The Mark of Death Upon the Land
Even in Paradise an occasional calamity can occur. Agate’s misfortune appeared in the form of a drought. To the west of the plain built by the ancient Niobrara River, the Rocky Mountains began to rise again. This renewed uplift, after millions of years of relative quiet, eventually led to an even drier climate and a replacement of the savanna with a landscape of unbroken grasslands from the mountains to the Mississippi River and beyond. Trees then could survive only on canyon slopes along the courses of the few large remaining rivers that crossed the plains. Those rivers flowed toward the central lowlands of North America, once an embayment of the Gulf of Mexico. This Mississippi Embayment, as it is called by geologists, extended as far north as the present location of Cairo, Illinois.
During the first rumbles of this upheaval there were occasional instabilities in the weather of the Great Plains. From the fossil evidence of Carnegie Hill, University Hill, and the Stenomylus quarry, we can see that drought touched the land.
What happens when disaster stalks the land? That question, so pertinent to an understanding of fossil deposition at Agate, can be answered best by looking at the normal scheme of life. Animal populations are cyclic, increasing rapidly to near the highest numbers which can be supported on available food supplies. If times are good, animal populations can be quite high. If the food supply decreases, massive dieoffs result. Successive cycles of plenty and poverty then produce high populations followed by dieoffs.
The fossil evidence suggests that a prolonged drought occurred during the Golden Age at Agate, resulting in death everywhere. The vast numbers of rhino skeletons preserved at Carnegie Hill and University Hill provide paleontological evidence that the drought must have lasted for several years.
Climates change slowly, and there are wet and dry cycles. Every rancher and farmer discovers this when he plows new land during a wet cycle, for sooner or later drier years catch up with him. He expects the optimum to be the standard; but he is badly hurt during average times, and really suffers when the dry years come. It is the same with populations of wild animals. When the times are good and the grass and trees are lush, fat, and green, more of the young survive and the whole population flourishes. The plant-eaters expand their herds and the meat-eaters increase to keep up with the better food supply the plant-eaters provide. In each case the standards for survival are lowered, and the less than perfect can survive and in turn produce young of their own. But when the water fails and plants refuse to grow, the herbivores starve and the carnivore population in turn declines. Nature is indifferent—neither cruel nor kind. When times are bad every species is improved, for the strongest and most tenacious survive to reproduce themselves. There are benefits to hardship.
So it was at Agate only a year or so after the day of our visit. The river died for a while. As with many rivers much of its water flowed beneath the surface, through the sand and gravel of the bed. When the ancient Niobrara died there was still water moving through the sands and filling the low spots in its bed. Some animals could dig down to it and survive, others could stake claims to the diminishing water holes. So the thirsty, suffering herds of Menoceras went to the river and found no water. The strong held the water holes. The smart dug into the sand and made their own water holes. The rest died. They died by the hundreds, and thousands. Mixed with the carcasses of Menoceras were other victims: occasional chalicotheres, giant pigs, oreodons, cats, dogs, and a variety of equally thirsty smaller animals. Perhaps most of the animals went farther up or down stream, or perhaps they chose not to die at the river. Whatever the pattern of dying might have been, we know that Menoceras left untold numbers of skeletons on the broad, flat, and dry bottom of the ancient Niobrara.
Finally the rains fell in the mountains to the west. The river filled with water again and ran in sheets across the plain. At Agate the millions of Menoceras bones and lesser numbers of the bones of other animals were swept for a few hundred meters downstream and into some sort of backwater or river lake—possibly a great meander, or an oxbow lake. There, like a gigantic mass of jackstraws, they were piled in a tangled mat 30 centimeters (12 inches) thick, covering an unknown number of hectares. All we really know is that they were moved far enough to get thoroughly jumbled, but not far enough to be badly broken or much eroded by the action of the water.
The mass of bones was soon buried by the sands and silts dropped by the reborn river, and by wind-carried debris swept off the parched land. Once buried, the bones were partially petrified by mineral water flowing beneath the surface. The land was built up a few hundred meters by sediments continually brought down from the mountains to the west. Eventually, continued uplifts of the Rockies and the Great Plains combined with erosional cycles to leave the modern Niobrara River. The two erosional remnants known today as Carnegie and University Hills were produced by the cutting of the modern river system. On the sides of these hills were exposed the tangle of bones which marked the site of ancient tragedy.
But this wasn’t the only scene of mass death to be preserved here in the fossil record. A few kilometers away an earlier drought took a toll of many other animals. The little gazelle-like camel Stenomylus tells the same story in scores of skeletons east of the Menoceras burial ground.
These graceful little camels may have died at the edges of their vanished water hole. The skeletons are mostly undisturbed except for a few pulled apart by meat-eaters. Scores of their dried out, mummified carcasses were buried about the same time as the rhinos on the river’s dry bottom. Like the Menoceras, the camels lay there for millions of years, intact in their death poses, the muscles in the backs of their necks pulling their heads back sharply into an unnatural position. There they lay until men discovered them.
Our imaginary journey into the past has reached its end. We have seen a day at Agate as it might have been 20 million years ago. We have watched the animals going about their daily lives during times of plenty and have seen it as it was later, when death’s heavy hand left a magnificent fossil heritage. This unique place is a window into the past, a window through which we can look back at any time and observe life at Agate millions of years ago.