The reptile was unearthed in rocks from northeastern China

The abnormal animal, belonging to a group of aquatic reptiles, was unearthed in northeastern China and dates to the time of the dinosaurs.

The specimen reveals that it must have been very young when it died and became fossilised, says lead researcher Eric Buffetaut.

Details of the fossil appear in the UK Royal Society journal Biology Letters.

This animal was a choristoderan, an extinct reptile that reached a length of one metre in adulthood and was characterised by a long neck - two in this case.

The animal's spinal column divided in two at the point where the neck emerges from the body. This formed two long necks that ended in two skulls.

Choristoderans seem to have been common aquatic reptiles during the Cretaceous Period (144 to 65 million years ago) in what is now northeastern China.

"To my knowledge, it is the only record of a vertebrate fossil showing that kind of malformation," Dr Buffetaut, director of research at the Centre National de la Recherche Scientifique (CNRS) in Paris, France told BBC News.

More than 400 two-headed snakes have been recorded

Two-headed snake

"So the chances of finding a fossil like this are extremely low."

The abnormality is known to occur with some frequency in modern reptiles; about 400 cases of two-headed snakes have been recorded in historic times.

It is thought this can occur as a result of injury to an embryo.

The specimen comes from Cretaceous rocks in the Yixian Formation of northeastern China. It is now held at the Shenzhen Museum in southern China.