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Scattered across the pristine isolation of Nevada are many productive Ordovician-age fossil localities roughly 490 to 440 million years old, and two of the more significant sites can be visited in the Toquima Range. At what many fossil seekers call Ordovician Canyon, for example, paleontology enthusiasts can find a plethora of well preserved invertebrate animal remains from the Middle Ordovician Antelope Valley Limestone, including silicified brachiopods, bryozoans, sponges, cystoid echinoderms,conodonts, trilobites, gastropods, pelecypods, cephalopods and ostracodes. A second site paleontology seekers refer to as Graptolite Summit gives collectors a chance to find a fossil type many hobbyists rarely see outside of a textbook or popular guide to paleontology--the intricately designed graptolite, a colonial organism most often referred to as an extinct variety of hemichordate (or primitive chordate). The specimens at Graptolite Summit occur in a rock deposit called the Vinini Formation, which has been dated by geologists as Early to Late Ordovician. The Vinini is an incredibly widespread unit throughout Nevada, a dominantly siliceous assemblage of shales, siltstones, cherts and quartzites that bear sporadic occurrences of abundant graptolites. With the possible exception of the type locality (where a geologic rock formation was first named and described in the scientific literature), this particular area surrounding Graptolite Summit is probably the most intensively investigated graptolite-yielding section in all of the Vinini Formation. Over the decades it has provided professional paleontologists and amateurs alike with myriads of identifiable graptolites, in addition to common inarticulate brachiopods and carapaces belonging to a peculiar species of extinct cructacean called Caryocaris, whose oval-to D-shaped exoskeletons up to an inch across appear to be confined throughout the world to shales in which graptolites are the dominant fossil specimens preserved.
Both fossil areas are easily and safely reached. Still, collectors must not be lulled into dangerous complacency. The important thing to remember is that this part of Nevada remains one of the most remote sectors in all the Great Basin. Should a genuine emergency emerge while in the deep backcountry, medical and mechanical assistance will certainly be a long time in arriving, even if your situation has been relayed to the authorities over Citizens Band Radio. For this reason, it is recommended that visitors travel to the fossiliferous regions in the Toquima Range only in a reliable four-wheel drive vehicle, obeying all of the necessary rules that apply to back country travel; carry plenty of water, emergency provisions, cold-weather clothing, spare fan belts, and medical supplies. And by all means notify the authorities in the nearest community of your whereabouts, remembering to check back in with them upon leaving the area. The Graptolite Summit locality rests directly atop the Lower to Upper Ordovician Vinini Formation, which is locally loaded with all kinds of interesting graptolite remaims. Here in the Toquima Range the Vinini Formation has been measured by geologists at some 6,000 feet thick. It is predominantly a siliceous accumulation of thin-bedded black chert, quartzite, red to black siltstone and shale, dark limestone, and even some minor interbeds of pillow lavas, presumably formed on the ancient Ordovician ocean floor when hot magmatic extrusions came in contact with obviously much colder marine waters; identical kinds of lavas develop today in ocean waters near sites of sea-floor-spreading, where the so-called Mid-Atlantic Ridge produces new Earth crust through the upwelling of superheated magmas.
The most efficient way to find graptolites here is to remove sizable chunks of the varicolored to black shaley siltstones, then carefully split the rocks along their natural bedding planes (remember to wear protective eye gear). In doing this, most collectors soon realize the despite such a prominent presence of graptolites in the rocks, the fossils are sometimes difficult to spot on the fine-grained matrix, a number of them appearing as small silvery sheens on the surface of the shales. Do not become discouraged. What you have discovered is what graptolite specialists have known for ages--that the vast majority of specimens project to the unaided eye what has become known as a "traditional graptolitic aspect of preservation." That silvery sheen found glinting out at you when the sunlight strikes the surface of the rocks at just the right angle represents a 475-million-year-old graptolite colony whose original skeleton has been compressed through geologic time. Most specimens range anywhere from a quarter to an inch and a half in length and, depending of course on the particular genera of graptolites unearthed, can present a fascinating variety of distinctive shapes and sizes to study. Phyllograptus graptolites, for example, one of the more obvious types found near Petes Summit, grew, oval, roughly football-shaped colonies a little over a half inch long. Also present are the blade-like Orthograptus and Climacograptus, plus wishbone-shaped Didymograptus and slingshot-like Dicranograptus. Other genera available in the Graptolite Summit rocks include Clonograptus, Tetragraptus, Isograptus, Glyptograptus, Dicellograptus, Paraglossograptus, Pterograptus, Amplexograptus, Durangograptus, Callograptus and Cardiograptus. In addition to the graptolite remains in the Vinini Formation near Graptolite Summit, two other fossil types can be encountered in the shales and siltstones--inarticulate brachiopods and Caryocaris crustaceans. Such remains are far less abundant than the graptolites, though. Collectors interested in finding them would be advised to explore as many of the shale deposits as possible, splitting heaps of the easily separated layers wherever you go. And don't be shy about exploring the little gullies and ravines in the Graptolite Summit district--many graptolites, for example, can be found in the poorly exposed shales and siltstones that seem to hide in the most improbable-appearing areas.
