Like detectives at a 12,000-year-old crime scene, the team has been attempting to piece together a life history of this extinct, furry, SUV-sized mammal. What did it eat? Why did it die? And why did sloths mysteriously become extinct along with over three dozen other large ice age animals?

The mystery has suddenly gotten much more complicated. The museum has announced the research team has recovered more than 30 smaller bones which have been confirmed as belonging to a juvenile version of the same species "probably about one year old," according to Greg McDonald, the world's foremost giant sloth authority and senior curator of natural history at the National Park Service in Ft. Collins, Colo.

"With 30-plus bones I would rank this the second-most complete juvenile Megalonyx ever found."

The bones were found about 10 feet away from where the first adult bones were discovered. "This is the first time an immature sloth of this species has been found associated with an adult," said David Brenzel, curator of the UI Museum of Natural History.

"The bones are surprisingly well-preserved. Ribs are fragile, if we can find 20 that look this good, the condition of any other bones that are still out there must be exceptional."

The new specimen was excavated in a series of weekend trips that began in April and ended June 11. According to Brenzel, more than two dozen volunteers from all over Iowa helped in the recovery effort. "The bones were sitting smack-dab on the bottom of the creek. We didn't want to lose them in a spring flood," he said.

Even before diggers found the new sloth, the site had yielded almost 90 bones from an adult -- the second most complete adult skeleton of this species ever recovered. Bob and Sonia Athen first discovered the bones in the summer of 2001 behind their home, along the West Tarkio Creek, which forms the border between their land and land owned by Dean and Loreta Tiemann of Lincoln, Neb.

The Athens began gluing the pieces together that winter. Sonia and her daughter Katie, a UI student, subsequently brought the bones to Iowa City, where Holmes Semken, emeritus professor of geoscience and leader of the sloth research project, identified them as the remains of Megalonyx Jeffersoni, or Jefferson's Ground Sloth.

Although other sites have previously produced remnants of juvenile sloths, none has ever been found directly associated with an adult. "When paired with the adult, the juvenile represents a one-of-a-kind find," Semken said.

Conclusions drawn from sloth specimens recovered from separate locales and different geologic times are highly speculative because unknown variables muddle the causes and effects, said McDonald. "This is unprecedented; two individuals living and dying in the same place at the same time form a Rosetta Stone for understanding these extinct animals," he said.

The discovery opens the door to research that has never been possible before. "Frequently all we can do is take physical measurements of the bones. What's so exciting is the potential window this opens into the maternal behavior of giant sloths," McDonald said.

The museum will have to extract and sequence the DNA to prove that the two finds constitute a "sloth family," but the close association of the fossils is compelling evidence that they are related, Brenzel said, adding that the museum also learned in May that it has been awarded an $8,114 National Science Foundation (NSF) grant for further excavation of the find.

He said the discovery will drive a major rewrite by the research team of a second grant proposal to fund further analysis of the fossils.

"This adds a whole new dimension to our NSF proposal," Semken said. Tests can indicate climate, diet, and possible migration patterns of the animals, among a host of other important characteristics. Together, the two individuals may help establish parameters for age of sexual maturity, growth rates and diet changes with age.

"The window into the biology and behavior of this extinct species is unprecedented," McDonald said.

The museum is currently preparing a temporary exhibit of the new bones. In a reference to the famous T-rex dinosaur at Chicago's Field Museum, UI Museum of Natural History director Pamela Trimpe said,

"This is Iowa's Sue. We're very grateful to everyone, but especially the volunteers and people of southwest Iowa who have made it possible to bring this incredible discovery to the attention of the world."

Related Links
University of Iowa
Museum of Natural History Sloth Information Site