Deep Time Snapshot

Miocene and Mammoths

Ever wonder what it’s like to meet a mammoth? It was possible—only a few tens of thousands of years ago on the Great Plains. We call this epoch in time the Pleistocene, spanning from 2.6 million to 11,700 years ago. In the span of geologic time, this is a miniscule segment in the history of Earth. The world was in the middle of an extreme ice age, and North America was almost in the same position we see it now. North America was also host to the largest ice sheet at the time: the Laurentide ice sheet, a forceful tool that carved and shaped the landscape as it spread and flowed.

The Pleistocene is famous for its fauna, including Smilodon, the saber-toothed cat that was 30 percent bigger than a modern African Lion. The Smilodon is one of the most extraordinary cats to have ever lived, thanks to its incredible musculature, jaw strength, and extra-long canine teeth. Paramylodon, the cousin of the South American Megatherium (which became the namesake of a research club in the 1870s—look out for this post!), was a giant ground sloth. Unlike today’s sloths, they walked on the ground on feet sporting claws 18 centimeters long and lived and reproduced in caves. Glyptodons were a weird animal that looked like a combination of an armadillo and a tortoise. It had a six-foot long tail and possibly an elephant-like trunk. Animators made use of these quirky creatures in their film, Ice Age.

These, among other animals of the Pleistocene, are not only found in the fossil record of the Great Plains, but also in the unusually diverse La Brea tar pits. Thousands of excellently preserved specimens have been found in the gooey oil-tar underneath Los Angeles, California. After a few animals got stuck, their plight attracted predators interested in an easy meal. Some of the predators got stuck themselves, bringing in even more critters, creating a cycle. As a result, the tar pits have provided a snapshot of life in the Pleistocene, life that could be seen not just in (not so) sunny California, but across the continent.

Even more well-known are mammoths—some of the largest mammals ever walking the Earth. In the cold, harsh environment of the ice age, mammoths found refuge in a stretch of grasslands near the Sierra Nevada Mountains. Columbian mammoth remains have been found near ancient equine, bison, and sheep remains in the Upper Missouri Basin, the Rocky Mountains, the Yellowstone Basin and more. Basically, if there was vegetation available for eating, mammoths lived there. Some studies have even found mammoth limb bones showing evidence of human modification—giving light to the claim that early humans shared a world with these giant animals.


Featured image courtesy of U.S. National Archive:

Mt. Moran and Jackson Lake from Signal Hill, Grand "Teton National Park," Wyoming.


See the following sources for more info:


Flowers, Mark., Mags. Lightbody, Dominic. Frisby, and British Broadcasting Corporation. Ice Age Giants. Land of the Sabre-tooth. VAST: Academic Video Online. London, England: British Broadcasting Corporation (BBC), 2013.

Hill, Christopher L. “Stratigraphic and Geochronologic Contexts of Mammoth (Mammuthus) and Other Pleistocene Fauna, Upper Missouri Basin (northern Great Plains and Rocky Mountains), U.S.A.” Quaternary International 142 143 (2006): 87.

Holen, Steven R. “Taphonomy of Two Last Glacial Maximum Mammoth Sites in the Central Great Plains of North America: A Preliminary Report on La Sena and Lovewell.” Quaternary International 142 143 (2006): 30.

Deep Time Snapshot

The Cretaceous Interior Seaway

Jurassic Park helped to give dinosaurs their fame, but this really wasn’t when dinosaurs ruled the Earth. The Cretaceous period, starting about 145 million years ago, was the real heyday of our giant mesothermic friends (most dinosaurs were not ectothermic, or cold-blooded!). Until their extinction around 66 million years ago, dinosaurs were spread across the globe, including in the Great Plains region. At this point, Pangea had broken apart and North America was starting to look like the massive continent we know today.

Even more interesting, the North American landmass was home to a Cretaceous inland sea. The Western Interior Seaway was a result of incredibly high sea level, where a shallow ocean covered a massive part of the continent. “At its maximum extent, the seaway extended for 4,800 km from the North Slope of Alaska to northern Mexico and was approximately 1,620 km wide from central Utah to Minnesota,” according to a U.S. Geological Survey report. This includes much of the Great Plains region.

