Region 2: The Great Plains

Paleozoic rocks occur at the surface in the Great Plains only because of tectonic forces that have uplifted the younger rocks and caused them to erode, exposing the older rocks beneath. The Black Hills of western South Dakota are a particularly striking example of this phenomenon. The center of the Black Hills consists of Precambrian igneous rocks, surrounded by a rim of Paleozoic sedimentary rocks. The oldest sedimentary rock formation in the Black Hills is the Deadwood Formation, a layer of late Cambrian sandstone that outcrops around the town of Deadwood. The Deadwood Formation contains abundant marine fossils, including trilobites, brachiopods, trace fossils (burrows) (Figure 3.14), and bony plates from one of the oldest known armored fishes (Anatolepsis). Cambrian trilobites and brachiopods are also known from rocks in central Montana (see Figure 3.2).

The Ordovician Whitewood and Mississippian Pahasapa limestones overlie the Deadwood, forming concentric rings farther from the core of the Black Hills. These younger layers also contain abundant and diverse marine fossils, including corals, snails, and cephalopods (Figure 3.15). Mississippian-aged rocks in this part of the Great Plains are correlated—determined to be the same age—mostly by using tiny fossils called conodonts as index fossils. Mississippian-aged rocks in North Dakota include the Bakken Shale, which is an important oil-producing layer. The oil comes from the altered remains of organisms that lived in a shallow sea.

Conodonts

Conodonts are tiny, tooth-shaped microfossils (0.2 - 5 millimeters long), found in Cambrian- through Triassic-aged marine rocks. They have long been among the most important index fossils in these rocks, allowing the latter to be dated through biostratigraphy. For many years, paleontologists did not know what kind of animal they belonged to, but in 1983 the discovery of a whole conodont animal in Scotland revealed that they belonged to small, fish-like animals that were distant relatives of bony fish.

Isolated conodont elements (Silurian).

Restoration of a live conodont animal. Length 2 - 4 centimeters (1 - 2 inches).

The Mississippian Bear Gulch Beds of central Montana reveal a rare preservational “window” into the marine life of this time. The Bear Gulch Beds consist of layers of fine-grained limestone (similar to the Jurassic Solnhofen limestone in Germany that preserves many spectacular fossils, including Archaeopteryx, the oldest known bird). These rocks are exposed at the surface only because of the Potter Creek Dome, an uplifted outcrop located about 30 kilometers (18.6 miles) northeast of the Big Snowy Mountains in Fergus County, Montana. Bear Gulch preserves one of the most diverse fossil fish assemblages in the world (Figure 3.16), as well as fossils of many beautifully-preserved soft-bodied organisms, including arthropods, snails (gastropods), sea stars, nautiloid cephalopods, brachiopods, sponges, worms, and algae (Figure 3.17).

In yet another concentric band even farther from the core of the Black Hills, fossiliferous early Jurassic-aged rocks outcrop in in eastern Wyoming, eastern Montana, and western South Dakota. These Jurassic sediments eroded from the highlands to the north and west, and record several cycles of sea level rise and fall across the region. These limestones, sandstones, and shales are rich in fossil cephalopods, oysters, and other marine invertebrates (Figure 3.18).

By the late Jurassic, the shallow sea had begun to retreat to the east, and marine deposits of the middle Jurassic were replaced by deltas and freshwater deposits. The Morrison Formation is a layer of late Jurassic-aged rock exposed across a wide swath of the Rocky Mountains and Great Plains (Figure 3.19) The silty sediments of the Morrison were deposited by eastward-flowing rivers sweeping across broad, swampy floodplains, and contain extraordinary accumulations of dinosaur bones, as well as fossils of land plants including conifers, cycads, and ginkgoes, and also fish, frogs, lizards, crocodiles, turtles, and small mammals. The Morrison Formation’s abundant dinosaurs include some of the most famous, such as Apatosaurus, Stegosaurus, Allosaurus, Diplodocus, Camarasaurus and many more (Figure 3.20).

Figure 3.19: Geographic extent of the Jurassic-aged Morrison Formation.

In the Cretaceous, global sea levels rose, spreading shallow epicontinental seas over much of the continent. The Western Interior Seaway stretched across the center of North America from the Gulf of Mexico to the Arctic Ocean, and from the foot of the still-forming Rocky Mountains to as far east as Iowa. It covered most of the Dakotas and Nebraska, as well as eastern Montana, east-central Wyoming, and eastern Colorado. An abundance of aquatic life thrived there for tens of millions of years, and most of the bedrock in those states is of Cretaceous age. Over the course of the Cretaceous, the shores of this seaway swept back and forth, resulting in the deposition of alternating layers of marine and terrestrial rocks. Deeper waters toward the center of the Seaway led to the deposition of chalk—a carbonate rock made up primarily of the shells of microscopic marine algae, called coccolithophores (Figure 3.21). Today, such sediments accumulate mainly in the deep sea; during the Cretaceous, when sea levels were much higher than today, chalk accumulated throughout the extensive shallow inland seas. The Cretaceous period is in fact named for the abundance of chalk that accumulated during this time. (The Latin word for chalk is creta.) The Western Interior Seaway was also home to huge marine reptiles, including plesiosaurs, mosasaurs, and turtles (Figures 3.22 - 3.24), which are frequently found as fossils in Cretaceous rocks in Nebraska and the Dakotas, as well as bony fish, sharks, and sea birds (Figure 3.25).

