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In the backcountry wilds of Nevada lie two truly classic
Early Triassic ammonoid localities. Both sites yield innumerable,
beautifully preserved ammonoids--an extinct order of cephalopods--in
what geologists, stratigraphers and fossil cephalopod researchers
alike refer to as the Lower Triassic Thaynes Formation, roughly
240 million years old. The Thaynes is exposed at several localities
in the rugged mountain ranges of the Great Basin, yet it is nowhere
as reliably fossiliferous as its stratigraphic development at
two specific, classic exposures in Nevada. Each of the Thaynes
localities is highly regarded among ammonoid specialists, of
course--and both are visited rather frequently by paleontologists
the world-over--but one of those two specific fossil sites, the
place that happens to produce the most prolific numbers of and
best-preserved cephalopods, is not only the finest Early Triassic
ammonite-bearing site in North America, it is also one of the
great Mesozoic Era cephalopod horizons in the United States,
in general--and this, despite the fact that the overall aerial
outcrop of fossiliferous sedimentary rock is confined to a meager
few hundred feet of limestone and shale deposited approximately
240 million years ago in a vast tropical sea. The entire section
lies within what ammonoid enthusiasts call the Meekoceras
beds--a unit of cephalopod-bearing rocks in which the ammonoid
Meekoceras gracilitatus is the most distinctive and characteristic
specimen.
What makes that single, specific ammonoid locality so special
in a paleontological sense is that nowhere in North America are
the world-famous Meekoceras beds exposed through anywhere
near the thickness that they are at the fossil-rich section.
Through roughly 175 feet of exposed strata, abundant ammonoids
representing the Meekoceras beds can be found. At the
most fossiliferous and famous of the Early Triassic geologic
sections, the extinct cephalopods occur in three of the seven
limestone beds in the lowermost portions of Lower Triassic Thaynes
Formation. Above that principally carbonate interval, the Thaynes
consists of several hundred feet of thin-bedded grayish brown
to tan shales in which organic remains of any kind are completely
absent. Recent geologic studies, though, have demonstrated that
the fossiliferous section consists of several faulted, fractured
and vertically displaced blocks, and at least one of the blocks
is overturned. The upshot here is that, contrary to the opinion
expressed by early geological investigators, there is only one
ammonoid-bearing horizon at the locality, not several separate
zones as previously determined--yet, that zone can still be correlated
with several other notable Meekoceras occurrences worldwide,
places such as the Olenek-Lena River Basin in Siberia; Okhostsk-Kolyma
Land, Siberia; Japan; Kwangai, China; Timor; New Zealand; Himalayas,
India; Salt Range, Pakistan; Barabanja, Madagascar; northern
Caucasus Mountains; Arctic Canada and former Yugoslavia.
At the most-special of ammonoid-bearing localities, the
uppermost and youngest fossiliferous bed in the Thaynes, member
"g", consists of 12 feet of gray limestone that tends
to weather into shades of dark brown. It is a fine to medium-crystalline
carbonate unit characterized by thick to irregular bedding, with
fragmental and complete ammonoid conchs throughout. While the
cephalopods are perhaps not as well preserved as in the oldest
member at the measured section, at least eight species of ammonoids
have been described from the rich interval, including Juvenites
septentrionalis, Owenities koeneni, Owenoites stokesi,
Parannanites mulleri, Pseudosageceras multilobatum, Meekoceras
gracilitatus, Flemingites russeli, and Wyomingites arnoldi.
Unit "f", just below the fossiliferous "g"
member, is a barren section of fine to medium ,crystalline light
gray limestone some 48 feet thick; it is difficult to distinguish
the two from a distance, but the uniformly unfossilferous nature
of "f" suggests that if you come upon it in the field,
you should walk up section a short distance, through the barren
carbonates, to intersect the productive limestones of member
"g" above it.
The next oldest unit in the Thaynes section, member "e"
is a rusty-brown weathering, fine to medium crystalline limestone
roughly 45 feet thick. Here can be found abundant remains of
a genuine "living fossil"--the inarticulate brachiopod
called Lingula, which is considered by most paleontologists
to represent one of the great survivors of geologic time, a species
that has persisted through the eons when many other, perhaps
more glamorous creatures such as the dinosaur, the trilobite
and the ammonite vanished from Earth many millions of years ago.
Resembling a slender fingernail, Lingula first appears
in the fossil record during the Early Cambrian Period, approximately
540 million years ago. It has survived, unchanged in physical
appearance, for all that time.
