The five equid skeletons were preserved by volcanic ash falling on Pompeii
It has baffled experts for years but now a Cambridge University researcher has established the true identity of a skeleton discovered at Pompeii.
Initial research suggested it was an unknown, possibly extinct, horse. It is in fact a type of donkey.
Susan Gurney discovered that horse and donkey DNA had been accidently merged in the original research.
"In hindsight, it's possible to recognise two separate strands of horse and donkey DNA," she explained.
The bones of five horses were discovered in a stable belonging to a rich Roman household at Pompeii.
Six years ago researchers analysed the skeletons, examining the mitochondrial DNA sequences (mtDNA) of each of the horses.
The Pompeii donkey is related to Somali wild asses, like these
To their excitement, one of the beasts seemed to have an exotic and mysterious DNA type they had never seen before.
Had they stumbled across the bones of an unknown horse breed which was now extinct?
Well, the answer was more prosaic.
Susan Gurney works with Dr Peter Forster on horse genetics at the University of Cambridge. She revisited the study and realised the original team had made an error in the laboratory.
Researchers had accidentally combined a donkey mtDNA sequence with that of a horse, and thus created a hybrid which had actually never existed at all.
Somali wild ass
Writing in the Journal of Cellular Biochemistry, Ms Gurney explains that the first 177 nucleotides (molecules which form the structural units of a DNA sequence) match existing patterns for donkeys. The remaining 193 nucleotides match those of an existing breed of horse.
"In addition, the horse DNA that appears to have been inadvertently mixed in with the donkey's genetic information is the same type as that found in another Herculaneum horse, which might be the source of the mistake," she explained.
Herculaneum is a smaller Roman settlement close to Pompeii which was buried during the same volcanic eruption.
However, the finding is still an important one.
The donkey DNA finds its closest match with the DNA of domestic donkeys that are related to the Somali wild ass, typically found in Italy today.
In other European countries, asses are often descended from the Nubian lineage instead.
So the ancient Pompeiian donkey DNA sequence, if confirmed, could represent the origins of that division, and provide valuable evidence that the Somali breed appeared in Italy at least as early as Roman times.