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.
Precambrian
Era
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 formed deep below the surface.
Origin of life
Life first stirred
in the vast Precambrian seas. Life began as single-celled and noncellular
organisms, resembling modern day bacteria and algae. Multicellular
forms evolved from these first organisms.
Paleozoic
Era
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.
Age of Fish
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 settled on the sea floor, dissolved, 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 dwelling diatoms and
sponges 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.
Vast tropical
lowlands
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.
Mesozoic
Era
Age of Reptiles
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 Xiphctinus, an early member of the teleost fish
group which today includes salmon. Herperornis, 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
turtle remains have been found near Buffalo Gap. In the Badlands
to the east, the remains of octopus-like ammonites have been uncovered.
Ancestral Rocky
Mountains
During the Pennsylvanian
Period, the ancestral Rocky Mountains rose and were erodes 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 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."
Cenozoic
Era
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 form 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 climate trend brought some species to extinction and
allowed northward colonization for others. Plant species and plant communities
became increasingly like those of the present.
Paleocene and
Eocene Epochs
During the early
Tertiary Period (Paleocene and Eocene Epochs), North America's climate
was semi-tropical. Broad-leafed 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.
Miocene and Pliocene
Epochs
The trend toward
a more arid climate continued during the Miocene and Pliocene Epochs.
The Rocky Mountains were nearly at their highest elevation and intercepting
most of the moisture from the Pacific. The Miocene plant community
looked more like today's plant community. In the Northern Great plains
region, grazing and browsing mammals were found together in savanna-type
habitat. 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 America'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 shortgrass 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 arrive
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 overhunting 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? |
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