Special Sites: Agate Springs
In 1885, rancher James Cook and his wife were taking a ride up the conical buttes behind their house, which lay in the valley of the Niobrara River in the far northwestern corner of Nebraska, when a glitter under a rock shelf caught Cook’s eye. The ground beneath the shelf was scattered with bone fragments, including the fossilized shaft of a leg bone. The Cooks took the bone home with them, but didn’t get around to reporting the find for a little while. It wasn’t until 1892 that Erwin Barbour, a geologist from the University of Nebraska, made it out to the Cooks’ ranch and realized that they had stumbled onto something quite remarkable.
Over the next few years, teams from the University of Nebraska, the Carnegie Museum, and AMNH removed thousands of fossil bones from quarries on the sides of the two buttes, which became known as University Hill and Carnegie Hill. One of the commonest animals found was Menoceras, a pony-sized rhinoceros. In places, bones of Menoceras formed a tangled layer of bones and skulls over a foot deep. A piece of this layer was excavated by AMNH paleontologist Albert Thomson in 1919 and brought back to AMNH for exhibit - the slab was 4 feet wide, 8 feet long, and 14 inches deep. It weighed approximately 3 tons. Thomson and his colleague George Olsen spent 2 months getting it out of the quarry and preparing it for transportation. The slab can be seen today in the Hall of Advanced Mammals at AMNH. This huge tangle of rhino bones is the result of a drought that took place along the ancient Niobrara around 20 million years ago. As the river dried up, the animals seem to have concentrated around the last remaining water and food sources. We don’t know exactly why the animals died but they were so numerous that hundreds and thousands died, probably due to a mixture of causes including reasons directly to the drought, such as hunger and starvation, and other causes such as disease, old age, and even fighting. Finally, when rain did fall in the mountains to the west, the river filled with water again. Sheets of water poured across the plains, gathering up the millions of Menoceras bones and sweeping them downriver and into some sort of backwater, possibly an oxbow lake. It seems that they were moved far enough to get thoroughly mixed up and jumbled, but not far enough to get broken or eroded by the action of the water. Again, we don’t know exactly how long the drought lasted, but it must have lasted several years because the skeletons show different degrees of weathering, suggesting that some had lain on the ground for years before burial, which others look as if the were buried almost instantly.
In addition to the slab, AMNH has over 200 cataloged specimens of Menoceras from Agate. What is unusual about this collection, and what makes it so valuable to scientists, is that all the animals come from the same population. In most cases with fossil vertebrates, individuals from the same species may be separated by hundreds of thousands, or even millions of years, despite being from the same site. In this case, the way that the bones are tangled up tells us that these animals lived and died at more or less the same time. This allows us to use the bones to study the sort of variation that exists within a population of a long-extinct species, without having to worry about how we tell this apart from the variation that exists between different populations. For instance there are two basic types of Menoceras skulls at Agate springs. Some of the skulls have a pair of bony lumps on the nasal bone that indicate two horns, positioned side-by-side were present above the nose. The remaining skulls have a smooth nasal bone, indicating that a horn was not present. If these two types of skulls were found in different fossil sites, we might conclude that they belong to two different species, a horned species and a hornless species. However, because they are from the same population and represented in nearly equal numbers it is more likely that these skulls represent the males and females of the same species. Because there are so many specimens available and because we can tell the sex of the skulls, the Agate Menoceras collection is a rare opportunity to examine other differences between males and females of an extinct species. The age of the skulls can be reasonably determined by the extent of wear on the teeth. It turns out that the males at Agate Springs died at younger ages than females, on average. This mortality pattern is similar to other large fossil rhinos collections and resembles mortality patterns of modern rhinos, where males frequently die from horn and tusk wounds inflicted during fights. Thus, we can infer than Menoceras had social behaviors that are very similar to living rhinos.