Their dominance continued through the Jurassic and Cretaceous periods and ended when the Cretaceous–Paleogene extinction event led to the extinction of most dinosaur groups 66 million years ago.
The fossil record indicates that birds are modern feathered dinosaurs, Throughout the remainder of this article, the term "dinosaur" is sometimes used generically to refer to the combined group of avian dinosaurs (birds) and non-avian dinosaurs; at other times it is used to refer to the non-avian dinosaurs specifically, while the avian dinosaurs are sometimes simply referred to as "birds".
Like extant organisms, fossils vary in size from microscopic, even single bacterial cells one micrometer in diameter, to gigantic, such as dinosaurs and trees many meters long and weighing many tons.
A fossil normally preserves only a portion of the deceased organism, usually that portion that was partially mineralized during life, such as the bones and teeth of vertebrates, or the chitinous or calcareous exoskeletons of invertebrates.
are the preserved remains or traces of animals, plants, and other organisms from the remote past.
The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous (fossil-containing) rock formations and sedimentary layers (strata) is known as the fossil record.
These are chemical elements, like carbon or uranium, that are identical except for one key feature -- the number of neutrons in their nucleus.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?