THE ASHFALL FOSSIL BEDS

A Twelve Million Year Old Catastrophe Frozen in Time

by A.D. Griesemer

Assembly Room, A. K. Smiley Public Library

Introduction

I have been fascinated by rocks for as long as I can remember.  Of course there were plenty of them lying around where I grew up, in southeastern Wisconsin. That is because this region was overwhelmed by continental glaciers, during the geologic period known as the Pleistocene, most recently between 70,000 and 10,000 years ago.  Glaciers are rather untidy forces of nature.  As they move across the countryside, their hundreds to thousands of feet of ice gouge and bulldoze the surface of mother earth.  In the process, they massively redistribute surface materials in their wake, and as they melt, haphazardly drop astronomical tons of ice carried debris along their gravity-guided path.   Almost anywhere you live in Wisconsin, you inhabit terrain formed in those pathways.  For any of you that may have a Midwestern background, especially the states surrounding the Great Lakes, you probably are aware of the kind of rocks and debris to which I refer.  For the tillers of the field, these frequently boulder-sized specimens commonly litter the ground, making plowing a hectic experience, and the inevitable clearing process a never ending job.  Faced with this menagerie of impressive specimens, it didn’t take me long (my mother was a magnificent instigator) to realize that rocks can actually be rather interesting. They can serve multiple purposes – the construction of buildings and roads, farmer’s fence lines, and from the carbonate varieties, the manufacture of cement and fertilizer, or simply as a source of ornamental and commercially useful minerals, and much more. However, most interesting to me, one particular type – sedimentary rocks, limestones, shales, and sandstones, the ones that are formed mostly in bodies of water as oozes, muds, and sand deposits, are capable of preserving organisms that lived in that environment as they died and became entombed.  We in the “business” call these preserved organisms, as you are probably aware - fossils!  What makes this relationship of rocks and fossils so interesting, is that the enclosing rocks are like the pages in a book.  The fossils serve as the corresponding illustrations, and like pictures, are worth a thousand words – at least as they reveal their life story to science.  Intellectuals as far back as Aristotle recognized the uniqueness of these oddities, but it wasn’t until after the Dark Ages that any serious understanding of their existence began to surface, and not until the late 18th to the early 19th centuries that anything close to scientific thought was brought to bear on the issue.  It is now clear that fossils are the best clues paleontologists (geologists that study nonhuman fossils) have to recreate the ecology of an area at the time the creatures lived.  The study of the relationships between the rocks, and their enclosed fossils is therefore called paleoecology.  I mention all this in introduction to the topic of this paper – the highly unique and astounding (because of the level of preservation) fossil site in northern Nebraska, near the small town of Orchard.  The site is called the Ashfall Fossil Beds, where over 300 specimens of large fossil mammals have been discovered that have lain undisturbed since their deaths 1l.8 million years ago.  It has been determined that these organisms all probably died within a span of less than two months. In geologic thinking, that is a “frozen” point in time.  Clearly, as you will see, their deaths were caused by a natural catastrophe.  All of this drama was created and preserved by an air-borne sedimentary blanket of tiny glass shards that buried these poor creatures as they huddled together on this northern Nebraska plain, gradually losing their ability to breathe.  From the pictures I shall show you later, you will be able to interpret for yourselves, the helpless agony these animals experienced, as the drama unfolded and their lungs slowly filled with glass.

The Setting - North America Forty Million Years after the Dinosaurs

The Ashfall site was initially dated radiometrically in the 1980s by using uranium fission track dating of the glass shards (which means they actually count the number of uranium scratches on the particles). From this process they found the shards most closely matched the ejecta of an eruption that occurred in southwest Idaho.  This site, referred to today as the Bruneau-Jarbridge Eruptive Center, had initially been dated at 10 plus million years.  However, using the more accurate argon/argon method in 1995, they were able to refine the dating, and found it to be 11.8 million years old.  This is just a small sliver of the evidence that indicates that the western portions of our country are presently experiencing a considerable amount of major structural realignment, including potentially catastrophic volcanism.  More about this later.

 In addition to the revised time frame, I shall endeavor to utilize other evaluations of the enclosing ash to paint a picture for you of our ever-changing continent including, its time related general climate, geographic configuration, river systems, land forms, and its relative pleasantness for life as we understand it.  Indeed, you may be surprised how much of the total picture a study of rocks can create, with reasonable accuracy.  Of course you will have to give me a little leeway with some of my conclusions, for let’s face it; they really are still just educated guesses.

