Mysterious bone circles made from the remains of dozens of mammoths have helped shed light on how ancient communities survived Europe's Ice Age.
Around 70 of these structures are known to exist in Ukraine and the west Russian Plains.
The bones at one site are more than 20,000 years old, new analysis from the University of Cambridge and other institutions suggests. This makes the circle the oldest such structure built by humans discovered in the region.
Researchers say the bones were most likely sourced from animal graveyards, and the circle was then hidden by sediment and is now one foot below current surface level.
The majority of the bones found at the Russian Plains site are from mammoths.
A total of 51 lower jaws and 64 individual mammoth skulls were used to construct the walls of the 30ft by 30ft structure and scattered across its interior.
Scientists also found small numbers of reindeer, horse, bear, wolf, red fox and arctic fox bones.
For the first time, the archaeologists from the University of Exeter found remains of charred wood and other soft non-woody plant remains within the circular structure near the modern village of Kostenki, about 500km south of Moscow.
They say this indicates people were burning wood as well as bones for fuel, and the communities who lived there had learned where to forage for edible plants during the Ice Age.
Kostenki 11 represents a rare example of Palaeolithic hunter-gatherers living on in this harsh environment. One possibility is that the mammoths and humans could have come to the area en masse because it had a natural spring that would have provided unfrozen liquid water throughout the winter - rare in this period of extreme cold.
The last Ice Age, which swept northern Europe between 75-18,000 years ago, reached its coldest and most severe stage at around 23-18,000 years ago.
Most communities left the region, probably because of lack of prey to hunt and plant resources they depended upon for survival, scientists say.
Bone circles were eventually also abandoned as the climate continued to get colder and more inhospitable.
Previously, archaeologists have assumed the circular mammoth bone structures were used as dwellings, but the new study suggests this may not always have been the case.
The research, conducted by academics from the University of Cambridge, University of Exeter, Kostenki State Museum Preserve, University of Colorado Boulder and the University of Southampton, is published in the journal Antiquity.