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The creature left locals in Singapore baffled after the leftovers were discovered on the banks of the MacRitchie Reservoir, Singapore's oldest reservoir.
Karen Lythgoe, who comes from Scotland, documented the discovery. She said: "There were some people already looking at it from the boardwalk, but that was too far away to see what it was.
"We though it was a crocodile from that position, but it didn't look quite right, so we went off the path to get a closer look. It wasn't a crocodile. It was like something you might see in a zoo - it looked prehistoric with its big jaws and teeth.
"I was shocked and intrigued at how it came to be in the reservoir."
Imran Kassim, 27, thought it was another predatory reptile. He said: "Honestly, it flat out looked like an alligator, especially because a portion of it's body was gone - probably eaten by a happy monitor lizard.
"I was pretty shocked to see it as I thought it was an alligator, and that trail is supposed to be safe other than the cheeky monkeys stealing food, and there have never been huge animals there.
"It was pretty damn strange, but it looked enough like a lizard with its jaws wide open, that I would have never guessed it was a fish."
The creature has now been identified by the city's water agency and National Parks Board who said that it was an alligator gar - a fish native to the southern United States, some 10,000 miles away.
Which only adds to the mystery...
The species has been called a 'living fossil' because so much of its biology can be traced back to its earliest ancestors.
As for how the creature ended up in Singapore, authorities can only assume that it was kept as a pet and released once it had grown too large.
According to local media, juvenile alligator gar can be purchased from local fish merchants.
The statement from the city's water agency and National Parks Board said: "We would like to remind everyone that the release of these animals will disrupt our delicate aquatic ecosystem and may also pose a risk to users of our water bodies."
Releasing animals into reservoirs and waterways is punishable with a fine of up to SG$3,000 (£1,600/$2,200) under Singapore law.
The Singapore water agency has now removed the carcass.
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