updated 3/31/2008 6:55:28 PM ET 2008-03-31T22:55:28

Excavators have unearthed what are believed to be some of the oldest prehistoric bones discovered in a cave in Wisconsin, according to a University of Wisconsin-Green Bay professor.

The bones have been dated to between 5,600 and 5,800 years old, said Dr. John Luczaj, an earth science professor. They were discovered in New Hope Cave at Cherney Maribel Caves County Park in January and February.

"We believe there are older bones yet to be discovered further into the cave from data collected during the exploratory excavations done during the 1990s," J.D. Skattebo, chairman and founder of the Friends of Maribel Caves, wrote in an e-mail.

Scientists have yet to identify the animal the two vertebrae came from, but it could have been a small mammal "bigger than a bat but smaller than a woodchuck," Luczaj said.

The vertebrae each were less than a centimeter in length.

Skattebo and other excavators discovered the bones beneath flow or float stone, which is created by calcium carbonate flowing into the cave from spring melt water, he said.

The cave's sediment is at least as old as the bones and probably similar in age to that in other caves in Northeastern Wisconsin, Luczaj said. The fossils could help researchers understand what happened in the entire cave system, he said.

"We can infer things about climate if we understand how old the deposits are and what was the environment that these organisms lived in," he said.

Excavators will resume work on the cave north of Manitowoc either in May or June, depending on weather conditions, Skattebo said.

"Our goal is to learn and understand why the caves are still there, how did they fill, and when and why," said Skattebo. "We are also studying the groundwater interaction with the water table."

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