Graptolites first appear in the geologic record during the middle stages of the Cambrian Period, some 505 million years ago. Even though they persisted all the way up to the late Mississippian age, or roughly 325 million years ago, most species of graptolites had already become extinct by the latest Devonian Period 35 million years earlier. Graptolites achieved their highest degree of success during the Ordovician Period, when they attained worldwide distribution by adapting with ingenuity to three distinct modes of life. One order of graptolite, for example--the fan to leaf-shaped dendroids--led a sessile life attached to the sea floor, apparently straining the marine waters for microscopic organisms. Another type developed a special flotation device which allowed the graptolite colony, termed a rhabdosome, to drift in the open ocean; and a third kind solved its own planktonic challenge by attaching itself to floating strands of seaweed to hitch a free ride through the open ocean in search of better feeding grounds; presumably it too strained the sea waters for microscopic particles of food. In all three examples of graptolitic adaption, the actual colonial animal lived inside the minute rows of cups called thecae that developed along each individual segment of the rhabdosome; technically, these segments are called a stipe. The tiny saw-tooth compartments that housed the graptolite animals along the stipe show to best advantage under magnifications of ten or more power. Thus, a good-quality hand lens is indispensable in order to gain a detailed and aesthetic appreciation of your finds. The exact zoological classification of graptolites has presented a serious challenge to paleontologists. Early investigators referred graptolites to such disparate groups as coelenterates or bryozoans; yet, there certainly was no unanimity of opinion among fossil specialists throughout the 19th century. The breakthrough came when some perfect, three dimensional specimens were etched out of cherts using powerful brews of acids around 1948. Paleontologists then realized that the graptolite colony most closely resembled the modern pterobranch, a tiny marine hemichordate, which by definition is a primitive chordate whose notochord (a spine-like notch) is restricted to the basal part of the head. That explanation seemed to satisfy most paleontologists. Even the basic idea that the graptolite was an extinct colonial organism went completely unchallenged until 1989 when Noel Dilly, a marine biologist in London, suggested that the graptolite had not died out, that a single species had survived the Paleozoic Era and was alive and well on Earth today. What Dilly had identified was a dime-sized colony dredged up by a French team from 800 feet off of New Caledonia in the South Pacific. Dilly called it Cephalodiscus graptiloides--the sole surviving member of the graptolitic race, he claimed. Dilly, who published his ideas in the Journal of Zoology a number of years ago, also reported that while on vacation he actually witnessed his "living graptolites" cavorting in the warm, shallow waters off the coast of Bermuda. He speculates that his living fossils are "survivors of the main group who hung on in places where there hasn't been massive change in the environment in over 300 million years." The suggestion is a novelty, at best. By Dilly's own admission the chemical structure of the graptolite rhabdosome and that of his living fossil is only "similar," not identical. What generated most of the early enthusiasm for the theory was that his Cephalodiscus apparently possesses an extended spine-like protrusion from the main colony, a structure similar to what paleontologists call a nema on fossil graptolites. Modern pterobranchs, with which the graptolite is most often compared, do not develop such a needle-like projection, or nema, so the recent identification of a colonial hemichordate that does seem to bear a nema created quite a short-lived stir among paleontologists. One of the main problems with the entire concept is that Dilly's Cephalodiscus graptiloides is not the only species in its genus, and it's the only one to produce a nema--a structure which may not be directly analogous to the structures graptolites developed.
