Back in May, a group of Dutch fishermen trawling the North Sea noticed that a baby porpoise had been caught up in one of their nets. When they dragged it out of the water, they realized it was dead. But there was something very odd about the little creature. As Sarah Gibbens reports for National Geographic, the deceased porpoise had two heads.

Worried that keeping a marine mammal would be illegal, the fishermen chucked the porpoise back into the water. But they snapped a series of photos first, and alerted researchers to their unusual find. The two-headed porpoise, which is in fact a set of conjoined twins, was described in a recent paper published in the Online Journal of the Natural History Museum Rotterdam.

Based on the photographs, the authors of the study were able to conclude that the twin porpoises were male, and that they had only recently entered the world when they met an untimely demise. Their dorsal fins were not yet erect, their umbilical opening had not closed, and both heads had hair on the rostrums, or beaks—all tell-tale features of newborns.

As Thia Gose points out in Live Science, the porpoise twins had two fully-formed heads, two pectoral fins, a single genital opening, and one fully-formed body. Based on the imagery, the researchers think that the little guys were symmetrically conjoined, a phenomenon that is believed to occur when two separate embryos fuse together, or a single embryo does not split completely.

Twins are rare for porpoises and cetaceans—and conjoined twins are even more rare. Erwin Kompanje, mammal curator at the Natural History Museum and one of the authors of the paper, told Gose that adult females simply are not large enough to carry more than one fetus. Conjoined twins are an even more exceptional occurrence. The precise number is not known, but according to the authors of the study, only nine other cases of conjoined cetacean twins have ever been reliably documented. Most were fetuses found during the dissection of pregnant females.

Though the recently discovered conjoined porpoises made it out of the womb, they likely died soon after birth because their tail did not stiffen, making them unable to swim, Kompanje told Georgina Hines of New Scientist. 

Researchers were able to glean quite a bit of information about the porpoise twins from the fishermen’s photos. But because the infants were tossed back into sea, experts were unable to subject the rare creature to extensive tests. “The specimen,” the authors of the study write, “is lost for science and natural history.”