Perhaps you have read or heard geologists speak on the topic of the mobile nature of the Earth’s continental masses, a movement, which has been a dominant factor in the evolution of our planet’s life forms over the past billion plus years.  This is due to the existence of 10 to 12 massive crustal plates that have brutally interacted with each other on an ongoing basis from very early in the Earth’s history; I spoke to this group on this topic in 1998, the concept called plate tectonics.  This concept is as basic to the science of geology, as evolution is to biology. It is believed that the plate movement is driven by the heat generated at the Earth’s core, our critical internal engine. In response to plate movement, extensive warping of the crust has occurred frequently throughout time, resulting in periodic oceanic flooding of continents on a massive scale.  The last extensive inundation of the North American continent occurred about 70 million years ago, ending about the same time the dinosaurs seemed to vanish.  North America from that time on began to take on a fairly modern look, with only periodic oceanic incursions along its margins.  The Cordilleran chain of mountains that dominate western North America today, a portion of which we call the Rocky Mountains, had their origin during the Age of the Dinosaurs.  However, by the time our Nebraska scene of this paper was evolving, they had been worn down to relatively flat plains, but were again in the process of being rejuvenated.  This is a very significant issue for our story, for those newly elevating land masses, strongly began to affect the climate in the midsection of the country. Part of the mid-continent change involved the creation of very large lakes (Salt Lake would be a puddle in comparison). For those of you who have traveled to Utah to enjoy their beautiful national parks, you probably have visited Bryce Canyon.  These unusually colorful beds are the remnants of the sediments deposited on the bottom of a huge lake that filled the void caused by the last continental sea’s demise.  Similar lakes in Colorado and Wyoming produced the massive amounts of organism-laden sediment that resulted in the creation of the oil shales we periodically hear about.  These resources are often touted as our eventual (though temporary) last salvation when the liquid black gold runs out.  It was a time of mild temperate climates throughout the region, with forests abounding, including the mighty redwoods, yes redwoods in mid America.  With the uplift that started about twenty five million years ago, the forests began to diminish, and the first known grasslands developed.  The climate was cooling slightly and a rain shadow was forming on the eastern side of the Rockies that favored grasses over trees.  Geologists have given a name to the period of this physiographic and climatic change, calling it the Miocene Epoch.  The Miocene lasted for about twenty million years, ending about five million years ago, being replaced by an epoch called the Pliocene.  Strangely enough, these names were given to rocks discovered near Paris, France; by the English geologist Charles Lyell (often called the father of modern geology).  He based his decision on where the boundary lines between the epochs existed, by determining the percentage of modern mollusk species he recognized as fossils in the rocks being examined.  For the Miocene, that percentage was about 20% less than the Pliocene, but still contained around 35% of our modern forms. 

Western North America during the Miocene was very active and structurally unstable due to the interaction of two of those independent plates that make up the Earth’s crust.  In this case, the two largest plates, the Pacific and North American plates were colliding resulting in major crustal activity in the form of volcanism and massive lava flows (up to 10,000 feet thick) in Washington and Oregon.   Huge blocks of the crust alternately formed depressions and uplifts in Utah, Nevada, Arizona and California (which we now call the Basin and Range Province), with the Sierras being the most famous uplifted block.  Slippage action between these plates (about 120 miles so far) is still moving parts of Mexico toward San Francisco at about five centimeters a year along the infamous San Andreas Fault, and caused Baja, California to begin its separation from the Mexican mainland. In addition, a broad general uplift along the Cordilleran front and the Andes in South America was the beginning of the new massive mountain chains that still so dominate these two continents today.  In the southwest, the Colorado River was being challenged to maintain its flow to the west, due to the uplift of the Colorado Plateau.  However, this ancestral river refused to be intimidated, and doggedly kept up with the uplift, cutting downward at a rate equal to the rise, thus forming the magnificent Grand Canyon. In the mid-continent, the ancestral Mississippi had begun flowing south contributing to the creation of a very thick wedge of sediment in the Gulf Coast that is now over five miles thick.  In the several millions of years since, this massive thickness of sediment, with its included organic content, experienced a conversion of the organic materials to liquid hydrocarbons, which became trapped in these porous sediments forming “pools” of oil.  When this phenomenon was discovered in the late 19th Century, the United States’ prominence in world affairs dramatically changed, for it allowed this country to leapfrog the rest of the industrialized world by giving us at that critical time, access to the world’s largest known store of petroleum.  Of course, now it has been nearly pumped dry from the easily accessible areas.  The recent discovery of five to fifteen billion barrels in the deep waters of the Gulf is still a part of the original discovery.  Those numbers sound impressive until you realize that the United States annually consumes five billion barrels of this rapidly vanishing energy resource.

