrian
Bonanza King Dolomite, whose massive, thick accumulations of
magnesium carbonate yield occasional, rare, "bonanza"
layers of trilobites, plus locally abundant Girvanella algal
bodies.
It should be noted, of course, that the stratigraphic terminology
used to describe Cambrian rock formations in the Mojave Desert
and western Great Basin can be confusing. For example, throughout
the northeastern Mojave Desert and southern Great Basin the three
lower to middle Cambrian rock formations that outcrop in the
Marble Mountains (Latham Shale, Chambless Limestone and Cadiz
Formation) correlate stratigraphically with the lower (oldest)
three members of the very widespread Carrara Formation; quite
recently, by the way, one specific Carrara locality, many miles
from the Marble Mountains, has generated a great deal of excitement
among amateur fossil seekers, since it represents one of the
few remaining places in all the Mojave and Great Basin deserts
where unauthorized visitors can still legally collect early to
middle Cambrian trilobites from the Carrara Formation.
Then, too, in western Nevada and the neighboring northern
Death Valley district of California, the stratigraphic nomenclature
changes once again. Here, the fossiliferous Latham Shale correlates
with the upper portion of the Harkless Formation, or the locally
recognized Saline Valley Formation, where it is present in the
famous Waucoba Spring section of Death Valley National Park.
Above the Harkless/Saline Valley complex, lies the Mule Spring
Limestone, whose massive bluish-gray accumulations of Girvanella-bearing
calcium carbonates correlate quite nicely with the Chambless
Limestone in the Marble Mountains. And the lower to Middle Cambrian
Cadiz Formation is equivalent to the Monola Formation in northern
Death Valley and the Emigrant Formation in adjacent western Nevada.
All of these numerous Cambrian rock formations can be dated
pretty accurately at some 518 to 515 million years old. At that
distant time, the Marble Mountain trilobite quarry was situated
near the equator in warm, shallow tropical sea waters that encouraged
a genuine proliferation of early marine plants and animals. Today
of course, thanks to the slow, sure inexorable power of Continental
Drift--working its geologic magic through millions upon million
of years--the trilobite-rich deposit occurs well north of the
equator in present-day North America, where in the midst of a
vast arid desert an uplifted, lithified bed of primal marine
ooze holds the bodies of innumerable extinct arthropods, the
trilobites, preserved for eons in a state of suspended animation,
attaining a grand kind of immortality.
Unfortunately, as is generally well-known among rockhounds
and fossil seekers, the Latham Shale trilobite quarry in the
Marble Mountains is no longer accessible to unauthorized visitors,
as it's now part of the federally protected Trilobite Wilderness
area. Still, persistent paleontology enthusiasts have been able
to discover additional fossil-bearing exposures of Latham Shale
that lie outside the wilderness boundaries. There, trilobite
hunters happily fall into their own individual styles of fossil-finding.
One major collecting method is to sit upon the talus slopes and
sift through the shale debris discarded by previous visitors.
The idea here is to try to locate specimens that may have been
left behind by hunters who in their rush to quarry the larger,
more dramatic head shields, overlooked the less-obvious, though
often better-preserved trilobites. A second and arguably more
efficient technique is to clear a small, localized area of the
overlying loose shale and then, with a quality chisel, in combination
with a geology rock hammer or sledge hammer (be sure to sear
safety glasses at all times), work your way through the shales,
layer by layer, splitting the rocks along their natural bedding
planes. This obviously involves a lot of work, but it is definitely
the most productive way to find fossils. Wrap the pieces of fossiliferous
shale in heavy-duty paper towels or even newspaper, to ensure
their safe transport back home. Very little cleaning of the specimens
is usually required, perhaps just a careful scrubbing with water
and an old toothbrush to remove any dirt or grime caking the
surfaces of the shales.
Occasionally you may want to try to expose an inviting-looking,
partially exposed head shield obscured by a layer of matrix on
a large slab of fossiliferous shale. Proceed with caution, though,
since the fossilized chiton carapaces are deceptively fragile;
they tend to crack or crumble if subjected to an improper chiseling
technique. Before attempting to work out a potentially exceptional
find, practice on less well-preserved specimens or those you
wouldn't mind losing should you make an accidental and fatal
slip of the hand.
There are of course loads of trilobites still remaining
to be found in the Latham Shale, even after many decades of intensive
collecting by both amateur paleontology enthusiasts and professional
invertebrate paleontologists.
The history of fossil explorations in the Marble Mountains
begins in the early 1900s when geologist N. H. Darton discovered
the southwesternmost exposure of fossiliferous Cambrian rocks
in the United States. Darton was thus the first to collect trilobites
in the Marble Mountains, but it's not clear whether his finds
came from the exact area of the present-day trilobite quarry.
For identification and age-assignment of the specimens, Darton
submitted his collection to perhaps the most famous and knowledgeable
Cambrian Period expert of all time, Dr. Charles D. Walcott, who
considered the fossils to be "undoubtedly of Cambrian age,
probably Middle Cambrian." That would indeed be correct,
if Walcott had identified trilobites collected exclusively from
the Cadiz Formation--but nobody knows for certain where in the
local stratigraphic column the fossils came from. If on the other
hand he examined collections Darton secured from what today we
call the Latham Shale, then Walcott was decidedly incorrect on
the specific geologic age-assignment. At any rate, the question
is probably moot, since the exact faunal succession of the Cambrian
Period had not yet been definitively worked out in the early
1900s.
