Millions of years have passed in the history of South Dakota and, as time has passed, the plants, animals, climate, and topography have changed. Fossil records along with geological records hold information that help paleontologists explain these changes. The geological record tells us how the surface of the earth was transformed, which helps explain the changes in the climate. These changes, in turn, affect the composition and distribution of the flora and fauna found in North America. Rock layers which preserve the fossil record can also provide clues to the prehistoric sequence of events. As we study the past, we find explanations for the current plant and animal composition of the Black Hills and the surrounding plains.
During the Precambrian Era, the earliest period in geological time, the Black Hills region was covered by a vast sea which deposited sediments for millions of years. Sedimentary rocks of the Precambrian include shale, sandstone, and limestone. Igneous rock, now exposed at the core of the Black Hills, were forming deep below the surface. Life first stirred in the vast Precambrian seas. Life began as single-celled and non-cellular organisms, resembling modern day bacteria and algae. Multi-cellular forms evolved from these first organisms.
When compared to the Precambrian Era, the fossil record of the Paleozoic Era represents an explosion of activity. During this time most of the major plant and animal groups appeared. Since the North American continent was near the equator, the climate was warm and tropical. For much of the Paleozoic Era, the Black Hills region was covered by shallow seas.
Early in the Paleozoic Era, marine life was dominant. Some marine species developed hard outer coverings. Trilobites, early arthropods with exoskeletons and jointed appendages, were numerous. Shellfish, like crabs and snails, were found along with soft bodied jellyfish. As the shelled marine animals died, their shells accumulated on the sea floor, and were compressed forming layers of limestone. Some of the shells of these invertebrates were preserved intact. Fossilized brachiopods and mollusks are found in the Pahasapa Limestone which was deposited in the Black Hills region during the Mississippian Period. Silica from sea water may be responsible for the chert nodules found within limestone. Fossilized plants of the early Paleozoic were calcareous algae and lichens. As life in the sea continued to expand, life emerged on land.
Sea levels fluctuated greatly during this time. The emergence of land permitted sea plants to migrate to this new habitat, followed by sea animals. The first fossil evidence of land plants are fragmentary remains dating to the Silurian Period. Dominant land plants include club mosses and members of the psilophytes, which are believed to be the ancestor of the ferns and the flowering plants. Primitive amphibians were the first land animals.
Toward the end of the Paleozoic Era, North America was part of a vast tropical lowland. The forest was dominated by horsetails, tall lepidodendroid trees with long grass-like leaves, ferns, and the earliest conifers. In South Dakota, the earliest evidence of the palm-like cycads is found in the Minnekahta Limestone.
At the close of this era, many plant and animal species became extinct. Absent in the fossil record of the next era are trilobites, certain brachiopods, most horsetails, and many ferns.
Reptiles dominated the Mesozoic Era. The first dinosaurs appeared very early in the Mesozoic and became abundant by the end of the Triassic Period. By the close of the Mesozoic, they were gone, and other much smaller reptiles, like turtles and crocodiles remained.
Fossil records from the Cretaceous Period, most of which are found in the Badlands, give us a glimpse of this period. Shallow seas covered the continent. Large marine reptiles like the Alzadasaur, of the suborder Plesiosauria, hunted fish in these waters. Also present were Xiphactinus, an early member of the teleost fish group which today includes salmon. Hesperornis, a large flightless bird, plunged underwater using powerful legs to propel itself forward in pursuit of a meal. With a 25 foot wingspan, the giant Pteranodon flew overhead. Snails and clams lived on the muddy sea floor. Sea turtles remains have been found near Buffalo Gap. In the Badlands to the east, the remains of octopus-like ammonites have been uncovered
During the Pennsylvanian Period, the ancestral Rocky Mountains rose and were eroded down again. Streams and rivers carried sediment from these mountains east to the plains. When the interior of the North American continent rose, a sea floor rich in alluvial sediments was exposed. These fertile plains supported an abundance of life in the Cretaceous Period. Cycads were present, as well as the sequoia tree, whose fossils have been found near Black Hawk, South Dakota. The lush vegetation provided food for Triceratops that roamed these plains.
At the end of the Mesozoic Era, the large reptiles, including the winged pterodactyls and the marine plesiosaurs, vanished. Small mammals, the ancestors of the insectivores and the marsupials, had evolved during the Jurassic. With the demise of the dinosaurs, the mammals became more abundant and diversified during the Cenozoic Era, the "Age of Mammals."
As continental plates collided, the Rocky Mountains uplifted during the Laramide Orogeny, a process that lasted 55 million years. The uplift of the Black Hills started between the final stages of the Cretaceous and Paleocene, and continued until a final uplift occurred during the Pleistocene.
