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Lily Wilder spotted the print at Bendricks Bay in Barry, the Vale of Glamorgan, earlier this month when she'd been out walking with her father.
It is just over 10cm long and thought to be from a dinosaur with a height of 75cm - although it is impossible to tell the type of creature that left it.
Lily's mum Sally said in a statement: "It was Lily and Richard (her father) who discovered the footprint," said mum Sally.
"Lily saw it when they were walking along and said 'daddy look'. When Richard came home and showed me the photograph, I thought it looked amazing.
"Richard thought it was too good to be true. I was put in touch with experts who took it from there."
National Museum Wales palaeontology curator Cindy Wells said of Lily's Wilder's discovery: "This fossilised dinosaur footprint from 220 million years ago is one of the best-preserved examples from anywhere in the UK and will really aid palaeontologists to get a better idea about how these early dinosaurs walked. Its acquisition by the museum is mainly thanks to Lily and her family who first spotted it.
"During the Covid pandemic scientists from Amgueddfa Cymru have been highlighting the importance of nature on people's doorstep and this is a perfect example of this. Obviously, we don't all have dinosaur footprints on our doorstep but there is wealth of nature local to you if you take the time to really look close enough."
The Bendricks is a stretch of coastline that lies between Barry and Sully in the Vale of Glamorgan, and is regarded an important paleontologist site and a site of special scientific interest.
The south Wales group of the Geologists' Assocation has said it is 'the best site in Britain for dinosaur tracks of the Triassic Period'.
The footprint that Lily found is 220 million years old, having been preserved in mud.
According to the BBC, the dinosaur that left it has been described as a 'slender animal', which would have walked on its two hind legs and actively hunted other small animals or insects.
Similar prints found over in the United States are known to have been made by a dinosaur known as the Coelophysis.
National Museum Wales said in a statement: "Its spectacular preservation may help scientists establish more about the actual structure of their feet as the preservation is clear enough to show individual pads and even claw impressions."
Ben Evans, from British Institute for Geological Conservation, added: "Working together with Amgueddfa Cymru - National Museum Wales has allowed this amazing specimen to be safely recovered and re-homed alongside other specimens from this site.
"This beach is a site of special scientific interest and while we encourage people to visit and use it responsibly, collection of rocks, minerals and fossils from this site is not permitted. The BIGC, Amgueddfa Cymru or Natural Resources Wales should be contacted in the event of a new discovery."
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