A fisherman discovered a sand tiger shark jawbone in the Mississippi River, leading scientists to question how it got there.
The angler found the jawbone in Minnesota, just west of Grand Rapids, and brought the mysterious jaw to the Department of Natural Resources office on May 15 to find out what it was, KARE reports.
He also posted a photo of the jawbone to Facebook, where he mentioned there was still what appeared to be flesh on it when he found it. The flesh led him to believe the jaw couldn't have simply been a decoration.
DNR biologists determined that the 9- to 10-inch jawbone belonged to a sand tiger shark, which couldn't live in the river because it needs salt water to survive. After the DNR identified the bones, it returned them to the fisherman.
Sand tiger sharks may look aggressive, with their large bodies around 6 to 10 feet long and jawbones filled with sharp teeth, but are actually docile sharks that only attack humans if provoked, according to National Geographic.
The brownish-gray and spotted creatures tend to stay close to the shore, which is where the "sand" part of their name comes from. They live in warm or temperate ocean waters, and feed on small fish, crustaceans and squid. Sand sharks are the only species of shark known to come to the surface to take a gulp of air -- they keep the air stored in their stomachs, helping them to stay afloat in the water without moving to more effectively catch prey.
The species has a low reproduction rate, and is listed as vulnerable. Sand tiger sharks are protected in many of the areas where they live.
While a sand shark could not survive in the freshwater of the Mississippi River, other species of shark are capable of living in freshwater lakes and estuaries. The bull shark is the only shark that can live in both the ocean and in freshwater, according to Shark Savers.
Fishermen in Maryland were surprised to find themselves catching an 8-foot-long bull shark in the Potomac River in 2015, The Washington Post reports. While seeing a bull shark in the river isn't unheard of, wildlife authorities said the creature was a big catch.
Murphy Brown, 21, said her father and grandfather were "pretty shocked" to find the bull shark in their net.
"My dad said in his whole life -- and he's been fishing since he could walk -- he's never seen anything that big before," Brown said. "It was mind-blowing to be there and see it."
"You hear about other people making a big catch or you see it and you think, 'wow,' but then it happens to you," she added. "It was pretty crazy."