Dinosaurs did not all live together
Stegosaurus, Tyrannosaurus and early dinosaurs belong to different chapters of history. The familiar phrase “the age of dinosaurs” covers such a long interval that the distances between its inhabitants are easy to overlook.
Much of December
The earliest widely accepted dinosaurs are known from about 230 million years ago. The extinction at the Cretaceous-Paleogene boundary happened about 66 million years ago. In a model that compresses 4.54 billion years into one year, these points fall approximately on December 13 and December 26. More than a week and a half of the model calendar lies between them.
That interval spans about 164 million real years. It was not one unchanging world. Continents moved, environments changed, and whole groups of organisms appeared and disappeared. The opening marker represents early known dinosaurs; older forms of uncertain classification prevent us from treating it as the exact birth date of the group.
Stegosaurus never met Tyrannosaurus
Stegosaurus lived in the Late Jurassic, around 150 million years ago. Tyrannosaurus rex lived near the end of the Cretaceous, about 68 to 66 million years ago. Using these approximate dates, more than 80 million years separate them. About 66 million years have passed since the end of the interval in which T. rex lived.
T. rex is therefore closer to us in time than to Stegosaurus. This comparison remains true despite reasonable rounding of the dates. It does not mean that humans lived alongside tyrannosaurs. Even the “shorter” interval is enormous beside the roughly 300,000-year history of Homo sapiens.
What actually ended 66 million years ago?
Non-avian dinosaurs became extinct at the end of the Cretaceous. Birds, which are dinosaurs, survived and are still alive today. The simple claim that “dinosaurs went extinct” therefore needs qualification. The timeline marker describes the loss of particular groups during a global crisis, not the disappearance of every dinosaur descendant.
A world in which mammals only appear after that extinction is equally misleading. Earlier mammaliaforms, such as Morganucodon, coexisted with dinosaurs. The precise boundary between mammals and their close relatives depends on classification, so this animal's card does not present one certain date for the origin of all mammals.
Find these events in Earth's year
These dates come from the same records as the interactive timeline. Open an event to explore its position and explanation.
- about December 26Chicxulub impact and extinctionabout 66 million years agoView on timeline
- about December 13Early dinosaursabout 230 million years agoView on timeline
- about December 15Early mammaliaforms: Morganucodonabout 205 million years agoView on timeline
- about December 19The time of Stegosaurusabout 150 million years agoView on timeline
- about December 26 → about December 26The time of Tyrannosaurus rexabout 68–66 million years agoView on timeline
Questions about the scale of time
The interval from the earliest widely accepted dinosaurs about 230 million years ago to the extinction of non-avian dinosaurs about 66 million years ago falls approximately between December 13 and December 26. Birds are dinosaurs and remain alive today.
Yes, in terms of time. Stegosaurus lived about 150 million years ago, while T. rex lived about 68 to 66 million years ago. More than 80 million years separate them, longer than the roughly 66 million years from the end of T. rex's time to the present.