They were fearsome warriors who cut off the heads of their enemies and displayed them for all to see, bringing them back from battle hanging around their horses’ necks. But now research has confirmed that the Gauls did not merely sever the heads of their foes, they appear to have embalmed them to boot.

Experts say they have found traces of conifer resins on the remains of skulls discovered at the iron age settlement of Le Cailar in the south of France – a discovery they say backs up ancient reports that the Celtic Gauls preserved their grisly trophies.

“In fact the ancient texts told about us the head [being] embalmed with cedar oil … thanks to our chemical analysis we know that this information is right,” said Réjane Roure, co-author of the study from Paul Valéry University of Montpellier.

Previous finds at other sites have included a sculpture of a mounted warrior, not only with sword and spear but a head slung around the neck of the horse, while the gruesome practice is also noted in a number of ancient texts, and supported by discoveries of human skulls with marks of decapitation, and even nails inside them.

However, whether the Gauls did indeed embalm the severed heads was less clear.

Writing in the Journal of Archaeological Science, Roure and colleagues describe how they analysed human skulls found with weapons in an area of Le Cailar where they would have been widely visible – suggesting they would have been on display. The team took samples from 11 human skull fragments, noting many of the skulls showed cut marks of decapitation and signs that hint at the removal of the brain. They also tested five bones from animals found in the same area.

The analysis revealed traces of a host of substances on the human fragments, including fatty acids and cholesterol, much of which the team say are characteristic of degraded human, plant or animal fats. The animal bones also showed traces of cholesterol.

However the team found that six of the eleven human skull fragments bore traces of substances called diterpenoids – telltale signs that the bones had been in contact with conifer resin. Such traces were not found on the animal bones.

The researchers say the findings add weight to ancient reports that, after severing the heads of their enemies, Celtic tribes embalmed them – pointing to ancient Greek writers Strabo and Diodorus of Sicily who both record that a Greek called Poseidonios claimed to have seen such sights in Gaul. While these texts note that cedar oil was used, the team say it might have been a resin with a similar smell, as it is not clear if cedar trees were growing in the area in the third century BC.

Roure said the purpose of the preservation might have been to make sure the face and features of the enemy remained on show. “The ancient texts said only the most powerful enemies and the most important enemies were embalmed – maybe that was to be able to say ‘see that face, it was some big warrior’,” she said. She added the texts also said that the Gauls never gave back such heads “even for an equal weight of gold”. “We think that means sometimes some people tried to buy the heads,” said Roure.

The authors say it is not clear exactly how the embalming process was carried out, but that the heads might have been dipped in the resin, or it could have been poured over them, and might have been applied more than once as time went on. It is also unclear whether the process was also carried out on revered locals, or was reserved solely for enemies.

Dr Rachel Pope, an expert in European prehistory at the University of Liverpool who was not involved in the study, said the research was exciting.

“We knew from statues that the display of human heads was popular in Mediterranean France – akin to a broader tradition at this time involving the display of weapons. The evidence now, from this site, is that human heads were indeed embalmed,” she said. “Now we have the science that supports earlier archaeology, as well as a greater understanding of where the classical texts and the archaeology meet.”