Ashfall and the birds of a younger Nebraska

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.

[Grévy's zebra.]

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.

* * *

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.

* * *

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.

* * *

Also described from Ashfall is Anchigyps voorhiesi, described on Ashfall's website as an "eagle-like scavenger".  The generic name Anchigyps means "almost-vulture", and the bird is perhaps more significantly viewed as intermediate between typical accipitrids and what we know today as the Old World vultures, perhaps even directly ancestral to today's griffon vultures. (The specific epithet voorhiesi honours Michael Voorhies, the discoverer of the fossil bed.)

[Griffon vulture.]

Anchigyps is known from a single specimen consisting primarily of leg bones, though an ulna (a bone of the outer wing) and a fragment of mandible ("beak" in common parlance) helped to confirm the connection with modern Gyps vultures. Compared to its modern relatives, and consistent with its intermediate status, the redtail-sized Anchigyps was probably more adept at powered flight than extended soaring; we can imagine it perched in trees on the savanna, perhaps making an occasional sortie after small live prey, but spending most of its time waiting for carrion-feeding opportunities

* * *

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.

"Tarsometatarsus with phalanges in place" means, of course, the lower leg and toes—which in the secretarybird are pretty distinctive, plenty enough to justify the description of a new species from a single specimen. Like the modern version, the false secretarybird would almost certainly have been flighted but would have nevertheless been primarily terrestrial, stalking the grasslands on its long legs in pursuit of snakes and other prey, engaging them with a mix of kicking and stamping.

[Secretarybird.]

Whereas Balearica and Anchigyps can be confidently linked to modern birds in a genealogical sense, Apatosagittarius is more enigmatic. Its resemblance to the modern secretarybird Sagittarius represents convergence rather than true affinity: two distinct taxa happening upon a coincidentally similar "engineering" approach to life in similar conditions. "Life finds a way," as someone once said.

* * *

On our most recent visit, in mid-September, Jessa and I paused at a spot just outside the Rhino Barn marked with several red flags, and one yellow. Blue pitcher sage and erect goldenrod bloomed amongst the little bluestem, side-oats grama, and Indian grass, and the red leaves of sumac glowed from across the ravine. The only birds in evidence were a couple of barn swallows, surfing on the wind and enjoying some of the last of late summer's bounty of insects. The wind had been only a minor hindrance to my casting in the relatively sheltered environs of the trout stream that morning, but blew unobstructed here, keeping the flags in constant motion. 

The yellow flag commemorates the site of Voorhies' initial find, the baby rhinoceros, and the reds indicate the locations of major specimens uncovered during the 1978-79 National Geographic dig. The Rhino Barn does not extend here because the fossil bed has already been worked down to the lowest extent of the ash layer. No bones remain, just the flags to mark their previous resting places.

Though described after the fact, Balearica, Anchigyps, and Apatosagittarius were all uncovered in 1978 and 1979, during the National Geographic excavation—right here, in the only part of Ashfall where the rhinos, etc. were encased in plaster and removed. 

Charles Darwin devoted an entire chapter of the Origin to "the imperfection of the geological record", lamenting:

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.

The birds of Ashfall provide an excellent example of what Darwin had in mind. Quite apart from the relatively rarity of the specialised conditions that promote the fossilisation of any animal's remains, birds, with their light, hollow bones, are typically under-represented (compared with other taxa) in the fossil record. A wetland such as the one preserved at Ashfall must have attracted birds in great numbers, and conditions were certainly conducive, even ideal, for the preservation of fossils, and yet so far only three avian species have been described from Ashfall. Significantly, each of those three is a relatively large-bodied bird, and even so, two of the three are known from only partial skeletons. None has yet been found anywhere else.

It's almost certain that there are still undiscovered avian fossils at Ashfall, and it's even just possible that some may yet be found. The best place to look, however, will never be searched: the center of the watering hole, directly under the trove of rhinos, camels, and horses. (Ground-penetrating radar is no help here, as the nature of the Ashfall site is such that the fossils have the same density as the surrounding substrate.) We must therefore be especially grateful for the few birds that have been found at Ashfall: each unique, each bearing interesting relationships to living birds, and each a window onto a strange yet familiar past.

[Details from visitor centre mural by Mark Marcuson.]

References

Miocene hawk converges on Secretarybird

A Late Miocene Accipitrid (Aves: Accipitriformes) from Nebraska and Its Implications for the Divergence of Old World Vultures

Credits

Grévy's zebra, grey crowned crane, griffon vulture, and secretarybird photos are from Wikipedia. All others by Jessica & Mark Farrell-Churchill. 

[Hesperotestudo in bronze by Gary Staab.]

[Teleoceras, also by Gary Staab.]

[Looking down the valley from the Rhino Barn. My stretch of Verdigre Creek is a few miles away.]