Fossils of the Great Plains

Paleozoic fossils

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, which are 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). Cambrian trilobites and brachiopods are also known from rocks in central Montana.

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.

Mississippian Bear Gulch fauna

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.

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. 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, as well as fossils of many beautifully preserved soft-bodied organisms, including arthropods, snails (gastropods), sea stars, nautiloid cephalopods, brachiopods, sponges, worms, and algae.

Cretaceous fossils

Cretaceous marine fossils

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. The Western Interior Seaway was also home to huge marine reptiles—including plesiosaurs, mosasaurs, and turtles, which are frequently found as fossils in Cretaceous rocks in Nebraska and the Dakotas—as well as bony fish, sharks, and sea birds.

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).

Brachiopods declined dramatically in abundance and diversity at the end of the Paleozoic, their ecological niches being filled in many cases by bivalves, or mollusks with shells made up of two valves (parts) connected at a hinge. Examples of modern bivalves include clams, mussels, and oysters. Inoceramids, relatives of living oysters, were diverse and abundant during the Cretaceous. Inoceramids lived on flat parts of the sea floor. The largest species of inoceramid could reach diameters of up to 1.5 meters (5 feet)! 

Ammonoids also became diverse and abundant and are especially common fossils in Cretaceous rocks of the Dakotas, Wyoming, and Montana. The late Cretaceous Pierre Shale, which is exposed widely across this area, is especially famous for its beautifully preserved ammonoids. The slight younger Fox Hills Formation yields equally impressive specimens.

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. 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.

Abundant tiny fossils called foraminifera 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.

Cretaceous terrestrial fossils

Early Cretaceous Lakota Formation

Fossil Cycad National Monument is a national monument that no longer exists. The monument was established in 1922 to protect an unusual concentration of Early Cretaceous fossil cycadeoids (an extinct group of Mesozoic plants that resemble modern cycads) preserved in the Lakota Formation near Minnekahta in southwestern South Dakota. Cycadeoids were plants that had pinnately compound leaves (leaves with a central axis and lateral pinnae or leaflets) and thick stems covered with leaf bases; they produced cones and seeds, but no flowers. The fossil stems, which were once abundant in the Minnekahta region, resemble and are sometimes mistaken for fossil beehives.

Unfortunately, most of the fossil cycadeoids around Minnekahta were collected by fossil dealers and scientists; in fact, most specimens may have been removed before Fossil Cycad National Monument was created. The monument was dissolved in 1957. The Lakota Formation also yields vertebrate fossils, including a few species of dinosaurs.

Late Cretaceous Two Medicine Formation

In the late 1970s and 1980s, nests of dinosaur eggs—some containing embryos—were discovered in rocks of Montana’s Two Medicine Formation. Adult skeletons of the same dinosaur were found as well. The dinosaur was named Maiasaura, meaning “good mother lizard.” Maiasaura was a medium-sized hadrosaur (duck-billed dinosaur) about 8 meters (26 feet) long. It was bipedal and plant-eating. Maiasaura was the first dinosaur ever found associated with nests of babies, and its embryos were the first dinosaur embryos ever found.

One of the sites where these fossils were found was named Egg Mountain; it is located west of Choteau in Teton County, Montana. The fossils showed that Maiasaura made a shallow nest in the ground about 2 meters (6.5 feet) in diameter, in which it laid clutches of 30 to 40 eggs. Because different nests had different-sized babies, paleontologists suggested that the babies were living in the nests when they died and were being cared for by their parents. Maiasaura has only been found in Montana and is the Montana state fossil.

Other dinosaurs found in the Two Medicine Formation include the herbivores like ceratopsians (horned dinosaurs), Gryposaurus (a hadrosaur), Orodromeus, and Scolosaurus (an armored dinosaur). Predators include Daspletosaurus and Troodon; Troodon eggs have also been found. 

