The Big Muddy: The Mowry Sea of the Early Cretaceous Period

Today the broad expanse of the Great Plains, which forms the central part of North America, is a vast area of farms, prairies, and deserts. Here, immense herds of buffalo once roamed and wild mustangs galloped. However, the land did not always look like a swelling sea of grass. During the prehistoric past this was, quite literally, a sea. In fact, the central part of North America has been inundated underwater at several points throughout prehistory.

For example, during the middle to late Jurassic Period, most of central North America was underneath a shallow body of saltwater called the “Sundance Sea”. Here lived oysters, fish, hybodont sharks, belemnites, and at least three species of marine reptiles. When this sea was at its fullest-size about 165-160 million years ago, North America was shaped like a U, with the sea in the middle. However, as sea levels dropped and as tectonic forces pushed the North American landmass upwards above sea level, the Sundance Sea gradually shrank until it completely disappeared by the end of the Jurassic Period. Please see Prehistoric Times issue #112 (Winter 2015) for an article about this Jurassic seaway.

Many people with knowledge of paleontology will recall that later, during the middle to late Cretaceous Period, central North America lay underneath another body of saltwater called the “Western Interior Sea”. This was much larger and deeper than the earlier Sundance Sea, and it completely split North America in half. Here lived famous marine creatures such as the giant plesiosaur Elasmosaurus, the giant mosasaur Tylosaurus, the enormous sea turtle Archelon, and several species of huge carnivorous fish.

However, the formation of the Western Interior Sea was not a quick overnight process; it took many millions of years for the middle of North America to be re-flooded again. Beginning in the early Cretaceous Period, sea levels started to rise again. Soon, the northern parts of North America, in the areas that now form northwestern Canada, gradually became submerged underwater. Over the course of millions of years, sea levels continued to rise until much of central Canada and most of the central United States were at the bottom of the ocean. This northern shallow water body was called the Mowry Sea, and it formed the earliest stage of the development of the Western Interior Sea.

The Mowry Sea is known from multiple layers of rock which were deposited during the early to middle parts of the Cretaceous Period. Each of these layers signifies a time when the sea expanded and shrank. By about 100 million years ago, the Mowry Sea had taken on much of the area that the Sundance Sea had previously occupied sixty million years earlier (1). It would not be until the sea broke through the southern boundary of North America, and turning the continent into two large islands, that the Mowry Sea would become re-named as the Western Interior Sea.

The Skull Creek Shale is the lowest level of marine sediment deposit, and therefore constitutes the earliest stages of the creation of the Western Interior Sea. It is identifiable by a very dark shale, almost black in color, that stands out from the rest of the sediment layers, and measures about 250 feet thick. There have been very few fossils found within this layer, consisting mostly of brachiopod and pelycopod shells as well as isolated fish bones and teeth (2).

The Newcastle Sandstone, which lies on top of the Skull Creek Shale, is a mixture of sandstone, claystone, and siltstone. Paleontologists hypothesize that this layer represents a time when the sea temporarily shrank and the landscape turned into a shallow brackish water estuary, similar to the Mississippi Delta. Fossils found within this layer are more abundant than in the underlying Skull Creek Shale, and consist of the freshwater snail Viviparus and five species of saltwater clams (Corbula, Mactra, Protocardia, Tellina, and Thracia) (3).

The Mowry Shale, which overlies the Newcastle Sandstone, is a dark gray siliceous shale dated to the middle Cretaceous Period (4). This layer can be found in exposed outcrops throughout much of the northern and central Rocky Mountain region (5).

As you can see, much of the geology of the Mowry Sea consists of shale. Shale is formed from clay and silt particles, which remain suspended in the water column for prolonged periods of time until they eventually settle to the bottom. Other rocks within these layers are siltstone and sandstone. So we can assume that the Mowry Sea had very muddy murky water with very poor visibility. The Mowry Sea was also “hypoxic”, which is a fancy way of saying that the water had very little oxygen in it. Modern-day analogies to such an environment would be the Mississippi Delta, the waters of New York Harbor, and the western half of the Long Island Sound (6).

