Dinosaur experts have extracted samples of what appear to be soft tissues from a Tyrannosaurus rex fossil bone.
T. rex is perhaps the most famous dino and Montana has yielded excellent fossil specimens
The US researchers tell Science magazine that the organic components resemble cells and fine blood vessels.
In the hotly contested field of dino research, the work will be greeted with acclaim and disbelief in equal measure.
What seems certain is that some fairly remarkable conditions must have existed at the Montana site where the T. rex died, 68 million years ago.
Normally when an animal dies, worms and bugs will quickly eat up anything that is soft.
Then, as the remaining bone material gets buried deeper and deeper in the mud, it gets heated, crushed and replaced by minerals - it is turned to stone.
The form, and nothing else, is all that is left of the original. On the outside, the hindlimb fossil designated MOR (Museum of the Rockies specimen) 1125 has this appearance.
But when Dr Mary Schweitzer, of North Carolina State University, dissolved away the minerals, she found something extraordinary inside.
She discovered transparent, flexible filaments that resemble blood vessels. There were also traces of what look like red blood cells; and others that look like osteocytes, cells that build and maintain bone.
The soft structures move back into position after flexing
"This is fossilised bone in the sense that it's from an extinct animal but it doesn't have a lot of the characteristics of what people would call a fossil," she told the BBC's Science In Action programme.
"It still has places where there are no secondary minerals, and it's not any more dense than modern bone; it's bone more than anything."
Dr Schweitzer is not making any grand claims that these soft traces are the degraded remnants of the original material - only that they give that appearance.
She and other scientists will want to establish if some hitherto unexplained fine-scale process has been at work in MOR 1125, which was pulled from the famous dinosaur rocks of eastern Montana known as the Hell Creek Formation.
"This may not be fossilisation as we know it, of large macrostructures, but fossilisation at a molecular level," commented Dr Matthew Collins, who studies ancient bio-molecules at York University, UK.
"My suspicion is this process has led to the reaction of more resistant molecules with the normal proteins and carbohydrates which make up these cellular structures, and replaced them, so that we have a very tough, resistant, very lipid-rich material - a polymer that would be very difficult to break down and characterise, but which has preserved the structure," he told the BBC.
But if there are fragments, at least, of the original dinosaur molecules, their details could provide new clues to the relationship between T. rex and living species, such as birds.
Inevitably, people will wonder whether the creature's DNA might also be found. But the "life molecule" degrades rapidly over thousand-year timescales, and the chances of a sample surviving from the Cretaceous are not considered seriously.
"I actually don't work with DNA and my lab is not set up to do that," said Dr Schweitzer. "Our goal is more to look to see what we can find with respect to the proteins that are coded by the DNA.
"To a large degree, most of the chemical studies that have been done suggest proteins are more durable than DNA and they have almost the same kind of information because they use DNA as their template."
Dr Collins added: "I would agree that proteins are the molecules to go for - they are the major macromolecules in bone.
"We've got some very interesting research coming out from a number of labs looking at stable isotopes (different forms of the same atom) in bones and clearly information about diets which comes from such isotopes may now be amenable from these dinosaur materials."
However, he cautioned that the great age of MOR 1125 may put such detail beyond the investigating scientists.