In the summer of 1857, the conflict between the Sioux and the U.S. Army was heating up, and Lieut. G.K. Warren was commissioned to lead a mapping expedition deep into Nebraska Territory. The geographic and scientific goals of this expedition included the exploration of the natural history of the region. A young geologist, F.V. Hayden, accompanied Warren on this expedition. Hayden’s eye for geology and fossils served them well once the expedition reached the Sandhills. As a prolific collector and a member of the Megatherium Club (boosters of a young Smithsonian Institution), Hayden made sure that some of the most prominent eastern paleontologists were able to analyze and report on important fossils that the expedition found in the Loup and Niobrara River valleys.
At one stop along the North Fork of the Loup River, near what is now Seneca, Hayden found teeth and bones. He had probably seen similar fossils during his work with James Hall, the first state geologist of New York. The small collection, as later described by Joseph Leidy of the Philadelphia Academy of Natural Sciences, included teeth of a jaguar, mastodon, and horse. A second location in the Niobrara valley of southern Sheridan County yielded a large mammoth tooth. Leidy, rarely cautious when it came to naming new species, used this specimen to describe a new species of large primitive mammoth, the imperial mammoth.
Leidy’s concept of the imperial mammoth would soon fall into disuse, a casualty of better collections and, eventually, analyses of ancient DNA from fossils. However, large mammoths remain emblematic of Nebraska paleontology. The 1922 discovery of a very large mammoth on a Lincoln County farm became the centerpiece of the newly christened Elephant Hall in the University of Nebraska State Museum. Nicknamed “Archie” and reconstructed in bronze, it now stands in front of the museum to the delight of Husker fans and fourth graders who walk under its massive, upturned tusks.
Since the mid-nineteenth century, the Nebraska Sandhills have hosted dozens of paleontological expeditions. Fossils from these expeditions reside in many eastern natural history museums, from the American Museum of Natural History to Harvard and Yale. Yet the largest collections continue to be held by the University of Nebraska State Museum, whose paleontologists led (and continue to lead) expeditions to key localities in Sheridan, Morrill, Garden, Brown, and Cherry Counties. A modern landscape of stabilized dunes, alluvial gravels, silts, and marls preserves the deep history of Sandhills plant and animal communities.
The Sandhills were not always dunes, nor were they always grasslands. The presence of herbivores with high-crowned teeth (useful for eating abrasive grasses) in regional paleontological assemblages have been key to understanding the timing and development of grassland ecosystems in North America. From modest Miocene beginnings, warm season grasses gradually became larger components of the landscape during the Pliocene, reaching modern levels approximately 3 million years ago.
Some of the earliest Pleistocene sites in the Sandhills preserve animals of this earlier time and represent a transition from the warm mid-Pliocene to cooler glacial conditions. Giant camels, diminutive antelopes, llamas, stegomastodons, and borophagine dogs from the Lisco and Broadwater localities along the North Platte River are holdovers from these Miocene forests, when rhinos and oversize tortoises roamed the region. The Lisco quarry is famous for its giant camels and is estimated to be 4.5 million years old. Hundreds of extinct horses and a relatively complete skeleton of a stegomastodon were found at the slightly younger Broadwater quarry (3.5 million years old). Both sites represent a world that could come again. Over the next few decades, atmospheric CO2 levels and temperatures will increase, reaching levels that were last experienced during this mid-Pliocene transition.
The first time glaciers from Canada made it to the mid-latitudes marked a one-way shift in the animal and plant communities of the Great Plains. Browsers occupying the open forests of the Pliocene moved elsewhere or went extinct, to be replaced by cold-tolerant taxa of the Ice Age. These communities included mammoths, musk oxen, horses, and camels, and a broad range of cold-adapted small mammals such as jackrabbits, voles, and muskrats. It is these smaller species that help us tell time during the Ice Age, as the teeth of younger taxa quickly adapted to diets in colder, drier landscapes. Sites near Mullen on the North Fork of the Loup River document this change from Pliocene survivors to early Ice Age faunas. Some of the earliest harbingers of a 2.5-million-year cooling period were not mammoths and musk oxen, rather they were jackrabbits and muskrats!
The Ice Age was not always cold. During this time the climate swung between warm and cold, dragging the ranges of animals and plants to the south during cool periods and back to the north as it warmed up. Collecting sites in the ancestral Niobrara River valley near Hay Springs, Rushville, and Gordon—made famous by Leidy’s imperial mammoth—provide a picture of the Sandhills landscape as a cool, lush, relatively open forest. Ranchers of today would have recognized many of the animals that occupied the Sandhills at this time, as they were the ancestors of many modern forms. In addition to mammoths, stilt-legged horses and flat-headed peccaries occurred in these sites, as did the iconic saber-toothed cat, Smilodon fatalis.
In recent years new techniques, such as the recovery of DNA from fossil teeth and bones, have rewritten our understanding of some of these species. These studies suggest that mammoths were surprisingly similar throughout their range and that the genetic differences between woolly and Columbian mammoths were minor, despite measurable morphological differences. On the other hand, mastodon populations had evolved separately for much longer, and dire wolves are more closely related to earlier forms like Armbruster’s wolf (found in the Sheridan County sites) than to modern gray wolves.
The end of the Ice Age happened quickly. Continent-wide glaciers covered Canada at the peak of the last glacial episode (~20,000 years ago) but had melted north of the U.S. border within a few millennia. By twelve thousand years ago many of the largest animals were extinct, leaving a landscape occupied by a single remaining species of megafauna, bison. This period of warming also corresponds to some of the earliest evidence of humans in the Great Plains.