Although our continent may have had approximately the same basic geographic outline it does today, during the Miocene, North America had more oceanic flooding than it had experienced in more than 25 million years.  The oceans invaded the east and Gulf coast regions about 120 to 360 miles respectively, and close to 60 miles on the west coast.  Sorry, there would have been no dry land available for New York City, Florida, Houston, San Diego, Los Angeles, the San Joaquin Valley, or Seattle.  Of course, global warming may duplicate that feat within the next century or two.

Just so you don’t think that North America was hogging all the activity, in other parts of the world, during this time Australia was just beginning to separate from Antarctica, and India had slipped away from Africa crashing headlong into Asia causing the majestic range of mountains we call the Himalayas.  Greenland moved away from Europe, and the beautiful Mediterranean Sea was about to form, being a left-over from a previous sea called the Tethys. For a period of time it had become a massive inland lake, until a breakthrough at Gibraltar occurred during the Miocene rejoining it to the Atlantic.
In general, the Earth was beginning to look quite modern.  By the end of the Miocene, all the continents were about in their present positions, only the land bridge between North and South America had yet to form.  As a trade-off, the Bering Sea land bridge was in existence which allowed for some interesting animal migrations in both directions. Part of this modernization in geography included the splitting off of Antarctica from the rest of the continental masses, resulting in the creation of circumpolar oceanic circulation.  This meant that the vertical movements of oceanic waters which had kept world climates rather moderate were significantly reduced.  That resulted in a cooler and growing Antarctica, the enhancement of seasonality, and a general cooling world-wide.

So you can see that even though the Earth is well over four billion years old, it is still a very dynamic planet and a thriving element in the Sun’s cadre of eight planets (nine if you are stuck on keeping Pluto in the pack).  Again, all of this is possible due to 7000 degree centigrade heat that still is being generated at the Earth’s core with the ongoing disintegration of its inherited radioactive minerals.  Of course, eventually this engine will run out of fuel, and when this happens in another billion years or so (no one really knows), life on this planet will cease to exist – the Earth as a planet will die geologically, the magnetic shield will disappear, and with it, all life as we know it. Perhaps we will end up looking like our neighbor Mars. But don’t fret; maybe life on Venus will be possible by that time!

But we are diverging from our story about those poor unfortunate mammals that were roaming the northern plains of Nebraska at just the wrong time.  Let us first examine the environmental setting that attracted the animals we find as fossils in the ten feet of volcanic ash that occurs nested in a small pocket hundreds of miles from any known volcano past or present.

As indicated above, the climates of the Miocene were changing.  The Cordilleran mountain chain was rising, Antarctica had become isolated, cooling was the norm across most of the evolving world, and in the northern plains, the rain shadow caused by the rising mountains to the west was creating a drying effect in the mid-continent – resulting in the evolution of the world’s first extensive grasslands.  Of course, those animals best adapted to this new environment took over the scene, and of particular interest to us in our attempt to interpret the Ashfall story, are the larger mammalian herbivores, antelopes, horses, camels, and rhinos, especially rhinos. Ancestors of these mammal groups had been living in North America for at least thirty million years, but had remained mostly small to medium sized, but things changed dramatically when the grasses began to dominate the Plains.  Not only did the vastness of the grasslands increase the diversity of the grass-eating fauna, increased as well.   At the Ashfall site, for instance, several species of horses and camels have been found.  It is interesting to note, that although horses had their origin in North America, they migrated to Asia over the Bering land bridge in the Miocene. In both worlds they evolved into the horse of today, but died out in the New World, only to be returned to our shores by the Spaniards five hundred years ago.  Other mammals of note that shared these pasture lands, included giant hogs (six feet tall at the shoulder) saber toothed cats, bear/dogs, mastodons, raccoons, weasels, beaver, and deer.  The Plains perhaps could be best described as being like the savannas of Africa – full of diverse life living in a pastoral environment.  North America in general was a very conducive place for the evolution of a rich and diverse fauna and flora, and winter brought no snow storms to the northern plains.  But as I have indicated above, things weren’t so pleasant and tranquil from the Cordilleran ranges west due to the collision of those two massive crustal plates, one oceanic and one continental.  Since the continental plates are made of lighter and less dense materials, the oceanic plates slip beneath their continental equivalents, causing the deep oceanic trenches that are found along western South America, and the Aleutians.  That kind of action is not as prominent along the west coast of the United States because of the San Andreas Fault, where the plunging action has been transformed into a slipping motion.  Of course, the result of this movement means that in a few million years, Los Angeles will be passing San Francisco on its way north.  All of this action has been accompanied by considerable volcanism including the erupting of volcanoes and in some cases, the relatively quiet outpouring of unstoppable lava flows. If you have visited the large island of Hawaii, you have probably seen a small example of nature’s volcanic might at work.