Figure 3.21: A microscopic view of chalk, showing that it is composed almost completely of the shells of protists called coccolithophores. Scale bar = 4 nanometers (4 x 10-9 meters; about 0.0000001575 inches).

Marine invertebrates in these Mesozoic seas were very different from those that had filled the seas of the Paleozoic. Rugose and tabulate corals were replaced by scleractinians—modern corals (Figure 3.26). Brachiopods declined dramatically in abundance and diversity at the end of the Paleozoic, their ecological niches being filled in many cases by bivalves. In the Cretaceous, two bizarre groups of clams were particularly abundant: rudists formed reefs, while inoceramids lived on flat parts of the sea floor (Figures 3.27 - 3.28). Inoceramus was a large, usually flat, thick-shelled bivalve with tightly interlocking shells. The largest species could reach diameters of up to 1.5 meters (5 feet)! Inoceramids were relatives of living oysters—among today’s most common and well-known bivalves that cement themselves to the bottom—and were diverse and abundant during the Cretaceous. Ammonoids also became diverse and abundant, and are especially common fossils in Cretaceous rocks of the Dakotas, Wyoming, and Montana (Figure 3.29). The late Cretaceous Pierre Shale, which is exposed widely across this area, is especially famous for its beautifully preserved ammonoids. Most ammonoids are coiled flat, in a single plane. One fascinating aspect of ammonoid evolution, however, was the appearance of shells with bizarre shapes, called heteromorphs (“different shape”). These unique ammonoids were especially prevalent in the Cretaceous period. The shells of heteromorphs were uncoiled or three-dimensionally (helically) coiled (see Figure 3.29B - D). Since there are no similar life forms today to which to compare them, it has been difficult to figure out how they lived—most current paleontological thinking suggests heteromorphs floated or swam.

Figure 3.27: Rudists were unusual cone- or cylinder-shaped bivalves that clustered together in reef-like structures and went extinct at the end of the Mesozoic era. They ranged in size from a few centimeters to more than 50 centimeters (1.5 feet) tall.

Ammonoids

Ammonoids are a major group of cephalopods that lived from the Devonian to the end of the Cretaceous. Both nautiloids (the group that today contains the chambered nautilus) and ammonoids have chambered shells subdivided by walls, or septa (plural of septum). These shells are frequently, but not always, coiled. The term “ammonoid” refers to the larger group of these extinct cephalopods, distinguished by complex folded septa. Within ammonoids, “ammonites” is a smaller sub-group, distinguished by the extremely complex form of their septa. Ammonites were restricted to the Jurassic and Cretaceous periods. The form of the septa in nautiloids and ammonoids is not visible in a complete shell; it is most often seen in the trace of the intersection between the septum and the external shell. This trace is called a suture. Sutures are usually visible in fossils when sediment has filled the chambers of a shell, and the external shell has been broken or eroded away.

Ammonite shell break-away cross-section; surface plane of a septum and sediment-filled chamber.

Abundant tiny fossils called foraminifera (Figure 3.30) are found throughout Cretaceous sediments of the Western Interior Seaway. Foraminifera, or “forams,” as they are frequently called, are single-celled organisms (protists) with shells made of calcium carbonate. They live in the ocean in huge numbers, both at the bottom and floating in the water column, and are extremely important as index fossils and paleoenvironmental indicators.

During the late Cretaceous period (67 - 65 million years ago), the area that is now southeastern Montana, northeastern Wyoming, and northwestern South Dakota was a broad floodplain to the east of the developing Rocky Mountains, leading into the shallow marine Western Interior Seaway. The sediments deposited in these transitional environments (Figure 3.31) contain the remains of organisms that lived both on land and in the sea. The terrestrial layers, deposited by meandering rivers, contain abundant plant fossils—including numerous flowering plants (Figure 3.32), which had just begun to colonize the landscape. Terrestrial deposits also contain abundant fossils of land-dwelling animals that lived near the seaway, including insects, freshwater snails and clams, turtles, pterosaurs, small mammals (Figure 3.33), birds, amphibians, and, most famously, dinosaurs (Figure 3.34).

