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Vista across Dyer Formation outcrops, White River National Forest, Colorado. DMNS photograph.

Denver Museum of Nature and Science

High-Altitude Mass Extinction

At 11,000 feet of elevation in Colorado's Rocky Mountains are remains of a 360 million-year-old tropical sea. The bounty of this sea is preserved in fossils and limestone rocks tucked away in a remote corner of the White River National Forest. Looming over the chiseled gorge of Glenwood Canyon, these rocks are little known except by local geologists and fossil hunters. We've begun to study these strata because they were deposited at the end of the Devonian Period, when one of Earth's biggest extinction events occurred. As deep-time detectives, scientists at the Denver Museum of Nature and Science are using the fossils, minerals, and structures in these rocks to learn why and how this ancient ecosystem met its unfortunate demise.

The key geologic clues are housed in a suite of rocks called the Dyer Formation. The Dyer is remarkable because its strata provide evidence of what lagoons, tidal flats, beaches, and sub-tidal bays were like prior to and during the end-Devonian extinction event. The fossils from each of these environments provide snapshots of life from different parts of a giant extinct coastal system. By walking along and up through the stairstep-like exposures of the Dyer Formation, we can explore this ancient ecosystem, determining how sea life varied along the coast, and how its ecosystems changed through time. Some of the fossils that provide clues to ancient animals that inhabited this system include armored fish plates, shark teeth, conodonts, brachiopods, sponges, stromatolites, stromatoporoids, gastropods, bryozoans, and cephalopods. Many of these creatures lived atop the seafloor whereas others burrowed underground or swam overhead.



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Dyer Formation fossils (from left): shark tooth, conularid, snail. DMNS photographs.

Based on studies of similar rocks exposed in nearby areas of Utah, Wyoming, and New Mexico, geologists have determined that the Dyer Formation's inhabitants lived in a shallow inland sea that extended eastward from a large Pacific-sized ocean. This sea existed at a time when the continent of North America straddled the equator. Sometimes this is hard to imagine when one gets caught in a July snowstorm atop one of the Dyer Formation's 11,000-foot high outcrops! These tropical, equatorial seabeds were long ago uplifted during the formation of the Rocky Mountains, and are now exposed on one of Colorado's highest, snowiest, and paleontologically most beautiful plateaus.



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More Dyer Formation fossils (from left): seastar, sponge, turban snail, bryozoan fossils. DMNS photographs.


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Plate from giant armored fish; arrow indicates where it fits into skull. DMNS photographs.

Mass Extinctions: When Life Takes a Sudden Right Turn

To decipher the story of the Dyer Formation, we need to know a bit about extinctions. In the history of life on earth, new types of organisms constantly arise due to genetic mutations. The process in which new types of organisms, or species, arise is called 'speciation' (a species is a scientific name for an organism, such as Homo sapiens or Tyrannosaurus rex). In contrast, some types of organisms die, never to return again. This process is called extinction, and is always occurring.

Like the background hum of a highway or the din of passenger chatter in an airport, the everyday extinguishing of life is called "background extinction". The average longevity for individual species is around two million years. Yet occasionally watershed moments occur, when global catastrophes wipe out myriad species at a rate that is much faster than they can be replaced by new species. These massive and rapid kill-offs are called mass extinction events—periods of time when the rates of extinction massively outstrip levels of background extinction. There is neither a set formula that defines mass extinction nor specific time frame in which it plays out. Some occur geologically rapidly, like an ancient thermonuclear winter whereas others are slower, reflecting geologic-scale suffocation events.

Fortunately life always recovers, with Earth's environments being repopulated—but with ecosystems being occupied by a different cast of characters than inhabited them before the extinction event. Since animals arose approximately 550 million years ago, there have been five major mass extinctions:

Extinction Event
Millions of Years Ago
Era
Estimated Species Lost
End Ordovician
444
Paleozoic
60%-70%
Late Devonian
374-358
Paleozoic
70%
Late Permian
251
Paleozoic
90%
End Triassic
200
Mesozoic
70%-75%
End Cretaceous ("K-T")
65
Mesozoic
75%

Late Devonian Extinction: A "Perfect Storm" of Complex Global Events?

The Late Devonian extinction occurred in several pulses, with the latest and perhaps grand-daddy of them all occurring during what geologists call the Famennian stage of the Devonian Period. It is this Famennian extinction that is hypothesized to be contained in the uppermost strata of the Dyer Formation.

The cause of the Devonian's end-Famennian extinction is not known. Several events may have contributed, perhaps acting in concert. A massive sea level lowering has been documented from the end-Famennian, and may have been caused by changing climate and growth of polar ice sheets. Shoreline retreat associated with this event could have exposed reefs and other immobile organisms to air, to battering waves and to suffocating muds from advancing deltas and marshes. Submerged land-bridges, like the Transcontinental Arch, may have become emergent again, cutting off one part of the ocean from another. Growth of exposed continental areas may have also altered weathering of soils and rocks and changed atmospheric conditions. All of these events could have stressed and killed organisms on a global scale, and no one factor has yet been implicated as a sole cause of the mass extinction.

With so many intriguing events occurring, it is no surprise that geologists are clambering across Colorado's high-mountain outcrops of the Dyer Formation. There they search for clues, with hammer and lens at hand.

Article and photographs provided by C. James Bullecks, Linda K. Soar, and James W. Hagadorn (Department Associates and Curator, Denver Museum of Nature and Science).



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More Dyer Formation fossils (from left): brachiopods and nautiloid. DMNS photographs.


2013 Paleozoic Partner feature articles:

| January: Fossils of the 2013 National Fossil Day Artwork | February: Paleontological Research Institution, Museum of the Earth | March: Falls of the Ohio State Park | April: Field Museum of Natural History, Mazon Creek Collection | May: Prehistoric Trackways National Monument | June: Cincinnati Museum Center | July: Glacier Bay National Park & Preserve | August: University of Michican Museum of Paleontology, Silica Formation Fossils | September: Yale Peabody Museum of Natural History, Beecher's Trilobite Bed | October: Guadalupe Mountains National Park | November: Utah Geological Survey, Millard County Cambrian Fossils | December: Denver Museum of Nature and Science, High-Altitude Mass Extinction |

Last updated: November 26, 2013