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Sloth bones provide clues in dating Gray Fossil Site

SAM WATSON • Feb 6, 2007 at 10:27 AM

ETSU paleontologist Steven Wallace, National Park Service expert Greg McDonald and Blaine Schubert (left to right) display a sloth bone found at the Gray Fossil Site. Tony Duncan photo.


JOHNSON CITY - Using a fossilized ground sloth bone unearthed last summer, a National Park Service expert hopes to help East Tennessee State University learn more about the age of the Gray Fossil Site.

"What I'm hoping to do is, by studying the sloth from the Gray site, compare it with the other known discoveries and see if we can find a time range for the age of the site," Gregory H. McDonald said before presenting a lecture on his research goals Monday at ETSU. "We know that there were no known sloths in North America before 9 million years ago.

"Just by finding the sloth, we know the site was 9 million years old or younger. But if we can get more specific on the type of the sloth, that allows us to narrow the time frame even more."

ETSU paleontologists already have estimated the Gray site's age at 4.5 million to 7 million years old based in part on specimens of teleoceras, a short-legged species of rhinoceros.

McDonald, senior curator of natural history for the Park Museum Management Program based in Fort Collins, Colo., said the sloth study should complement information derived from the teleoceras.

"We certainly know that rhinos disappeared from North America 4.5 million years ago," McDonald said. "So the fact that we have rhinos means that it (the Gray site) has to be older than that. The fact that there's a sloth means it can't be any older than 9 (million years old)."

Sloth specimens found at the Gray site represent a smaller animal than giant ground sloth remains found in other digs on the continent or smaller tree sloths that still live in Central and South America.

"The largest of the ground sloths are elephant sloths ... but based on the size of the femur - the leg bone they got last summer - I would estimate that we're looking at an animal roughly in the size range of a black bear," McDonald said. "One of the things we know about this particular sloth in North America is that it got larger through time, so the earliest sloths here in North America are small.

"By the time they became extinct at the end of the Ice Age, they were about four times bigger."

So measurements of the fossilized sloth remains found in Gray could serve as time indicators, he said, as researchers try to estimate where the species fits into the fossil record.

"We don't have the bones yet that are diagnostic to the species level, so we are operating one step up at what we call the genus level," McDonald said.

He said the earliest North American ground sloths likely migrated from South America in the early stages of the Isthmus of Panama's formation via "island hopping" - swimming or floating on debris from island to island. Later, when the isthmus was fully formed, animals walked over the dry land bridge between continents.

"For a long time, South America was an island continent like Australia is today," McDonald said. "So it had some rather unusual animals on it, just like we think of kangaroos as being strange.

"But between 2 million and 3 million years ago, the Isthmus of Panama formed, and we started seeing an exchange of animals back and forth across North and South America.

"So animals that we think of being South American today, like llamas and tapirs and peccaries, started in North America and went south. Some of those strange South American animals, like 'possums, which we have around here today, came north."

Along with the sloth femur unearthed in a rhino dig last summer, the Gray site has produced two other ground sloth specimens, an ankle bone and a hand bone.

Steven Wallace, an ETSU paleontologist and director of the university's Don Sundquist Center of Excellence in Paleontology, said University of Tennessee researchers collected one specimen shortly after the site was discovered in 2000, and ETSU unearthed the second two summers ago.

"This (the sloth femur) is the first one we've really been able to get some serious data on," Wallace said, adding that broken specimen had to be assembled in ETSU's paleontology lab.

McDonald's visit was sponsored by the Sundquist Center of Excellence. Next, the center will bring in Northern Arizona University's Jim Mead for a similar study on an alligator specimen found in Gray. Mead will speak in a public lecture 4 p.m. Monday at ETSU's Brown Hall Room 261.

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