In 1907 Darton published a brief announcement of his find
in the Journal Of Geology, volume 15, number 5--a paper entitled,
"Discovery of Cambrian Rocks in Southeastern California."
At that time Darton referred to the area of discovery as the
Iron Mountains, a name soon to be discarded, interestingly enough,
by geologist Clifton W. Clark, who published the first detailed
geological study of the region in 1921. In that paper, published
in the University of California Publications Bulletin of the
Department of Geological Sciences, volume 13, number 1, Clark
wrote that he preferred the name "Bristol Mountain"
to "Iron Mountain," because this was the name given
on "the official map of San Bernardino County." While
this would appear to be a sound and logical approach to deciding
the name of an important geographic area, evidently that "official"
map was far from the definitive source. Twelve years later, in
1933, John C. Hazzard and Colin H. Crickmay published their "Notes
on the Cambrian Rocks of the Eastern Mohave Desert, California,"
University of California Publications in Geologic Science, volume
23, number 2, in which we find the "common name" given
to this area is now the "Marble Mountains;" and besides,
report Hazzard and Crickmay, "this has likewise been used
on the topographic maps made by the Los Angeles Water Bureau."
End of discussion. If the Los Angeles Water Bureau of 1933
could ignore the evidence of the "official" map of
San Bernardino County and conclude that the region should be
called the "Marble Mountains"--not the Iron Mountains
or even Bristol Mountain--there can be little room for doubt
regarding the authenticity of the name.
A great bulk of scientific writing has been published on
the fossils of the Marble Mountains. Likely candidates for the
best reference works available include-- "The Lower Cambrian
Olenellidae of the Southern Marble Mountains, California,"
by Joseph F. Riccio, Bulletin of the Southern California Academy
of Sciences, volume 51, part 2, May-August, 1952; "Fauna
of the Cambrian Cadiz Formation, Marble Mountains, California,"
by John F. Mason, Bulletin of the Southern California Academy
of Sciences, volume 34, number 2, 1935 (Even though Mason specifies
that his studies focused on the fossils of the Cadiz Formation,
his descriptions of specimens leaves little doubt that he was
actually investigating the faunas of what was later understood
to be the Latham Shale.); "Early Cambrian faunas from the
Marble and Providence Mountains, San Bernardino County, California,"
by J. D. Mount, Bulletin of the Southern California Paleontological
Society, volume 6, number 1, 1974; "Characteristic of Early
Cambrian Faunas from Eastern San Bernardino County, California,"
by J. D. Mount, Paleontological Tour of the Mojave Desert, California-Nevada,
Southern California Paleontological Society Special Publications,
number. 2, 1980, pp. 19-29; and "Cambrian Fossils from the
Mojave Desert," by C. E. Resser, Smithsonian Miscellaneous
Collections, volume 81, number 2, 1928, which bears a noted distinction:
it's the very first systematic discussion of fossils from the
trilobite quarry in the Marble Mountains.
The Marble Mountains trilobite quarry certainly used to
be a great place to bring children and others who wished to experience
the joys of fossil collecting for the first time. The trilobite
specimens were easily spotted in the shales, and because they
were so exceptionally abundant everyone was almost certain to
take home a memorable find. Camping sites there tended to be
dry, as most folks who wished to spend time collecting brought
along their own food, water and wood, pitching tens or parking
their cars along reasonably flat, level areas considerably downslope,
well-removed from the actual fossil quarry.
The trilobite quarry in California's Marble Mountains holds
a special place in my heart. I was taken there as a youngster
on my very first fossil-hunting trip (long before the federally
established Trilobite Wilderness, of course). We arrived in the
dead of night, a winter wind howling from the north, the temperature
hovering around freezing; and we pitched our tent on the rock-strewn
ground near a prominent, convenient parking area amidst the rugged
desert terrain. I found sleep difficult to come by that night,
but it wasn't the cold that kept me alert. Nothing as "commonplace"
in southern California as a wind chill factor in the teens could
have prevented sleep. I was all revved up, ready to find those
trilobites right then and there, with a flashlight if necessary.
I lay awake listening to the savage whipping of the walls of
our canvas tent, envisioning numerous perfect trilobites specimens
in my hot little hand, knowing that they represented some of
Earth's oldest identifiable remains of an animal with hard parts--a
creature who 518 million years ago had actually witnessed its
primordial environment through crystal eyes of calcite, an amazing
adaption in the progression of life on our planet.
That following morning we hiked up the footpath to the
quarry, against the crazy frigid sting of a Mojave Desert wind.
All along the trail I kept inspecting the exposed shales, trying
to imagine what a real trilobite in the rocks would look like.
I had often poured over their pictures in books--but, to genuinely
hold one, to actually touch the once-living representative of
such awesome prehistory was anticipation of the most electrifying
kind.
At last we arrived, and the fossil quarry was before us.
I started probing through the shales left behind by others. Trilobites
began to show up almost everywhere I happened to look. This was
indeed, a fossil-hunter's paradise. With each head shield that
came to light, I inspected the sad stony eyes of a creature whose
descendants survived for almost 300 million years.
It didn't seem fair that the first of these great animals,
the olenellids, should die out while their lucky relatives endured
for eons to come. There was an injustice here I firmly decided,
although the uncluttered mind of a child could not then have
known of the Darwinian struggle for survival of the fittest and
other such impressive explanations for the olenellids' disappearance.
I never did find that perfect, complete trilobite I had
dreamed of during my first fossil hunt in the Marble Mountains.
But it didn't matter. That single experience charged me with
a life-long fascination with fossils and the stories they can
tell us of distant, long-vanished ages.
|
