The extinction at the end of the Cretaceous Period, about 66 million years ago, wiped out all non-avian dinosaurs and many other forms of life. Yet dinosaurs did not completely disappear, because one branch had already evolved into birds. This makes modern birds living dinosaurs, not just dinosaur relatives.
Paleontology helps scientists reconstruct this story using fossils, rocks, anatomy, and evolutionary evidence.
Key Facts
- The end-Cretaceous mass extinction occurred about 66 million years ago.
- Modern birds are avian dinosaurs, descended from theropod dinosaurs.
- Non-avian dinosaurs went extinct, but the avian dinosaur lineage survived.
- The Chicxulub impact crater is about 180 km wide and is linked to the extinction event.
- Relative age can be found by rock layers: older layers are usually below younger layers.
- Percent survival = surviving species / original species x 100
Vocabulary
- Non-avian dinosaur
- A dinosaur that is not part of the bird lineage and went extinct at the end of the Cretaceous Period.
- Avian dinosaur
- A member of the dinosaur group that includes modern birds and their extinct birdlike ancestors.
- Theropod
- A mostly meat-eating group of bipedal dinosaurs that includes Tyrannosaurus, Velociraptor, and the ancestors of birds.
- Mass extinction
- A rapid global event in which a large percentage of Earth's species die out over a relatively short geological time.
- Fossil evidence
- Preserved remains, traces, or chemical signs of past life that scientists use to study ancient organisms.
Common Mistakes to Avoid
- Saying all dinosaurs went extinct is wrong because birds are living avian dinosaurs that survived the end-Cretaceous extinction.
- Calling pterosaurs dinosaurs is wrong because pterosaurs were flying reptiles from a separate evolutionary group.
- Assuming feathers evolved only for flight is wrong because many feathered dinosaurs likely used feathers for warmth, display, camouflage, or brooding before powered flight evolved.
- Treating one fossil as complete proof is wrong because paleontologists build conclusions from many lines of evidence, including anatomy, rock age, footprints, eggs, and chemical clues.
Practice Questions
- 1 The Chicxulub crater is about 180 km wide. If a map scale is 1 cm = 30 km, how many centimeters wide should the crater be drawn on the map?
- 2 A fossil bed contains 48 identified species before an extinction layer and 12 species above it. What percent of the original species survived?
- 3 Explain why modern birds can be called dinosaurs even though they look very different from animals like Triceratops or Tyrannosaurus.