English: A young adult Tyrannosaurus rex skull on display in the ontogeny exhibit at the Museum of the Rockies in Bozeman, Montana.

This fossil was found in 2000 in the Charles M. Russell National Wildlife Refuge in Garfield County, Montana.

Nicknamed "B-Rex" for its discoverer, paleontological preparer Bob Harmon, this fossil is famous for containing partially-fossilized soft tissue in its femur -- allowing scientists to study, for the first time ever, the actual DNA structure of dinosaurs. In 2016, paleontologists announced that they had found medullary bone -- which means the B-Rex was a female, and egg-producing.

Ontogeny means "the study of a species as it changes during its growth and life." Many animals not only grow larger, but change shape and add or lose features as they grow older. When it comes to fossils, it's easy to think that a juvenile is a different species. The study of ontogeny is also important because it helps us understand animals as they grow older, and what diseases, problems, or changes their bodies underwent at adults and in old age.

Tyrannosaurs lived from 68 to 66 million years ago. T. rex was the last of the tyrannosaurids (bipedal predators with large skulls and miniscule forearms). On average, a T. rex was about 40 feet in length, 13 feet tall at the hips, and weighed 10.5 tons.