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(CNN)A creature known as a "living fossil" -- the alligator gar -- has found its way to Kansas for the first time in documented history.
Angler Danny Lee "Butch" Smith caught the 4.5-foot, 39.5-pound (1.37-meter, 17.92-kilogram) fish September 20 on a routine fishing trip in the Neosho River, according to the Kansas Department of Wildlife and Parks. The alligator gar's fossil records date back to nearly 100 million years ago, hence the nickname "living fossil fish."
Smith has seen gar before -- longnose, shortnose and spotted gar are local to Kansas -- but nothing like this one. That's because alligator gar are not native to Kansas waters.
"When it came up out of the water the first time ... I was shocked, I was stunned. I've seen gar jump, but nothing like this one did." Smith said. "This is a once-in-a-lifetime deal, I'm sure."
Unlike other gar typically seen in Kansas, alligator gar have a broader snout and resemble an American alligator. The rare fish can be found from southwestern Ohio and southern Illinois to the Mississippi River drainage basin, and extend south of the Gulf of Mexico, according to the United States Fish & Wildlife Service.
That's one well-traveled gar
How the alligator gar got to Neosho River remains a mystery.
In his 26 years working with the Kansas Department of Wildlife and Parks, regional fisheries supervisor Sean Lynott said this is only his second or third time encountering a species not native to the river.
Introducing non-native species is harmful because they can bring diseases and disrupt the ecosystem. It's also illegal in Kansas to release fish into public waters unless the fish was caught from those waters.
"We don't know where this fish comes from. It's a lot easier to move fish if you move water, so we have that concern as well. Where is that water coming from?" Lynott said.
Alligator gar from conservation programs in other states are tagged, but Lynott said they did not find a tag on this one. Other gar in the US are found primarily around larger river systems, such as the Mississippi River, but Lynott said there aren't connecting waterways for the gar to naturally get to the Neosho River.
Go fishing -- anything can happen
Biologists will work to figure out the fish's origins through other means, such as genetic identification and microchemistry tests. Lynott said their best guess right now is the gar was released by either an aquarium or someone who had the gar as a pet.
"We're pretty confident that this fish would not have ended up in the Neosho River unless it was transported and released by man," Lynott said.
Lynott said the agency has 30 days to determine whether it will acknowledge this finding as a state record. The group has not come to a consensus yet, but the Kansas Department of Wildlife and Parks plans on posting its findings on ksoutdoors.com.
Smith, who's lived in Kansas -- and fished there -- his whole life, called this experience a mystery. But he was back on the water the next day, after replacing his boat's oar, which was damaged by the gar.
"Get out and enjoy the outdoors. ... You can't catch a fish sitting on the couch," Smith said. "Anything can happen if you spend enough time on the water."