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:

 

See the following sources for more info:

http://geology.teacherfriendlyguide.org/index.php/fossils-sc/fossils-region4-sc

https://www.geosociety.org/documents/gsa/timescale/timescl.pdf

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.

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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:

Find the photo here

 

See the following sources for more info:

http://plainshumanities.unl.edu/encyclopedia/doc/egp.na.001

http://palaeo.gly.bris.ac.uk/palaeofiles/lagerstatten/

http://www.ucmp.berkeley.edu/paleozoic/paleozoic.php

https://www.britannica.com/science/geologic-time

“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.