However, in spite of its oxygen-deficient water, the Mowry Sea was not lifeless. Numerous animal fossils have been found within the Mowry Shale. There were at least two species of ammonites named Metengonoceras and Neogastroplites (7). Several species of bony fish have been found here, identifiable mostly through isolated fossil fish scales (8).  One of these was Holcolepis, composed of several species, and possibly resembled a modern-day herring in appearance. Another fish, much larger and much more formidable, was Hypsodon granulosus. Hypsodon was a large bony fish closely related to Xiphactinus, a large carnivorous fish found in the Western Interior Sea, and would have looked similar – so similar, in fact, that Xiphactinus was formerly classified as a species of Hypsodon (9).

Skeleton of Xiphactinus. Photograph by Chip Clark of the Smithsonian Institution (March 8, 2017). Public domain image, Wikimedia Commons.  https://commons.wikimedia.org/wiki/File:Xiphactinus_fossil_SI.jpg.

At least one species of modern shark, Carcharias amonensis, also swam in these waters (10). The genus Carcharias is the same genus as the modern-day Sand Tiger Shark (Carcharias taurus). This shark is distinctive for its ability to gulp air in order to maintain buoyancy (11). However, this habit might have also been helpful if it lived in water with very low levels of dissolved oxygen. Carcharias amonensis existed from the Cenomanian to the Turonian Stages of the Cretaceous Period and its fossils, mostly teeth, have been found in Kansas, South Dakota, Colorado, and Utah (12).

Sand Tiger Shark (Carcharias taurus). Photo by D. Ross Robertson of the Smithsonian Institution (November 11, 2016). Public domain image, Wikimedia Commons.
https://commons.wikimedia.org/wiki/File:Carcharias_taurus_SI.jpg.

The Mowry Sea grew slowly but steadily throughout the Cretaceous Period until, around 90 million years ago, it eventually broke through the southern part of the land and connected itself to the Gulf of Mexico. North America was now split in half, forming the island of Appalachia in the east and Laramidia in the west, and the Western Interior Sea was officially born.

Source citations:

  1. J. D. Stewart and Marjorie Hakel (2006), “Ichthyofauna of the Mowry Shale (Early Cenomanian) of Wyoming”. In New Mexico Museum of Natural History and Science Bulletin 35. Late Cretaceous Vertebrates from the Western Interior. Spencer G. Lucas and Robert M. Sullivan, eds. Albuquerque: New Mexico Museum of Natural History and Science, 2006. Page 161.
  2. Arthur B. Campbell, U. S. Department of the Interior Geological Survey Bulletin 1082-1: Geology and Mineral Deposits of the St. Regis-Superior Area Mineral County, Montana. Washington: United States Government Printing Office, 1960. Pages 647-648.
  3. Arthur B. Campbell, U. S. Department of the Interior Geological Survey Bulletin 1082-1: Geology and Mineral Deposits of the St. Regis-Superior Area Mineral County, Montana. Washington: United States Government Printing Office, 1960. Pages 649-650.
  4. Arthur B. Campbell, U. S. Department of the Interior Geological Survey Bulletin 1082-1: Geology and Mineral Deposits of the St. Regis-Superior Area Mineral County, Montana. Washington: United States Government Printing Office, 1960. Pages 651; J. D. Stewart and Marjorie Hakel (2006), “Ichthyofauna of the Mowry Shale (Early Cenomanian) of Wyoming”. In New Mexico Museum of Natural History and Science Bulletin 35. Late Cretaceous Vertebrates from the Western Interior. Spencer G. Lucas and Robert M. Sullivan, eds. Albuquerque: New Mexico Museum of Natural History and Science, 2006. Page 161.
  5. Douglas A. Sprinkel et al (2012), “Cedar Mountain and Dakota Formations around Dinosaur National Monument: Evidence of the First Incursion of the Cretaceous Western Interior Sea into Utah”. Utah Geological Survey Special Study 143. Page 12.
  6. J. D. Stewart and Marjorie Hakel (2006), “Ichthyofauna of the Mowry Shale (Early Cenomanian) of Wyoming”. In New Mexico Museum of Natural History and Science Bulletin 35. Late Cretaceous Vertebrates from the Western Interior. Spencer G. Lucas and Robert M. Sullivan, eds. Albuquerque: New Mexico Museum of Natural History and Science, 2006. Page 161.
  7. Arthur B. Campbell, U. S. Department of the Interior Geological Survey Bulletin 1082-1: Geology and Mineral Deposits of the St. Regis-Superior Area Mineral County, Montana. Washington: United States Government Printing Office, 1960. Page 652.
  8. J. D. Stewart and Marjorie Hakel (2006), “Ichthyofauna of the Mowry Shale (Early Cenomanian) of Wyoming”. In New Mexico Museum of Natural History and Science Bulletin 35. Late Cretaceous Vertebrates from the Western Interior. Spencer G. Lucas and Robert M. Sullivan, eds. Albuquerque: New Mexico Museum of Natural History and Science, 2006. Page 161.
  9. T. D. A. Cockerell (1918), “Some American Cretaceous Fish Scales, with Notes on the Classification and Distribution of Cretaceous Fishes”. U. S. Geological Survey. Shorter Contributions to General Geology, professional paper 120. Pages173-176; 177-179.
  10. Douglas A. Sprinkel et al (2012), “Cedar Mountain and Dakota Formations around Dinosaur National Monument: Evidence of the First Incursion of the Cretaceous Western Interior Sea into Utah”. Utah Geological Survey Special Study 143. Page 12.
  11. The Ultimate Guide: Sharks. The Discovery Channel, 1996.
  12. Douglas A. Sprinkel et al (2012), “Cedar Mountain and Dakota Formations around Dinosaur National Monument: Evidence of the First Incursion of the Cretaceous Western Interior Sea into Utah”. Utah Geological Survey Special Study 143. Page 12; Cretaceous Atlas of Ancient Life: Western Interior Seaway. “Carcharias amonensis”. https://www.cretaceousatlas.org/species/carcharias-amonensis/.

Bibliography:

Books:

  • Campbell, Arthur B. U. S. Department of the Interior Geological Survey Bulletin 1082-1: Geology and Mineral Deposits of the St. Regis-Superior Area Mineral County, Montana. Washington: United States Government Printing Office, 1960.

Articles:

  • Cockerell, T. D. A. (1918). “Some American Cretaceous Fish Scales, with Notes on the Classification and Distribution of Cretaceous Fishes”. U. S. Geological Survey. Shorter Contributions to General Geology, professional paper 120. Pages 165-203. https://pubs.usgs.gov/pp/0120i/report.pdf.
  • Sprinkel, Douglas A.; Madsen, Scott K.; Kirkand, James I.; Waanders, Gerald L.; Hunt, Gary J. (2012). “Cedar Mountain and Dakota Formations around Dinosaur National Monument: Evidence of the First Incursion of the Cretaceous Western Interior Sea into Utah”. Utah Geological Survey Special Study 143. Pages 1-20. https://ugspub.nr.utah.gov/publications/special_studies/SS-143/SS-143.pdf.
  • Stewart J. D.; Hakel; Marjorie (2006). “Ichthyofauna of the Mowry Shale (Early Cenomanian) of Wyoming”. In New Mexico Museum of Natural History and Science Bulletin 35. Late Cretaceous Vertebrates from the Western Interior. Spencer G. Lucas and Robert M. Sullivan, eds. Albuquerque: New Mexico Museum of Natural History and Science, 2006. Pages 161-163.

Websites:

Videos:

  • The Ultimate Guide: Sharks. The Discovery Channel, 1996.


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