Immediately below the productive Lingula zone lies
the second ammonoid-rich layer, unit "d". It is a fine
to medium-crystalline light-gray limestone, massively bedded
with slabby partings, some 15 feet in thickness. It is everywhere
crammed with plentiful cephalopodal remains, mostly fragmental,
but the coquinoid nature of member "d" keeps many collectors
busy for hours at a time, gently cracking the organic-rich carbonates
to free the prized ammonoids within. Kummel and Steele identified
eight species of ammonoids from the horizon: Juvenites septentrionalis,
Aspenites acutis, Owenites koeni, Inyoites stokesi, Paranannites
mulleri, Meekoceras gracilitatus, Wyomingites aplanatus, and
Preflorianites toulai.
Units "c" and "b" are both poorly exposed--and
unfossiliferous. They have a combined thickness of roughly 30
feet, consisting of pale gray limestone and occasional micaceous
calcareous shales that tend to weather into platy slabs.
But the underlying unit "a", which reaches a
maximum development of some 33 feet, is abundantly fossiliferous
with both broken and complete cephalopods. As a matter of fact,
approximately half or more of the light-gray, brown-weathering
limestone, characterized by frequent limonitic flecks and partings,
is composed of ammonoid remains. It is, indeed, a stunning deposit
that has been visited by innumerable fossil collectors over the
past decade. As a consequence, horizon "a" has suffered
a noticeable decline in outstanding ammonoid specimens remaining
to be collected; the perfect fossils have become increasingly
difficult to find, especially the larger, showy shells, several
inches in diameter, for which commercial dealers pay top dollar.
There is obviously no way to prevent commercial fossil collectors
from visiting the area, but one can only make the observation
that if the trend continues, within three to five years there
won't be much left to find there except unidentifiable fragments.
Member "a" of the Thaynes Formation is justifiably
a world-famous ammonoid deposit; some 23 species have been described
from it, including Dieneroceras (three species), Xenocelities
(two species), Juvenites (two species), Meekoceras
gracilitatus, Hemiaspinites obtusus, Flemingites russeli, Anaflemingites
silberlingi, Preflorianites toulai, Pseudospidites wheeleri,
Owenites koeneni, Paranannites apenensis, Prophingites slossi,
Parussuria compressa, Lanceolites compactus, Aspenites acuts,
Wyomingites whiteanus, Arctoceras tuberculum, Arctoprionites
sp. and Pseudosageceras multilobatum.
A second major Early Triassic ammonoid also occurs in Nevada.
While not as world-famous as the primary locality just discussed,
it does bear abundant, nicely preserved cephalopods of identical
geologic age as those found elsewhere in the Thaynes Formation
of Nevada--though curiously enough, the remote, yet accessible
area apparently has been visited far less frequently by paleontology
enthusiasts: at last visit the fossiliferous section was still
in essentially pristine condition, even though the ammonoidiferous
horizon--amateurs call it the Meekoceras beds, while professional
ammonite specialists refer to the same series of fossil-bearing
strata as the Tardus and Romunderi Zones--has been known to fossil
hunters since at least the late 1800s. Famed ammonoid specialist
James Perrin Smith visited the locality in the early 1900s and
took away loads of identifiable cephalopods; of course, sporadic
numbers of fossil enthusiasts have since found their way to the
productive deposits, including a smattering of commercial collectors
(who for the benefit of all conscientious collectors must keep
their distance, or the Bureau of Land Management will surely
close it all down, enacting severe restrictions on who can keep
what they find there)--yet, even after decades of semi-regular
visitation the area remains rich with well preserved Early Triassic
ammonoids, roughly 240 million years old. The ammonoids occur,
of course, in the Lower Triassic Thaynes Formation, which is
sporadically exposed throughout a specific geographic area of
Nevada. In California, noteworthy fossiliferous outcrops of ammonoid-bearing
Early Triassic strata also occur at Union
Wash (Inyo County, in the shadows of Mount Whitney; the ammonoids
there can be found in the Union Wash Formation). And Early Triassic
ammonoids occur in western Utah, as well--see Kevin Byland's
page, Fossil Cephalopods In
Utah.
Both fossil localities in the Early Triassic Thaynes Formation
described here provide collectors with numerous well-preserved
invertebrate animal remains: ammonoids and brachiopods from the
Lower Triassic Thaynes Formation. They are remote localities,
but a system of well-graded dirt roads provides surprisingly
easy access to the fossil zones. Most conventional cars in perfect
working order should have no difficulty reaching the productive
sites.
At the Nevada ammonoid zones in the Early Triassic Thaynes
Formation, it is intriguing to realize that as you hold in your
hand an ammonoid specimen collected from the famous Meekoceras
beds, an identical species of extinct cephalopod may be weathering
out thousands of miles away, in China, in Madagascar or in Siberia--a
species that swam through the same Mesozoic sea at an identical
moment in geologic time, some 240 million years ago, is now preserved
on distant continents.
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