In 1902 a paleontologist named Barnum Brown found an exciting new fossil.
He found it while he was on a fossil-hunting expedition in Montana, at a site filled with ancient rocks.
Brown was a very well-known and experienced fossil collector who had traveled all over the world hunting for dinosaur fossils.
And he had just discovered the remains of what is now considered the most famous dinosaur that ever lived: the Tyrannosaurus rex.
The new fossil was located in solid rock.
In the early days of paleontology, the methods for excavating, or digging out, a fossil were not very sophisticated.
So Brown had to use dynamite to blast the rock into pieces.
Then these broken rock fragments, which weighed hundreds of kilograms, were shipped to the Museum of Natural History in New York City.
At that time the Museum of Natural History had one of the best dinosaur fossil collections in the world.
Henry Osborn, a museum curator and expert paleontologist there, studied Brown's newly discovered fossils.
Osborn wrote the first scientific paper on the new dinosaur, and he also gave it its name: Tyrannosaurus rex or "tyrant lizard king."
The Tyrannosaurus rex—or "T. rex," as it was soon nicknamed—certainly seemed to deserve its name.
The skeletons Osborn studied revealed a huge and terrifying creature.
In adult form it was 15 meters from head to tail.
It walked on its two back feet and stood almost seven meters high, sometimes weighing over six tons.
With a huge skull, and about sixty long, sharp teeth, the dinosaur could tear off more than two hundred kilograms of flesh in a single bite!
Osborn also realized that the Tyrannosaurus rex had lived during the late Cretaceous period.
The Cretaceous period began 136 million years ago and ended 65 million years ago.
During this time palm trees, ferns, and flowering plants grew, but grasses had yet to evolve.
Huge flying reptiles called pterosaurs flew overhead while frogs and turtles moved along below in the land and water.
The only mammals in existence at the time were very small, similar in size and shape to mice or rats.
In this ancient world, the giant Tyrannosaurus rex was probably truly a lizard king!
Six years after his first discovery, Barnum Brown found another Tyrannosaurus rex at the Montana site.
This second skeleton had more bones than the first one, so it was another exciting and useful discovery for the field of paleontology.
Since Brown's early discoveries, more T. rex skeletons have been found.
In 1990 paleontologist Susan Hendrickson collected a skeleton that was the biggest and most complete skeleton ever.
It was named Sue in Hendrickson's honor, and eventually sold to a Chicago museum for lots of money—about 8.4 million dollars!
In 2000, five more T. rex skeletons were excavated from the Montana site where Barnum Brown had discovered the first ones.
Some of these newer dinosaurs were very well-preserved, adding more information to experts' growing knowledge.
As a clearer picture of the T. rex emerged, scientists learned that it had probably reached its full size at around age twenty, as humans do.
Its front legs were short but quite strong, and its back legs had three toes with claws.
The T. rex probably ate smaller plant-eating dinosaurs, maybe hunting by hiding behind trees, waiting to attack its unsuspecting prey.
With its sharp teeth, the T. rex could easily rip into food.
But the dinosaur probably didn't just hunt for food.
Instead it likely scavenged food as well, stealing it from other dinosaurs, and eating their leftovers.
Bite marks found on the bones of several T. rex suggest that the T. rex may have even dined on other T. rex dinosaurs!
Scientists have debated how quickly the T. rex could move.
They think it probably moved pretty fast—about 16-32 kilometers per hour.
But because of its huge size, the Tyrannosaurus rex was not as fast as some smaller dinosaurs.
Scientists have also carefully examined the T. rex's forearms.
The arms were quite short compared to its massive body, and had two fingers attached.
These short arms might have been long enough for the dinosaur to grab prey.
But they were definitely too short to reach its mouth.
So it's been difficult to figure out exactly how the T. rex used these small limbs.
Today, over one hundred years after Barnum Brown's exciting discoveries in Montana,
paleontologists have more sophisticated tools for the excavation and study of fossils.
But when it comes to the T. rex, they still have many more questions than answers.
Perhaps future fossil excavations will reveal more about this amazing and massive creature—Tyrannosaurus rex, the tyrant lizard king.