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In rare circ*mstances, the actual remains of animals, plants, and microorganisms may be preserved as fossils in which they are essentially unaltered from their original state. The most remarkable aspect of preserved remains fossils is that they often include parts of organisms that are not often fossilized; e.g., soft tissues including skin, hair, and feathers of animals, and foliage or seeds of plants. These processes may also produce fossils of entire organisms.
Four fossilization processes can preserve remains of organisms:
Drying
Freezing
Entrapment in Amber
Entrapment in Asphalt
Overall, fossils formed via these processes are rare in the geologic record because the conditions required to form them are unusual. There are relatively few fossils of these types, particularly relative to fossils that have been permineralized, replaced, or recrystallized, or formed as molds or casts via impressions or compressions.
Additionally, since these fossils are typically not found encased in sedimentary rocks like many other fossils, they are more likely to be Pleistocene or Holocene in age, although fossils in amber that are Mesozoic in age have been found.
Drying
Drying or desiccation involves the dehydration of tissue so that decay or scavenging does not occur. It is similar to mummification. Drying can preserve skin and organs as well as hair and feathers.
Most fossils that have been preserved by desiccation are found in dry caves in arid environments as in the American Southwest, most notably in Grand Canyon National Park. Caves provide important habitat for some animals and can also preserve their remains if they die in them.
Drying can preserve both body fossils and coprolites (dung), a type of trace fossil.
Park Examples
Grand Canyon National Park, Arizona
Grand Canyon National Park has a remarkable record of desiccated Pleistocene fossils found in the many dry caves found in its limestone units, particularly the Muav Formation and Redwall Limestone. Some of these caves have produced vertebrate, invertebrate, and plant fossils, as well as fossil dung and guano.
Rampart Cave in the western Grand Canyon contains the best known Pleistocene fossils recovered from the park, including a diverse assemblage of vertebrate fossils as well as thick deposits of the dung of Shasta ground sloths (Nothrotheriops shastensis). The oldest layer in the cave contains sloth dung that is between 40,000 and 24,000 years old and the younger deposit is between about 13,000 and 11,000 years old. Unfortunately, approximately 70% of the dung deposit was consumed by a fire in 1977.
Desiccated bats and other mammals such as ringtails have been more recently discovered in other Grand Canyon dry caves. Two caves contain as many as thousands of desiccated bats of multiple species. These well-preserved bat specimens range in age from more than more than 45,800 years old (e.g., beyond the limit of radiocarbon dating) to modern, indicating use of the caves by bats over that time interval. One of the reasons that these fossils are so significant is that bats are rarely preserved in the fossil record.
Learn More
Pleistocene Life and Landscapes—Grand Canyon
Fossils of the 2019 National Fossil Day Artwork—Grand Canyon National Park: 100 Years of Fossil Discovery
Ancient bat remains illustrate the role of caves as habitat anchors in the temporally dynamic landscape of the Grand Canyon
Santucci VL and Tweet JS (ed). 2020. Grand Canyon National Park: Centennial paleontological resource inventory (non-sensitive version). Natural Resource Report. NPS/GRCA/NRR—2020/2103. National Park Service. Fort Collins, Colorado.—[IRMA Portal] [NPSHistory.com]
Guadalupe Mountains National Park
Like Grand Canyon, Guadalupe Mountains National Park contains dry caves that contain Pleistocene and Holocene fossils that have been preserved by desiccation. Caves in the park contain bird feathers, fossils of small mammals, and the dung, skin and hair of Shasta ground sloths.
Glen Canyon National Recreation Area
A large alcove in the Navajo Sandstone called Bechan Cave in Glen Canyon National Recreation Area contains important Pleistocene fossils preserved by drying. The cave’s name comes from the Navajo word for ‘big feces.”
The cave contains extensive deposits of dung, mostly from mammoth, but also from rabbits, bighorn sheep or mule deer, and Shasta ground sloth. Preserved hair has also been recovered from the cave, mostly from mammoths.
Packrat Middens
Packrat middens, mounds of debris that these large rodents have collected that also contain their nests, may contain a variety of Pleistocene and Holocene fossils including twigs and other plant debris, bones, and dung. Packrat middens in dry alcoves, rock crevices, and underneath rock ledges may contain organic material thousands of years old that is well preserved, particularly in the American southwest.
Packrat middens have been found in 40 western NPS units ranging from Glacier National Parks to Big Bend National Park. National parks throughout the Colorado Plateau including Arches, Canyonlands, Capitol Reef, and Zion, and well as national monuments like Colorado, Natural Bridges, and Navajo, contain packrat middens whose fossils provide important paleontological and climatic information that spans tens of thousands of years. For example, bison bone fragments and debris from Douglas fir and limber pine, species that are not currently present in the deserts of southeastern Utah, have been found in a midden in Arches National Park.
