Ancient Amphibians Discovered in Iowa

In May 1985 an important new fossil bed was discovered in southeast Iowa. A unique 12- to 20-inch thick bed of shale was found in a farmer-owned limestone quarry. The shale is filled with well preserved fossil amphibian bones. It ranks high on the list of important fossil finds for all of North America.

Early Evolution

Not much is known about early tetrapods (four-footed animals). Amphibians are the earliest known tetrapods. They were the first animals with backbones (vertebrates) to go from living in water to living on land. All other land vertebrates, including reptiles, birds and mammals, are descendents of them.

Early amphibians resembled salamanders. They had long bodies and short limbs. Many early amphibians were quite large. Some were six to ten feet long. They were covered with bony scales. The size and shape of their teeth show that they were predators. Their diet probably consisted mainly of fish.

These amphibians lived in Iowa during the Mississippian age. Streams and small pools of water covered the area. These made it an ideal place for them to feed and reproduce. The small pools also became sites where their skeletons collected. Some bones became buried under layers of mud. The mud preserved them from weathering and decay.

Numerous Ancient, Rare Fossils in Iowa

One reason geologists are so interested in this bed is because there are so many fossils. They have found jaws with teeth, shoulder pieces, hip bones, front and rear legs, feet bones, vertebrae and ribs. Some bones have been flattened. Others have kept their original shapes. There are large and small fossils. The largest pelvis probably came from an animal that was over six feet long.

Geologists are also excited about the age of these fossils. Most amphibian fossils have been found in younger rocks from the Pennsylvanian or Permian age. Only about a dozen places have amphibian fossils from the older Mississippian age. Most of these are in the British Isles. Two are in North America.

The Iowa amphibians are probably older than the other ones in North America. They help fill in a long gap in the fossil record. The fossils may provide clues to the evolution of early tetrapods. Discovery of this site is a major event.

Used with permission from the Iowa Department of Natural Resources Geological Survey. Adapted from their Web site and original article in Iowa Geology 1986, No. 11, Iowa Department of Natural Resources.



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