The tail section of the ten-metre long Lower Cretaceous period Iguanodon was handed to curators at Sunderland Museum, who believe the find defies the age of the Tyne and Wear landscape.
"It's really quite a puzzle as to how the bone got there," says Sylvia Humphrey, the Keeper of Geology at Tyne & Wear Archives & Museums.
"Dinosaur bones are younger than the rocks of this area, as this region is on the Permian strata, which is 250 million years old.
"The rocks of this region are far too old for it to have lain here, so it has been lost or dropped by someone in the past.
"We think, although we can never be sure, that it is a piece of vertebrae from an Iguanodon, and may originate from the Wealden [Sussex] area.
"It has similarities to material from the collection of the Natural History Society of Northumbria on display at the Great North Museum: Hancock in Newcastle."
The anonymous finder has loaned the piece to the venue, leaving experts from the Natural History Museum intrigued.
"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," suggests Dr Angela Milner, from the Palaeontology Department.
"The bone is the solid part of vertebra from the tail of an Iguanodon-like dinosaur. It is not complete enough to identify it more precisely."
The Iguanodon was the first dinosaur to be recognised when it was identified by 19th century doctor Gideon Mantell, whose collections are held by the Natural History Society of Northumbria and display at the Great North Museum.
Fossil hunters often find similar bones in the South-East rock formation, which takes its name from the Weald region of Kent and has gained a reputation for dinosaur discoveries.