“Bite-Size Bits of Local, National, and Global History”
“Bite-Size Bits of Local, National, and Global History”
Near Iowa City in Johnson County, Iowa — The American Midwest (Upper Plains)

Devonian Fossils

Devonian Fossils Marker image. Click for full size.
Photographed By Connor Olson, June 25, 2020
1. Devonian Fossils Marker
Inscription.  Fossils can be seen in most of the limestone layers exposed in the Gorge, and these provide clear evidence of ancient life that once inhabited a shallow sea. Fossils comprise more than half of some layers, especially the coral-rich “Rapid Biostrome" seen at the Biostrome Plaza (biostrome: literally "life layer"). The Rapid Biostrome can be traced over 150 miles from Davenport to the Minnsota border. Corals and sponges (stromatoporoids) are some of the most spectacular of these fossis, and in fact, the name Coralville derives from the abundance of fossil corals seen in the area. Some corals are represented by complex colonies made up of over 1000 individual animals (colonial corals), whereas other coral skeletons were secreted by a single coral animal (solitary "horn" and "cup" corals). Growth rings can be seen in many coral fossils, reflecting seasonal changes in growth rates similar to modern tree rings. The extinct stromatoporoid sponges possessed hard lime skeletons that show lumpy or branching forms.

A slab containing a bony plate from a large armored fish called an arthrodire an early jawed fish that reached 30 feet in

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length) was removed from the Gorge following the 1993 flood and is on display with other Gorge fossils in the Visitor Center, including slabs of crinoids. Smaller tooth-like and bony plates from other early jawed fish are occasionally seen by careful observers at the Gorge. Brachiopod shells are the most abundant fossil seen the Gorge, and Favosites (the "honeycomb coral") is one of the most common colonial corals, especially in the Rapid Biostrome.

Idiostroma Limestone
This polished slab of Devonian limestone shows an abundance of fossil stromatoporoid sponges with scattered small corals and other fossils. Stromatoporoids are an extinct group of sponges that possessed hard lime skeletons. Beds rich in stromatoporoid fossils e seen at many places around Coralville Lake, and the slabs seen in these interpretive displays come from layers of the Cedar Valley Group known as the "Idiostroma beds," named after the abundance of small broken branches of a particular type of stromatoporoid. Lumpy and more massive forms of other stromatoporoid sponges are also seen the slabs. In addition, twigs and lumps of the colonial "honeycomb" coral (Favosites) are evident, some encrusted by stromatoporoids.

Caption: The Devonian Diorama at the University of low Museum of Natural History's low Hall on the University campus, shows the colorful profusion

Devonian Fossils Marker image. Click for full size.
Photographed By Connor Olson, July 25, 2020
2. Devonian Fossils Marker
of life that once existed here.
Topics. This historical marker is listed in these topic lists: Anthropology & ArchaeologyDisastersEnvironment. A significant historical year for this entry is 1993.
Location. 41° 43.309′ N, 91° 31.933′ W. Marker is near Iowa City, Iowa, in Johnson County. Marker can be reached from Prairie Du Chien Road Northeast. Touch for map. Marker is at or near this postal address: 2850 Prairie Du Chien Rd NE, Iowa City IA 52240, United States of America. Touch for directions.
Other nearby markers. At least 8 other markers are within 4 miles of this marker, measured as the crow flies. Site Geology (here, next to this marker); The Coralville Dam (a few steps from this marker); View from the Overlook Plaza (a few steps from this marker); An Ancient Sea (a few steps from this marker); Samuel Jordan Kirkwood (approx. 3.9 miles away); Old Brick (approx. 4 miles away); Mormon Handcart Brigade Camp (approx. 4.1 miles away); St. Mary's Church (approx. 4.1 miles away). Touch for a list and map of all markers in Iowa City.
Devonian Fossil Gorge image. Click for full size.
Photographed By Connor Olson, July 25, 2020
3. Devonian Fossil Gorge
Credits. This page was last revised on June 7, 2021. It was originally submitted on June 6, 2021, by Connor Olson of Kewaskum, Wisconsin. This page has been viewed 242 times since then and 82 times this year. Photos:   1, 2, 3. submitted on June 6, 2021, by Connor Olson of Kewaskum, Wisconsin. • Mark Hilton was the editor who published this page.

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Dec. 1, 2023