Based on the extent of the covering of ash, both in thickness and geography, there is reason to believe that what occurred in Idaho 11.8 million years ago was what is loosely called a super-volcano.  It is now known that the Bruneau-Jarbridge volcanic center, of which our first National Park, Yellowstone is a part, has not only been active for several million years, but has been active as recently as 600,000 years ago. The extent of this system was not appreciated until the 1970’s when geologists recognized that Yellowstone Lake was flooding on the south end and apparently rising on the north.  Why?  With a great deal of investigation, using especially the input from twenty-two seismographs, that were monitored by the University of Utah, they were able to determine to their amazement that Yellowstone Park was essentially the center of a huge caldera, 18 to 30 miles wide, with a growing bulge on its north side!  Such systems do not form familiar volcanic cones; they form massive depressions when they erupt.  That is because the rising magma becomes trapped, forming an immense dome of liquid rock and gases, which when they finally break through to the surface, literally and completely blow their top. The last one known to have done so on this planet was 74,000 years ago in Sumatra, forming Lake Toba, which is 30 by 60 miles in size.  It has been estimated that eruption lowered the global temperature by 5 degrees Celsius, which would mean a 15 degree loss for places like central Europe – which would have a devastating impact on everything living, and trying to grow.  We are talking about an event that would have ejected 10,000 times more volcanic debris and ash than Mt. St. Helens.  With all the monitoring that has been done in the past 30 years, they now know that the magma chamber that temporarily lies trapped below Yellowstone, is 25 miles long and 12 miles wide, about half the size of the Park, and 6 miles thick.  They also found out that this caldera system has been the site of several eruptions in the past, including 2 million years ago, 1.2 million years ago, and as I said earlier, 600,000 years ago.   -  do you see a pattern?  This can be a bit disconcerting.  The only thing geologists can do is to continue to monitor their seismographs, which helps predict the status of the magma mass and perhaps its intent.  It appears to be modestly quiet at present, they say. 

The only corresponding evidence that the Sumatra event was a colossal super-volcanic episode 74,000 years ago is to look to the fossil record.  Interestingly enough, completely independently, biologists working with mitochondria, which have the unique ability to store the history of an organic entity through the female line, they came across an astonishing bit of evidence.  By working with this element of our genetic make-up, these researchers were able to conclude that the human race had gone through a symbolic “bottleneck” which they felt would have resulted in only 5 to 10 thousand people surviving, world wide, somewhere between 70 -80,000 years ago!  They based this on the surprising evidence that all humans today appear to be amazingly similar genetically – in other words, there appears to be much less than expected DNA diversity in the world’s human population, which would indicate we all came from a very small population.  Apparently these dates are not universally accepted, but nevertheless, isn’t this complimentary evidence amazing!?   Yes, these people are suggesting that during that time frame, the human race almost became extinct!  Michael Rampino, an environmentalist at N.Y.U. is quoted as saying “Civilization will start to creak at the seams” when Yellowstone erupts again, and they say not if, but when.  I must add that I do not know of any other evidence among other animal or plant groups that substantiate the timing of this Sumatra event.  But there seems little doubt, that a repeat of the 11.8 million year event in Idaho, would devastate most of North America, and seriously affect the entire planet.

