Several old publications mention a polar bear fossil that was found circa 1916 in Breck Smith Cave which is 8 miles west of Lexington, Kentucky. Apparently, 3 women who were exploring the cave discovered the bones, and they took them to an “authority” at the University of Kentucky. He identified them as polar bear. Unfortunately, the specimen was never described in the scientific literature, and as far as I can determine, the ownership of this unique fossil has long been forgotten. Because the presence of polar bears in northern Kentucky would have interesting ecological implications, I would like to locate the fossils and have a modern expert examine them. If anyone knows where they are, please contact me.
Polar bear killing a seal. If the fossil specimen from Breck Smith Cave was correctly identified, seals must have also been present in the Ohio and upper Mississippi Rivers during the Ice Age. Walrus fossils have been excavated from 2 sites in Michigan. As far as I know, those are the closest pinniped fossils to Breck Smith Cave.
From the available information I don’t know for sure who the “authority” was that identified the polar bear fossil (or fossils), but I suspect it was Arthur Miller, head of the Kentucky University Geology Department from 1892-1925. I strongly suspect he misidentified the specimen because the associated fossils imply a temperate environment. Bones of raccoon, gray fox, woodchuck, wolf, bison, and man were found in the cave with the supposed polar bear fossil. Today, polar bears are exclusively found in arctic marine environments. I think it’s more likely the specimen is from a grizzly bear (Ursus arctos) or even a black bear (Ursus americanus). Grizzly bears are closely related to polar bears and occasionally interbreed with them in the wild and in captivity. A grizzly bear fossil was found at Welsh Cave as I related in my last week’s blog entry about the Kentucky Bluegrass Country. However, there must have been a specific diagnostic feature for the scientist who identified the specimen to suggest the fossil specimen was from a polar bear. Polar bears are almost entirely carnivorous, and their teeth are quite different from those of grizzly and black bears. If he made the diagnosis based on teeth, he may have been correct. There’s also the slight possibility that a man carried the polar bear fossil from the arctic to Kentucky. Maybe he considered it a magic talisman of the great white bear.
Let’s assume the identification is correct, and it was not carried for a thousand miles by people traveling from the arctic to Kentucky. How did polar bears, a species that depends largely on seals and whale carcasses, live in northern Kentucky? Even during the Ice Age, northern Kentucky was not exactly polar bear country. The Laurentide Glacier reached southeastern Ohio during the Last Glacial Maximum, but northern Kentucky was mostly prairie with fingers of boreal forests then. Glacial advance during the Illinois Ice Age 100,000 years earlier than the most recent one was greater but still barely reached the Ohio River. If polar bears did range into northern Kentucky during the Ice Ages, seals, their favored prey, must have been present on the Ohio and upper Mississippi Rivers. Fossil hunters may want to look for seal specimens on the river bottoms here.
It’s not surprising that a scientist could have misidentified fossils from the Ursus genus. Recent studies of the Ursus genome show a close relationship between black bears, grizzly bears, and polar bears. (Grizzly bears and brown bears are one and the same species.) The first study of polar bear DNA determined they diverged from grizzly bears ~130,000 years ago. Coincidentally and conveniently, this corresponds with the earliest known polar bear fossil which is from Norway and dates to ~130,000 BP. A second study of polar bear DNA wasn’t as convenient–it suggested polar bears diverged from grizzly bears closer to ~600,000 years BP and as a species was much older than from what’s known in the fossil record. Polar bears live in an environment where fossils are rarely found. Most arctic fossils are deeply emerged in frigid waters, probably never to be found. The most recent study of Ursus genetics looked at the whole genome, and it paints a much more complicated picture. This study found that grizzly, black, and polar bears all diverged from a common ancestor between 4-5 million years ago. This corresponds temporally with the beginning of the Pliocene when Ice Ages began occurring, causing continental changes in the environment and creating differentiating habitats for new species of bears. Though black bears and grizzly bears diverged ~5 million years ago, the genetic evidence indicates these 2 separate species occasionally interbred until about 100,000 BP. The genetic evidence also indicates that although grizzly bears and polar bears diverged as long ago as 5 million BP, these 2 species have periodically interbred in the past and they still do.
A second generation polar bear-grizzly bear hybrid. A genome wide study suggests grizzly and polar bears diverged about 5 million years ago but periodically interbreed. The genome of brown bears living on the Alexander Islands in Alaska is made up of 5%-10% polar bear DNA. Curiously, grizzly and black bears also diverged about 5 million years ago and periodically interbred until about 100,000 years BP. Maybe that explains why Pleistocene black bears were as big as modern day grizzlies.
Polar bears probably interbreed with grizzly bears more frequently during phases of global warming when their favorite habitat shrinks, and they come into contact more often. Hybrids raised by grizzly mothers have a greater chance of survival because they learn to survive in a more varied habitat. Middle Pleistocene polar bears were more genetically diverse than they are today, perhaps because the cycles between Ice Ages were shorter during this time period than they were in the Late Pleistocene.
References:
Brown, Joseph Stanley
GSA Bulletin 33 1922
Cooper, C.L.
“The Pleistocene Fauna of Kentucky” within The Paleontology of Kentucky edited by W.R. Jilson
The University of Kentucky Press 1931
Webb, Miller et. al.
“Polar and Brown Bear Genomes Reveal Ancient Admixtures and Demographic Footprints of Past Climate Change”
PNAS 2012
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