A large boulder now sitting in front of the Forest Service offices in Spearfish is not just a rock. It contains a fossil from one of the very earliest periods of life on Earth, and it was found in the Black Hills.
The three-ton boulder contains the fossilized remains of a creature that once roamed the seas that covered the Black Hills area 450 million years ago.
The fossil is encased in what geologists call Whitewood dolomite, which is similar in composition to limestone. The rock is composed primarily of the bodies of millions of microscopic animals whose bodies were deposited on the sea floor.
When the microscopic animal remains sank to the sea bottom, the body of the creature was preserved with them.
Karl Emanuel, North Zone geologist for the Black Hills National Forest, said the boulder was found during a logging operation along upper Elk Creek about 20 years ago. Tony Balistreri was working with the logging crew when he noticed the exposed fossil on one of the surfaces of the boulder. He and the other loggers made the decision to move the rock to an area away from the logging activity.
It sat there until last year when Forest Service geologists decided to move it to a more protected location. It took a large semi-truck and some heavy equipment to move the 6,000 pound boulder to Spearfish on Aug 2.
Emanuel said the creature encased in the rock is from the genus called endocerida. It’s so rare, it hasn’t yet been given a species name.
“It has yet to be classified,” said Emanuel.
It had a long, hard, shell, shaped like a cone. At the large end of the cone, its exposed, fleshy body featured tentacles similar to a squid. According to Emanuel, it was a carnivore that hunted the ocean bottom for other small creatures such as worms and other cephalopods. It moved about in a manner similar to a squid. In other words, it moved backwards.
The fossil in the boulder on display outside the Forest Service offices in Spearfish includes a four-foot-long portion of the creature’s conical shell.
Emanuel said the era in which the creature lived is called the Ordovician, a very remote period in the history of life on Earth.
“There were no plants on land, and there were no creatures on land,” he said.
The earth during that period would have been difficult for a current observer to recognize. The earth’s atmosphere was not breathable, and the surface of the earth would have had an appearance similar to the present surface of Mars, said Emanuel. But the seas were full of life. Worms and crab-like creatures were beginning to move about the sea floor, and underwater plants were abundant.
The remarkable thing about this particular fossil, said Emanuel, is that it preserves a portion of the remote history of this incredibly ancient planet and that it was found in the rock layers that make up the Black Hills. The Black Hills, in fact, contain a record of much of the history of life on Earth.
“It’s a significant fossil,” said Emanuel.
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