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The only two-headed shark fetus ever recorded was discovered in the Gulf of Mexico in 2011.
Researchers have proved that the first two-headed shark fetus ever recorded was actually a single shark with two heads, rather than conjoined twins.
The two-headed bull shark was found off the coast of Key West in 2011 and is the subject of a new research paper published in the Journal of Fish Biology.
"It is quite rare, but much more rarely observed," said Dr. Michael Wagner, a researcher at Michigan State University and co-author of the study told the NY Daily News.
"There's no way to know what the cause was."
Dr. Wagner described the condition as "axial bifurcation," a developmental abnormality where the embryo begins to split into two different organisms, but does so incompletely.
"Each head has five pairs of gills and gill openings, a single pair of eyes, a single pair of nares and a mouth with well-developed dentition," the study reads.
The strange shark was found when a commercial fisherman captured a pregnant female bull shark from the Gulf of Mexico. The two-headed fetus was found inside along with several normal fetuses, which were released safely.
Even though the fetus was near term, the two-headed shark died shortly after it was detached from the umbilical cord and was preserved at the Florida Keys Community College.
The bull shark was caught in the same body of water as the 2010 BP Deepwater Horizon oil spill, leading to questions about whether contamination or pollutants caused the abnormality.
Dr. Wagner called making that leap "unwarranted" and said there was "no evidence to support that cause or any other.
"It's good that we have this documented as part of the world's natural history, but we'd certainly have to find many more before we could draw any conclusions about what caused this."