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University of Iowa News Release


May 13, 2009

Museum of Natural History announces new sloth bone discoveries

The University of Iowa Museum of Natural History's Tarkio Valley Sloth Project has discovered a bone from a type of giant Ice Age sloth never before recorded in Iowa, called Paramylodon harlani.

Ice Age sloth expert Greg McDonald, the senior curator of natural history for the National Park Service's Park Museum Management Program and a consultant on the Tarkio Valley project, identified the approximately 5-inch long bone as the animal's fifth metacarpal, which once connected its wrist to its little finger, during a visit to the museum last week.

"Greg saw that bone, and you could see his face light up," said Holmes Semken, project leader and emeritus professor in the UI Department of Geoscience.

Southwestern Iowa property owner Bob Athen found the metacarpal nearly two years ago just downstream from the project's current riverbed excavation site near Shenandoah, Semken said. At that time, it was mistakenly identified as part of a Megalonyx jeffersonii sloth, three of which -- an adult, a toddler, and a baby -- have been uncovered at the site.

McDonald, whose visit was made possible by a recent National Science Foundation grant for the project, checked all the bone identifications as part of his analysis while on campus. He said his goal was to establish an accurate catalog of the bones as a basis for further investigations.

A presentation on the excavation that will display and discuss key findings will be shown at the Greater Shenandoah Historical Society Museum, 405 West Sheridan Ave., on Friday and Saturday, May 29 and 30.

"Of all of the Megalonyx sites discovered, the Tarkio specimens will provide the basis for the most comprehensive analysis of sloth paleobiology and ecology of any to date," McDonald said.

Both Paramylodon and Megalonyx were nearly elephant-sized Ice Age mammals that became extinct about 12,000 years ago. But Paramylodon sloths were equipped with broader, triangular claws for digging rather than the sharp claws of the Megalonyx, which were used for grabbing at woody vegetation like tree branches, McDonald said.

The new sloth identification comes on the heels another major excavation discovery at the original site: On April 25, three more major Megalonyx bones were discovered. Semken and David Brenzel, another primary investigator for the project, identified the findings on site as a scapula (shoulder blade) from the baby sloth -- only the second recovery from this specimen -- and two rib bones from the older toddler sloth.

"We have a nearly complete rib cage now for the juvenile sloth," Semken said, adding that the baby sloth scapulas are the among the animals' most fragile bones. "Someone a lot smarter than me is going to have to figure out how these bones have managed to survive virtually unblemished while the more robust limb bones appear to be missing."

The April 25 bone discoveries are the first in nearly three years for the project, which began in 2001. Semken said the group's excavation efforts were halted in 2007 and 2008 because of flooding, and the excavation last December turned up empty-handed.

"We were concerned that the fossil deposit was running out after the disappointing results from the dig in December, but it's clear that we have encountered the bone bed again," Semken said. "Discovering these bones was a thrill for everyone. We plan to get back out there as soon as the water in the river goes down, hopefully this month or the next. We have a lot more sloth to find."

McDonald estimated that the approximately 110 bones recovered from the adult Megalonyx represent about 60 percent of the full skeleton, making it the second most complete of its kind. Only six other semi-complete skeletons of this species have ever been found, none of which are associated with younger animals. The older juvenile is also the second most complete of its kind, with more than 40 bones recovered. The skeletons have the added research benefit of being buried in sediments that will provide valuable environmental data about the climate and ecology during the sloths' lives.

Museum of Natural History Education and Outreach Coordinator Sarah Horgen stressed the opportunity this project provides for students.

"The students are a key part of this truly groundbreaking research," Horgen said. "They help us excavate, they help us clean the bones, and they have the chance to do their own research. For an aspiring archeologist, it's really a once-in-a-lifetime opportunity."

The Tarkio Valley Sloth Project, a joint effort between the UI Museum of Natural History, Department of Geoscience and Office of the State Archaeologist with volunteers and students from across the Midwest, began in 2001 when Bob and Sonia Athen uncovered bones from Megalonyx jeffersonii in the bed of the West Tarkio Creek behind their home near Shenandoah. Soon, more bones were found on the property of the adjoining landowners, Dean and Loreta Tiemann. Both the Athens and the Tiemanns graciously agreed to permit excavation and donate the fossils of this rare species to the University of Iowa on the condition that students be involved in all aspects of the project: excavation, bone preservation and research.

The University of Iowa Museum of Natural History features three permanent galleries exploring natural history and emerging environmental research in Iowa and beyond. For more information call 319-335-0606 or visit

STORY SOURCE: University of Iowa News Services, 300 Plaza Centre One, Iowa City, Iowa 52242-2500

MEDIA CONTACTS: Sarah Horgen, UI Museum of Natural History,, 319-335-0606; George McCrory, University News Services, 319-384-0012,; Writer: Maggie Anderson