As the mountains rose in the west, the interior climate changed. With each pulse upward, the Rocky Mountains collected more and more of the available moisture. The air continuing east brought less precipitation to the plains region. As the North American continent moved northward away from the equator, temperatures decreased and seasonal variations in climate increased. In Cenozoic times, the North American climate was warmer than present, though cooler and drier than during the Mesozoic. This climatic trend brought some species to extinction and allowed northward colonization for others. Plant species and plant communities became increasingly like those of the present.
During the early Tertiary Period (Paleocene and Eocene Epochs), North America's climate was semi-tropical. Broad-leaved evergreen forests with palms flourished. Four-toed Eohippus horses consumed the soft leaves and shoots, using teeth adapted for such succulent vegetation.
Near the end of the Eocene the climate became cooler and drier. Plant communities were becoming less tropical. Grasslands were present, though their distribution was limited. This arid trend persisted into the Oligocene Epoch. Savannah-type habitat replaced the denser forests. Rivers and forested river bottoms traversed the land. The landscape of the Oligocene accommodated many species of animals. The titanotheres, huge beasts with large, long horns on their snouts, browsed in the forested drainages and grazed in the rising grasslands. Swift herds of oreodonts, small sheep-sized mammals with fang-like teeth, browsed in the savannas. The Oligocene horse, Mesohippus, possessed three-toed feet which enabled it to walk on the soft soil in river bottoms and glades. The saber-toothed cat preyed upon the small, but fleet Mesohippus. Lizards and snakes were present. Land turtles fed on the abundant plants surrounding ponds.
The trend toward a more arid climate continued during the Miocene and Pliocene Epochs. The Rocky Mountains were nearly at their highest elevation and the intercepting most of the moisture from the Pacific. The Miocene plant community looked more like today's plant community. In the Norther Great Plains region, grazing and browsing mammals were found together in savanna type habitats. The titanotheres were extinct. Horses, rhinoceros, and camels inhabited the savannas, meadows, and wooded streams. Catfish, frogs, turtles, and beavers were present in the scattered ponds.
Pleistocene and Holocene Epochs
The Pleistocene Epoch began two to three million years ago and ended around 10,000 years ago. This was the "Ice Age", when glaciers advanced and retreated across the North American Continent. Glacial advances displaced vegetation zones to lower latitudes and lowered the range of forests in the mountains. Seasonal variation was less extreme. The composition, diversity, and structure of North American's modern plant and mammalian communities were shaped by the end of this time period.
Much of the information concerning the composition of the rising Great Plains plant community is from the last glacial episode. The last continental glacier reached its maximum 12,000-14,000 years ago. The Black Hills region was never glaciated, but the continental ice sheet came within 150 miles, reaching what is now the Missouri River. Spruce forests were present in park-like stands across South Dakota. The Mammoth Site in Hot Springs has evidence of a plant community that has been described as a cold steppe-grassland with rare scattered trees. Though most of the vegetation consisted of grasses and sedges, there were many other species represented. These included saxifrage, parsley, mustard, sagebrush, spruce, birch, juniper, and oak. Some mammals associated with the Black Hills included the mammoth, peccary, wolf, coyote, and the giant short-faced bear.
End of the Ice Age
When the last continental ice sheet retreated, the Northern spruce gradually followed northward. Some remained as relict stands in suitable habitat in the canyons and the valleys of the Black Hills. Pines slowly migrated to the Black Hills from the west. Early Holocene short-grass prairie can be found just southwest of the Black Hills and contains grass species that are also located in the high altitude in the Black Hills. During the Holocene Epoch, a period of extreme arid conditions known as the Altithermal contributed to the rise of more drought tolerant plants, like blue grama and buffalo grass. The vast grassland plains expanded during this period. As the spruce declined, the more xeric ponderosa pine became dominant.
As the composition and distribution of vegetation changed, the distribution of animals also changed. Ranges of many species moved northward with the retreating ice sheet. Animals that could not digest the new coarse vegetation of the grasslands, like the horse, mammoth, rhinoceros, and camel became extinct in North America. Other species, like the bison, deer and elk, moved into the available habitat and survived.
Humans entered the picture in South Dakota during the Pleistocene. Paleontologists debate the possible influence that humans had on the extinction of large mammals like the mammoth. The theories range from the loss of habitat and the decline of forage to over-hunting by man.
The plant composition and distribution of today resulted from the changes in climate, the ability of species to take advantage of newly opened spaces, and the rate of plant population migration. The Black Hills region had an array of habitats available for plants to establish themselves. Animal species adapted to the changing plant community, migrated, or became extinct. Many plant and animal species from the north, south, east, and west that had migrated into the South Dakota region remained. Today the Black Hills is a place of great biodiversity because of this past.
The environment of the present will one day be the paleontology of the future. The topography, climate, and wildlife and plant distributions are still changing. What will the Black Hills be like a million years from now.