Yes, we now can predict that the northern plains did experience just such a catastrophe during the mid-Miocene.  During that event, a caldera in Idaho, over several weeks or months, spewed out an enormous volume of dust and ash that with just the right prevailing winds, brought death to the totally unsuspecting animals going about their business on the northern plains, and perhaps over a much broader area.  Unfortunately the only surviving evidence exists in Nebraska. Let us now examine that evidence entombed in that ten feet of tiny glass shards, and try to interpret the site, before, during and after that catastrophe.

Opening Nebraska’s Twelve Million Year Old Time Capsule

Fossil hunting is usually not a very dramatic and often, not even a very exciting event.  Someone estimated that only one in a million living organisms ends up as a fossil, and when a fossil hunter is lucky enough to find that one specimen, it more likely than not is badly broken, disarticulated, partially complete, and perhaps exists only as an impression of the organism.  And by far the majority of fossils discovered by paleontologists are of the invertebrate variety (no backbone); of interest to some (such as your presenter) who can be enthralled by such creatures as clams, snails, crinoids, trilobites, corals, and of course the oh so common brachiopods, that have inhabited the seas of this planet for over a billion years.  Unfortunately most of the notoriety in the field goes to paleontologists who hunt for much more exotic backboned animals, such as dinosaurs, saber toothed cats,  mammoths, of course primates, and if you include paleoarchaeologists, humans!  Since these larger creatures are harder to find, due to their size, which limits their generic biomass at any one time, and since they usually do not die in water where they can be more easily buried, any find is cause for newspaper attention, at least locally, and usually loads of public interest.  This is often the bane of invertebrate paleontologists, but we just have to learn to live with the reality of the human psyche and predilection for their own vertebrate class – mammals.

For nineteen years I worked at the University of Nebraska State Museum which built its reputation on the collection of just such four-footed exotic creatures.   For the past one hundred and fifty years, fossil hunters have scoured the hills and valleys of Nebraska seeking and unearthing fossil mammals that lived on those evolving plains during the last thirty five million years. Museums all over the world have examples of this rich fossil fauna, as specimens were commonly traded, especially in the early Twentieth Century, among the major natural history museums in an effort to enhance their exhibit potential.   These range from tiny antelopes and deer less than a foot tall to giant giraffe-like camels, and the massive imperial mammoths that reached twelve feet tall at the shoulder.  The critical issue in all of these finds, scientifically speaking, is not that these strange creatures existed, or that they seem out of place in our frame of reference, it is that their remains provide us with a critical storehouse of information about what it was like environmentally speaking when they inhabited the areas in which they are found.  They all are story tellers, and paleoecologists are the interpreters of these stories.  The basic tools the paleoecologists use is the knowledge biologists provide concerning like animals living today, and an understanding of the geologic history of the region.  In other words, there is a lot of speculation, guesses if you prefer, but I can tell you from experience, it is a very enjoyable challenge to pursue.  And in most cases, no one can prove you wrong – easily.

That was certainly true in 1971 when a comrade of mine, Mike Voorhies, the Curator of Vertebrate Paleontology at the University of Nebraska State Museum stumbled across an extraordinary discovery on the banks of Verdigre Creek in northern Nebraska.  He had been surveying this general area for several years looking for outcrops that might yield fossils.  He was about to go back to camp (where his wife was waiting with supper), when he decided to look at an outcrop of volcanic ash in an area that had recently experienced a heavy summer shower.  His trained eyes immediately saw a gleaming white specimen sticking out of the bank far above his head.  What he found on closer examination, was the foot long jaw of a small rhinoceros.  This in itself was not surprising, but when he dug deeper into the bank he found that not only was the jaw there, so were the skull and neck vertebrae!  Now he got really excited, for this meant that there was a good chance that the animal was all there and fully articulated – as I indicated above, a major rarity! 