Latest Cretaceous Hell Creek and Lance formations

During the latest part of the Cretaceous period (67–66 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. In the northwest-central region, latest Cretaceous environments are preserved in the Hell Creek Formation in Montana, North Dakota, and South Dakota. In Wyoming, the upper part of the Lance (or Lance Creek) Formation is the same age and yields similar fossils.

The sediments deposited in these transitional environments 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. These plant fossils include numerous flowering plants, which greatly diversified during the Late Cretaceous. Terrestrial deposits also contain abundant fossils of land-dwelling animals that lived near the seaway, including insects, freshwater snails and clams, amphibians, turtles, pterosaurs (an extinct group of flying reptiles), birds, and small mammals.

Dinosaur diversity

These formations are most famous for their multiple bone beds containing abundant dinosaurs, including Tyrannosaurus rex, the giant horned Triceratops, the “ostrich dinosaurs” Struthiomimus and Ornithomimus, the armored Ankylosaurus and dome-headed Pachycephalosaurus, and the large hadrosaurs (duck-billed dinosaurs) Edmontosaurus and Anatotitan. They also preserve abundant fossil plants.

Tyrannosaurus rex

The first skeleton of Tyrannosaurus rex was discovered in the Hell Creek Formation of Montana by Barnum Brown in 1902. To date, dozens of additional T. rex specimens have been found (estimates of the total number vary) from as far north as Alberta and Saskatchewan in Canada to as far south as New Mexico and maybe Texas in the U.S. Many T. rex specimens are held by museums in the United States and Canada, although a few are held by institutions overseas and some are owned by private collectors.

Tyrannosaurus rex has captivated people since the early twentieth century due to its enormous size and ferocious appearance. The largest specimen so far discovered, "Scotty" from Saskatchewan, Canada, was more than 12 meters (more than 40 feet) long and may have weighed more than 8,800 kilograms (nearly 20,000 pounds). Tyrannosaurus rex could stand more than 3.6 meters (about 12 feet tall) at the hips. Tyrannosaurus rex is generally thought to have been a predator, although some have argued that it was primarily a scavenger. (It should be noted, however, that many carnivores will both hunt and scavenge.)

Today, the most famous T. rex specimen from the Hell Creek Formation is undoubtedly "Sue." Sue was named for Sue Hendrickson, who discovered the dinosaur in 1990 on the Cheyenne River Reservation near Faith, South Dakota. After a court case to settle who owned the dinosaur, it was sold to the Field Museum of Natural History in Chicago for more than $8 million in 1997. Another T. rex from South Dakota, "Stan," was sold for more than $30 million in 2020. Stan is slated to go on display in Abu Dhabi.

The high prices commanded by these dinosaurs have spurred debate about the commodification of fossils. While some countries (like Canada) prohibit or strictly regulate the collection, sale, and export of fossils, laws governing the collection, ownership, and sale of fossils in the United States depend on who owns the land on which they are found. 

Some other T. rex fossils on display in the U.S. include another of Barnum Brown's specimens at the American Museum of Natural History in New York City and "The Nation's T. rex" (also known as "Wankel's T. rex") at the Smithsonian Institution in Washington, D.C. In the northwest-central region, "Montana's T. rex" (also called "Peck's Rex") is on display at the Museum of the Rockies in Bozeman, Montana. All of these specimens were found in Montana; the Nation's T. rex and Montana's T. rex were found on federal land and are thus publicly owned. Casts (high-quality reproductions) of T. rex specimens are also on display at museums around the country.

