Experts from the Denver Museum of Science and Nature confirmed Wednesday that a rib bone found by a construction worker last month in Snowmass — before the mammoth was found — is from Bison antiquus, or ancient bison, the predecessor of modern-day bisons.
Also Wednesday at the dig in the bottom of the Ziegler Reservoir just outside of Snowmass Village, museum workers began to dig out both a huge mastodon skull and a seven-foot-long curving tusk thought to be from an adult mammoth.
The ancient bison is now the third prehistoric species found at the site, along with one Columbian mammoth and at least three mastodons. All are thought to be at least 12,800 years old and could be much older.
“Unlike the mammoths, the bison did not become extinct,” said Dr. Steven Holen, curator of archaeology at the Denver museum, an expert on mammoths and the director of the Snowmass dig. “They were about 25 percent bigger and had straighter horns.”
A construction worker from Gould Construction found the bison bone before the first mammoth bone was found on Oct. 14. He and other crew members thought the rib bone was from either a cow or a buffalo and didn’t think all that much of it at the time.
But the worker kept the bone and showed experts how he found it in the same peat layer where the mammoth was found. That led to the bone being confirmed Wednesday as a bison antiquus rib.
The other buzz at the site was when Don Brandborg, a trained volunteer for the museum, began working to dig out what was later determined to be a huge mastodon skull, the first found in Colorado.
“I kept coming back and it kept getting bigger and bigger and bigger,” said Holen.
The top of the skull, and its tusks, had been knocked off by a bulldozer last week. Crews marked the site and on Wednesday morning, they began to start excavating the area.
“I hit pay dirt,” Brandborg said in a statement released by the museum.
The skull was sitting upright in the dirt and Dr. Holen said it appeared the teeth were still in the skull, which could help determine the age of the animal and perhaps how it died.
Nearby, other workers found a long twisting tusk buried in the dirt. It was broken in the middle and museum crews built a plaster cast around the tusk so they could safely lift it out of the ground.
Dr. Holen said it was too soon to tell, but given its length and curvature, the tusk could be from a mammoth — as mastodon tusks were straighter and shorter.
So far, five mastodon tusks have been found at the site, along with the small tusks from a juvenile Columbian mammoth, which was the first ancient creature identified in the reservoir.
Under a tent where a grid has been laid out over the remains of the young mammoth, workers on Wednesday found two more limb bones and two more ribs from the mammoth. The additional bones further indicate that the whole animal may be resting in the layer of peat.
“It was a really great day at the site,” Dr. Holen said.
Today, a mastodon expert from the University of Michigan, Dr. Dan Fisher, is expected to arrive in Snowmass Village. He is a close friend of Dr. Holen’s and is excited to be on site given that this is only the fourth time remains of mastodons have been found in Colorado.
“To find several mastodons in a lake at this altitude is very significant,” Holen said. “We didn’t really understand that mastodons lived up here.”
Ziegler Reservoir is a 10- to 15-acre, glacially-formed lake sitting at 8,850 feet on a flat section of the ridge that separates the Brush Creek and Snowmass Creek valleys.
“It is very amazing and one of the greatest sites ever in Colorado in terms of paleontology,” Dr. Holen said.