'Full-on berserker mode': Archaeologists solve mystery of medieval soldiers with gruesome square holes in their skull

BERLIN — Archaeologists have finally determined which weapon caused mysterious square holes in the skulls of soldiers who died in the 1361 Battle of Visby.

Close analysis of the bone fractures and cutting-edge motion-capture technology revealed that medieval combatants worked in groups to methodically incapacitate their opponents and then deliver a final, fatal blow with a spiked club.

"We have to understand the complexity of medieval combat; it's not a simple process," Patrick Randolph-Quinney , a biological anthropologist at Uppsala University in Sweden, said in a research presentation Aug.

5 at the 25th European Paleopathology Association meeting in Berlin.

The research has not yet been published in a peer-reviewed journal.

Randolph-Quinney and colleagues studied skeletons from a mass grave related to the Battle of Visby , which was fought in July 1361 on the Swedish island of Gotland.

The Danish king Valdemar Atterdag (also known as Valdemar IV) aimed to plunder the city of Visby, which was a major hub of the Hanseatic League that had become wealthy through its control of trade routes between Western Europe and the Baltic.

Atterdag's soldiers faced off against a similar number of Gotlander defenders, but Atterdag was victorious.

The battle resulted in 1,800 Gotlandic casualties to 300 Danish dead.

The men who died at the Battle of Visby were buried in their armor immediately after the fighting concluded.

Their final resting place — a mass grave close to the battlefield — was first excavated in the early 20th century.

But experts still have questions about what actually happened in the battle and how medieval combat worked.

"The conventional historical narrative suggests that the defenders of Visby were predominantly poorly trained, poorly equipped young boys and elderly men who were no match for the might of the Danish army," Randolph-Quinney told Live Science.

One of the 700-year-old skulls from a combatant in the Battle of Visby that had mysterious square holes. (Image credit: Patrick Randolph-Quinney) To better understand the lives and deaths of the men in the mass grave at Visby, Randolph-Quinney launched a project in collaboration with experts in conflict archaeology, ancient DNA , medieval history, and medieval weapons and armor .