Radiometric dating relies on the properties of isotopes.
These are chemical elements, like carbon or uranium, that are identical except for one key feature -- the number of neutrons in their nucleus.
The observation that certain fossils were associated with certain rock strata led early geologists to recognize a geological timescale in the 19th century.
The development of radiometric dating techniques in the early 20th century allowed geologists to determine the numerical or "absolute" age of the various strata and thereby the included fossils.
Birds, at over 10,000 living species, Through the first half of the 20th century, before birds were recognized to be dinosaurs, most of the scientific community believed dinosaurs to have been sluggish and cold-blooded.
Most research conducted since the 1970s, however, has indicated that all dinosaurs were active animals with elevated metabolisms and numerous adaptations for social interaction. Evidence suggests that egg laying and nest building are additional traits shared by all dinosaurs.
While dinosaurs were ancestrally bipedal, many extinct groups included quadrupedal species, and some were able to shift between these stances.
Today's knowledge of fossil ages comes primarily from radiometric dating, also known as radioactive dating.
When paleontologist Mary Schweitzer found soft tissue in a Tyrannosaurus rex fossil, her discovery raised an obvious question -- how the tissue could have survived so long?
The bone was 68 million years old, and conventional wisdom about fossilization is that all soft tissue, from blood to brains, decomposes.
Like other tyrannosaurids, Tyrannosaurus was a bipedal carnivore with a massive skull balanced by a long, heavy tail.
Relative to the large and powerful hindlimbs, Tyrannosaurus forelimbs were small and they retained only two digits. rex have been found in North American rock formations dating to the last three million years of the Cretaceous Period at the end of the Maastrichtian stage, approximately 68.5 to 65.5 million years ago; it was among the last dinosaurs to exist prior to the Cretaceous-Tertiary extinction event. rex have been identified, some of which are nearly complete skeletons.