The ancient Iberian practice of severing the heads of the dead, driving a giant nail through their skulls and putting them on display may have been more complex than we thought.
This is the conclusion of an international team of researchers who used isotope analysis to determine whether the Iron Age skulls were of locals (and therefore being venerated in death) or outsiders (and likely nailed up as a display of intimidation and power), as had both been previously argued.
The researchers found that it appears to have been a mixture of the two—hinting at both varied motivations behind the nailed head ritual, as well as providing the first direct evidence of human movement patterns in the Iberian Peninsula during the Iron Age.
"This result suggests that the practice of severed heads was applied in a different way at each site," paper author and archaeologist Rubén de la Fuente-Seoane of the Autonomous University of Barcelona said in a statement. This, he adds, "seems to rule out a homogeneous [uniform] symbolic expression."
In their study, the researchers investigated seven severed and nailed skulls, likely all male, that had been recovered from two Iron Age sites: Puig Castellar and Ullastret.
At Puig Castellar, the skulls were found along the inner face of the wall that surrounded the settlement—suggesting that they were displayed as a demonstration of power to intimidate foes and maintain order through fear within the settlement as well.
Two of the skulls from Ullastret, meanwhile, were found in a street in the middle of the ancient city, suggesting that they may have been displayed on the wall or doorway of adjacent houses, perhaps in celebration of the deceased, the team explained.
The third Ullastret skull, meanwhile, was found in one of the settlement's external walls, suggesting that it could instead represent more of a war trophy.
To determine where each of the skulls had come from, the team first analysed the stable oxygen and strontium isotopes in the dental enamel of each of the specimens.
The ratio of different isotopes preserved in teeth can shine a light on a person's sources of food and water, which vary in isotopic composition from place to place.
Thus by comparing the isotope values from the skulls with reference values for each site—calculated from local sediment and vegetation samples—the team was able to determine which skulls were from local men, and which were from further afield.
"Our premise in approaching the study was that if they were war trophies, they would not come from the sites analysed—while if they were venerated individuals, these would most likely be local," de la Fuente-Seoane explained.
"At Puig Castellar the isotope values of three of the four individuals differs significantly from the local strontium reference, which suggests that they were probably not from the local community.
"In contrast, Ullastret revealed a mixture of local and non-local origins."
While more research is needed to be sure, de la Fuente-Seoane concluded: "It suggests that the selection of individuals for the severed heads ritual was more complex than initially thought."
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Reference
De la Fuente-Seoane, R., López-Onaindia, D., Codina Falgas, F., De Prado, G., Álvarez, C. F., Rovira Hortalà, M. C., Díaz-Zorita Bonilla, M., Nieto-Espinet, A., & Subirà, M. E. (2025). Territorialisation and human mobility during the Iron Age in NE Iberia: An approach through Isotope Analyses of the Severed Heads from Puig Castellar (Barcelona, Spain) and Ullastret (Girona, Spain). Journal of Archaeological Science: Reports, 62. https://doi.org/10.1016/j.jasrep.2025.105035
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