Up to 1.7 billion T. rex dinosaurs lived on Earth, a new study found. But scientists aren't sure where all the bones went.

• New calculations suggest that 1.7 billion T. rexes lived on Earth from 65.5-68 million years ago.
• Scientists have uncovered fewer than 100 fossilized T. rexes, so there may be more to discover.

A new study estimates 1.7 billion Tyrannosaurus rexes roamed the earth between 66 million and 68 million years ago, but scientists have recovered fewer than 100 of them.

So the big question is: Where are all the dino bones?

This dichotomy between how many T. rexes lived and how few fossils we have of them shows us just how rare fossilization is and how much more we have to learn about these majestic creatures.

How scientists count T. rex's total population

It turns out, estimating the total number of T. rexes requires some complicated calculations, Charles Marshall, a paleontologist at the University of California, Berkeley, told Insider.

And scientists have yet to settle on a concrete number.

Marshall was the lead author of an earlier study that estimated 2.5 billion T. rexes once roamed Earth. That was about 32% higher than the latest estimate.

But the author of the new study and evolutionary ecologist at the Johannes Gutenberg University of Mainz, Eva Griebeler, saw an opportunity to improve upon the model that Marshall and his team built.

Marshall and his team created a model factoring in several different variables, including T. rex population density, average body mass index, average life span, geographic range, age of sexual maturity, number of eggs laid, survival rates, and generation time.

In the new study, Griebeler said she "applied a much more realistic survivorship curve" to bring the total number of T. rexes down to 1.7 billion.

"It looks like what she's done is very clever and a significant advance beyond the data that we use in our study," Marshall said. "I've got no complaints. It looks like a better set of survivorship curves than we used."

It's impossible to know for sure exactly how many T. rexes lived during their roughly 2 million years on Earth, but Marshall's model and Griebeler's research provide a reasonable estimate other scientists can build on.

Where are all the T. rex bones?

Of the roughly 1.7 billion, or so, T. rexes who roamed our planet, scientists have only uncovered a few hundred fossils, equating to fewer than 100 total dinosaurs.

And only about 30 of them are anything close to complete skeletons, George Stanley, a retired paleontologist and professor emeritus at the University of Montana, told Insider.

"If you discover a T-rex tooth during an excavation, that's a big deal for your day or week's work," Stanley said.

Paleontologists are more likely to find fragments of isolated bones, teeth, and claws than a perfectly preserved skeleton of a T. rex or any other dinosaur. And even then, finding any kind of fossil is rare.

That's because the conditions have to be just right for fossilization to occur in the first place.

To become a fossil, the body of an animal must be buried, and in most cases, animal remains are consumed or destroyed by other animals shortly after death, Richard Holtz, a vertebrate paleontologist and principal lecturer at the University of Maryland, told Insider.

"Only the smallest fraction of individuals make it to become a fossil, and only a small fraction of those are discovered," Holtz said.

And even if the remains aren't eaten or carried off by other animals, fossilization is also heavily dependent on the environment.

A dinosaur living in the mountains or the open plains, for example, is much less likely to become a fossil because burial is more difficult in those areas than in the lowlands where sediment from the mountains runs off, Holtz said.

"Only individuals that lived and died in the right spots are likely to be covered over by sand or silt or clay and thus possibly preserved," Holtz said.

Scientists actually have more T. rex fossils than many other dinosaurs

T. rexes lived in the lowland floodplains with plenty of sediment for burial, which is why scientists actually have more fossils of T. rex and know more about them than many other dinosaurs, Holtz said.

Despite their prime conditions for fossilization, if Giebeler's calculations are correct, scientists have only found about 0.0000002% of the T. rex that lived on Earth.