Secrets of the Very Long Dead: Ashfall Fossil Beds State Historical Park

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Excavations are ongoing at Ashfall Fossil Beds State Historic Park. (Paul A. Johnsgard)

By Paul A. Johnsgard

One day in the summer of 1971, University of Nebraska’s paleontologist Mike Voorhies and his wife, Jane, were walking along a streambed tributary of Verdigre Creek, in Antelope County, gathering data for a planned geological map. Mike knew the area well, having grown up in the small town of Orchard, only eight miles away. Walking along the streambed ravine, he noticed an exposed layer of ash about a foot in thickness partway up the face of a steep ravine.

(As a geological aside, this pale grayish ash layer is part of the widespread evidence of a volcanic explosion that had occurred nearly twelve million years ago, during late Miocene times. The ash originated in what is now southwestern Idaho, where a gigantic “hotspot” of magma once erupted. Because the North American tectonic plate has drifted slowly to the west over the past twelve million years, the hotspot that produced the Ashfall eruption is currently located under Yellowstone National Park, where it is responsible for all of that region’s thermal features and earthquake-prone landscape.)

As Mike walked along the ravine, he noticed a small, fossilized jaw and teeth. After some careful excavation, the entire skull of a baby rhinoceros about a foot in length slowly emerged. Further excavation exposed some of the neck vertebrae, leading him to believe the entire skeleton might be present. Returning the next day, the baby rhino’s entire skeleton emerged, as did three more, including one of an adult rhinoceros.

In spite of his excitement, time and equipment didn’t then permit additional digging, but Mike recognized this as his find-of-a-lifetime. Under his direction in 1977 a museum crew began to remove the layer of sandstone above the volcanic deposits. From an exposed area of about two hundred square feet the group collected several more rhino skeletons. With the support of a National Geographic grant, Mike brought a group of eight students back in 1978, and they began large-scale excavations over an area of about six thousand square feet.

By 1979 some two hundred skeletons of various mammals had been exposed from the volcanic matrix, in one part of which twelve rhino skeletons were clustered into an area not much bigger than the size of an average living room. The site had once been a watering hole that had gradually filled with ash. Lung failure, caused by inhaling the volcanic dust, eventually killed all the unlucky animals that had huddled together in it.

Besides the rhinos, which were robust, short-legged animals belonging to a genus (Teleoceros) known as the barrel-bodied rhinoceros, there also were five genera of horses, three camels, and two members of the dog family. There was also a small saber-toothed deer closely resembling the modern musk deer (Mochus) of Asia, whose distinctive protruding canine teeth are used by males for fighting. There were also smaller mammals, turtles, and a few birds. The smallest animals evidently died soonest, as their remains are largely situated at the lower level of the volcanic layer, with the middle-sized and larger animals being sequentially located above.

Most of the fossils found so far are of species that had previously been discovered, but the Ashfall site is unique in the number and completeness of specimens and the extremely high details of bone preservation. The tiny particles of volcanic dust served as a perfect casting material, preserving tiny body parts, such as the tiny middle ear bones of larger mammals and the windpipes of cranes. Other remarkable finds include feather impressions, an unborn rhino calf inside the skeleton of its mother, a bird skeleton with small pebbles that had once served for grinding food in its gizzard, and fossil grass seeds from the throat of a rhino. More than fifty species of plants and animals have been identified from the site.

No doubt the prize finds have been the numerous intact skeletons of rhinos, most of which lay crouched with their legs tucked under them or lying on their sides. More than a dozen were found tending young as they died. The clustered grouping suggests that this rhinoceros was much more social than are the present-day species of rhinos, which tend to be solitary animals. Evidently, like elephants, these rhinos formed herds of adult females and calves, accompanied by single adult males. Of about one hundred skeletons, only seven were adult males, with adult females outnumbering them by a ratio of more than six to one. The staggered ages of the young indicates that they were born seasonally, rather than throughout the year.

Some of the more notable finds, at least for bird lovers, were the skeletons of an extinct crane species that, at least in terms of its skeletal features, is nearly identical to the modern African crowned cranes. The two living species of crowned cranes are believed to be the most “primitive” of all fifteen living cranes, with some unique features such as long hind toes that allow them to perch in trees. In contrast to the more advanced cranes, the windpipes of crowned cranes extend directly from the gullet to the lungs, rather than forming a loop that penetrates the keel of the breastbone. This loop greatly extends the length of the windpipe, enhancing the birds’ vocal resonating abilities and probably increasing the volume of their calls. The discovery of crowned cranes at Ashfall also proves that primitive cranes once ranged to North America, where now only the sandhill crane and whooping crane exist.

