Scientists say they have made a surprising discovery – a fossil bone of an extinct turtle species scientifically known as Atlantochelys mortoni.

Atlantochelys mortoni lived in what is now North America during the upper Cretaceous period, about 75 million years ago.

The prehistoric creature was about 3 m (10 feet) from tip to tail, making it one of the largest sea turtles ever known. It may have resembled modern loggerhead turtles, but was much larger than any sea turtle species alive today.

For more than 160 years, only a partial humerus of Atlantochelys mortoni was known, but the second part of the same bone was discovered in 2012 by amateur paleontologist Gregory Harpel.

A team of paleontologists headed by Dr Ted Daeschler of Drexel University immediately recognized the fossil as a humerus – the large upper arm bone – from a turtle, but its shaft was broken so that only the distal end, or end nearest to the elbow, remained.

They also thought the fossil looked extremely familiar. They joked that perhaps it was the missing half of a different large, partial turtle limb housed in the collections at the Academy of Natural Sciences of Drexel University. That bone also had a broken shaft, but only its proximal end, nearest to the shoulder, remained. The coincidence was striking.

“I didn’t think there was any chance in the world they would actually fit,” said team member Dr Jason Schein from the New Jersey State Museum.

There was no reason to think a lost half of the same old bone would survive, intact and exposed, in a New Jersey streambed from at least the time of the old bone’s first scientific description in 1849, until Harpel found it in 2012.

The older bone was also without a match of any kind, making a perfect match seem even more farfetched.

It was originally named and described by famed 19th-century naturalist Louis Agassiz as the first, or type specimen, of its genus and species.

In the intervening years, it remained the only known fossil specimen from that genus and species.

“Sure enough, you have two halves of the same bone, the same individual of this giant sea turtle. One half was collected at least 162 years before the other half,” Dr Daeschler said.

Now, the scientists are revising their conventional wisdom to say that, sometimes, exposed fossils can survive longer than previously thought.

“The astounding confluence of events that had to have happened for this to be true is just unbelievable, and probably completely unprecedented in paleontology,” Dr Schein said.

The fully assembled humerus of Atlantochelys mortoni now gives the scientists more information about the massive sea turtle it came from as well.

The scientists believe that the entire unbroken bone was originally embedded in sediment during the Cretaceous period, 70 to 75 million years ago, when the turtle lived. Then those sediments eroded and the bone fractured millions of years later during the Pleistocene or Holocene, before the bone pieces became embedded in sediments and protected from further deterioration for perhaps a few thousand more years until their discovery.

The description of the newly discovered fossil bone of Atlantochelys mortoni appears in the Proceedings of the Academy of Natural Sciences of Philadelphia.

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Daeschler T et al. ‘Humerus of Atlantochelys mortoni.’ Proceedings of the Academy of Natural Sciences of Philadelphia, published April 2014