Fossils of Iowa
by Jean Cutler Prior
Many people have their beginning interest in geology stimulated by finding fossils. Holding the shell of a sea-dwelling organism found in an Iowa rock, far from the nearest ocean, makes us think about the vast changes that have occurred over the Earth's surface and the great length of geologic time that has passed. Studying fossils helps us appreciate the history of life on Earth. They provide a link between geology and biology that is valuable to the study of global changes and how life adapts. Fossil remains also are an important tool in dating different rock layers, and in comparing the sequence of strata from place to place across broad areas. Iowa has many well known fossil-bearing rock formations, and fossils from around the state have found their way into museums around the world. These pages help to identify a few of Iowa's many fossils that may be found by careful observation of road cuts, quarries, stream banks and other exposures of earth materials.
Bottom-dwelling corals lived in reef-like communities in warm, clear, tropical seas covering Iowa. Many species were colonial, living together in a mass of individual skeletons of lime, resembling a honeycomb. Distinctive colonial forms from eastern Iowa include the "chain coral" (left), Pachyphyllum (middle), and Lithostrotionella (right). They were especially abundant in Devonian and Silurian seas, 375 to 425 million years ago.
Corals also lived alone in curved, cone-shaped skeletons unattached to other individuals. This fossil "horn coral" housed the animal's soft tissues, including tentacles which filtered food particles from sea water.
These shells are among the most common fossils found in Iowa. Brachiopods lived inside the protective cover of two hinged shells, attached to the floor of warm, shallow seas that once covered the state. These eastern Iowa specimens are about 375 million years old (Devonian).
Shells of marine animals are often preserved as fossils. This coiled shell from Winneshiek County was inhabited by a snail. The sluggish, bottom-dwelling mollusk scavenged or grazed the ancient sea floor about 440 million years ago (Ordovician). The snail moved on a flat muscular foot and could withdraw inside its shell for protection.
This jaw belonged to a 2 to 3 ft-long placoderm, a primitive fish partly covered by bony plates that gave it an armored appearance. The black color of this 375 million-year-old (Devonian) specimen from Black Hawk County results from mineralization of the fossil bone.
"Stroms" are extinct organisms related to sponges. They constructed skeletons of lime and lived in various shaped colonies that resembled layered mats, branches, and rounded masses. This Floyd County specimen, with its prominent nodes, lived 370 million years ago (Devonian) .
Crinoids lived anchored to the sea floor by flexible, rooted stems. Segments of the rounded stems are commonly found as fossils. Famous localities in Iowa include the LeGrand and Burlington areas.
Often called "sea lilies," crinoids are actually animals related to starfish. This 350 million-year-old (Mississippian) specimen from Marshall County shows the arms, which in life would filter sea water for food particles.
Prized by collectors, whole trilobites usually display a three-lobed, oval-shaped, segmented skeleton, often with distinct eyes. They belong to an extinct group of bottom-dwelling, hard-shelled arthropods that scavenged the sea floor. These Scott County specimens are 375 million years old (Devonian).
"Moss animals" were also colonial, filter-feeding organisms that inhabited the sea floor. A well known bryozoan (Archimedes) consisted of concentric rows of lace-like fronds attached to a corkscrew-shaped axis. The preserved core is seen in this Lee County specimen (340 million years old, Mississippian).
These squid-like animals lived in chambered shells and could propel themselves by ejecting water from a tube near their head. The shell's partitions were filled with gas, enabling the animal to regulate its buoyancy. These straight-shelled cephalopods (left), from Marion County are 300 million years old (Pennsylvanian). The coiled cephalopod (right) is a 365 million-year-old (Devonian) specimen from Butler County, and a distant relative of the chambered nautilus seen in today's oceans.
These black fossil seeds are from Scott County. They grew at the end of a frond on a fern-like tree about 300 million years ago (Pennsylvanian).
Tooth shapes of these 15,000 year-old molars indicate mastodons browsed tree branches, while mammoths grazed grasses. Fossils of these extinct Ice Age (Pleistocene) creatures resembling elephants have been widely found across Iowa.
Like gastropods and cephalopods, clams are also mollusks that live in a protective shell. This Plymouth County specimen lived on a sea floor 90 million years ago (Cretaceous). Clams were abundant in these waters, the last of the great inland seaways to cover Iowa.
This pelvic bone belonged to a 3 to 4 foot-long proto-anthracosaur, a rare, primitive amphibian that lived 330 million years ago (Mississippian) and was found in Keokuk County.
The fossil foliage of seed ferns (small fossils) was found in Dallas County, and the scale-tree (Lepidodendron, large fossil) in Muscatine County. About 310 million years ago, these plants were common in the coastal swamps that produced Iowa's coal deposits (Pennsylvanian).
Photographs by Paul VanDorpe