|Text: Field Trip||Images: On-Site||Image: Ammonoid||Links: My Web Pages|
Numerous 248 million year-old ammonoids can be found in the shadows of Mount Whitney--at 14,505 feet the highest point in the contiguous United States--near Lone Pine, California. The extinct cephalopods occur at Union Wash along the western flanks of the Inyo Mountains, directly east of the mighty Sierra Nevada, whose impressive ice-sculpted peaks dominate the western skyline. At Union Wash, which happens to be one of the major drainage courses for the western slopes of the Inyos, geologists have identified more than 2,300 feet of Early Triassic strata belonging to the appropriately named Union Wash Formation. Within this thick and relatively undeformed sequence of marine-originated siltstones, mudstones, shales and limestones, ammonoids are common to locally abundant at two separate horizons.
And both fossil-bearing layers are currently accessible to interested amateur collectors, though the most famous and ammonoid-rich area does happen to lie within the designated Southern Inyo Wilderness, established in December 1978.
Visitors to Union Wash may of course continue to explore that specific, celebrated cephalopod horizon--called by ammonoid enthusiasts worldwide the Meekoceras beds (named for the most characteristic species present in the bed)--but those who choose to investigate the extensive fossil deposit (hiking is required to reach it, since motor vehicles are not allowed to enter a designated, federal wilderness area) must not remove specimens from the bedrock deposits; only freely weathered ammonoids and chunks of fossiliferous rock already eroded off the exposed strata may be collected by unauthorized visitors. Always check with the local rangers, though, before collecting even surface paleontological specimens within a formally established Wilderness region: most places that have been placed into that kind of federal protection program are completely off-limits to any manner of fossil gathering. In order to conduct a formal paleontological dig at Union Wash--removing ammonoid-bearing material from the bedrock for scientific study--one must secure a special-use permit issued by the Bureau of Land Management. Without exception, the permit is granted only to personnel representing either a museum or a fully accredited university.
Even though it is not as widely exposed, or nearly as fossiliferous as the justifiably famous Meekoceras ammonoid beds, a second ammonoid horizon at Union Wash also continues to provide amateur cephalopod seekers and professional invertebrate paleontologists with loads of identifiable fossil specimens--the so-called Parapopanoceras zone, which at last field check occurs outside the Southern Inyo Wilderness boundaries.
The most abundant ammonoid encountered in the Parapopanoceras zone is the species for which the layer was named, Parapopanoceras haugi. Closely resembling a tiny coiled gastropod, Parapopanoceras ammonoids measure only a few millimeters in diameter and are most efficiently examined under powers of 10X or greater magnification. Larger, more readily identifiable ammonoids described from the interval include Hungarites vatesi, Paranannites oviformis, Trilolites pacifica, Keyserlingites (sp.), Acrochorduceras inyoense, Xenodiscus bittneri and Xenodiscus multicamaratus. In addition to the ammonoids, a few other fossil varieties have also been described from the Parapopanoceras horizon. These include an orthocone nautiloid cephalopod related to Orthoceras sp., pelecypods, and several species of conodonts--minute tooth-like structures, unrelated to modern jaws, that served as a feeding apparatus in an extinct lamprey eel-like organim (seen only in the insoluble residues of Union Wash limestones treated with dilute acetic acid).
The first paleontologist to study the Parapopanoceras zone at Union Wash was Triassic ammonoid specialist James Perrin Smith, who published his findings in 1914 in the classic monograph Middle Triassic Marine Invertebrate Faunas Of North America, United States Geological Survey Professional Paper 83; in a preliminary statigraphic overview of Triassic ammonoids from the western United States, originally issued on July 29, 1904, Smith wrote that a Mr. H. W. Turner had discoverd the Parapopanoceras beds at Union Wash in 1899--mentioning, too, that in 1900 and 1903 he'd accompanied Turner to collect additional ammonoid material from the cephalopodicly important Union Wash horizon. Smith concluded that the Parapopanoceras ammonoid fauna, while similar to forms recognized from the Mediterranean region, most closely resembled species already described from localities in the Arctic and India. Smith believed that the beds were lowermost Middle Triassic in geologic age, but more recent paleontological studies indicate that this is not the case, that the Parapopanoceras zone actually lies at the very top of the Early Triassic, approximately 248 million years old--yielding specimens that in the context of geologic time chronology lived "only" three or four million years after the greatest mass extinction in history, the traumatic end time Permian Period, when fully 96 percent of all life on Earth died out.
After investigating the moderately prolific Parapopanoceras zone in Union Wash, it's time for a trip farther on down section, into even older portions of the lower Triassic Union Wash Formation, to visit the world famous Meekoceras beds.
