Rare malformations can cause animals to grow two heads – and it seems that kind of thing has been happening for millions of years. In 2006, paleontologists in China described a two-headed fossil from the age of dinosaurs .

Hyphalosaurus is a small, aquatic reptile that lived more than 120 million years ago, and is known from thousands of fossils. But one special specimen sports two complete heads and necks.

"Starting from the level of the pectoral girdle, the vertebral column divides into two cervical series, forming two long necks that end in two skulls," the researchers wrote in 2006.

The fossil show a malformation called axial bifurcation, which can occur during development when an embryo begins to split into twins, but doesn't complete the process. While rare, two-headed anima

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