In the late 9th century, a powerful army of Vikings from across Scandinavia joined forces to achieve a common goal: invade and conquer Anglo-Saxon England. Now, archaeologists think they may have identified the remains of hundreds of these marauding Norsemen, according to a new report published in the journal Antiquity.

In the 1970s, archaeologists discovered a mass grave containing hundreds of skeletons on the grounds of St. Wystan's, a historic Ango-Saxon church in Repton, Derbyshire. Excavations that continued into the 1980s revealed that the mound contained 264 bodies, buried together in what appeared to be a partially leveled Anglo-Saxon chapel. Men comprised 80 percent of the remains, with several exhibiting signs of violent injury. Some graves held Scandinavian-style funerary goods, including a pendant of Thor's hammer and a Viking sword. One contained four children—possibly sacrificial offerings. The researchers also found the vestiges of a large defensive ditch.

© Martin Biddle

© Martin Biddle

The researchers thought the mound was a Viking Great Army burial site; Anglo-Saxon records say Scandinavian combatants wintered in Repton in 873-874 CE, after forcing the local king into exile, and coins found at the site date to the same era.

Radiocarbon dating, however, suggested that some remains were actually from the 7th and 8th centuries CE. This meant that the skeletons would have been buried over the course of several centuries—some of them before the Vikings' arrival. The age of skeletons remained a point of contention among archaeologists for years.

© Martin Biddle

© Martin Biddle

The current study found that those dates were wrong. University of Bristol archaeologist Cat Jarman re-evaluated the skeletons using a new form of carbon dating. She found that the bones did all date back to the late 9th century, contradicting initial tests. This mistake wasn't due to poor research methods, but to the Vikings' fish-heavy diets, she said.

"The previous radiocarbon dates from this site were all affected by something called marine reservoir effects, which is what made them seem too old," Jarman explained in a press statement. "When we eat fish or other marine foods, we incorporate carbon into our bones that is much older than in terrestrial foods. This confuses radiocarbon dates from archaeological bone material, and we need to correct for it by estimating how much seafood each individual ate."

Jarman says that pinpointing the age of the Repton burial mound helps illuminate the history of the earliest Viking raiders, who went on to become part of a considerable Scandinavian settlement in England. "Although these new radiocarbon dates don't prove that these were Viking army members, it now seems very likely," she said. "It also shows how new techniques can be used to reassess and finally solve centuries-old mysteries."