By Peggy Mihelich | CNN
Originally published on March 8, 2007

HELL CREEK FORMATION, Montana (CNN) -- In a desolate landscape, under a hot sun, college students are on their hands and knees poking and brushing away at the ground. They are here to solve a mystery buried for 65 million years, trapped in rock and layers of sediment. They are dinosaur detectives.

"This dinosaur is stuck in with plants, it's all strewn about. And how it's strewn really tells a story about this dinosaur" said Museum of the Rockies crew chief Nels Peterson.

Peterson, along with research students Gina Sorrentino and Luke Padgett, came to eastern Montana for the summer under the direction of the Museum of the Rockies curator of paleontology, John "Jack" Horner, to find dinosaur bones that help paleontologists understand how diet and behaviors changed over a dinosaur's lifetime.

Horner and his detectives look for multiple specimens of the same dinosaur -- from babies to adults. Over the years he's collected the life history of Triceratops and is working on the life history of the king himself, Tyrannosaurus rex. (Watch and learn more about the king of the dinosaurs -- 3:00 )

"You don't want to set your mind to find too many things because as soon as you set your mind to find something, you're probably going to overlook something," Horner said.

That's an open-minded approach that's helped him find the first dinosaur embryos and the duck-billed plant-eater he named Maiasaura.

Right age rock
Even though dinosaurs lived all over the world, their fossilized remains can only be found in places where the right age rock is exposed at the surface of the ground. In the United States that includes parts of Utah, Wyoming and Montana.

"You wouldn't walk into a wheat field expecting to find a dinosaur," Horner said.

Take Michigan, for example. Even though Michigan had dinosaurs at one time, the right age rock isn't there anymore. The rocks exposed to the ground are 300 million years old -- too old for dinosaurs.

Dinosaurs lived between 235 million years to about 65 million years ago. Fossils are found in places where sand and silt built up around their remains and then turned to rock -- preserving the bones.

Sixty-five million years ago, the Hell Creek Formation was a subtropical paradise where dinosaurs lived and died. Today it's a badland where erosion occurs faster than plants can grow. Strong winds pick the landscape clean, leaving behind bits and pieces of the past.

"You walk around and look for bone sticking out of the ground. It's easy to just find bone laying on the ground out here," Peterson said.

Peterson and his crew usually spend 10 hours a day out in the field doing hard physical labor in an unforgiving environment. Daily temperatures soar over 100 degrees Fahrenheit. The hills are steep and slippery, and there are rattlesnakes lurking about.

Still, nobody complains -- they are all eager volunteers.

Bones vs. rocks
"It's really exciting to find that first corner, or that first little edge of bone," said Sorrentino, a graduate student at Stony Brook University. "It's not like 'Jurassic Park' where they are digging up perfectly articulated skeletons that just died yesterday.

"This was a big river back in the time when the dinosaurs lived here. Their carcasses got washed down the river and they got scavenged, pulled apart. ... If something gets buried immediately or right away, you may get the whole skeleton."

But most of the time they find only pieces. Even so, it's a thrill, Sorrentino said.

And how do you tell a bone from a rock?

"It just takes knowing what bones look like in the first place. Being able to recognize the shapes they make," Sorrentino said. Another way is sound. A light tap on a bone produces a high-pitched sound, whereas a rock will make more of a thud.

Sorrentino said the best bones for identifying a specific dinosaur are the skull and teeth, and it helps if you have an expert eye like Horner's.

Early in the summer, the group found a Triceratops, a horned plant-eating dinosaur.

"We've got a Triceratops horn. There's a Triceratops in there and the frill, the big shield, that it had at the back of their head," Sorrentino said.

Chisels, dental picks and paintbrushes are used to separate the sediment from the bones. It's a delicate process that requires a light touch and lots of patience -- it can take weeks to successfully excavate a site.

The bones are then covered in some dirt and wrapped in plaster bundles called jackets. They are hauled out on foot and put on a truck for transport back to the lab in Bozeman.

Under the microscope
"When it comes right down to it, paleopathology really is like dinosaur CSI," said Montana State University doctoral student Ewan Wolf. Wolf works in the basement of the Museum of the Rockies, X-raying fossils. He looks for signs of breaks that have healed and for signs of disease.

Meanwhile, researchers in a room next door to Wolf painstakingly slice dinosaur bones paper thin for examination.

"Fossil bone, despite being hundreds of millions of years old, is amazingly preserved at the microscopic level," said Museum of the Rockies histology technician Ellen-Therese Lamm.

Looking at a bone from the outside can tell researchers how dinosaurs looked, behaved, and were related to each other. At the microscopic level, they can find out about their growth history, diet and longevity.

In 2003, paleontologist Mary Schweitzer at North Carolina State University cut open one of the T-rex bones retrieved by Horner and found medullary bone. Medullary bone is a thin layer of vessels found in the leg bones of modern-day pregnant birds. Schweitzer compared the leg bone of a female ostrich to the leg bone of Horner's T-rex. Schweitzer concluded the T-rex must have been a pregnant female -- a rare discovery that also helps link birds to dinosaurs.

Most scientists now believe birds evolved from Maniraptors -- meat-eating dinosaurs that lived 150 million years ago. Recent findings of a Dromaeosaur, a small, fast, meat-eating dinosaur (closely related to the Velociraptor) with primitive feathers supports the theory.

The end
Another popular puzzle is why dinosaurs became extinct. Was it the breakup of the continents, the Earth's changing climate or an asteroid strike off the Yucatan peninsula? No one knows for sure, but they were around for 150 million years -- much longer than humans.

Back in the Hell Creek, University of the South geology student Luke Padgett is standing on top of a hill overlooking what was once dinosaur country.

"We're not really any closer in understanding the end," he said.

Padgett scans the horizon for a black layer of sediment known as the K-T boundary which marks the end of the age of dinosaurs.

The K-T boundary contains a high amount of iridium, commonly found in meteorites.

"We know by that there was an asteroid impact at the end of the Cretaceous. At that layer we can see really easily out here with no trees. And there's an interesting pattern. There's dinosaurs below it and no dinosaurs above it."

But Padgett thinks it's not the end of the dinosaurs that matters as much as how cool they were when they were alive and what they can tell us about evolution.