A fishing vessel in the North Sea between the UK, the Netherlands, and Scandinavia found quite the catch last month: the first-ever documented two-headed harbor porpoise.

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The dead conjoined porpoise twins were caught up in the GO9 Onderneming fishing vessel's trawl net on May 30, according to the Natural History Museum of Rotterdam's journal Deinsea.

The Dutch museum said the ship's workers were "astonished" to find that the animal had what appeared to be two heads. They took pictures and then threw it overboard. The crew thought it would be illegal to keep the dead porpoise, so the actual specimen is now lost to the ocean.

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But the photos were sent to marine mammal scientists who wrote about the case last week, saying "conjoined twins in wild mammals are extremely scarce." The photos appeared to show only the second-known case of twinning in this species and the first known case of conjoined twins. "That makes these Siamese twins porpoises extra special," the museum noted.

Overall, whales and dolphins are rarely found in nature as twins. So the conjoined twins -- which in this case meant the porpoises shared one body -- was even more surprising of a find.

"Descriptions of conjoined twins in whales and dolphins are extremely rare," the scientists wrote. "We were aware of only nine published cases."

The twin harbor porpoises found off the coast of the Netherlands likely had a birth defect. Scientists said the marine mammal was likely male and recently birthed, but died shortly thereafter.

The museum called the discovery an "exceptional" find.