Part of a series of articles titled Park Paleontology News - Vol. 15, No. 2, Fall 2023.
Article
Paleontology of Ashfall Fossil Beds National Natural Landmark
Justin Tweet, NPS Partner—Paleontologist
American Geosciences Institute
Photo by R. Otto, University of Nebraska State Museum.
Introduction
Although the popular image of a fossil dig usually involves a hot, dry desert or colorful badlands, Nebraska is one of the best areas of the United States for finding fossils from the Cenozoic, also known as the Age of Mammals (66 million years ago to the present). One of the most outstanding sites is Ashfall Fossil Beds in Antelope County, northeastern Nebraska. Excavations and other studies here since the 1970s have uncovered hundreds of excellently preserved skeletons of extinct rhinos, horses, camels and other animals, buried in volcanic ash dating to approximately 11.93 million years ago. Visitors can see some of these skeletons still in the ground, some of them almost perfectly articulated.
Photo by R. Otto, University of Nebraska State Museum.
The Fossil Beds
Detailed study of the rocks and fossils gives a dramatic picture of the events that led to the fossil beds. Geologically speaking, the fossil beds are within the Ash Hollow Formation, dated to around 12 to 5 million years ago within the Miocene Epoch. 11.93 million years ago, there was a waterhole at the site, set within a broad savannah that was home to many kinds of mammals, some familiar in appearance, others not, and most from groups no longer native to North America. At this time, a massive volcanic eruption occurred hundreds of miles away in what is now southwestern Idaho. The volcano responsible was part of the same system that today is beneath Yellowstone National Park.
Huge amounts of volcanic ash were ejected and spread in a plume across North America; as much as a foot or so (30 centimeters) fell in the area around Ashfall. Smaller animals were killed early on, suffocated by the ash; their fossils are found at the bottom of the ash layer and are often smashed or broken up due to trampling by larger animals in the waterhole. Larger animals with bigger lungs lived longer but were killed by the effects of breathing the ash, which left telltale marks on their bones. The last to succumb were a herd of rhinos, Teleoceras major (illustration)], that had congregated around the water source and eventually became too weak to leave it. Their skeletons are the least damaged because there was nothing left to trample them. Meanwhile, blowing wind deposited more ash in the waterhole, eventually covering the carcasses.
Photo by V. Santucci, National Park Service.
Excavations at the site show these intervals of death and burial, with the smallest animals at the bottom, then horses and camels a few inches higher, and finally the rhinos. More than 20 species of animals have been recovered from the ash, with many others found throughout the sands deposited before the waterhole and eruption (although not nearly as well-preserved). They include a turtle and a large tortoise (demonstrating this area did not regularly go below freezing); several species of lizards found as gut contents in birds of prey; a snake; four bird species, including 40 crane skeletons, some with preserved feathers and gut contents; two species of dogs; a mole; a mouse (also as gut contents); four species of camels; a musk deer; five species of horses; and the rhino Teleoceras major, by far the most abundant animal found here. Many of the rhino skeletons are preserved in three dimensions, showing their broad hippo-like bodies, and some have fossils of grass fragments in their mouths and bodies. Study of the 100+ rhinos indicates they likely represent several small herds with individuals of various ages and sizes, mostly females. The bones of large predatory mammals such as bone-cracking dogs are rare, but their presence is also known from trace fossils such as dung, tooth marks, and foot prints.
Photo by R. Otto, University of Nebraska State Museum.
History of the Site
Ashfall Fossil Beds is situated in rolling farmland drained by nearby Verdigre Creek. There are reports of people finding fossil bones in the immediate area of the fossil beds going back to the 1920s, including a partial rhino skull found in a sandstone bed below the ash in 1953. It wasn’t until the 1970s that the fossil beds were uncovered by scientists. In 1971 erosion by heavy rains exposed a skeleton of a baby rhino, which was found by paleontologist Michael Voorhies. Expeditions in the late 1970s collected more than 120 articulated skeletons from the site.
In 1986, the Nebraska Game and Parks Foundation purchased the fossil site, leading to the creation of Ashfall Fossil Beds State Historical Park in 1991. The site is managed by the University of Nebraska State Museum and is open to visitors from early May through early October. The fossil site itself was declared a National Natural Landmark in 2006. A few years later, in 2009, the Hubbard Rhino Barn was opened at the site. This structure covers an area of partially exposed bonebed, allowing visitors to see fossil skeletons still embedded in the ash and paleontologists at work documenting, preparing, and conserving specimens. Research, collection, preparation, and other work are ongoing, and much more remains to be discovered. The full extent of the waterhole is not known to date, and it is likely other fossil species will be found here in the future.
Photo by R. Otto, University of Nebraska State Museum.
Photo by V. Santucci, National Park Service.
Administered by the National Park Service, the National Natural Landmark Program recognizes and supports the voluntary conservation of exemplary biological and geological sites that illustrate the nation’s natural heritage, including those that contain significant paleo resources. National natural landmark sites are designated by the Secretary of the Interior and are owned by a variety of public and private stewards. To date, significant paleo resources have contributed, in whole or in part, to the designation of 59 national natural landmark sites across the country. These landmarks represent the range of geologic history from very early marine life 510 million years ago to more recent (40,000 years ago) mammals.
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Last updated: September 26, 2023