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Scattered across the pristine Great Basin isolation of Nevada are many productive Ordovician-age fossil localities roughly 485 to 444 million years old, and two of the more significant sites can be visited in the rugged 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 ostracods.
And a second site that paleontology buffs refer to as Graptolite Summit provides visitors with 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 (enough to provide one gallon per person per day), 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 remains. 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.
Most of the graptolites occur in pastel-colored shaly 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 shaly 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 locally abundant graptolite specimens.
The most efficient way to find graptolites here is to remove sizable chunks of the varicolored to black shaly 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 470-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 Graptolite Summit, grew oval to 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.
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 to species level 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 bath, dissolving away the limestones to leave intact, perfect specimens in the residues.
In addition to brachiopods, gastropods and cystoid echinoderms, other specimens identified from the Antelope Valley Limestone include ostracods, trilobites, conodonts (acetic acid must be used to dissolve out the phosphatic conodont elements), cephalopods, sponges, pelecypods and bryozoans. Many of the fossil groups tend to occur within their own individual rock layers, to the exclusion of other types of organisms. Not a few of the protruding limestone ledges for example yield profuse ostracods, while others bear plentiful trilobite remains, 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--unrelated to modern jaws--that originally served as a unique feeding apparatus of an early, primitive lamprey eel-like animal.
The Toquima Range localities offer collectors a superlative selection of well preserved Middle Ordovician fossil specimens some 470 million years old. Along with several correlative time-equivalent localities in Utah and eastern Nevada, Ordovician Canyon may well be one of the most fossiliferous Middle 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 areas 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.
|Click on the images for larger views. At left, two paleontology enthusiasts stand at the mouth of Ordovician Canyon, Nevada; view is westward to the distinctive, bold outcrops of the fossil-bearing Middle Ordovician Antelope Valley Limestone; at right, a collector (faintly visible at middle right of image) prospects the talus slopes just north of the mouth of Ordovician Canyon for Middle Ordovician invertebrate fossils in the Antelope Valley Limestone.|
Click on the images for larger pictures. Left to right--Looking northwestward from the fossil graptolite locality in the Middle to Upper Ordovician Vinini Formation near Graptolite Summit, Nevada. A enthusiast of Early Paleozoic Era paleontology at lower left. The pastel-colored to black shaly siltstones which sandwich a prominent bed of whitish-brown quartzite (a knob of the quartzite is visible at the extreme upper right) near Graptolite Summit furnish paleontology enthusiasts with loads of fascinating graptolite remains, an extinct variety of hemichordate that reached its zenith of adaptation in the Ordovician Period.
Right-- The prominent outcrops of the Middle Ordovician Antelope Valley Limestone immediately north of the mouth of Ordovician Canyon, Nevada, yield common to abundant silicified invertebrate fossils, including brachiopods (arguably the most common fossil type encountered), echinoderm debris, trilobites, sponges, ostracods, conodonts, bryozoans, pelecypods, cephalopods and gastropods. Note the seeker of Ordovician paleontology at lower right-center.
|Click on the images for larger views. Here are two fossil-types collected from the Toquima Range, Nevada: a brachiopod and a graptolite. From left to right: left--a silicified pedicle valve from the brachiopod Hesperorthis affin. H. matulina Cooper (resting on its natural limestone matrix), 10mm across, from the Middle Ordovician Antelope Valley Limestone, Ordovician Canyon, Nevada. Right--an oval graptolite colony, 17mm long, called Phyllograptus anna from the Lower to Upper Ordovician Vinini Formation near Graptolite Summit, Nevada.|
|Click on the images for larger views. Here are two varieties of graptolites from the Middle Ordovician Vinini Formation exposed near Graptolite Summit, Nevada. At left is a dendroid-type graptolite, 15mm long, called Callograptus sp.; at right is the slingshot-like graptolite called Dicranograptus spinifer Elles and Wood, 22mm long, which has been naturally preserved with an unusual reddish coloration, presumably through replacement by the mineral limonite.|
|Click on the images for larger views. Here are two varieties of graptolites from the Middle Ordovician Vinini Formation near Graptolite Summit, Nevada: at left is a Climacograptus sp. graptolite, 33mm long; at right are several thread-like Clonograptus flexilis (J. Hall) graptolites, the longest of which is 20mm.|
|Click on the image below for a larger view. Here is a slab of shaly limestone from the Middle Ordovician Antelope Valley Limestone, Ordovician Canyon, Nevada, which bears several silicified pedicle valves of the brachiopod Plectorthis cf. P. perplexus (Ross); all of the specimens are roughly 10mm across. Right--a silicified pedicle valve from the brachiopod Plectorthis cf. P. perplexus (Ross), 10mm across, (still on its natural limestone matrix) from the Middle Ordovician Antelope Valley Limestone, Nevada.|
|For information on what can and cannot be collected legally from America's Public Lands, take a look at Fossils On America's Public Lands and Collecting On Public Lands--brochures that the Bureau Of Land Management has allowed me to transcribe.|