As she scraped cold dirt from the remains of an extinct bison, Pam Groves wrinkled her nose at a rotten-egg smell wafting from gristle that still clung to the animal’s bones. She lifted her head to scan the horizon, wary of bears that might be attracted to the flesh of a creature that gasped its last breath 40,000 years ago.

In the type of discovery they have dreamed about for years, Groves and Dan Mann, both researchers at the University of Alaska Fairbanks, in summer 2012 found in the thawing bank of a northern river almost the entire skeleton of a steppe bison that died during the last ice age.In adventurous work sponsored by the Bureau of Land Management, Mann and Groves have been boating down lonely northern rivers for 15 years looking for scattered bones of ice age mammals, always hoping to find a complete skeleton or mummy of a mammoth, horse, or American lion. In mid-June, on a familiar stretch of river that flows northward on Alaska’s North Slope, they rounded a river bend and saw the skull of a large bison trapped against a willow shrub. Upside-down skullGroves described the scene: “We were paddling downriver, battling through a nasty squall of hail and wind, thinking about our camping spot about a mile downriver,” she said. “When the hail was just breaking up, we saw the upside-down skull with the lower jaw still attached. The teeth were really white. They stood out.”The pair landed their inflatable canoe at the base of the 60-foot bluff. Even before they stepped out in their rubber boots, Groves spotted other bones that told her this wasn’t an ordinary site. Though this ever-changing wall had yielded many bones over the years, those were scattered remains of ice-age creatures separated by meandering river action and the crumbling and re-forming of permafrost-cemented bluffs. Mann said their typical discoveries resemble “Pleistocene in a blender.”“It’s really unusual to find bones that are still articulated (together),” he said. “We’ve never found anything this intact before. I think it’s really exciting when we find single lion bones.”Groves and Mann spent the next four hours carefully removing soil from the skull. When Mann lifted it out, the spread of the steppe bison’s horns was 43 inches. The record Boone and Crockett modern bison has a horn tip spread of 27 inches. Fairbanks expert on Pleistocene animals Dale Guthrie estimated this bison, which Mann and Groves have nicknamed “Bison Bob,” was a 12-year-old male.After stowing the skull safely in the front of their canoe, Groves and Mann camped for the night. When they returned to the site, they saw more bones sticking out of frozen bluff sediments. Bones of each leg were still connected by ligaments. Reddish-brown hair clung to some bones.“When I saw the fur, that’s when I really got excited,” Mann said.Transporting skull by helicopterThe pair worked the bluff for the next three days, pulling up buckets of river water to thaw the pieces of their rare find. They ended up finding every piece of the animal’s skeleton except for a left shoulder blade.With a few hundred bison parts attracting blowflies in their camp, Mann and Groves awaited a helicopter they had called to transport the bison to somewhere safer.Shortly after their find, they used a satellite phone to try and contact archaeologist Mike Kunz, their longtime collaborator at the Bureau of Land Management. Kunz is interested in the ancient Paleoindians that once lived on Alaska’s North Slope and what might have caused both them and bison to disappear about 10,000 years ago. Bad weather kept Kunz pinned down at the archaeological site he was digging several hundred miles away, but a helicopter arrived near Mann and Groves and transported Bison Bob to a North Slope BLM camp. There, someone put the bones in a predator-proof case.