Rare mammoth fossil being excavated on Santa Rosa Island. (National Park Service)

One thing is certain about the 13,000-year-old mammoth skull just excavated off the coast of California.

"This mammoth find is extremely rare and of high scientific importance," Don Morris, a retired archaeologist who studied the remains found at Channel Islands National Park, said in a statement. "I have seen a lot of mammoth skulls and this is one of the best preserved I have ever seen."

But much else about the fossil is still a mystery — one that Morris and his colleagues are trying to solve.

For starters, scientists aren't sure what species the creature is. The skull is too small to belong to one of the massive, 14-foot Columbian mammoths that appeared in North America roughly 1 million years ago and migrated to the Channel Islands between the two most recent ice ages.

But the skull is also too large to belong one of the pygmy mammoths that evolved from the Columbians as a result of insular dwarfism — the phenomenon by which island species evolve to be smaller because of scarce resources and an absence of predators. Those creatures grew to be just 6 feet tall.

Even more confusing, the mammoth's tusks give conflicting evidence about its age when it died. The right one is nearly five feet long and coiled in a manner characteristic of fully grown mammals. But the left tusk is shorter and sloped, making the mammoth seem like a juvenile.

Did the skull belong to a teenage Columbian mammoth who still needed a few years to grow? Was it an unusually large, adult pygmy? Or some sort of transition species between the two? Scientists think the third option is the least likely. Morris and his colleagues hope they might find some answers in the mammoth's teeth; by measuring their size, spacing and the thickness of the enamel, researchers can estimate the animal's age at its death.

Geologist Dan Muhs of the U.S. Geological Survey, who also worked on the excavation, said that the new skull suggests there may have been multiple mammoth migrations from the California coast to the Channel Islands' shores — one during the most recent ice age, which started about 30,000 years ago, and one during the previous glacial period about 120,000 years before that.

The expanded polar ice caps would have locked much of the Earth's water up at the poles, causing sea levels to drop. At the time, when the Channel Islands were a single land mass, mammoths probably swam toward it holding up their trunks as snorkels, much as modern elephants do today. When sea levels rose, the low lying parts of the island flooded until only the mountain tops were exposed, creating the island chain 100 miles due west of Los Angeles that we know today.

The mammoth skull was found by National Park Service biologist Peter Larramendy in a 13,000-year-old rock layer on Santa Rosa Island, the second-largest land mass in the island chain. Interestingly, it's the same age as the human femurs known as "Arlington man," which were found on Santa Rosa and are among the oldest human remains found in North America.

In about 3,000 years, humans would spread throughout the continent and the mammoths would be extinct, including on the Channel Islands. The new remains could help untangle a possible link between those two facts, and help scientists resolve an ongoing debate about whether humans really hunted North America's mammoths to extinction.

"There's a possibility the mammoths died out before humans arrived, and it's possible humans ... hunted them to extinction," Muhs told CNN. "But there's a third possibility that at the end of the last glacial period, mammoths could have been under stress with limited food resources with sea levels rising at the islands. Then the arrival of humans delivered the final blow."

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