Dinosaur mummies

Several dinosaur “mummies” have been found in Cretaceous beds in the northwest-central region. These exceptionally preserved fossils were formed when a dinosaur was buried suddenly, preserving impressions of skin and other traces of soft anatomy. Well-known Edmontosaurus mummies from the Lance Formation of Wyoming are on display at the American Museum of Natural History (New York City) and the Naturmuseum Senckenberg (Frankfurt, Germany). Another Edmontosaurus mummy called "Dakota" from the Hell Creek Formation is held by the North Dakota Heritage Center and State Museum in Bismarck, North Dakota. "Leonardo," a Brachylophosaurus mummy from the Judith River Formation of Montana (the Judith River Formation is similar in age to the Two Medicine Formation and older than the Hell Creek Formation) is on display in the Great Plains Dinosaur Center, Malta, Montana.

Cretaceous-Paleogene (K-Pg) extinction

The boundary between the Cretaceous and Paleogene periods is found at the top of the Hell Creek and Lance formations, separating them from the overlying Paleogene Fort Union Formation. Detailed work in the late twentieth century showed that this boundary is now marked by a concentration of the element iridium. Most geologists think that the iridium came from a large comet or meteorite crashing into the Earth in the Yucatán region of Mexico. This impact was likely the primary cause of the mass extinction that marks the end of the Cretaceous period.

Cenozoic fossils

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. Nevertheless, 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. 

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. 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. 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. North American Land Mammal Ages (NALMAs) are used to describe the relative ages determined by fossil mammals.

Paleogene fossils

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. 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.

Paleogene and early Neogene mammals from the northwest-central region include a great diversity of hoofed mammals, as well as carnivores and primates. Paleocene and Eocene mammals and plants are particularly common and diverse in the Williston Basin of the Dakotas and the Bridger and Uinta Basins of Wyoming and Utah.

White River Group

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. Fossil-bearing Oligocene rocks of the White River Group also extend beyond South Dakota into other states, including Colorado, Nebraska, and Wyoming.

These rocks contain some of the most abundant, diverse, and well-preserved Paleogene fossil mammals found anywhere in the world, such as extinct relatives of modern groups like camels and horses and groups that left no living descendants, including the pig-like entelodonts ("hell pigs") and the sheep-like oreodonts. Oreodonts were the most abundant mammals in the savannah-like environment. Nevertheless, some of the most common vertebrate fossils in Cenozoic sediments in Nebraska and the Dakotas are tortoises. Tortoises are a group of turtles that live on land; they have short, strong legs used for digging burrows. 

Neogene fossils

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 21–19 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, including the small “gazelle-camel” Stenomylus, the short-legged rhinos Menoceras (the first rhino with horns) and Teleoceras, the carnivorous “beardogs” Daphoenodon and Ysengrinia, the fierce-looking giant entelodont (hell pig) 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.

Agate Fossil Beds

Most of the fossils in the Agate Fossil Beds quarries were excavated from a bone bed that is as much as 60 centimeters (2 feet) thick in some areas. Particularly famous are accumulations of skeletons from the rhino Menoceras (see the image at the top of this page). 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.

Quaternary fossils

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. 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.

Giant bison

Bison first appear in North America in the late Pleistocene, around 200,000 years ago, having migrated from Asia across the Bering Land Bridge, a land bridge that existed between eastern Russia and Alaska when sea levels were lower during the Pleistocene. 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! 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.

The Columbian mammoth

Mammoths were close cousins of modern elephants that lived across North America and Eurasia for several million years. At least three of different kinds of mammoths lived in North America until they mostly disappeared around 10,000 years ago (mammoths persisted on some isolated, far northern islands, going completely extinct only about 4000 years ago).

The most familiar kind of mammoth is the woolly mammoth, Mammuthus primigenius, which lived in colder climates close to the glacial front. Woolly mammoths lived in both Eurasia and North America; in North America, their range extended from Alaska to the northeastern United States. The larger Columbian mammoth (Mammuthus columbi) ranged throughout most of the lower 48 U.S. states and into Mexico. The small pygmy mammoth (Mammuthus exilis) was found only on the Channel Islands off the coast of southern California. 

The Mammoth Site

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 camels, llamas, giant short-faced bears, wolves, coyotes, and prairie dogs. 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.