A miniature two-headed creature from the age of the dinosaurs has been discovered in China, astonishing scientists who never imagined that so rare a mutant could be preserved for 100 million years.

The skeleton of a young aquatic reptile, known as Sinohydrosaurus lingyuanensis, is the first fossil ever unearthed which has two heads and necks, showing that a developmental abnormality seen in modern snakes, turtles, lizards and crocodiles also afflicted their ancient forebears.

The find is remarkable because it is extremely rare for any dead animal to be preserved as a fossil: the vast majority decompose without leaving a trace of their existence.

For a specimen as rare as a mutant with two heads to survive defies the laws of probability, scientists said.

The two-headed creature measures only 7 centimetres (2.75in) from heads to tail, indicating that it must have died in infancy or while still a foetus. Had it lived to adulthood, it would have grown to 1 metre (3.3ft) in length.

The extraordinary creature seems to have suffered from a malformation known as axial bifurcation, which leads to the development of a duplicate head and neck on a single body.

It is well-known among modern reptiles and was first reported by Aristotle. About 400 cases of two-headed snakes are known to science and it has also been noted in crocodiles, lizards and turtles.

Two-headed reptiles rarely survive in the wild — the heads have independent brains and tend to fight over food — but snakes and turtles with the condition have been known to live for several years in captivity.

One of the most famous examples is a two-headed albino black rat snake, known as “We”, which is kept at the City Museum in St Louis, Missouri. We was born in 1999 and put up for sale by the museum, on eBay, earlier this year. It attracted no worthwhile bidders. The snake is now sponsored by a Florida biotech company that uses snake venom to develop new drugs.

A two-headed turtle has also survived at the Natural History Museum, Geneva, for 10 years. Similar two-headed creatures are thought to have inspired many of the monsters of ancient legend.

The Hydra was a water serpent with nine heads, each of which would grow back if cut off. It was eventually killed by Hercules during one of his twelve labours. Cerberus was a three-headed dog that guarded the entrance to the Underworld.

The fossilised two-headed reptile was discovered in the Yixian formation of northeastern China, which has yielded many of the most remarkable dinosaur finds of recent years.

Scientists have ruled out any possibility that the specimen is a fake and details of the discovery are published today in the journal Biology Letters.

“This two-headed reptile seems to be unique in the fossil record,” said Eric Buffetaut of the French National Centre for Scientific Research, who led the study team.

“This simply isn’t something you would expect to find. It is rare enough for anything to fossilise, and cases like this are unusual even among modern animals. The chances of finding a fossil like this are almost impossibly small.”

Mutant Myths

# Cerberus The mythological three-headed dog that guarded the entrance to Hades, the underworld, in Greek and Roman myth. Subdued and captured by Hercules as one of his 12 labours

# The Hydra Water beast with many serpent heads. When one was cut off, two would grow back. Killed by Hercules, who solved the problem by burning each neck stump after severing each head

# Maya two-headed monster Double-headed dragon used often in Maya art and culture. It is thought to signify the power of the Earth and natural disasters

# Ghidorah or Ghidrah Three-headed monster from Japanese film, defeated by Godzilla

# Fluffy A modern variation on Cerberus: a three-headed dog kept by Hagrid to guard the Philosopher’s Stone in the first Harry Potter novel and film