This has turned out to be an once-in-a-lifetime discovery and very significant scientific opportunity for Mike.  Due to a variety of circumstances, mostly financial, the site could not be excavated until 1977.  At that time, using a small university crew, they were able to establish that the site was quite extensive, and were able to excavate a representative area of twenty square meters.  What they discovered was a volcanic ash bed ten feet thick, overlain by a two to three foot capstone of tough river-deposited sandstone which served as a protective covering for the soft ash below.  Probing with steel rods proved that the site was at least three hundred square meters in size, indicating clearly that the excavation would take serious financing.  At this point Mike approached the National Geographic organization about the site seeking their financial assistance.  They agreed and provided Mike with enough funds for two full field seasons.  It became very obvious that this truly was going to be the find of a lifetime.  By 1980 the quarry produced over two thousand casts from two hundred individuals, weighing more than forty tons.  The interesting thing was that the dominant animals at this site were not camels or horses, or antelope like the savannahs of Africa, instead, the dominant form appears to have been a barrel chested, short legged rhinoceos, now called Teleoceras major.   Although the rhinoceros was the dominant form found at the site, there were at least ten other mammalians found in the ash, as noted above, including three species of camels, and five species of horses.  The Miocene has often been called the era of the horse, for it has proved to have been a very critical time period in horse evolution.  Although none of the horses at the time were large (pony size at most), they were very diverse, and at a critical stage in their development, most notably, moving from a three to one toed stance on their front feet.  Both kinds of horses (three and one toed) are found at the Ashfall site. The diversity and quality of preservation found at Ashfall, has caused one leading paleontologist to claim that the site “represents one of the most important assemblages of Tertiary horses in existence anywhere and from any time Period”.

Returning to the rhinos, another interesting discovery at this early stage was that the ratio of females to males at the site was about six to one - about the same as is found in the modern large ungulate herds in Africa.  In other words, these animals were gregarious, with the dominant males probably forming harems.  This certainly does not mimic the living rhinos of Africa or Asia, which seem to live very solitary lives.  To support this concept, another paleontologist about fifty years earlier had found only males in another Nebraska quarry containing the same rhinoceros, again similar to the herds of large mammalian herbivores in Africa, where younger males commonly live a bachelor life away from the main herd, staying isolated until they feel they are ready to challenge a dominant male.  In addition, based on tooth wear, the calf groupings at the site, seem to be about a year apart, suggesting that these rhinos mated seasonally, again very different from the rhinos living today, which mate sporadically throughout the year. Yes, all this information can be gleamed from just looking at what nature has preserved for us in the form of siliceous skeletons.

However, don’t be misled by the good preservation of this material to think that it is easy to handle.   First of all the ash itself is terrible material to work with.  The good thing about it is that it is soft, and can easily be dug into.  In fact when a person stomps on the ten foot thick bed of ash, it feels spongy and almost as if there is a rebound effect.  That is because the ash particles do not compress well, even after twelve million years.  That said, ground water moves with relative ease through the ash, and over that span has replaced a great majority of the calcium in the mammal bones with silica, which made the bones very hard, but also left them brittle.  Mike described the handling of the broken bones saying that it was like “trying to lift a sack of thin, cracked wine glasses out of a brier patch without shattering them”.  To deal with this brittleness, after they have removed the ash away from the bone with picks, dental tools and brushes, they soak the bones with a solution of plastic preservative, giving them many coats.  Once they feel the bones are stable, they coat them in plaster, again using several coats, sometimes including two by fours or other stabilizers to hold larger bones or complexes of bones together before attempting to lift them from the site.  In another interesting side-light, there is another famous Nebraska Miocene quarry on the Nebraska/Wyoming border called the Agate Fossil Bed National Monument. At this site, the bones of a wide variety of mammals, including a different species of rhino have been found.  These bones were more permanently replaced by silica, and became very solid.  Not only were they hard, but the ribs were found to produce a ringing sound when hit with something hard.  Henry Ryder, the head preparator at the Nebraska State Museum at the time (in the 1950’s) actually put together an arrangement of rhinoceros ribs that produced a reasonably complete musical scale.  He actually built a device resembling a xylophone with some rhino ribs, and would play this device for parties.   He gained such notoriety that he was asked to appear on the Ted Mack Amateur Hour!

As you can probably imagine, once a crew finds a site of this quality, the excitement level rises dramatically.  They will work from sunup to sundown to excavate and learn the extent of the find, geographically, geologically, and paleontologically.  In this particular instance, the field crew was rewarded with some extraordinary experiences.  The first to find specimens of this quality is exciting enough, but in this case some really unusual finds surfaced as the dig progressed.  The ash is so fine and the detail so good, that one crew member found fossil feather impressions, another found the remains of an unborn fetus in the skeleton of its mother, a preparator found identifiable grass seeds in the throat of a rhino, and a graduate student unearthed a bird skeleton with polished gizzard stones in its throat, and another found the last meals of two cranes, one had swallowed a lizard, and the other, a mouse.  Even in that first year they were rewarded with an unusual find.  Pushed up against the pelvis of a large adult, they found the skeleton of a small calf, positioned as if in its death throes, it was trying to nurse.  This together with the later find of the unborn calf, allowed them to be able to distinguish between the males and females, since those associated with the calves all had significantly smaller tusks.