Although ammonoids are notably prevalent in the Meekoceras ammonoid beds, not all sections of the limestones are abundantly fossiliferous. Most of the fossils are preserved in localized concentrations as fragmentary and complete calcium carbonate steinkerns of the original invertebrate animals. But be sure to collect only those specimens that have already weathered out of the limestones. Unless you've secured the necessary BLM collection permit, don't conduct any digging into bedrock within the federally designated Southern Inyo Wilderness Area.
The Meekoceras beds at Union Wash were discovered in 1896 by pioneering paleontologist Charles Doolittle Walcott during one of his epic expeditions to the Western states in search of fossiliferous Early Cambrian exposures. Which means, ultimately, that Walcott--one of the most observant and successful field paleontologists in all of human history (he who discovered the world-famous middle Cambrian soft-bodied Burgess Shale fauna of Canada, for example)--actually walked right by those younger Parapopanoceras beds in Union Wash, failing to note the ammonoid-bearing zone that H. W. Turner would eventually discover in 1899. Walcott subsequently donated his collection from Union Wash to James Perrin Smith, who determined that the ammonoids were of Early Triassic geologic age, or roughly 248 million years old. Based on the presence of Meekoceras gracilitatus in the fossil collections from Union Wash, Smith assigned the entire fauna to the formally recognized Meekoceras zone, a major cephalopod horizon known from several localities around the globe, such as the Arctic Circle, Siberia, Japan, China, Timor, New Zealand, India, Pakistan, Madagascar, the northern Caucasus Mountains and the former Yugoslavia. The Meekoceras beds have also been identified at a handful of correlative sites in the United States, including northeastern Nevada, southeastern Idaho and northern Utah. It is in fact the oldest Mesozoic ammonoid bed yet discovered in North America, and is the third-oldest known from that geologic age in the world. Only the Otoceras and Genodiscus ammonoid zones precede it in the worldwide stratigraphic record of the Triassic Period, the oldest division of the Mesozoic Era.
Smith published his findings on the ammonoid fauna at Union Wash in 1932 in USGS Professional Paper 167, Lower Triassic Ammonoids of North America. He noted that the most distinctive variety recovered from the Union Wash limestone layers was Meekoceras gracilitatus, the species for which the zone was originally named in the first place. Other genera described from Smith's Meekoceras bed at Union Wash include Ophiceras (four species); Owenites (four species); Xenodiscus (four species); Anasibirites (three species); Sturia (two species); Lanceolites (two species); Clypeoceras (two species); Lecanites (two species); Inyoites; Proptychites; Aspenites; Flemingites; Pseudosageceras; Prophingites; Danubites; Juvenites; and six additional species of Meekoceras. Smith concluded that most of the ammonoid species at Union Wash showed close affinities to similar types recovered from localities in India and Timor; hence, he concluded they are Asiatic varieties, while the younger Parapopanoceras zone yields species that are more closely related to types discovered in the Arctic and Asia, with only a general similarity to the well-known Early Triassic faunas of the Mediterranean region.
Union Wash remains one of the great Early Triassic localities in North America. It's a place where at least two distinct fossiliferous horizons yield a rich association of 248 million year-old cephalopods. Even though the incredibly productive Meekoceras beds presently lie with a federally protected wilderness area, amateurs and professional paleontologists alike may still hike to it and find plenty of ammonoids to take home--remembering of course to keep only loose, freely eroded specimens; don't dig into the strata within a wilderness zone without a BLM collecting permit.
While finding ammonoids at Union Wash, it is inspiring to gaze back westward to the Sierran skyline across the Owens Valley, watching the glacier-incised canyons take on crevasse-like shadowing as the sun dips below snowy peaks whose elevations average over 14,000 feet--a great mountain range born from Jurassic-age batholithic magmatic intrusions of liquid rock, some 100 million years younger than the ammonoids you hold in your hand.
|The view is east to the Inyo Mountains at the mouth of Union Wash; the stratified reddish-brown and grayish rocks in foreground and along the slopes at middle left of the image represent limestones and shales of the ammonoid-bearing lower Triassic Union Wash Formation; grayish strata along skyline at middle and upper left are carbonate rocks that lie within the middle to upper Pennysylvanian Keeler Canyon Formation, which yields many species of fusulinids (an extinct single-celled animal that secreted a distinctive football to wheat-shaped shell), crinoid stems, bryozoans and brachiopods.|
|A view back westward across Owens Valley to the Sierra Nevada from Union Wash, where common to locally abundant ammonoids occur at two major fossil horizons in the lower Triassic Union Wash Formation, some 248 million years old. Slope at left foreground consists of limestones and shales of the lower Triassic Union Wash Formation.|
|A Meekoceras gracilitatus (White) ammonoid from the lower Triassic Union Wash Formation, Union Wash, Inyo County, California. Specimen is 54mm in diameter. This is the ammonoid for which the famous Meekoceras Beds horizon was named.|