Evidence of this inland sea lies in the paleontology—the fossils—of the region. Geologists have found an incredibly diverse variety of marine fossils all over the landlocked states, even here in Arizona. The fossils found are not just shallow water creatures, though some extensive coral reef remains have been studied. Giant turtles, ammonites, crinoids, sharks and fishes have been discovered in areas that are now deserts and grasslands. On top of that, giant marine beasts resembling the Loch Ness monster, like Mosasaurs and Plesiosaurs who needed large expanses of salty ocean water to survive, have been found. Right along side these are the bones and tracks of dinosaurs who tromped on the muddy shores.

The rock that hosts the marine fossils also tells the tale of deep ocean covering North America. Chalk, limestone and dolostone are carbonate rocks, created when microscopic sea-critters died and accumulated on the ocean floor, eventually compacting into solid rocks.

By the end of the Cretaceous, the Rocky Mountains were beginning to form, and the Western Interior Seaway was retreating. The dropping sea level and receding inland sea led way to swampy environments, and later, the formation of coal.


Featured photo courtesy of U.S. National Archives:

Aerial View Upstream of Boom Site in Monument Valley, Utah Where Oil Spill Into the San Juan River Was Contained before Flooding Caused Overflow of Oil and Debris Into Lake Powell, 10/1972


See the following sources for more info:

Roberts, Laura N. Robinson, Laura N. Robinson. Kirschbaum, Kirschbaum, Mark A., and Geological Survey, Issuing Body. Paleogeography of the Late Cretaceous of the Western Interior of Middle North America: Coal Distribution and Sediment Accumulation. U.S. Geological Survey Professional Paper; 1561. Washington: Denver, CO: U.S. G.P.O.; U.S. Geological Survey, Information Services, 1995.

Deep Time Snapshot

Early History—Plains Evolution

The Earth formed with misty beginnings—most of its history is lumped into a section of time called the Precambrian. The Precambrian includes both the Archean and Proterozoic eons, which span from the formation of Earth about 4.6 billion years ago to the beginning of the Phanerozoic eon 541 million years ago. The first signs of life have been found as fossilized evidence from the Precambrian, before humans, dinosaurs and even most multicellular life existed. Earth’s atmosphere had not yet developed the oxygen-rich characteristics observed today, leaving these organisms to survive on anaerobic processes. The landmasses, though different than the modern continents, formed early and were molded by an episodic cycle throughout deep time.

That’s right, Pangea was not the only supercontinent—only the most recent. Evidence shows that six major supercontinents formed and disassembled long before Pangea, with other more minor assemblages occurring in between. The landmass we call North America was active in the continuing supercontinent cycle of Earth, though at first it was combined with other bits of land like Scotland and Greenland under a different name, Laurentia. As the continents crashed together and were pulled apart, the shape of Laurentia evolved into the familiar pattern of North America, including the ancient Great Plains region.

Things got exciting in the time after the Precambrian, starting with the Cambrian explosion. Often referenced for its trilobites (the ancient cockroaches of the sea), the Cambrian is famous for its detonation in the diversification of life. By the 500-million-year mark, the first marine vertebrates began to appear, and about 100 million years later, the first evidence of plants on land. The Phanerozoic era was the jump start of all life on Earth today, leaving behind some of the world’s most extensive and well-preserved paleontological deposits. Some of these, like the House Range in Utah and Mazon Creek in Illinois, are still treasure troves nestled into the vast expanses of the Great Plains.

Animals began the transition onto land around 360 million years ago; mammals did not appear until about 200 million years ago. Humans and their most recent ancestors have only been around for a fraction of that time: approximately 200,000 years, the tail end of the Cenozoic era. The earliest records suggest that Paleo-Indians may have arrived in the Great Plains region 15 thousand years ago, some even extending that number to 38,000 years ago. Carbon-dated remains from Wyoming provide evidence of humans at least as early as 8,500 BC. This range has led to the belief that humans may have shared time with now extinct megafauna—mammoths, giant sloths, and saber-toothed cats.


Featured photo courtesy of U.S. National Archives:

Fossils Are Seen Along Trail from Spanish Bottom, in the Maze, 05/1972

Find the photo here


See the following sources for more info:

“Chapter 7 – The Supercontinent Cycle.” In Earth as an Evolving Planetary System, 201-35. 2016.

Mac Niocaill, Conall, and Mark A. Smethurst. “Palaeozoic Palaeogeography of Laurentia and Its Margins: A Reassessment of Palaeomagnetic Data.” Geophysical Journal International 116, no. 3 (1994): 715-25.

Wedel, Waldo R. “Prehistory and Environment in the Central Great Plains.” Transactions of the Kansas Academy of Science (1903-) 50, no. 1 (1947): 1-18.