Experts are puzzled as to how a dinosaur bone thought to belong to a 10 metre long Iguanodon ended up in a pensioner's back garden.
The Sunderland man found the football-sized vertebra while digging among tree roots and took it to his local museum.
Staff there believed the bone came from a dinosaur and that has since been confirmed by experts at the National History Museum. But Sunderland stands on rocks that are far older than the dinosaur, which means the bone must have been deposited there later on.
Many of the Iguanodons unearthed in the UK were found in the East Sussex area, 300 miles south of Sunderland. They lived 115-130 million years ago. Experts will never know how the bone made the journey, but glacial disturbance or human interference were considered most likely.
Jo Cunningham, manager of Sunderland Museums, said: "We're very grateful to our museum visitor for bringing this amazing find in to us; it will always remain a mystery as to how it found its way there, and if they hadn't been digging up their garden it could have lain undiscovered.
"The person who found it wishes to remain anonymous, but has kindly agreed to loan it to Sunderland Museum & Winter Gardens so that the people of the region can enjoy this unusual find."
Sylvia Humphrey, keeper of geology at Tyne & Wear Archives & Museums, was there when the pensioner took it to Sunderland Museum & Winter Gardens.
She said: "It's really quite a puzzle as to how the bone got there. Dinosaur bones are younger than the rocks of this area, as this region is on the Permian strata, which is 250 million years old."
Dr Angela Milner, from the Palaeontology Department at the Natural History Museum, London, said: "The bone is the solid part (the centrum) of vertebra from the tail of an Iguanodon-like dinosaur. It is not complete enough to identify it more precisely.
"The rocks around Sunderland are much too old to contain dinosaur bones so there are only two explanations as to how it got there - either by glacial transport or a one-time souvenir from the south coast of England where Iguanodon bones are not infrequently found by fossil hunters."