Visitors to Ashfall are able to watch scientists continuing the excavation work. (Paul A. Johnsgard)

Several other species of fossil birds have also been found at Ashfall, including a hawk (Apatosagittarius) with convergent similarities to the modern African secretary bird. Another major recent discovery is a primitive vulture-like hawk, Anchigyps voorhiesi, which seems to connect anatomically the typical hawks and eagles with the Old World vultures, and which was named in honor of Mike Voorhies.

In common with these fossil birds, modern rhinos, camels, and wild horses now only occur in the Old World. This pattern shows how the distributions of many mammals and birds have shifted over the past twelve million years, and how evolution has molded them during the intervening time span. For example, three of the five species that were found at Ashfall had feet with three well-developed toes that were probably useful for walking in soft terrain or in dodging predators. One species (Protohippus) also had three toes, but two of them were reduced in size and perhaps of no functional value. Another species, the large, single-toed Pliohippus, closely approached the modern horse, having broad hooves that are best adapted to running fast over hard surfaces, but probably had limited cornering abilities.

In contrast, the toes of camels have evolved to allow walking over relatively soft substrates. Modern camel feet have feet with two toes and undivided soles, helping to maximize each foot’s surface area and to help spread their substantial body weight. Other than the rhinos and horses, camels were the most common large mammals found at Ashfall, with several dozen found. Most of them are of a type called Procamelus that, as the name implies, is believed to have been ancestral to the true Old World camels.

Although initially researchers had to excavate the site in the open air, in 2009 a 17,500-square-foot building, the Hubbard Rhino Barn, replaced a much smaller earlier structure. Within the building, a walkway allows visitors a close view of the excavated skeletons, and of scientists continuing the excavation work.

Along the walls a large series of paintings are displayed, showing remarkably lifelike reconstructions of many examples of the area’s rich fossil legacy. They are part of a larger series of about fifty paintings done by Mark Marcuson, depicting some of Ashfall’s most iconic fossil animals. Mark also painted the large mural in the visitor center, showing the doomed rhinos in their watering hole. A group of Pliohippus horses is leaving the watering hole, while other horses, camels, and cranes can all be seen nearby, as well as some distant elephants. Although so far none has been found at Ashfall, elephants have been found in nearby Niobrara excavations.

Mark Marcuson is one of the finest museum artists in America; his spectacular work can also be found throughout the State Museum on the University of Nebraska-Lincoln campus (“Elephant Hall”), and he has done commissioned paintings for several books. An extensive exhibit of his artwork from childhood onward is on public exhibit at the State Museum until early next year.

The Ashfall Fossil Beds were named a state park in 1991, and a National Natural Landmark in 2006. The park is located two miles east and six miles north of Royal, in northern Antelope County. Its visitor center includes a preparation laboratory, interpretive displays, and small gift shop with a good selection of books. Surrounding the excavation is a large area of restored mixed-grass prairie, where wildflowers and typical grassland birds such as western meadowlarks, dickcissels, and upland sandpipers can usually be seen.

The 360-acre park is open Monday through Saturday, 9:00 a.m. to 5:00 p.m., Memorial Day to Labor Day, with limited hours in May, September, and October. Paleontologists from the University of Nebraska State Museum and interpretive staff are present; the Nebraska Game and Parks Commission oversees the park and its maintenance. Admission is $5 for adults and $3 for children over five years old. Holders of current Nebraska state park permits are admitted free; daily or annual park permits are on sale at the site. Information can be obtained at (402) 893-2000 or at www.ashfall.unl.edu.

References

Bouc, K. (Coordinator). “The Cellars of Time: Paleontology and Archeology in Nebraska.” Nebraska History 75(1) (1994):1–162.

Garbino, M. “World Tour: Ashfall Fossil Beds, Nebraska.” Smithsonian Magazine 42(9) (2012):48–49.

Voorhies, M. R. “Ancient Ashfall Creates a Pompeii of Prehistoric Animals, Dwarfing the St. Helens Eruption.” National Geographic 159 (1) (1981): 66-75.

Image Credits: Paul A. Johnsgard

Immigration in Nebraska