Middens may consist of either loose debris or be by a hardened crystalline substance known as amberat, which is made by the drying of packrat urine. Preservation of plant and animal debris in middens is particularly good where amberat is present.
Learn More
Studying the Past and Predicting the Future Using Rat Nests
Freezing
Freezing can lead to the fossilization of entire animals, including muscles, tendons, organs, and skin. If animal or plant remains are frozen before they decay or are destroyed by scavengers, they can be preserved in an unaltered state as long as they remain frozen.
To date, no fossils preserved by freezing have been identified in units of the National Park System, although some parks have conditions that may yield frozen fossils.
The conditions that preserve fossils via freezing are geologically rare. Frozen animal and plant remains or pollen are most likely to be present in areas with permafrost, active glaciers, or in ice caves.
Permafrost consists of ground that remains frozen for a period of time greater than two years, but may extend for much longer periods. The best known frozen fossils are the mammoths that have been recovered from Siberia and some places in Alaska. Many parks in the Arctic and Subarctic have permafrost, including Bering Land Bridge National Preserve, Gates of the Arctic National Park and Preserve, and Denali National Park and Preserve.
Animal and plant remains may be frozen in glaciers, particularly in crevasses, and they may become exposed as glaciers melt, especially due to climate change. Many national parks in Alaska have active glaciers, including Wrangell-St. Elias and Glacier Bay national parks and preserves. Mount Rainier and Glacier national parks in the lower 48 also contain active glaciers.
Ice caves are caves in rock that contain ice year round. Ice caves are present in at least four national park areas:
Craters of the Moon National Monument, Idaho
El Malpais National Monument, New Mexico
Lava Beds National Monument, California
Sunset Crater National Monument, Arizona
Although fossils have not been identified in any of the ice caves in these parks, a frozen twig has been recovered from an ice cave on private land immediately adjacent to El Malpais National Monument.
Learn More
NPS Glaciers—Frozen Rivers of Ice
NPS Caves and Karst—Ice Caves
Volcanic Resources Summary—El Malpais National Monument
Entrapment in Amber
Amber is hard, brittle fossil resin that is usually derived from conifers. Though quite rare, amber can enclose and preserve insects and other organic material such as mushrooms or flowers. Typically, the remains of the original organism decays within the amber, so fossils preserved in amber are generally molds.
The only Triassic aged amber in the Western Hemisphere has been found in the Petrified Forest Member of the Chinle Formation in Petrified Forest National Park. The pieces of amber were found as small pebbles in paper shale and paper coal (e.g., shale and coal that weather into paper-thin layers). They are detrital particles that were transported in rivers or streams, and then deposited in calm water environments outside the main channels.
No fossil inclusions have been identified in these amber pieces.
Entrapment in Asphalt
Organisms may also become fossilized when they become in entrapped in natural asphalt seeps, like at Rancho La Brea in Los Angeles, a Natural National Landmark. Asphalt seeps form where hydrocarbons flow onto the Earth’s surface and the lighter or volatile components either degrade or escape into the atmosphere, leaving tar-like substances behind.
Fossils in asphalt deposits are not known from national park sites, but elaterite (a type of dark brown asphalt) seeps are present in the Permian White Rim Sandstone in Canyonlands National Park. The fossiliferous Buckthorn Asphalt Quarry is located next to Chickasaw National Recreation Area.
The La Brea Tar Pits contains the largest and most diverse assemblage of extinct Pleistocene plants and animals in the world. The tar pits were predator traps formed when pools of water overlay the tar and unsuspecting animals became entrapped.
Learn More
National Natural Landmarks—Rancho La Brea
Featured Parks
Craters of the Moon National Monument and Preserve (CRMO), Idaho—[CRMO Geodiversity Atlas] [CRMO Park Home] [CRMO npshistory.com]
Denali National Park and Preserve (DENA), Alaska—[DENA Geodiversity Atlas] [DENA Park Home] [DENA npshistory.com]
El Malpais National Monument (ELMA), New Mexico—[ELMA Geodiversity Atlas] [ELMA Park Home] [ELMA npshistory.com]
Glen Canyon National Recreation Area (GLCA), Arizona and Utah—[GLCA Geodiversity Atlas] [GLCA Park Home] [GLCA npshistory.com]
Grand Canyon National Park (GRCA), Arizona—[GRCA Geodiversity Atlas] [GRCA Park Home] [GRCA npshistory.com]
Guadalupe Mountains National Park (GUMO), Texas—[GUMO Geodiversity Atlas] [GUMO Park Home] [GUMO npshistory.com]
Lava Beds National Monument (LABE), California—[LABE Geodiversity Atlas] [LABE Park Home] [LABE npshistory.com]
Petrified Forest National Park (PEFO), Arizona—[PEFO Geodiversity Atlas] [PEFO Park Home] [PEFO npshistory.com]
Sunset Crater National Monument, Arizona—[Geodiversity Atlas] [Park Home]
Fossils and Paleontology