What Mike feels confident about is that he has discovered the equivalent of a living community, which is very rare in the fossil record.  In other words, he feels he has a good representation of the natural living grouping of animals that inhabited this area 11.8 million years ago.  Looking only at the rhino specimens, the number of young adults and calves, again comparing it to modern large herbivore herds, suggests this was a very healthy herd, which can be interpolated to suggest the community was healthy as well. This is a good example of a reasonable scientific guess.   The reason paleontologists rarely have the opportunity to find such a site, especially for land animal populations, is that when these animals die, nature through numerous means is very adept at dispersing and decomposing what is left behind.  This is accomplished by oxygen, predators, wind, water in the form of rivers and streams, and the breakdown of the organic material by the sun, several small invertebrates and a myriad of other micro-organisms.  In the paleontological world, there is a word for this miracle preservation of a complete community; it is a German term, lagerstatten.  It has been suggested that the Ashfall site is the only mammal-yielding lagerstatten known to exist in North America that is older than two million years.  That fact alone makes Ashfall a very significant paleontological site – for here the animals that lived together died together as a true community – a true time capsule of the northern Nebraska plains.

To complete what is known of the rest of the picture, let us examine the remainder of the once thriving community.  There are five other vertebrate types that left behind evidence of their presence in this area, a small saber tooth deer,  simple pond turtles and  frogs, a large elephant-like animal (a shovel tusker) a migrant from Asia, and finally two and perhaps three types of dogs.   In a near-by quarry two other significant vertebrates add information to the evolving picture for paleoecologists.  This companion quarry yielded specimens of a large, up to four foot long tortoise (similar to the ones from the Galapagos Islands), and an alligator!  Yes an alligator!  Now it doesn’t take a genius to recognize both of those reptiles represent a significant clue to help paleoecologists propose the paleo-climate for that Nebraska plain twelve million years ago.   

So how should we paint the picture of that catastrophic moment, just prior to the inundation by that deadly cloud of jagged glass dust?  What does all the evidence mentioned above, suggest was the environment of the time?  Well, when we put it all together, it seems pretty clear that the northern Nebraska plain was most likely subtropical, and dominated by lush grass, with some trees along the stream and river channels.  The presence of alligators and large tortoises is a pretty good clue to that conclusion.  Although this next suggestion may be a bit of a stretch, the most common herd animal may have been the barrel chested rhino Teleoceras, which lived in modest sized matriarchal herds, including a few dominant males.  Everything from the excavation suggests that the immediate scene before the volcanic blizzard hit was a small ox-bow lake or pond of a few acres in size that was probably flooded seasonally, drying up in the heat of summer. The only proof that there was standing water at all is the presence of pond turtles, frogs, and microscopic diatoms (a fresh water alga, with siliceous skeletons).  The evidence suggests that there were perhaps several invasions of ash, since the ash beds appear to be layered.  There is also evidence that the deaths took place over a period of several weeks.  This conclusion resulted from the recognition that there was a consistent layering of the animal types.  Since the turtles and birds are found at the bottom of the section of ash, the conclusion is that they died first, they were followed by the smaller mammals, the horses and camels, and finally in the upper most fossil bearing beds came the rhinos. This order was consistent throughout the quarry. There is also evidence that the land was uninhabitable for weeks after the last invasion for the final four to five feet of ash is totally barren of fossils. Although there are no large carnivores in the fauna,  tooth marks have been found  on the larger bones suggesting that at least in the early stages, carnivores were trying to salvage some food, before it all was totally buried beneath the thickening coating of ash.  The death postures suggest that the animals simply lay down quietly as breathing became more and more difficult.  They are found with their legs either tucked beneath them or they are lying on their sides.  Mike has pinned down the cause of death, at least for the rhinos to Marie’s Disease – hypotrophic pulmonary osteo-dystrophy.  This was determined by the recognition of abnormal patches of highly porous superficial boney material on the larger bones, which is known to occur in large mammals that experience lung failure.

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