A Nova Scotia man out for a casual beach stroll has stumbled upon what's being described as one of the most significant fossil discoveries in the province's history.

Patrick Keating and his dog, Kitty, were walking along the Northumberland Shore in early July when they came across a rib cage, backbone and the partial sail belonging to an extinct, sail-back reptile — the first discovery of its kind in Nova Scotia.

"It wasn't fully exposed, but five or six of the ribs, which are sort of white in colour, were exposed through the mud in the bank of the shore and they caught my eye as I glanced around," Keating said Thursday at the unveiling of the fossils at the Museum of Natural History in Halifax.

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"When I detached it from the mud, I realized it was almost like a carcass-shaped, fossilized skeleton. It was exciting to me."

Mr. Keating took the fossil home, cleaned it up and showed it to a few people. Since his brother's children are younger, Mr. Keating suggested his sibling take the kids and the discovery to the museum to find out what it was.

"They were virtually beside themselves, calling it an extraordinarily significant find," Mr. Keating said of museum staff.

A family visit to the same spot a week later yielded another rare find: the skull from the creature, which experts believe lived some 290 million to 305 million years ago.

The discovery will be on display at the museum for two weeks starting Saturday.

Deborah Skilliter, the museum's curator of geology, said the fossils have opened a window into the ancient world and staff will now work to learn more about the reptile's life and death.

She estimated the Keatings discovered about 45 per cent of the reptile's fossilized remains.

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"This one we think is a juvenile, a baby, so about two to three feet in length, tip to tail, and 30 to 40 pounds," she said.

"But some of these guys could grow up to 4.7 metres in length and up to 300 pounds. So they were the top predators of their time. Some were meat eaters, some were vegetarians.

"We have to do some preparation work to reveal more of the teeth, but right now it looks like this little guy was a carnivore."

The provincial government said paleontologists knew the reptiles existed in the area because footprints were found in 1994 in Nova Scotia's Colchester County.

A spokesman for the Heritage Department said the exact location of Keating's fossil find isn't being disclosed in order to protect the site and any future discoveries.

The government said a rare, fossilized dragonfly wing has already been discovered at the site since the other fossils were found.

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Nova Scotia is renowned for its rich fossil deposits.

The province's fossil cliffs at Joggins, where tidal action continually uncovers fossilized plants and trees, is a designated UNESCO World Heritage Site.