Near Iowa City in Johnson County, Iowa — The American Midwest (Upper Plains)
Devonian Fossils
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
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that reached 30 feet
in 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
Photographed by Connor Olson, July 25, 2020
2. Devonian Fossils Marker
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 of life that once existed here.
Location. 41° 43.309′ N, 91° 31.933′ W. Marker is near Iowa City, Iowa, in Johnson County. It 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.
Regionally, this marker is in Eastern Iowa. It is also in the American Midwest and in the Corn Belt. Globally, it is in North America, the Western Hemisphere, the Western World, and the Anglosphere. Historically, it finds itself in what was once the territory of the Mississippian Culture and also the Louisiana Purchase.
Other nearby markers. At least 8 other markers are within
Credits. This page was last revised on June 7, 2021. It was originally submitted on June 6, 2021, by Connor Olson of Menomonee Falls, Wisconsin. This page has been viewed 576 times since then and 40 times this year. Photos:1, 2, 3. submitted on June 6, 2021, by Connor Olson of Menomonee Falls, Wisconsin. • Mark Hilton was the editor who published this page.
