Ashfall Fossil Beds State Historical Park, between the towns of Royal and Orchard in Antelope County, Nebraska, bills itself as the nation's "most unexpected state park" and "established 11,830,000 years ago". My first visit there was over two decades ago, but the story of my visit, like the story of Ashfall itself, begins in Idaho.
My friend Gale and I, both former hacksite attendants with The Peregrine Fund, traveled to Boise in the summer of 1999 for the celebration of the peregrine's delisting. While in Idaho, we were graciously hosted by Gale's former colleague Greg McDonald at his place in Bliss, which happened at the time to have a dead Grévy's zebra (Equus grevyi) buried in the front yard. The zebra was sent by a zoo, already deceased, and actually begins to make sense when you consider that Grévy's zebra bears certain similarities to the extinct Hagerman horse (E. simplicidens), and that Greg worked just down the road as the superintendent of the Hagerman Fossil Beds National Monument. The worms and beetles presumably having finished their work by now, the zebra's skeleton is most likely on display right now at Hagerman.
Greg McDonald likes his tea hot. His method, not approved by the folks at Twinings, is to bring the kettle to a furious boil and then leave it there for a good five minutes before steeping the tea. I soon came to think of it as "geothermal tea", because when we weren't scalding ourselves and watching black-chinned and broad-tailed hummingbirds on the patio behind his trailer house, we were driving around on an impromptu geological tour of southern Idaho (in the course of which we saw no peregrines, but several prairie falcons). Greg's palaeontological charge, the Hagerman horse, thrived in the Pliocene and Pleistocene epochs, and so Greg made sure we learned a lot about the Pleistocene-era Bonneville Flood. But his interests and knowledge are wide-ranging, and he also described the area's deeper seismic and volcanic history.
The geological hotspot that is currently, thanks to tectonic drift, under Yellowstone was at one time located in Greg's neighbourhood, southeast of Boise. Several of the apparent buttes we saw there are actually volcanic remnants, lava plugs exposed by the subsequent erosion of their cinder cones. And here at last is the connection: During the Miocene, not quite twelve million years ago, a massive volcanic eruption in southern Idaho—similar in style to the Mount St. Helens blast, but much larger—sent a plume of volcanic ash drifting eastward toward Nebraska. So, after our sojourn in Idaho, Gale and I had to make a pilgrimage to Ashfall on our way back to Lincoln.
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In 1971, another palaeontologist, Michael Voorhies, noticed a thick layer of volcanic ash, recently exposed by heavy rains, on a hillside overlooking a tributary of Verdigre Creek. Emerging from the ash was the complete skull of a baby rhinoceros, and upon excavating the site Voorhies found the rest of the skeleton—plus several more. A major dig, undertaken with the support of the National Geographic Society, took place in 1978 and '79, and made Ashfall famous within the palaeontological community.
The volcanic ash drifting eastward from Idaho dusted death over a vast landscape. Birds, with their delicate respiratory systems, would have succumbed first to the airborne silicates choking the air; then other animals in rough order of body size. Large-bodied mammals like rhinos were the last to die; they suffered long enough that their skeletons show signs of long-term oxygen deprivation. Life was utterly extinguished here for years or even decades.
The Ashfall site is palaeontologically rich because it was a low-lying wetland: the water attracted dying animals desperate for relief in a dusty, dessicated, and desolate moonscape, and the low terrain collected more than its fair share of ash, which buried the corpses of the dead and facilitated their preservation as fossils.
The land surrounding the fossil bed was purchased by the Nebraska Game and Parks Foundation in 1986, and designated as Ashfall Fossil Beds State Historical Park five years later. Excavations continue under the auspices of the University of Nebraska State Museum, and under the physical shelter of the .4-acre "Rhino Barn", where fossils can be left in situ and visitors can interact with palaeontologists and (more often) graduate students. Interesting discoveries continue to be made here, in part because of the in situ approach. Just recently, an intern at Ashfall discovered tracks attributable to the bone-crushing dog Epicyon, which was already known from its bite marks on some specimens, though none of its own bones have yet been found at Ashfall.
[The Rhino Barn.]
The most numerous fossils at Ashfall are of Teleoceras major, the barrel-bodied rhinoceros, with over a hundred specimens documented thus far. Other mammals present include three genera of camels, three genera of canids, and five genera of horses (including both three-toed and one-toed types), plus a few oddities such as sabre-toothed deer and oreodonts. (When Jessa asked me about oreodonts, I replied that the dentition of these animals was specialised for scraping the creme filling from sandwich cookies. I was rewarded with a "Why do I even talk to you?")
Reptile fossils at Ashfall, though less numerous, are nonetheless instructive. A single specimen of Sternotherus, a "stinkpot" or musk turtle, confirms the site's status as a more or less permanent watering hole. (The complete absence of fish fossils, however, indicates that it was not a stream and not connected to other bodies of water.) And the presence of giant tortoises (Hesperotestudo), along with other evidence including seeds of subtropical grasses—some of those seeds recovered from in between the teeth of horses, camels, and rhinos!—speaks to a much warmer climate than that currently prevailing.
I didn't get back to Ashfall for a long while after that initial visit with Gale in 1999. But in recent years, Jessa and I have returned to Ashfall occasionally while in the area to fly-fish at Verdigre Creek. And while the rhinos, camels, and horses are of course compelling, I have grown increasingly interested in a set of lesser-known fossils: the birds of Ashfall.
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The throngs of crane-watchers who visit the central Platte Valley each spring might be interested to know that cranes were here even during the Miocene, but perhaps surprised that they are not the ancestors of today's sandhill and whooping cranes. The visitor's centre at Ashfall houses the type specimen of Balearica exigua, a close relative of the black crowned crane (B. pavonina) and the grey crowned crane (B. regulorum), both found today in sub-Saharan Africa. Crowned cranes lack the "inflated" sternum or breastbone of the typical cranes, which acts as a resonating chamber and contributes to the evocative calls that are such a big part of the crane-watching experience in Nebraska.
The crowned cranes are the smallest of the living cranes, and exigua was smaller still than both pavonina and regulorum. Neither of the extant species is migratory, and it's quite possible that exigua was resident in what is now Nebraska, not just a seasonal visitor like the sandhills and whoopers.
[Grey crowned crane.]
The type specimen is beautifully preserved and almost entirely complete. Between the bird's ribs can even be seen the fossilised jaw of a lizard, presumably the crane's last meal before the grey cloud descended and snuffed the bird out.
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From my perspective as a falconer and raptor enthusiast, the most interesting avian fossil to emerge from the Ashfall deposit is Apatosagittarius terrenus, the false secretarybird. To quote the abstract from the paper announcing its discovery:
A falconiform Miocene fossil bird from North America, represented by a nearly complete tarsometatarsus with phalanges in place, is that of a hawk that converged on the living Secretarybird Sagittarius serpentarius. It is described as a new genus and species, Apatosagittarius terrenus. The palaeoenvironment indicates a Miocene setting similar to the present day habitat of the living Secretarybird, characterized by extensive grasslands and savannas.
Now let us turn to our richest geological museums, and what a paltry display we behold! That our collections are imperfect is admitted by every one. The remark of that admirable palaeontologist, Edward Forbes, should never be forgotten, namely, that very many fossil species are known and named from single and often broken specimens, or from a few specimens collected on some one spot.