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Ancient bison skulls unearthed by flood show evolution story

The ancient skulls came to light again in October when flood waters receded. (Submitted Photo)

PIERRE (AP) -- More than 100 bison skulls unearthed by the 2011 Missouri River flood are giving scientists more information about how the modern bison evolved.

The 101 skulls found in October by the U.S. Army Corps of Engineers date from 10,000-5,000 years ago and reveal the changing face -- and horns -- of the bison.

"The skulls themselves represent quite a long span of bison evolution. We have some that are probably 10,000 years in age. I think the majority is going to be right around 6,000 to 5,000 and then there are some very modern ones," Mike Fosha, assistant state archaeologist, said.

The skulls demonstrate rapid evolution in bison, mainly driven by a changing climate.

The earliest bison in the "new world" came appeared about 2 to 2.5 million years ago, Fosha said. Those bison had adapted to voracious predators and the cold that came with the first glacial event by having long horns.

"So they had these huge, huge horns. And what they would use them for was protection against predation and to move snow around to get down to the grass," Fosha said.

But as the snow melted and some large prey, such as the cave bear, became extinct, the need for large horns lost its merit. Much of the food the bison ate was processed to keep the animals head up, so nature started selecting smaller and smaller animals, he said.

The world then went through three large glacial advances which continued to change the climate from cold to warm.

"There were certain peaks where bison were very, very numerous. There weren't a lot of them 10,000 years ago or 12,000 years ago because climate was changing and grasses were just becoming more and more available," Fosha said. "When that happened, we started getting large herds of bison. When that climate changed again, we went down to very, very small numbers and again during the ice age 5,000 years ago that would have brought cooler weather, a lot of grasses and bison populations would have started to explode again. It is that period, I think, is what we are seeing the majority of bison from."

This is not the first time a group of bison skulls have been found in the river. A group of 25 to 30 skulls were found near the Fort Randall Dam a few years ago, but finding 101 of them is significant, Fosha said.

As for why so many bison bones have been found in the river, bison were just thirsty, he said.

During the pre-steamboat era, fur trappers would travel up stream in the spring and be forced to paddle around hundreds of dead floating bison.

Fosha said researchers suspect during frozen periods many bison died in the river trying to get a drink of water. As a large herd gathered on a frozen portion of the river, it would collapse, drowning them.

As those bodies decomposed the skulls would and other bones would collect in deep pools.

The skulls were revealed during the large releases by the Corps. Fosha said the pulses of water scoured the bottom of the stream bed and cause it to erode downward, likely freeing the skulls and other bone pieces.

The skulls were found in one general area, but that is not unusual. The river sorts items by weight and size, Fosha said. Other smaller skeletal pieces may be farther down the river, while heavier pieces would lie upstream.

While the skulls are not likely to reveal new information, Fosha said, they should confirm for researchers facts about bison they know, but have not been able to demonstrate well.

However, if the skulls date to a period where bison were not living in the northern plains, researchers have their work cut out for them to come up with an explanation, Fosha said.

"We are going to see if the relative ages fit our presumed models," he said.

The skulls are now at the South Dakota School of Mines and Technology where they will be cleaned and stabilized before they are studied.

The skulls were found by the Corps while doing flood assessment, Rick Harnois, senior field archaeologist at the Oahe project, said. It is not unusual for the Corps to find items and they periodically monitor several areas for unearthed items.