The world's most complete lower jaw bone of an ancient marine predator has been found in outback Queensland.

The 1.6-metre long mandible of a Kronosaurus queenslandicus is thought to be more than 100 million years old.

The Kronosaurus was an 11-metre-long marine reptile with a crocodile-like head and body with powerful flippers.

The fossil was found by grazier Robert Hacon in January while spraying for weeds on his cattle property, Euraba, east of Julia Creek.

The drought had killed the grass, which helped unearth the bone.

At first Mr Hacon did not think anything of the bone and even drove over it.

But after driving just 100 metres, curiosity got the better of him and he went back.

The tip of the Kronosaurus jaw, with a 50c coin inside one of the tooth sockets. ( Supplied: Kronosaurus Korner )

"I was kicking a few stones around with my feet ... I looked over a little bit and I thought 'oh my God what have I got here'," he said.

Mr Hacon did not know exactly what he had found, but knew it was some type of bone.

"I was very much elated because I knew instantly I had found something of great significance," he said.

"It was so perfect. It was just like it had been killed a couple of weeks before and the crows picked it clean."

Most intact fossil of Kronosaurus queenslandicus

Mr Hacon uncovering the mandible of the Kronosaurus. ( Supplied: Kronosaurus Korner )

The fossil has been put out on display this week at the Kronosaurus Korner Museum in Richmond.

Museum Curator Dr Tim Holland said it was an amazing discovery because all the other Kronosaurus jaw bones found had been eroded, damaged or were incomplete.

"It pretty much gives us the first really good, accurate idea of what a Kronosaurus jaw looks like," he said.

"It was the most complete Kronosaurus mandible in the world."

This specimen stands at 1.6 metres long but Dr Holland said a fully grown jaw would reach about 2.6 metres.

Dr Holland is in the process of writing a scientific paper on the fossil and all the new information that it has brought to light.

He said the dinosaur was a daunting creature which could have easily bitten in half any of the animals that it shared its environment with.

"The jaws of the Kronosaurus was approximately twice the power of a large saltwater crocodile and we know from fossilised stomach content associated with other Kronosaurus specimens that the animals ate turtles, sharks and giant squids," he said.

"The front section of the lower jaw has these really amazing long grooves that would accommodate teeth overhanging from the upper jaw.

"This hasn't really been well described before in any of the scientific literature so that is really exciting."