The parchment used to make thousands of pocket Bibles in the 13th century was not created from the skin of unborn calves, scientists have confirmed. Instead, they were made from calf, goat or sheep skin.

The mystery of what the uterine vellum – a thin form of parchment used in these Bibles – was made from has long been debated by researchers. Thousands of pocket Bibles were produced in Medieval Europe and the uterine vellum was what made it possible.

Pocket bibles were mostly produced in Paris and Bologna. Their creation probably arose to allow for more widespread preaching and evangelism – having a portable text would have been a big advantage for friars spreading Christianity. It is estimated that around 20,000 of these Bibles were produced.

An international team of scientists led by the University of York analysed the uterine vellum to find out how the parchment was made and what from. "One of the outstanding controversies in the field of codicology concerns the origin and production of so-called 'uterine vellum'," they wrote in the journal PNAS. "Researchers in the field of manuscript studies have long disputed the origin of this ultrafine writing material."

The team said some have suggested the parchment must have been made from the hides of smaller, thinner-skinned mammals such as rabbits or squirrels, while others put forward the idea of calve or lamb foetuses. They said: "Use of the Latin term abortivum in many sources has led some scholars to suggest that the skin of foetal calves or sheep was used. Others have argued that it would not be possible to sustain herds if so many pocket Bibles were produced from foetal skins, arguing instead for unexpected alternatives, such as rabbit."

To find out, Sarah Fiddyment and her colleagues analysed the parchment by extracting collagen from it. Mass spectrometry allowed them to establish the source of the species it came from. In total, they analysed 220 samples from 72 13th century pocket Bibles.

All were found to have come from calf, goat or sheep – with no evidence of rabbit or squirrel. Geographic patterns suggested producers would make the parchment from whatever was locally available. The team concluded: "These results suggest that ultrafine vellum does not necessarily derive from the use of abortive or newborn animals with ultrathin hides, but could equally well reflect a production process that allowed the skins of maturing animals of several species to be rendered into vellum of equal quality and fineness."