After collecting graptolites near Graptolite Summit, visitors will want to visit the spectacular fossil exposures of the Antelope Valley Limestone at Ordovician Canyon in the Toquima Range. Here, the Middle Ordovician Antelope Valley Limestone is roughly 950 feet thick, yielding prodigious numbers of fossilized shelly creatures. The productive limestone layers near the mouth of the canyon (referred to as the Mill Canyon Sequence by geologists; two miles from the mouth, geologists call the strata the June Canyon Sequence) consist principally of silty to finely crystalline limestones that weather into shades of dark gray, medium gray, grayish orange, grayish yellow, yellow gray, brownish orange and yellowish orange. The most fossiliferous exposures occur northeast of the mouth of Ordovician Canyon, but productive horizons can be discovered through the canyon corridor up to two to two and a half miles west of the mouth.
In addition to brachiopods, gastropods and cystoid echinoderms,
other specimens identified from the Antelope Valley The Toquima Range localities offer collectors a superlative selection of well preserved Middle Ordovician fossil specimens some 475 million years old. Along with several specific localities in Utah, Ordovician Canyon may well be one of the most fossiliferous Ordovician sections in all the Great Basin. Add to that the profusion of fascinating graptolites near Graptolite Summit and you have an extensive fossil field that begs to be explored--preferably during mid Spring through early Fall when the weather conditions most reliably favor a comfortable experience. Both Toquima Range fossil localities lie within a designated United States national forest. This means that they are administered by the United States Forest Service, not the Bureau of Land Management, even though the sites occur on public lands. In the past, hobby fossil collecting has been allowed to go on here without the need of a special use permit. Just to be on the safe side, though, you might want to contact the local Forest Service office before any visit is made to the Toquima Range. |
Most
of the graptolites occur in pastel-colored shaley siltstones
of the Vinini Formation. Numerous scientific crews have worked
these fossiliferous siltstones in the vicinity of Graptolite
Summit over the decades, periodically entrenching the thin-bedded,
poorly exposed sedimentary layers to a depth of several feet
in search of productive graptolite layers. Depending on the degree
of erosion inflicted by wintertime's snow drifts in the Toquima
Range, remnants of their abandoned excavations may be visible
just up slope from a prominent bed of whitish-brown quartzite
interbedded in the section. The quartzite layer is called a key
marker bed by stratigraphers, because it conveniently separates
the lower member of the Vinini Formation from the upper member.
All of the rich graptolite horizons in the vicinity of Graptolite
Summit occur within a rather restricted interval of shaley siltstones
and shales some 400 to 600 feet thick, a productive section that
happens to straddle that massive, distinctive bed of quartzite.
In general, rocks to the southeast of the quartzite marker bed
are younger than those to the northwest, but the fact remains
that most of the shales and siltstones that sandwich the quartzite
horizon yield rare to abundant graptolite specimens.
Many
collectors like to concentrate their attention along the moderate
talus slopes immediately north of the mouth. Here can be found
infrequent to relatively common brachiopods and gastropods, in
addition to abundant cystoid echinoderm debris, or small crinoid-like
ossicles whose precise identification is impossible owing to
the fragmentary nature of the material. Since these easily accessible
exposures have been probed by eager collectors for decades, the
richest limestone layers, those yielding the greatest diversity
and abundance of specimens, can now be found only in the rugged
terrain farther north of the road. In this area the Antelope
Valley Limestone is more reliably fossiliferous, yielding a genuinely
remarkable assemblage of nicely preserved remains--all of them
thoroughly silicified, by the way, replaced by silicon dioxide.
Such a style of preservation means that collectors can immerse
the fossiliferous calcium carbonate matrix in a diluted acid
batch, dissolving away the limestones to leave intact, perfect
specimens in the residues.
Limestone
include ostracodes, trilobites, conodonts (acetic acid must be
used to dissolve out the phosphatic conodont elements), cephalopods,
sponges, pelecypods and bryozoans. Most of the fossil groups
appear to occur within distinct and separate zones within the
Antelope Valley Limestone. Some of the protruding limestone ledges,
for example, yield many ostracodes, while others bear plentiful
trilobite fragments, brachiopods, sponges, gastropods, or cephalopods.
Conodonts, on the other hand, may show up in the residues of
limestones collected throughout the entire thickness of the formation,
appearing as minute (only one to three millimeters in length,
or less than an eighth of an inch) tooth-like specimens that
originally served as a unique feeding apparatus of an early,
primitive eel-like chordate.