Just north of Cambridge, England, as archaeologists excavated the site of a planned highway, they expected to make some unique discoveries. But one particular find left them stumped: more than 8,000 frog and toad bones, all buried together in a ditch.
In total, the Observer’s Dalya Alberge reports that the bones came from roughly 350 amphibians, the majority of which were members of common frog species. Researchers from the Museum of London Archaeology (MOLA) found the massive grave in a 46-foot-long ditch beside an Iron Age roundhouse at Bar Hill, where a settlement stood during the middle and late Iron Age (400 B.C.E. to 43 C.E.), per BBC News.
“In my experience, mainly working on sites from London, we don’t get that many frogs,” Vicki Ewens, MOLA’s senior archaeozoologist, tells the Observer. “To have so many bones coming from one ditch is extraordinary.”
What were the frogs doing in the ditch? The archaeologists are unsure but say the animals likely weren’t eaten, as the bones bear no charring or cuts. It’s possible they were boiled, which wouldn’t have left any marks.
A more likely scenario is that the frogs chose to gather in this spot. Nearby, the researchers found evidence that people in the area were processing crops, which would have drawn beetles and aphids—choice snacks for frogs.
Even if the frogs were lured to this spot, that doesn’t explain why they died there. One possibility is that they fell into the ditch and got stuck, the scholars say. Another is that the frogs all froze during the winter. Land-based frogs typically burrow underground below the frost line during the winter to hibernate. While the cold-blooded critters can endure low temperatures, particularly harsh winters can make the ground cold enough to kill them.
Another possible explanation is that the frogs died of a disease like ranavirus, which wiped out a large number of frogs in England in the 1980s, per the Observer. The highly infectious ranavirus, which can survive outside a host body for more than two months, causes internal bleeding that kills adult amphibians within days or weeks.
“This is a puzzling and unexpected find, which we are still trying to fully understand,” says Ewens in a statement. “This accumulation of frog remains may have been caused by a number of different factors, possibly interacting over a long period of time. We just aren’t sure yet what these were.”
Also unclear is whether frogs played a special role in European Iron Age societies. Many other ancient cultures held them in high regard. The Mesopotamians and ancient Egyptians associated frogs with fertility, water and renewal, and the Egyptians gave their fertility goddess, Heqet, the head of a frog. The Aztecs similarly viewed toads as a source of fertility, in the form of the earth mother goddess Tlaltecuhtli. The ancient Chinese ascribed symbolism to both frogs and toads in a number of ways.
In a 2018 blog post for University College London, archaeologist Hannah B. Page explained that the connection between frogs and fertility stems from “the importance ... past civilizations gave to the rivers that flowed through their lands,” from the Nile to the Tigris and Euphrates.
Page added, “[T]he frog, as a watery symbol of the life-giving waters, was then depicted in reliefs, sculpture[s] and objects.”
The frog bones were one of many finds made at Bar Hill between 2016 and 2018, when archaeologists completed some 40 excavations spanning 578 acres—all part of a massive highway expansion plan. Researchers are still in the process of analyzing their finds, including the frog remains.
As they learn more about the area, according to the statement, the team hopes that “this new knowledge will shed light on the reasons behind the death of so many amphibians. Until then, this will remain a prehistoric frog mystery.”