In the early twentieth century, the first skeletons of Tyrannosaurus rex were discovered in one of these floodplain deposits, the Hell Creek Formation. The boundary between the Cretaceous and Paleogene periods was also identified at the top of this sandstone layer. Detailed work in the late twentieth century refined the placement of the boundary; it is now marked by a concentration of the element iridium, usually at the top of the Hell Creek but sometimes in the lower part of the overlying Fort Union Formation (Figure 3.35). The iridium is thought by most geologists to have come from the impact of a large comet or meteorite, which was likely a primary cause of the mass extinction that marks the end of the Cretaceous period.

Figure 3.31: Simplified stratigraphy of the western margin of the Western Interior Seaway across Montana and North Dakota during the late Cretaceous period.

The Hell Creek Formation is most famous for its multiple bone beds containing abundant dinosaurs, including (in addition to T. rex) the giant horned Triceratops, the “ostrich dinosaurs” Struthiomimus and Ornithomimus, the armored Ankylosaurus and dome-headed Pachycephalosaurus, and the large hadrosaurs Edmontosaurus and Anatotitan (see Figure 3.34). Several dinosaur “mummies” have also been found in the Hell Creek beds (Figure 3.36). These exceptionally preserved fossils were formed when a dinosaur was buried suddenly, preserving impressions of skin and other traces of soft anatomy.

Figure 3.35: The Cretaceous - Paleogene (K - Pg) boundary (red arrow) along Interstate 25, Raton Pass, Colorado.

The early and mid-Cenozoic was a time of significant tectonic activity in the Northwest Central States. The Western Interior Seaway disappeared by the end of the Paleocene, and most of the region became dry land, yet extensive sedimentary deposits represent lakes, rivers, and floodplains. The uplift of mountain ranges to the west was a source of sediment that was transported and deposited by rivers in basins throughout the Great Plains and Rocky Mountain regions. Thick ash beds were also deposited in this area by periodic volcanic eruptions throughout the Cenozoic. The shrinking Western Interior Seaway is represented in the Dakotas by the Paleocene Cannonball Formation, which contains abundant and diverse mollusks, shark teeth, and occasional land plants (Figure 3.37).

Cenozoic rocks preserve a fossilized view of ecosystems dramatically different from those of the Cretaceous. The dinosaurs disappeared, and mammals replaced them as the dominant large vertebrates on land. During this time, the environment was initially warm and humid, with widespread tropical and subtropical forests, but as global temperatures fell in the late Eocene and Oligocene, the climate of the Northwest Central became more arid, and grasslands replaced forests in many areas. Fossil land mammals are particularly common in Cenozoic sediments of the Great Plains, beginning with the Paleocene and Eocene, and continuing through the Miocene. These fossils can be found in thick sequences of layered sedimentary rock, which accumulated in lakes and rivers that fed into the numerous basins across the region (Figure 3.38). The abundant fossil mammals preserved in these rocks form one of the most complete records of mammal evolution known anywhere in the world. Mammals are so numerous and diverse here that they are commonly used as index fossils. (It is unusual to use vertebrates for biostratigraphy, because they are frequently much rarer than invertebrate fossils.) There is even a special series of terms—known as the North American Land Mammal Ages—used to describe the relative ages determined by fossil mammals.

Figure 3.38: Sedimentary basins of the Northwest Central.

Paleogene and early Neogene mammals from this region include a great diversity of hoofed mammals, as well as carnivores, and—surprisingly— primates (Figures 3.39 - 3.40). Paleocene and Eocene mammals, as well as fossil plants, are particularly common and diverse in the Williston Basin of the Dakotas and the Bridger and Uinta Basins of Wyoming and Utah (Figure 3.41). Oligocene mammals are best known from the White River Badlands—an area of western South Dakota with a deeply eroded landscape, through which the White River flows. The rocks here are late Eocene to Oligocene in age (about 40 - 30 million years old), and were deposited by rivers moving through an environment much warmer and wetter than it is today. These rocks contain some of the most abundant, diverse, and well-preserved Paleogene fossil mammals found anywhere in the world (Figure 3.42), including extinct relatives of modern groups, such as camels and horses; groups that left no living descendants, such as the pig-like entelodonts; and the sheep-like oreodonts, which were the most abundant mammals in this savannah-like environment.

Miocene mammals are abundantly preserved in a number of deposits, including the spectacular Agate Fossil Beds of western Nebraska, and the Ashfall Fossil Beds of northeastern Nebraska. Many of these fossil beds formed as a result of rapid burial in volcanic ash. The spectacular fossils exposed at Agate Fossil Beds National Monument and Ashfall Fossil Beds State Historical Park reveal that during the early Miocene (about 2119 million years ago), this area was a grassy savanna, similar to those in today’s East Africa. Miocene Nebraska was filled with diverse and abundant large mammals (Figure 3.43), including the small “gazelle-camel” Stenomylus, the short-legged rhino Menoceras (the first rhino with horns), the carnivorous “beardog” Daphoenodon, the fierce-looking giant entelodont Daeodon (formerly Dinohyus), the bizarre clawed herbivore Moropus, and the “land beaver” Paleocastor, which dug spectacular long spiral burrows known today as the trace fossil Daemonelix (Figure 3.44).

Most of the fossils in the Agate Springs quarries were excavated from a bone bed that is as much as 60 centimeters (2 feet) thick in some areas. Particularly famous are clusters of skeletons from the rhino Menoceras (Figure 3.45). These animals may have preferred to spend most of their time lying in shallow ponds or stream courses. When a multi-year drought occurred and the food supply disappeared, the Menoceras remained at the waterhole where they died. When water flowed again, the seasonal stream buried their bodies beneath layers of mixed sandy sediments and volcanic ash.

Some of the most common vertebrate fossils in Eocene and Oligocene sediments in Nebraska and the Dakotas are not mammals, however, but tortoises. Tortoises are a group of turtles that live on land, and have short, strong legs used for support and digging burrows. In contrast, most turtles live in the water and have webbed feet to help them swim efficiently, but will venture onto land occasionally to lay eggs. Two of the most common types of fossil tortoise are Stylemys from the Oligocene of Nebraska and Hesperotestudo from the Miocene of Nebraska and Kansas (Figure 3.46).

The cooling temperatures that affected all of North America at the beginning of the Pleistocene epoch, around 2.5 million years ago, brought an influx of new mammals to the Great Plains. These included mammoths, rodents, bison, and musk oxen.

Bison first appear in North America in the late Pleistocene, around 200,000 years ago, having migrated from Asia across the Bering Land Bridge. The oldest and largest of the several species that evolved here was the “giant bison,” Bison latifrons, which had horns measuring up to seven feet tip-to-tip, a shoulder height of 2.5 meters (8.2 feet), and may have weighed over 2000 kilograms (4400 pounds)—up to twice the size of the modern American bison (Figure 3.47). The giant bison became extinct around 20,000 - 30,000 years ago, at the beginning of the last glacial maximum. Bones of this species, as well as those of other extinct species such as Bison antiquus, are common throughout the Great Plains, especially in Nebraska.

Mammoths were close cousins of modern elephants that lived across North America and Eurasia for several million years. A number of different kinds of mammoths lived throughout North America (including the Rocky Mountains and Great Plains regions), until they became extinct around 10,000 years ago. The most familiar kind of mammoth is the woolly mammoth, Mammuthus primigenius, which lived in colder climates close to the glacial front (Figure 3.48). Farther south, other large mammoths were abundant. The Columbian mammoth, Mammuthus columbi (Figure 3.49), and the imperial mammoth, Mammuthus imperator, were previously thought to have been separate species, but paleontologists have recently concluded that they actually belonged to the same species. At Mammoth Site in Hot Springs, South Dakota, the skeletons of at least 60 mammoths (mostly Columbian) are preserved together with the bones of many other ice age mammals, including camel, llama, giant short-faced bear, wolf, coyote, and prairie dog. This extraordinary fossil assemblage formed around 26,000 years ago, when a cavern in the Minnelusa Limestone collapsed, creating a sinkhole into which the animals fell.

Mastodons and Mammoths

These two kinds of ancient elephants (or, more technically, proboscideans) are frequently confused. Both were common during the Pleistocene, but they had different ecological preferences and are usually found separately. Mammoths are close cousins of modern African and Asian elephants; mastodons are more distant relatives, from a separate line of proboscideans that branched off from the modern elephant line in the Miocene. Mastodons have a shorter, stockier build and longer body; mammoths are taller and thinner, with a rather high "domed" skull. In skeletal details, the quickest way to tell the difference is with the teeth: mastodons have teeth with conical ridges, a bit like the bottom of an egg carton; mammoths, in contrast, have teeth with numerous parallel rows of ridges. The teeth are indicative of the two species' ecological differences. Mastodons preferred to bite off twigs of brush and trees, while mammoths preferred tough siliceous grasses. Thus, mastodon teeth are more suitable for cutting, while mammoth teeth are more suitable for grinding.

A mastodon tooth, suitable for chewing twigs and tree leaves. About 20 centimeters (8 - 9 inches) long.

A mammoth tooth, suitable for grinding grass and soft vegetation. About 25 centimeters (almost a foot) long.

Large mammals are not the only life forms preserved from the ice age. Freshwater and land mollusks are common to abundant in many soft sediment deposits across the Great Plains (Figure 3.50).